Reliable Design of Ammonia and Urea Plants

2012 AMMONIA TECHNICAL MANUAL Paper 4b Page 1 of 16

Reliable Design of Ammonia and Urea Plants

During the design of ammonia and urea plants many aspects have to be taken into account besides

process performances. The plant reliability is a factor as important as process performances in determining the future plant profitability. To design a reliable plant it is necessary to take into

account many aspects, such as the process mechanical design of all parts, instrumentation, process control, and the plant layout. It is also essential to have a good quality control in every phase of the

project. This paper describes the design choices that are crucial in order to reach the highest standard of reliability in these plants.

Pasquale Talarico

CASALE GROUP – Switzerland

Andrea Scotto CASALE GROUP – Switzerland

Introduction

s Reliability is a broad scope, understanding what it is, comes first. Exploring the literature, it is possible to find many definitions of reliability, like:

• The ability of a system or component to perform its required functions under stated conditions for a specified period of time.

• The idea that something is fit for a purpose with respect to time;

• The capacity of a device or system to perform as designed;

• The resistance to faults or failures of a device or system;

• The ability of a device or system to perform a required function under stated conditions for a specified period of time;

• The probability that a functional unit will

perform its required function for a specified interval under stated conditions.

• The ability of something to "fail well" (to fail without catastrophic consequences and is restorable in a reasonable period of time).

In our opinion the more accurate statement is: "The high reliability of plants is a choice and not an accident of fortune". In fact, once a plant is designed and built there is very little that can be done to reduce operating costs, and reliability is a significant part of them, because they are substantially established during the plant's design. If low operating costs is the target, this chart makes it clear that they are designed into the plant and

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2012 AMMONIA TECHNICAL MANUAL Paper

equipment during feasibility, design and construction phases. A part from general statements and standard activities which are usplantetc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a process licensor. Therefore the paper describeapproach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type of plan As an example, it is well known to the majority of the specialists of the fertilizer sector that corrosion phenomena are of paramount importance for the reliability high pressure Urea plants but in case a low pressure carbonate pumps fails the overallwell with the same result of losing the overall production. On ammonia plant, that is a sequence of different unit, it is important for the reliability that all of them, also the sections or equipment generally undervalued like process condensate pumps, turbine condensate pumps, medium


equipment during feasibility, design and construction phases. A part from general statements and standard activities which are usplant's design phase like Hetc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a process licensor. Therefore the paper describeapproach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type of plants. As an example, it is well known to the majority of the specialists of the fertilizer sector that corrosion phenomena are of paramount importance for the reliability high pressure Urea plants but in case a low pressure carbonate pumps fails the overallwell with the same result of losing the overall production. On ammonia plant, that is a sequence of different unit, it is important for the reliability that all of them, also the sections or equipment generally undervalued like process condensate pumps, turbine condensate pumps, medium


equipment during feasibility, design and construction phases.

A part from general statements and standard activities which are us

s design phase like Hetc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a process licensor.

Therefore the paper describeapproach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type

ts.

As an example, it is well known to the majority of the specialists of the fertilizer sector that corrosion phenomena are of paramount importance for the reliability high pressure Urea plants but in case a low pressure carbonate pumps fails the overallwell with the same result of losing the overall production.

On ammonia plant, that is a sequence of different unit, it is important for the reliability that all of them, also the sections or equipment generally undervalued like process condensate pumps, turbine condensate pumps, medium


equipment during feasibility, design and construction phases.

A part from general statements and standard activities which are usually performed during a

s design phase like Hetc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a

Therefore the paper describeapproach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type

As an example, it is well known to the majority of the specialists of the fertilizer sector that corrosion phenomena are of paramount importance for the reliability high pressure Urea plants but in case a low pressure carbonate pumps fails the overall urea plant may stop as well with the same result of losing the overall



equipment during feasibility, design and

A part from general statements and standard ually performed during a

s design phase like HAZOPetc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a

Therefore the paper describes the reliable design approach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type

As an example, it is well known to the majority of the specialists of the fertilizer sector that corrosion phenomena are of paramount importance for the reliability high pressure Urea plants but in case a low pressure carbonate

urea plant may stop as well with the same result of losing the overall





AZOP, SIL analysis etc., this paper has the objective to provide additional and unusual approach to plantreliability as it may be seen from the eyes of a

s the reliable design approach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type




2012 AMMONIA TECHNICAL MANUAL Paper 4b



, SIL analysis etc., this paper has the objective to provide additional and unusual approach to plant's reliability as it may be seen from the eyes of a

s the reliable design approach for Ammonia and Urea plants that should be undertaken by the various engineering disciplines through the illustration of some specific examples which are out of the well know and recognized critical area of these type




pressure low temperature exchcompressor seal gas system is according to recognize state of the art because their fault result anyway in undesired plant shut Having this approach in mind, the paper would provide some specific examples related to the more significant involved in the design of Ammonia and Urea plants. In particular it will be illustrated the process design as well as the mechanical, machinery, instrument and piping design. 1. The process design is having for the operating reliability of the plants: as an example the proper selection of operating conditions, the control system and the material of construction is a paramount step in the further development of the project. Furthermore the by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In factfeature, under certain conditions, can prevent the plant to trip in case of human error or instrumentation failure. Entering more in the specific approach highlighted above the following paragraphs provide some specific examples of how the process dUrea plant reliability. Ammonia The ammonia plant is quite complicated petrochemical gas plant that foresee different chemical reactions and operations working on range of operative condition that -27

pressure low temperature exchcompressor seal gas system is according to recognize state of the art because their fault result anyway in undesired plant shut

Having this approach in mind, the paper would provide some specific examples related to the more significant involved in the design of Ammonia and Urea plants. In particular it will be illustrated the process design as well as the mechanical, machinery, instrument and piping design.

Process Design

The process design is having for the operating reliability of the plants: as an example the proper selection of operating conditions, the control system and the material of construction is a paramount step in the further development of the project.

Furthermore the by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In factfeature, under certain conditions, can prevent the plant to trip in case of human error or instrumentation failure.

Entering more in the specific approach highlighted above the following paragraphs provide some specific examples of how the process design can contribute to Ammonia and Urea plant reliability.

Ammonia

The ammonia plant is quite complicated petrochemical gas plant that foresee different chemical reactions and operations working on range of operative ondition that 27°F) up to elevated temperature (1000°C


Having this approach in mind, the paper would provide some specific examples related to the more significant disciplines which are typically involved in the design of Ammonia and Urea plants. In particular it will be illustrated the process design as well as the mechanical, machinery, instrument and piping design.

Process Design


Furthermore the operating flexibility originated by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In factfeature, under certain conditions, can prevent the plant to trip in case of human error or instrumentation failure.

Entering more in the specific approach highlighted above the following paragraphs provide some specific examples of how the

esign can contribute to Ammonia and Urea plant reliability.

Ammonia Process

The ammonia plant is quite complicated petrochemical gas plant that foresee different chemical reactions and operations working on range of operative ondition that spans

) up to elevated temperature (1000°C


Having this approach in mind, the paper would provide some specific examples related to the

disciplines which are typically involved in the design of Ammonia and Urea plants. In particular it will be illustrated the process design as well as the mechanical, machinery, instrument and piping design.

Process Design


operating flexibility originated by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In factfeature, under certain conditions, can prevent the plant to trip in case of human error or instrumentation failure.


esign can contribute to Ammonia and Urea plant reliability.

rocess Design

The ammonia plant is quite complicated petrochemical gas plant that foresee different chemical reactions and operations working on range of operative

spans from cryogenic () up to elevated temperature (1000°C




The process design is having a fundamental role for the operating reliability of the plants: as an example the proper selection of operating conditions, the control system and the material of construction is a paramount step in the further development of the project.

operating flexibility originated by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In factfeature, under certain conditions, can prevent the plant to trip in case of human error or


esign can contribute to Ammonia and

esign

The ammonia plant is quite complicated petrochemical gas plant that foresee different chemical reactions and fourteenoperations working on range of operative

from cryogenic () up to elevated temperature (1000°C

Page 2 of

pressure low temperature exchangers, compressor seal gas system is according to recognize state of the art because their fault result anyway in undesired plant shut-down.



a fundamental role for the operating reliability of the plants: as an example the proper selection of operating conditions, the control system and the material of construction is a paramount step in the further

operating flexibility originated by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that can boost the plants reliability. In fact feature, under certain conditions, can prevent the plant to trip in case of human error or



The ammonia plant is quite complicated petrochemical gas plant that foresee seven

fourteen operations working on range of operative

from cryogenic (-33°C) up to elevated temperature (1000°C

of 16

angers, compressor seal gas system is according to recognize state of the art because their fault

down.


disciplines which are typically involved in the design of Ammonia and Urea plants. In particular it will be illustrated the process design as well as the mechanical,

a fundamental role for the operating reliability of the plants: as an example the proper selection of operating conditions, the control system and the material of construction is a paramount step in the further

operating flexibility originated by the proper selection of a scheme that can allow to safely operate the unit within a sufficiently wide span of pressure, temperature and fluid composition ranges is an aspect that

this feature, under certain conditions, can prevent the plant to trip in case of human error or



The ammonia plant is quite complicated seven unit

operations working on range of operative 33°C /

) up to elevated temperature (1000°C / /




1832°F ) as well as from low pressure (1 bar / 15 psi) to high pressure (150 bar / 2175 psig). The know-how to manage all the wide range of operative condition and fluids to be handled is of the most importance. Such know-how covers all the engineering discipline as described in the following paragraphs starting from the process design that have to:

• Define of the most appropriate flow scheme and operative parameters of each equipment / unit according to the "state of the art" of the available proven technology.

• Define of the most appropriate design condition and material selection for all equipment / units of the whole plant.

• Define of the sparing philosophy for all machines.

• Define of the required instrumentation for monitoring and control the plant operation within the defined operative parameters along the plant.

• Define of the required instrumentation and emergency shut-down system for preventing operation outside the defined operative parameters that may offset the reliability and safety operation of any equipment, units or plant sections.

• Definition of the safety relief system able to manage any possible overpressure that may be caused by plant mal-operations, utility failure or failure of process control and emergency interlock system.

As an example the design of a suitable steam network system and in particular the control system for the let-down station and associated valves is important for minimizing the network steam pressure fluctuation in case of partial unit /section shut-down avoiding to triggering undesired total plant shut-down.

Urea Process Design As it is well know the modern urea processes are recycle type of process, therefore a temporary or sudden inefficiency of the synthesis loop, or more in detail of the urea reactor, would have a significant impact on the overall urea plant performances and under certain conditions may even lead to a plant shut-down. Typically the best way to overcome this problem is to provide a stable recycle flow of carbamate to the high pressure section thus preventing the further increase of water in the synthesis loop that typically ends up in further lowering the reactor efficiency. This kind of problem can be safely and environmentally addressed providing a pressurized carbamate tank with a sufficient hold up to manage the described transient situation. In fact the excess of carbamate generated by the synthesis loop can be stored under pressure, thus preventing unnecessary ammonia venting to atmosphere, maintaining or even reducing the carbamate flow to the reactor. Once the conditions of the reactor are recovered the excess carbamate can be slowly recycle to the synthesis without affecting the plants operation and production. 2. Pressure Equipment Design The pressure equipment in petrochemical process plants is designed to last from 10 to more than 25 years, according to the operating conditions, without maintenance and it is usually not spared. Therefore their reliability is one of the key factor for the entire plant reliability since their failure usually imply a plant shut-down and quite often personnel and plant safety is concerned.




Reliability concerns in the design and construction of equipment have been incorporated in the requirements of various codes or practice of design, which are based upon the wide experience and knowledge of professional experts and specialists in the industry, backed up by the experience of local plant managers, engineers and operators who have direct experience in the relevant plant operation. But anyway equipment reliability have to be taken into special consideration from the beginning of the whole project and should be developed in each step of the project as an integrated aspect of design, not considered as a separate issue referring only to the mechanical features. This is particularly true for Ammonia and Urea plants that involve high pressure, elevated temperature and corrosive conditions, often mixed together, which requires reliability considerations for equipment to be already incorporated in the process concept. The most important factors affecting equipment reliability are the metallurgical and mechanical features and the aspects to be considered are:

• Definition of the best design solutions for the defined conditions and material selection.

• Full development of the defined design solutions according to the best available technologies.

• Selection of the most appropriate supplier of any equipment, with particular care for the critical items, which can assure not only the best performance but also the most proven design from a reliability and safety point of view.

• Perform all the required test at the equipment supplier to guarantee a correct manufacturing quality.

• Proper installation and commissioning. • Compliance with the monitoring and

maintenance procedures to preserve equipment characteristics and prevent failures and early deterioration.

In all these steps reliability aspects shall be one of the leading targets. It should be noted that metallurgical and mechanical aspects cannot be treated separately, since any material choice involves specific mechanical features and vice versa. Whereas the fundamentals are common, Ammonia and Urea equipment shall face different conditions which impose significant differences in their design to reach the desired reliability. Equipment for Ammonia Plants The Ammonia process is characterized mainly by gaseous fluids, mixtures of hydrogen, methane, steam, air, carbon mono and dioxides and ammonia. Gaseous mixtures are generally not corrosive, therefore carbon and low alloy steels are extensively used in ammonia plants, but if they are at high temperature and/or high pressure they can become highly aggressive and require special design and material selection to assure reliable operations. The most challenging sections are the reforming and the synthesis sections. In the reforming section the high temperatures, in some part above 900°C (1650°F), are a challenge in itself, but the presence of hydrogen, carbon mono and dioxide and steam in the gas can lead to phenomena such as metal dusting, which can have catastrophic consequences. The design of equipment in this section involves very different solution and materials, from the use of special metals for high temperature, to




the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that avoid the temperature range dusting is possible. The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to precautionand installation, a very specific experience along the supply chain from the designer to the Mreliability In the synthesis section the material selection is mainly governed by two phenomena rthe particular cohydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to avoid problems related to phenomena: hydrogen Hydrogen The term hydrogen attack is used to indicate several phenomena linked to the damages caused by hydrogen.ones for ammonia plants are: •


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that avoid the temperature range dusting is possible. The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to precautionand installation, a very specific experience along the supply chain from the designer to the Manufactures is required to reach the expected reliability In the synthesis section the material selection is mainly governed by two phenomena rthe particular cohydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to avoid problems related to phenomena: hydrogen Hydrogen The term hydrogen attack is used to indicate several phenomena linked to the damages caused by hydrogen.ones for ammonia plants are: • High Temperature Hydrogen Attack, which

in an Ammonia plant typically occurs in areas such as secondary reformers, waste heat boilers/exchangers, high temperature shift converters, methanator and synthesis loop The conditions under whiattack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which permissible operating limits as a function of steel type, hydrogen partial pressure and temperature.


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that avoid the temperature range dusting is possible.

The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to precautions during design, constrand installation, a very specific experience along the supply chain from the designer to the

anufactures is required to reach the expected reliability.

In the synthesis section the material selection is mainly governed by two phenomena rthe particular cohydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to avoid problems related to phenomena: hydrogen

Hydrogen Attack

The term hydrogen attack is used to indicate several phenomena linked to the damages caused by hydrogen.ones for ammonia plants are:

High Temperature Hydrogen Attack, which in an Ammonia plant typically occurs in areas such as secondary reformers, waste heat boilers/exchangers, high temperature shift converters, methanator and synthesis loop equipment The conditions under whiattack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which permissible operating limits as a function of steel type, hydrogen partial pressure and temperature.


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that avoid the temperature range dusting is possible.

The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to

during design, constrand installation, a very specific experience along the supply chain from the designer to the

anufactures is required to reach the expected

In the synthesis section the material selection is mainly governed by two phenomena rthe particular conditionshydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to avoid problems related to phenomena: hydrogen attack and nitriding.

ttack

The term hydrogen attack is used to indicate several phenomena linked to the damages caused by hydrogen. The most considerable ones for ammonia plants are:

High Temperature Hydrogen Attack, which in an Ammonia plant typically occurs in areas such as secondary reformers, waste heat boilers/exchangers, high temperature shift converters, methanator and synthesis

equipment (Fig. 1).The conditions under whiattack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which permissible operating limits as a function of steel type, hydrogen partial pressure and temperature.


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that avoid the temperature range in which metal

The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to

during design, constrand installation, a very specific experience along the supply chain from the designer to the


In the synthesis section the material selection is mainly governed by two phenomena r

nditions of gas hydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to avoid problems related to the two concurring

attack and nitriding.

The term hydrogen attack is used to indicate several phenomena linked to the damages

The most considerable ones for ammonia plants are:


(Fig. 1). The conditions under whiattack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which permissible operating limits as a function of steel type, hydrogen partial pressure and


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that

in which metal

The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.For those items in addition to the needed

during design, construction, testing and installation, a very specific experience along the supply chain from the designer to the


In the synthesis section the material selection is mainly governed by two phenomena related to

of gas that containhydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to

two concurring attack and nitriding.


The most considerable


The conditions under which hydrogen attack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which permissible operating limits as a function of steel type, hydrogen partial pressure and


the extensive use of refractory, to water jacketing, from the selection of high alloys resisting to metal dusting, to design features that

in which metal

The most critical items are the primary reformer heater, the secondary reformer, the connecting transfer line and the downstream boiler system.

the needed testing

and installation, a very specific experience along the supply chain from the designer to the


In the synthesis section the material selection is elated to contains

hydrogen and ammonia at high temperature and pressure. These conditions require a specific material selection and equipment design, to

two concurring attack and nitriding.


The most considerable


ch hydrogen attack can occur are defined by a set of empirical curves, namely the Nelson Curves, reported on API 941, which set permissible operating limits as a function of steel type, hydrogen partial pressure and

•

•

Fig. 1. High temperatur

Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason Hydrogen assisted cracking can ocservice temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the synthesis section.

Hydrogen disbonding could affect components where an overlay osteel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical of Ammonia plants. There hequipment incorporating this design has suffered from cracking at the interface between the austenitic layer and the base metal after being in service for a period of time.

It could be connected to entrapped at the overlacooling down cycle or to a phenomenon called 'carbon migration' with carbon


Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason Hydrogen assisted cracking can ocservice temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the synthesis section.Hydrogen disbonding could affect components where an overlay osteel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical of Ammonia plants. There have been numerous instances where equipment incorporating this design has suffered from cracking at the interface between the austenitic layer and the base metal after being in service for a period of

It could be connected to entrapped at the overlay-base metal interface during a cooling down cycle or to a phenomenon called 'carbon migration' with carbon


Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason Hydrogen assisted cracking can ocservice temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the synthesis section. Hydrogen disbonding could affect components where an overlay osteel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical of Ammonia plants.

ave been numerous instances where equipment incorporating this design has suffered from cracking at the interface between the austenitic layer and the base metal after being in service for a period of

It could be connected to entrapped at the base metal interface during a

cooling down cycle or to a phenomenon called 'carbon migration' with carbon

Fig. 1. High temperature hydrogen attack

Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason Hydrogen assisted cracking can ocservice temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the

Hydrogen disbonding could affect components where an overlay osteel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical




Page 5 of

e hydrogen attack

Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason Hydrogen assisted cracking can occur at service temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the

Hydrogen disbonding could affect components where an overlay of stainless steel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical




of 16

e hydrogen attack

Hydrogen assisted cracking, which has also been detected in Ammonia plants vessels, in particular where the component had not been adequately post weld stress relieved during fabrication. For this reason

cur at service temperatures well below the relevant Nelson curve. Generally it occurs in low alloy with a chrome level above 2%, typically utilized in the hottest part of the

Hydrogen disbonding could affect f stainless

steel or Inconel 600 has been applied on a carbon or low alloy pressure resistant body, usually to provide protection against high temperature hydrogen attack, nitriding and hot corrosion from sulphides (H2S) typical







Nitriding In the presence of hot ammonia atmosphere, above a certain temperature depending on the type of steel, ammohard and brittle Fephenomenon is called Nitriding.develops on low alloy steels and on stainless steelsrate and higher temperatures compared alloy steels. On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, expossurface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the component failure


migrating from the alloy steel and concentrating at the austeniteinterface to produce a high hardness, crack susceptible martensitic zoThis degradation mechanism is not uncommon in Ammonia Plants with an Inconel 600 overlay on low alloy with a chrome level above 2%.

Nitriding In the presence of hot ammonia atmosphere, above a certain temperature depending on the type of steel, ammohard and brittle Fephenomenon is called Nitriding.develops on low alloy steels and on stainless steels, howeverrate and higher temperatures compared alloy steels. On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, expossurface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the component failure

Fig. 2.



Nitriding

In the presence of hot ammonia atmosphere, above a certain temperature depending on the type of steel, ammohard and brittle Fephenomenon is called Nitriding.develops on low alloy steels and on stainless

however, on rate and higher temperatures compared alloy steels.

On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, expossurface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the component failure

. Section



In the presence of hot ammonia atmosphere, above a certain temperature depending on the type of steel, ammonia reacts with iron to form a hard and brittle Fe-N interphenomenon is called Nitriding.develops on low alloy steels and on stainless

on the latter at a much reduced rate and higher temperatures compared

On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, expossurface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the component failure (Fig. 2).

Section - Nitriding progress throughcracks


migrating from the alloy steel and concentrating at the austeniteinterface to produce a high hardness, crack susceptible martensitic zone. This degradation mechanism is not uncommon in Ammonia Plants with an Inconel 600 overlay on low alloy with a chrome level above 2%.

In the presence of hot ammonia atmosphere, above a certain temperature depending on the

nia reacts with iron to form a N inter-metallic layer. This

phenomenon is called Nitriding.develops on low alloy steels and on stainless

the latter at a much reduced rate and higher temperatures compared

On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, expossurface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the

(Fig. 2).

Nitriding progress throughcracks


migrating from the alloy steel and concentrating at the austenite-alloy steel interface to produce a high hardness, crack

This degradation mechanism is not uncommon in Ammonia Plants with an Inconel 600 overlay on low alloy with a


nia reacts with iron to form a metallic layer. This

phenomenon is called Nitriding. Nitriding develops on low alloy steels and on stainless

the latter at a much reduced rate and higher temperatures compared with low

On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress concentration cracks can occur, exposing further surface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the

Nitriding progress through


migrating from the alloy steel and alloy steel

interface to produce a high hardness, crack

This degradation mechanism is not uncommon in Ammonia Plants with an Inconel 600 overlay on low alloy with a


nia reacts with iron to form a metallic layer. This

Nitriding develops on low alloy steels and on stainless

the latter at a much reduced with low

On pressure equipment this layer does not cause any problem until it remains compact and does not crack, but since a brittle material will reach more easily its rupture limit, in areas of stress

ing further surface to the nitriding atmosphere. This propagation, cycle after cycle, can lead to the

Nitriding progress through

For this reason in ammonia atmosphere, usually above 370low alloy steel are not used in contact with fluid and replaced with austenitic stainless steel or even non Nitriding occurs also on stainless steel usually above 400°Cductility orate, their use guarantees an acceptable reliability in this situation. Only the thinnest components require the selection of material such as Inconel 600 which is fully resistant Synthesis All the equipment section, but the most critical ones are the ammonia converter and downstream exchangers, because temperatures In ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10Ammonia catalyst, once reduced, should not come into contact with oxygen, since pyrophoricbetween catalyst changes without repair or inspection. In general, since the use of stainless steel or higher alloy steels would be too expensive, these items are made from low alloy steel, selehydrogen attack. To avoid nitriding usually either the temperature of the pressure components is kept below 370°C or a lining with a nitriding resistant material is applied, this second solution being less reldue to the possible occurrence of hydrogen disbonding.

For this reason in ammonia atmosphere, usually above 370-low alloy steel are not used in contact with fluid and replaced with austenitic stainless steel or even non-ferrous alloy.

Nitriding occurs also on stainless steel usually above 400°Cductility of stainless steel and their low nitriding rate, their use guarantees an acceptable reliability in this situation. Only the thinnest components require the selection of material such as Inconel 600 which is fully resistant

Synthesis

All the equipment section, but the most critical ones are the ammonia converter and downstream exchangers, because temperatures

In particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10Ammonia catalyst, once reduced, should not come into contact with oxygen, since pyrophoricbetween catalyst changes without repair or inspection.

In general, since the use of stainless steel or higher alloy steels would be too expensive, these items are made from low alloy steel, selected according to the Nelson curves to resist hydrogen attack.

To avoid nitriding usually either the temperature of the pressure components is kept below 370°C or a lining with a nitriding resistant material is applied, this second solution being less reldue to the possible occurrence of hydrogen disbonding.

For this reason in ammonia atmosphere, usually -380°C,

low alloy steel are not used in contact with fluid and replaced with austenitic stainless steel or

ferrous alloy.

Nitriding occurs also on stainless steel usually above 400°C (750°

f stainless steel and their low nitriding rate, their use guarantees an acceptable reliability in this situation. Only the thinnest components at the higherequire the selection of material such as Inconel 600 which is fully resistant

Synthesis Sections

All the equipment section, but the most critical ones are the ammonia converter and downstream exchangers, because temperatures and the highe

particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10Ammonia catalyst, once reduced, should not come into contact with oxygen, since pyrophoric. Therefore converters should operate between catalyst changes without repair or inspection.

In general, since the use of stainless steel or higher alloy steels would be too expensive, these items are made from low alloy steel,

cted according to the Nelson curves to resist hydrogen attack.

To avoid nitriding usually either the temperature of the pressure components is kept below 370°C or a lining with a nitriding resistant material is applied, this second solution being less reldue to the possible occurrence of hydrogen disbonding.

For this reason in ammonia atmosphere, usually (700-715


ferrous alloy.

Nitriding occurs also on stainless steel usually °F) , but considering the high

f stainless steel and their low nitriding rate, their use guarantees an acceptable reliability in this situation. Only the thinnest

the higher require the selection of material such as Inconel 600 which is fully resistant

ections

All the equipment is critical in the synthesis section, but the most critical ones are the ammonia converter and downstream exchangers, because combine

the highest

particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10Ammonia catalyst, once reduced, should not come into contact with oxygen, since

Therefore converters should operate between catalyst changes without repair or


cted according to the Nelson curves to resist

To avoid nitriding usually either the temperature of the pressure components is kept below 370°C or a lining with a nitriding resistant material is applied, this second solution being less reldue to the possible occurrence of hydrogen

For this reason in ammonia atmosphere, usually 715°F) carbon steel and


Nitriding occurs also on stainless steel usually , but considering the high


temperature usually require the selection of material such as Inconel 600 which is fully resistant to nitriding.

critical in the synthesis section, but the most critical ones are the ammonia converter and downstream

combine st ammonia content.

particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10Ammonia catalyst, once reduced, should not come into contact with oxygen, since





Page 6 of

For this reason in ammonia atmosphere, usually carbon steel and




temperature usually require the selection of material such as Inconel

to nitriding.


combine the highest ammonia content.

particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst changes, since it must run for 10-15 years. Ammonia catalyst, once reduced, should not come into contact with oxygen, since it is highly





of 16

For this reason in ammonia atmosphere, usually carbon steel and




temperature usually require the selection of material such as Inconel


the highest ammonia content.

particular reliability is essential for the ammonia synthesis converter, which is the reactor with the longest run between catalyst

15 years. Ammonia catalyst, once reduced, should not

it is highly Therefore converters should operate

between catalyst changes without repair or



To avoid nitriding usually either the temperature of the pressure components is kept below 370°C or a lining with a nitriding resistant material is applied, this second solution being less reliable due to the possible occurrence of hydrogen




The first solution usually callinternal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher reliability.Sienvironment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on highof the processmaterials at a suitable thermal insulation and/or gas flushing. This concept inherently increases the safety of high Even technology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the one guaran Also the synthesis loop heat exchangers located downstream the ammonia converter the same concerns as the ammonia reactor, especially the first exchanger after the reactor outlet. Whatever exchanger superprephilosophy should be the sametemperature lowto achieve this target are specific to each typeexchang The possibility to directly connect the exchanger to the converter, avoids the need for highpressure, highusual at the same time,


The first solution usually callinternal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher reliability.Since the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on highof the processmaterials at a suitable thermal insulation and/or gas flushing. This concept inherently increases the safety of high-pressure parts. Even with technology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the one guaran Also the synthesis loop heat exchangers located downstream the ammonia converter the same concerns as the ammonia reactor, especially the first exchanger after the reactor outlet. Whatever exchanger super-heater, pre-heater, philosophy should be the sametemperature lowto achieve this target are specific to each typeexchanger The possibility to directly connect the exchanger to the converter, avoids the need for highpressure, highusual source of trouble and leakage, and savesat the same time,


The first solution usually callinternal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher reliability.

nce the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on highof the process. Rmaterials at a lower temperature by means of suitable thermal insulation and/or gas flushing.

This concept inherently increases the safety of pressure parts.

with the improvtechnology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the one guaranteeing the highest reliability.

Also the synthesis loop heat exchangers located downstream the ammonia converter the same concerns as the ammonia reactor, especially the first exchanger after the reactor

Whatever exchanger heater, or

heater, or a gasphilosophy should be the sametemperature low. to achieve this target are specific to each type

er.

The possibility to directly connect the exchanger to the converter, avoids the need for highpressure, high-temperature piping, which is a

source of trouble and leakage, and savesat the same time,


The first solution usually callinternal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher

nce the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on high

Reliability is increased keeping lower temperature by means of

suitable thermal insulation and/or gas flushing.

This concept inherently increases the safety of pressure parts.

the improvtechnology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the

teeing the highest reliability.


Whatever exchanger is selector a boiler, a gas-gas exchanger, the design

philosophy should be the same Of course the design solutions

to achieve this target are specific to each type

The possibility to directly connect the exchanger to the converter, avoids the need for high

temperature piping, which is a source of trouble and leakage, and saves

at the same time, material and pressure drop.


The first solution usually callsinternal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher

nce the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on high

eliability is increased keeping lower temperature by means of


This concept inherently increases the safety of

the improved technology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the



is selected either a boiler, or boiler feed water

gas exchanger, the design philosophy should be the same

f course the design solutions to achieve this target are specific to each type



material and pressure drop.


s for specific internal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher

nce the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to be kept as low as possible on high-pressure parts

eliability is increased keeping lower temperature by means of



construction technology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the


Also the synthesis loop heat exchangers located downstream the ammonia converter is subject to the same concerns as the ammonia reactor, especially the first exchanger after the reactor

ed either a steam boiler feed water

gas exchanger, the design philosophy should be the same:

f course the design solutions to achieve this target are specific to each type





for specific internal design, direct connections between equipment, which imply specific knowledge and design solutions, but guarantees an higher

nce the detrimental effect of the process environment on the materials increases in direct proportion to the temperature, the latter needs to

pressure parts eliability is increased keeping

lower temperature by means of suitable thermal insulation and/or gas flushing.


construction technology, some recent cases involving an additional hot wall ammonia converter suffered from weld cracking on the girth weld between shell and head, showing that the philosophy of keeping cold the pressure components is still the

Also the synthesis loop heat exchangers located subject to

the same concerns as the ammonia reactor, especially the first exchanger after the reactor

a steam boiler feed water

gas exchanger, the design keep

f course the design solutions to achieve this target are specific to each type of

The possibility to directly connect the exchanger to the converter, avoids the need for high-

temperature piping, which is a source of trouble and leakage, and saves,


The example, whether the gas is on the shell side or the tube side) depends on the plant configuration, but the use of Uincrease the exchanger reliability over the fixed tubestresses shall be considered, such as using fountainarranged radially instead of parallel to each other, as in a conventional Upermitting a symmetrical thermal stress distribution, while no paretaining wall of the gas channel is exposed to the hottest gases. For kettle type boilersdisposition usually is more reliable than the vertical onedeposit on the tubesheet c

Fig. 3.

When supermaterial is subject to nitriding temperatures, high alloy material specifically developed for stress corrosioover stainless steel, because of the high risk of stress corrosion inherent in this material.

The arrangementexample, whether the gas is on the shell side or the tube side) depends on the plant configuration, but the use of Uincrease the exchanger reliability over the fixed tube-sheet type.stresses shall be considered, such as using fountain-type Uarranged radially instead of parallel to each other, as in a conventional Upermitting a symmetrical thermal stress distribution, while no paretaining wall of the gas channel is exposed to the hottest gases.

For kettle type boilersdisposition usually is more reliable than the vertical onedeposit on the tubesheet c

Fig. 3. Horizontal type synthesis boiler directly connected to

When supermaterial is subject to nitriding temperatures, high alloy material specifically developed for stress corrosioover stainless steel, because of the high risk of stress corrosion inherent in this material.

arrangement example, whether the gas is on the shell side or the tube side) depends on the plant configuration, but the use of Uincrease the exchanger reliability over the fixed

sheet type. All design possibility tstresses shall be considered, such as using

type U-tubes, in which the tubes are arranged radially instead of parallel to each other, as in a conventional Upermitting a symmetrical thermal stress distribution, while no paretaining wall of the gas channel is exposed to the hottest gases.

For kettle type boilersdisposition usually is more reliable than the vertical ones, where boiling water contaminants deposit on the tubesheet c

Horizontal type synthesis boiler directly connected to

When super-heater are considered, since tube material is subject to nitriding temperatures, high alloy material specifically developed for stress corrosion resistance should be preferred over stainless steel, because of the high risk of stress corrosion inherent in this material.

of the exchanger itself (for example, whether the gas is on the shell side or the tube side) depends on the plant configuration, but the use of Uincrease the exchanger reliability over the fixed

All design possibility tstresses shall be considered, such as using

tubes, in which the tubes are arranged radially instead of parallel to each other, as in a conventional Upermitting a symmetrical thermal stress distribution, while no paretaining wall of the gas channel is exposed to

For kettle type boilers (Fig. 3)disposition usually is more reliable than the

, where boiling water contaminants deposit on the tubesheet causing corrosion.

Horizontal type synthesis boiler directly connected to ammonia converter

heater are considered, since tube material is subject to nitriding temperatures, high alloy material specifically developed for

n resistance should be preferred over stainless steel, because of the high risk of stress corrosion inherent in this material.

of the exchanger itself (for example, whether the gas is on the shell side or the tube side) depends on the plant configuration, but the use of U-tube type greatly increase the exchanger reliability over the fixed

All design possibility tstresses shall be considered, such as using

tubes, in which the tubes are arranged radially instead of parallel to each other, as in a conventional U-tube exchanger, permitting a symmetrical thermal stress distribution, while no part of the pressureretaining wall of the gas channel is exposed to

(Fig. 3), the horizontal disposition usually is more reliable than the

, where boiling water contaminants ausing corrosion.

Horizontal type synthesis boiler directly mmonia converter



Page 7 of

of the exchanger itself (for example, whether the gas is on the shell side or the tube side) depends on the plant

tube type greatly increase the exchanger reliability over the fixed

All design possibility to reduce stresses shall be considered, such as using

tubes, in which the tubes are arranged radially instead of parallel to each

tube exchanger, permitting a symmetrical thermal stress

rt of the pressureretaining wall of the gas channel is exposed to

, the horizontal disposition usually is more reliable than the

, where boiling water contaminants ausing corrosion.

Horizontal type synthesis boiler directly mmonia converter



of 16

of the exchanger itself (for example, whether the gas is on the shell side or the tube side) depends on the plant

tube type greatly increase the exchanger reliability over the fixed

o reduce stresses shall be considered, such as using

tubes, in which the tubes are arranged radially instead of parallel to each

tube exchanger, permitting a symmetrical thermal stress

rt of the pressure-retaining wall of the gas channel is exposed to

, the horizontal disposition usually is more reliable than the

, where boiling water contaminants

Horizontal type synthesis boiler directly


n resistance should be preferred over stainless steel, because of the high risk of




Other The same principles for design and materials selection should be used to guarantee the reliability of the of the plant, although temperature and pressures are lower. However it should be considered that each plant has its own specificities, different ambient conditions, local rules, types of feed gas and therefore the approach appropriate to ensure the best design and reliability. Special consideration should be given in thaspects to the COpossibility to choose different solution and the different corrosiveness involvedis a key aspect of the plant reliability. Equipment for The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water treatment sections use standastainless steels (AISIsynthesis loop is manufactured with special steels properly studied for urea application. It has to be borne in mind that different process choices greatly affect the material selection of the a different process scheme may require a completely different approach. The most common type of special material for the Urea synthesis is 316L


Other Sections The same principles for design and materials selection should be used to guarantee the reliability of the of the plant, although temperature and pressures are lower. However it should be considered that each plant has its own specificities, different ambient conditions, local rules, types of feed gas and therefore the approach appropriate to ensure the best design and reliability. Special consideration should be given in thaspects to the COpossibility to choose different solution and the different corrosiveness involvedis a key aspect of the plant reliability. Equipment for The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water treatment sections use standastainless steels (AISIsynthesis loop is manufactured with special steels properly studied for urea application. It has to be borne in mind that different process choices greatly affect the material selection of the equipment; therefore, the same equipment in a different process scheme may require a completely different approach. The most common type of special material for the Urea synthesis is 316L


ections

The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections of the plant, although temperature and pressures are lower.

However it should be considered that each plant has its own specificities, different ambient conditions, local rules, types of feed gas and therefore the approach appropriate to ensure the best design and reliability.

Special consideration should be given in thaspects to the COpossibility to choose different solution and the different corrosiveness involvedis a key aspect of the plant reliability.

Equipment for

The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water treatment sections use standastainless steels (AISIsynthesis loop is manufactured with special steels properly studied for urea application.

It has to be borne in mind that different process choices greatly affect the material selection of

equipment; therefore, the same equipment in a different process scheme may require a completely different approach.

The most common type of special material for the Urea synthesis is 316L


The same principles for design and materials selection should be used to guarantee the

equipment in all other sections of the plant, although temperature and pressures

However it should be considered that each plant has its own specificities, different ambient conditions, local rules, types of feed gas and therefore the approach one design fit all is not appropriate to ensure the best design and

Special consideration should be given in thaspects to the CO2 removal section, where the possibility to choose different solution and the different corrosiveness involvedis a key aspect of the plant reliability.

Equipment for Urea Plants

The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water treatment sections use standastainless steels (AISI-304L or AISIsynthesis loop is manufactured with special steels properly studied for urea application.



The most common type of special material for the Urea synthesis is 316L




However it should be considered that each plant has its own specificities, different ambient conditions, local rules, types of feed gas and

one design fit all is not appropriate to ensure the best design and

Special consideration should be given in thremoval section, where the

possibility to choose different solution and the different corrosiveness involved in each choice is a key aspect of the plant reliability.

lants

The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water treatment sections use standard commercial

304L or AISIsynthesis loop is manufactured with special steels properly studied for urea application.



The most common type of special material for the Urea synthesis is 316L 'urea grade






Special consideration should be given in thremoval section, where the

possibility to choose different solution and the in each choice

is a key aspect of the plant reliability.

The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the lowpressure, evaporation, vacuum and waste water

rd commercial 304L or AISI-316L), the

synthesis loop is manufactured with special steels properly studied for urea application.


equipment; therefore, the same equipment in a different process scheme may require a

The most common type of special material for urea grade'






Special consideration should be given in these removal section, where the

possibility to choose different solution and the in each choice

The urea process is characterized by the presence of quite aggressive process fluids like ammonium carbamate, ammonium carbonate and urea solution, which require the use of special stainless steels, especially under synthesis conditions (high pressure and high temperature). In principle, all urea processes utilize stainless steel, but while the low-pressure, evaporation, vacuum and waste water

rd commercial 316L), the

synthesis loop is manufactured with special steels properly studied for urea application.


equipment; therefore, the same equipment in a different process scheme may require a

The most common type of special material for ' (UG)

steel.Fig. 4.

This material is widely used in urea plants, mainly because of its excellent weldability, fair corrosionHowever, the large amount of passivation air required and limited corrosionharsher cthe most critical components The super22Nibetter corrosionand reweldability. This material is generally the basic selection when the presence of passivation air allows stainless steels to be used. This type of material has been used for many years in urea plants and has proven to be reliable within its operating performance of this material has always been satisfactory, even in the most critical items such as the HP stripper. As for 316L22specifications aimed to define the technicarequirements, acceptability criteria and

steel. Fig. 4. Active corrosion in 316L Urea

This material is widely used in urea plants, mainly because of its excellent weldability, fair corrosion-resistance and relatively low cost. However, the large amount of passivation air required and limited corrosionharsher conditions prevents its application for the most critical components

The super-22Ni-2Mo is an upgrade of 316Lbetter corrosionand re-passivation capacity and excelleweldability.

This material is generally the basic selection when the presence of passivation air allows stainless steels to be used.

his type of material has been used for many years in urea plants and has proven to be reliable within its operating performance of this material has always been satisfactory, even in the most critical items such as the HP stripper.

As for 316L22-2 CASALEspecifications aimed to define the technicarequirements, acceptability criteria and

Active corrosion in 316L Urea

This material is widely used in urea plants, mainly because of its excellent weldability, fair

resistance and relatively low cost. However, the large amount of passivation air required and limited corrosion

onditions prevents its application for the most critical components

-austenitic stainless steel type 25Cr2Mo is an upgrade of 316L

better corrosion-resistance, higher passivation passivation capacity and excelle

weldability.


his type of material has been used for many years in urea plants and has proven to be reliable within its operating performance of this material has always been satisfactory, even in the most critical items such as the HP stripper.

As for 316L-UG, since the introduction of 25CASALE has developed the relevant

specifications aimed to define the technicarequirements, acceptability criteria and

Active corrosion in 316L Urea material


resistance and relatively low cost. However, the large amount of passivation air required and limited corrosion

onditions prevents its application for the most critical components

austenitic stainless steel type 25Cr2Mo is an upgrade of 316L

resistance, higher passivation passivation capacity and excelle


his type of material has been used for many years in urea plants and has proven to be reliable within its operating performance of this material has always been satisfactory, even in the most critical items such

UG, since the introduction of 25has developed the relevant


Active corrosion in 316L Urea material


resistance and relatively low cost. However, the large amount of passivation air required and limited corrosion-resistance in

onditions prevents its application for the most critical components (Fig. 4)

austenitic stainless steel type 25Cr2Mo is an upgrade of 316L


This material is generally the basic selection when the presence of passivation air allows

his type of material has been used for many years in urea plants and has proven to be reliable within its operating performance of this material has always been satisfactory, even in the most critical items such



Page 8 of

Active corrosion in 316L Urea grade


resistance and relatively low cost. However, the large amount of passivation air

resistance in onditions prevents its application for

(Fig. 4).

austenitic stainless steel type 25Cr2Mo is an upgrade of 316L-UG. It has



his type of material has been used for many years in urea plants and has proven to be reliable within its operating limits. performance of this material has always been satisfactory, even in the most critical items such



of 16

grade


resistance and relatively low cost. However, the large amount of passivation air

resistance in onditions prevents its application for

austenitic stainless steel type 25Cr-UG. It has

resistance, higher passivation passivation capacity and excellent


his type of material has been used for many years in urea plants and has proven to be

The performance of this material has always been satisfactory, even in the most critical items such

UG, since the introduction of 25-has developed the relevant

specifications aimed to define the technical requirements, acceptability criteria and




qualification tests for base material to be used in CASALE equipment as well as all the requirements for welding 25-22-2. These specifications are the result of CASALE experience in the field of urea and are continuously updated on the basis of the improved experience and the technical progress. While austenitic stainless steels are susceptible to stress corrosion cracking (SCC) by chlorides which may be present in the utility fluid, especially in carbamate condensers, duplex stainless steels have high resistance to SCC. They have been used in some plants for urea synthesis components such as mixers, valves and piping for more than 30 years. But experience shows that common duplex stainless steels are not suitable for the harsher conditions, such as stripper or reactor components. Specific duplex steels have been developed for the urea process conditions, to extend their use to more critical components. These types of duplex usually have high levels of chromium, which is favorable to passivation of steel. As a consequence, duplex stainless steel is easier to passivate than austenitic steels, which means duplex stainless steel requires less dissolved oxygen compared to austenitic stainless steel. However, it has to be remembered that the dissolved oxygen content in the liquid phase varies, even if the oxygen feed is constant. The excessively low dissolved oxygen content, not oxygen feed, could cause active corrosion even in high-chromium duplex stainless steel. Therefore the quantity of oxygen feed must be determined on the basis of an analysis of the complete system. The passive corrosion rate of duplex stainless steel developed for urea application is generally comparable to that of 25Cr-22Ni-2Mo.

Special duplex materials developed to increase their already high resistance to erosion are selected for valves plugs, ejector nozzles, mixers and similar application. Titanium and Zirconium are a reactive metal which owe their corrosion-resistance to its ability to passivate. The addition of oxygen to the process is not required to passivate these metals. Moreover, they are neither subject to intergranular corrosion in urea synthesis conditions nor to stress corrosion cracking. Therefore they are ideal candidates for the most critical components in the urea synthesis. Titanium has been widely utilized in the past. Equipment made of this material is suitable for the highest temperature, even in absence of oxygen, but it is expensive and its life is limited due to erosion, especially in the case of strippers. In addition, reliability is lower compared with stainless steel equipment because of the construction difficulties; therefore, maintenance costs are higher. Zirconium is even more expensive than titanium and the construction requirements are also higher, as both welding and forming have more stringent purity requirements. This aspect lays many doubts over the reliability of a big welded construction made of this material, which has rarely been attempted. In any case the correct material selection by itself does not guarantee a reliable performance in the harsh environment of the urea synthesis. Careful material screening and specific construction procedures are keys requirements to achieve the requested corrosion resistance. Usually, detailed standards are developed by the licensor specific for each type of material, to guarantee the quality of the actual materials




used in construction and assure that its characteristics do not deteriorate during the manufacturing process. Material selection is never separate from design, because other parameters like mechanical properties, workability and weldability as well as economic considerations like prices, availability and delivery have to be taken into considerations. Even an accurate material selection cannot by itself guarantee the reliability of equipment, without correct design, quality construction, and proper operation and maintenance. In fact corrosion, even if under control, is always present. The special materials required for corrosion resistance are not generally suitable for the heavy-gauge construction needed to withstand the high operating pressures for the dimensions typical of modern plants. In general, such equipment would be too expensive, and for some materials the procedures involved in fabrication would impair the corrosion-resistance properties, such as stainless steel and, especially, duplex steels. For these reasons in nearly all applications the corrosion-resistant materials are applied as a lining to a pressure-resistant shell made of steel. Different choices are available, related both to the materials selected and to the design philosophy. The easiest way is to apply the corrosion resistant layer directly to the pressure bearing steel. That can be done by weld overlaying, or cladding. The main problem with this type of design is that any damage to the protective lining cannot be detected until also the pressure envelope is corroded, with a high possibility of catastrophic failure.

This solution has been used in the past, but it is not generally considered nowadays for the synthesis equipment. The common solution is to apply the corrosion-resistant material as a loose lining to the pressure casing. The loose lining seals the corrosive fluid, while the external casing has only a structural function. A leak detection system is provided between the internal layer and the pressure casing to inform of any damage to the corrosion-resistant lining, before the fluid can corrode the pressure components. This feature is essential for the reliability of the equipment, since early detection of any failure of the corrosion-resistant layer could prevent damage to the pressure shell that could result in the equipment collapsing or, at least, to damage that is difficult to repair. Automatic leak detection systems greatly improves the safety and reliability of equipment, permitting an early intervention, which usually prevent extended damages difficult to repair. Urea Reactor The reactor is a simple vessel with adequate volume to let the reaction progress, but the pressure, the large dimensions and the huge surface to be lined make it a critical construction, where each detail must be designed according to specific features to guarantee the expected reliability. The reactor has been layered with titanium, zirconium, austenitic stainless steels (316L-UG and 25Cr-22Ni-2Mo) and duplex stainless steel. The choice depends on the type of process and the specific experience of the licensor. 25Cr-22Ni-2Mo has been the typical choice for modern plant, giving excellent results in terms of reliability and endurance.




Stripper The stripper isequipment in the urea process. Different process designs have different requirements; that is why so many materials have been used in the past. The equipment itself is generally a fallingevaporator, and different detailsgenerally related to the different material requirements. Tube material is the most important choice.Reliability is mainly connected to the specific choice that each basic material selection involves. Notwithstanding the general basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of the equipment. Carbamate The design of this eqsubstantially according to the technology selected. Horizontal or vertical installation with process fluid tube side or shell side are commonly used.Since the reaction heat is usually utilized to generate steam stress corrosion can be a problem. In this case the use of a duplex material for tubes may be advantageous. Other The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections of the plant, altho


Stripper The stripper isequipment in the urea process. Different process designs have different requirements; that is why so many materials have been used in the past. The equipment itself is generally a fallingevaporator, and different detailsgenerally related to the different material requirements. Tube material is the most important choice.Reliability is mainly connected to the specific choice that each basic material selection involves. Notwithstanding the general basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of the equipment. Carbamate The design of this eqsubstantially according to the technology selected. Horizontal or vertical installation with process fluid tube side or shell side are commonly used.Since the reaction heat is usually utilized to generate steam stress corrosion can be a problem. In this case the use of a duplex material for tubes may be advantageous. Other Sections The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections of the plant, altho


Stripper

The stripper is equipment in the urea process.

Different process designs have different requirements; that is why so many materials have been used in the past.

The equipment itself is generally a fallingevaporator, and different detailsgenerally related to the different material requirements.

Tube material is the most important choice.Reliability is mainly connected to the specific choice that each basic material selection involves.

Notwithstanding the general basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of the equipment.

Carbamate Condenser

The design of this eqsubstantially according to the technology

Horizontal or vertical installation with process fluid tube side or shell side are commonly used.Since the reaction heat is usually utilized to generate steam stress corrosion can be a problem. In this case the use of a duplex material for tubes may be advantageous.

ections

The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections of the plant, altho


possibly the most critical equipment in the urea process.


The equipment itself is generally a fallingevaporator, and different detailsgenerally related to the different material

Tube material is the most important choice.Reliability is mainly connected to the specific choice that each basic material selection

Notwithstanding the general basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of

ondenser

The design of this eqsubstantially according to the technology


ections

The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections of the plant, although fluids are less corrosive


possibly the most critical equipment in the urea process.


The equipment itself is generally a fallingevaporator, and different details generally related to the different material


Notwithstanding the general similarity, each basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of

ondenser

The design of this equipment can vary substantially according to the technology


The same principles for design and materials selection should be used to guarantee the reliability of the equipment in all other sections

ugh fluids are less corrosive


possibly the most critical

Different process designs have different requirements; that is why so many materials

The equipment itself is generally a falling in design are

generally related to the different material


similarity, each basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of

uipment can vary substantially according to the technology





possibly the most critical

Different process designs have different requirements; that is why so many materials

The equipment itself is generally a falling-film in design are

generally related to the different material

Tube material is the most important choice. Reliability is mainly connected to the specific choice that each basic material selection

similarity, each basic material selection, from stainless steel to duplex to titanium imply a specific detailed design, where specific knowledge and experience is the key for a trouble free life of

uipment can vary substantially according to the technology

Horizontal or vertical installation with process fluid tube side or shell side are commonly used. Since the reaction heat is usually utilized to generate steam stress corrosion can be a problem. In this case the use of a duplex



and pressures lower.In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is not true, making the difference band failure.this area than in the synthesis loop because less diligence has been exercised in materials selection. Although common stainless steels can be used, the appropriate limits on chemical compositiomust be observed and quality checks carried out in many components to avoid corrosion which, when starts, can be fast. A typical example is the use of welded stainless steel tubes for exchangers. Whereas the base material can be suitable for the operaweld impair the corrosion resistance, leading to localized corrosiontube should not be used in most of the Urea plant equipment.

Fig. 5.

Thegeneral, standard components can be selected but not all standard components are suitable.

and pressures lower.In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is not true, making the difference band failure.this area than in the synthesis loop because less diligence has been exercised in materials selection.

Although common stainless steels can be used, the appropriate limits on chemical compositiomust be observed and quality checks carried out in many components to avoid corrosion which, when starts, can be fast.

A typical example is the use of welded stainless steel tubes for exchangers. Whereas the base material can be suitable for the operating conditions, often the autogenous weld impair the corrosion resistance, leading to localized corrosiontube should not be used in most of the Urea plant equipment.

Fig. 5. Typical localized corrosion of a welded

The same is valid for all component selection: in general, standard components can be selected but not all standard components are suitable.

and pressures lower.In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is not true, making the difference band failure. Sometimes more troubles arise in this area than in the synthesis loop because less diligence has been exercised in materials


A typical example is the use of welded stainless steel tubes for exchangers. Whereas the base material can be suitable for

ting conditions, often the autogenous weld impair the corrosion resistance, leading to localized corrosiontube should not be used in most of the Urea plant equipment.

Typical localized corrosion of a welded

same is valid for all component selection: in general, standard components can be selected but not all standard components are suitable.

and pressures lower. In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is not true, making the difference b

Sometimes more troubles arise in this area than in the synthesis loop because less diligence has been exercised in materials


A typical example is the use of welded stainless steel tubes for exchangers. Whereas the base material can be suitable for

ting conditions, often the autogenous weld impair the corrosion resistance, leading to localized corrosion (Fig. 5)tube should not be used in most of the Urea

Typical localized corrosion of a welded tube


Page

In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is not true, making the difference between success

Sometimes more troubles arise in this area than in the synthesis loop because less diligence has been exercised in materials

Although common stainless steels can be used, the appropriate limits on chemical compositiomust be observed and quality checks carried out in many components to avoid corrosion which,

A typical example is the use of welded stainless

Whereas the base material can be suitable for ting conditions, often the autogenous

weld impair the corrosion resistance, leading to (Fig. 5). In general welded

tube should not be used in most of the Urea



Page 11 of

In general, but not always, common stainless steel and standard components can be used.Only with experience and deep process knowledge is it possible to know when this is

etween success Sometimes more troubles arise in

this area than in the synthesis loop because less diligence has been exercised in materials

Although common stainless steels can be used, the appropriate limits on chemical compositiomust be observed and quality checks carried out in many components to avoid corrosion which,



weld impair the corrosion resistance, leading to . In general welded




of 16

In general, but not always, common stainless steel and standard components can be used. Only with experience and deep process knowledge is it possible to know when this is

etween success Sometimes more troubles arise in

this area than in the synthesis loop because less diligence has been exercised in materials

Although common stainless steels can be used, the appropriate limits on chemical composition must be observed and quality checks carried out in many components to avoid corrosion which,



weld impair the corrosion resistance, leading to . In general welded







A typical case is the hydrolyser in the waste water treatment, where the stripping action of steam is used to removthe treated urea plant waste water condensate so as to maximize the hydrolysis of the urea content. That actually increases the corrosiveness of the fluid. Special test and chemical composition limits should prescribed for the staemployed. Further tests should be provided on the welds to assure the reliability of this equipment. 3. Several crucial choices shall be taken for the selection of the proper equipment. Hereafter some key points aregroups of machines: centrifugal compressors and centrifugal pumps. Centrifugal The required process performances are always the starting point for the compressor selection. Other points that need to be considered duthe selection process are the following:•


A typical case is the hydrolyser in the waste water treatment, where the stripping action of steam is used to removthe treated urea plant waste water condensate so as to maximize the hydrolysis of the urea content. That actually increases the corrosiveness of the fluid. Special test and chemical composition limits should prescribed for the staemployed. Further tests should be provided on the welds to assure the reliability of this equipment. 3. Rotating Equipment Design Several crucial choices shall be taken for the selection of the proper equipment. Hereafter some key points aregroups of machines: centrifugal compressors and centrifugal pumps. Centrifugal The required process performances are always the starting point for the compressor selection. Other points that need to be considered duthe selection process are the following:• Operating range, turndown capacity and

anti-A sufficient turndown capacity (required to permit an easy plant operation in all cases (i.e. startEORthe undesired opening of the antivalves. Anticarefully designed and a sufficient number of control loops for each compressor train shall be considered.


A typical case is the hydrolyser in the waste water treatment, where the stripping action of steam is used to removthe treated urea plant waste water condensate so as to maximize the hydrolysis of the urea

That actually increases the corrosiveness of the

Special test and chemical composition limits should prescribed for the staemployed. Further tests should be provided on the welds to assure the reliability of this equipment.

Rotating Equipment Design

Several crucial choices shall be taken for the selection of the proper equipment. Hereafter some key points aregroups of machines: centrifugal compressors and centrifugal pumps.

Centrifugal Compressors

The required process performances are always the starting point for the compressor selection. Other points that need to be considered duthe selection process are the following:

Operating range, turndown capacity and -surge

A sufficient turndown capacity (required to permit an easy plant operation in all cases (i.e. startEOR, etc.) and to avoid as far as possible the undesired opening of the antivalves. Anti-carefully designed and a sufficient number of control loops for each compressor train shall be considered.


A typical case is the hydrolyser in the waste water treatment, where the stripping action of steam is used to remove the NHthe treated urea plant waste water condensate so as to maximize the hydrolysis of the urea


Special test and chemical composition limits should prescribed for the staemployed. Further tests should be provided on the welds to assure the reliability of this


Several crucial choices shall be taken for the selection of the proper equipment. Hereafter some key points are discussed considering two groups of machines: centrifugal compressors and centrifugal pumps.

ompressors


Operating range, turndown capacity and

A sufficient turndown capacity (required to permit an easy plant operation in all cases (i.e. start

etc.) and to avoid as far as possible the undesired opening of the anti

-surge control system shall be carefully designed and a sufficient number of control loops for each compressor train shall be considered.


A typical case is the hydrolyser in the waste water treatment, where the stripping action of

e the NH3 the treated urea plant waste water condensate so as to maximize the hydrolysis of the urea


Special test and chemical composition limits should prescribed for the staemployed. Further tests should be provided on the welds to assure the reliability of this


Several crucial choices shall be taken for the selection of the proper equipment. Hereafter

discussed considering two groups of machines: centrifugal compressors

ompressors



A sufficient turndown capacity (required to permit an easy plant operation in all cases (i.e. start-up, shut

etc.) and to avoid as far as possible the undesired opening of the anti

surge control system shall be carefully designed and a sufficient number of control loops for each compressor train



and COthe treated urea plant waste water condensate so as to maximize the hydrolysis of the urea


Special test and chemical composition limits should prescribed for the stainless steel employed. Further tests should be provided on the welds to assure the reliability of this






A sufficient turndown capacity (Figrequired to permit an easy plant operation

up, shut-down, SOR, etc.) and to avoid as far as possible

the undesired opening of the antisurge control system shall be

carefully designed and a sufficient number of control loops for each compressor train



and CO2 from the treated urea plant waste water condensate so as to maximize the hydrolysis of the urea


Special test and chemical composition limits inless steel

employed. Further tests should be provided on the welds to assure the reliability of this



The required process performances are always the starting point for the compressor selection. Other points that need to be considered during


ig. 6) is required to permit an easy plant operation

down, SOR, etc.) and to avoid as far as possible

the undesired opening of the anti-surge surge control system shall be

carefully designed and a sufficient number of control loops for each compressor train

•

•

•

Fig. 6.

Material selectionStarting from the minimum process requirements the compressor material selection shall be properly checked considering problems of corrosion (i.e. wet CO2), but also allowable stresses. For this last point special consideragiven to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or millfrom solid forgings).

Mechanical behaviorDuring the selection process the rotordynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after purchase order award to the selVendor.

Quality controlThe requirements for inspections and tests shall be carefully defined during the design phase.

Fig. 6. Compressor perfor

Material selectionStarting from the minimum process requirements the compressor material selection shall be properly checked considering problems of corrosion (i.e. wet

), but also allowable stresses. For this last point special consideragiven to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or millfrom solid forgings).Mechanical behaviorDuring the selection process the rotordynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after purchase order award to the sel

endor. Quality controlThe requirements for inspections and tests shall be carefully defined during the design phase.

ressor perfor

Material selection Starting from the minimum process requirements the compressor material selection shall be properly checked considering problems of corrosion (i.e. wet

), but also allowable stresses. For this last point special consideragiven to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or millfrom solid forgings). Mechanical behavior During the selection process the rotordynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after purchase order award to the sel

Quality control The requirements for inspections and tests shall be carefully defined during the design

Page

ressor performance curves

Starting from the minimum process requirements the compressor material selection shall be properly checked considering problems of corrosion (i.e. wet

), but also allowable stresses. For this last point special consideration shall be given to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or mill

During the selection process the rotordynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after purchase order award to the sel

The requirements for inspections and tests shall be carefully defined during the design

Page 12 of

mance curves


), but also allowable stresses. For this tion shall be

given to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or mill

During the selection process the rotordynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after purchase order award to the selected


of 16

mance curves


), but also allowable stresses. For this tion shall be

given to impellers’ material. The tip speed shall remain well inside the limits of the applied material, of the references of the selected vendor and of the applied technology for the construction of the impellers (i.e. welded, brazed or milled

During the selection process the rotor dynamic issues shall be carefully evaluated. It is recommended to use always machines referenced for the same application with similar speeds and performances. At least a preliminary evaluation of the critical speeds and of the rotor stability shall be performed during the evaluation process. Final reports for lateral analysis, torsional analysis and stability analysis shall be reviewed after

ected





•

Centrifugal At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the centrifugal pumps. Other key points for the pump selection are the following:

•

•


Not only the final tests (i.e. mechanical running test, performance test etc.) shallconsidered. Inspections and tests on the raw materials (i.e. castings, forgings etc.) and during the manufacturing process (i.e. hydrostatic test, spin test, balancing test etc.) shall be also taken into account.

• Installation A properessential in order to achieve the best possible mechanical behavior and reliability. The required supervision shall be considered starting from the design and tender phase.

Centrifugal At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the centrifugal pumps. Other key points for the pump selection are the following:

• Operating A pump is properly selectedoperating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e. lower vibration levels, longebearings and mechanical seals etc.).

• NPSHa and NPSHrPumps shall be selected with a proper margin between NPSHa and NPSHr. Pump selection can be affected by an extremely low value of NPSHa. Some service in ammonia and urea plants are really senpump selection shall be carefully evaluated (i.e. SemiCarbamate Pumps).


Not only the final tests (i.e. mechanical running test, performance test etc.) shallconsidered. Inspections and tests on the raw materials (i.e. castings, forgings etc.) and during the manufacturing process (i.e. hydrostatic test, spin test, balancing test etc.) shall be also taken into account.Installation and cA proper installation and commissioning is essential in order to achieve the best possible mechanical behavior and reliability. The required supervision shall be considered starting from the design and tender phase.

Centrifugal Pumps

At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the centrifugal pumps.

Other key points for the pump selection are the following:

Operating pointA pump is properly selectedoperating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e. lower vibration levels, longebearings and mechanical seals etc.).NPSHa and NPSHrPumps shall be selected with a proper margin between NPSHa and NPSHr.

Pump selection can be affected by an extremely low value of NPSHa. Some service in ammonia and urea plants are really sensitive to this matter and for them pump selection shall be carefully evaluated (i.e. Semi-lean Solution Pumps, HP Carbamate Pumps).



and commissioninginstallation and commissioning is

essential in order to achieve the best possible mechanical behavior and reliability. The required supervision shall be considered starting from the design and tender phase.

umps

At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the centrifugal pumps.

Other key points for the pump selection are the

oint A pump is properly selectedoperating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e. lower vibration levels, longebearings and mechanical seals etc.).NPSHa and NPSHr Pumps shall be selected with a proper margin between NPSHa and NPSHr.

Pump selection can be affected by an extremely low value of NPSHa. Some service in ammonia and urea plants are

sitive to this matter and for them pump selection shall be carefully evaluated

lean Solution Pumps, HP Carbamate Pumps).



ommissioninginstallation and commissioning is

essential in order to achieve the best possible mechanical behavior and reliability. The required workhourssupervision shall be considered starting from the design and tender phase.

At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the


A pump is properly selectedoperating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e. lower vibration levels, longebearings and mechanical seals etc.).

Pumps shall be selected with a proper margin between NPSHa and NPSHr.



lean Solution Pumps, HP



ommissioning installation and commissioning is

essential in order to achieve the best possible mechanical behavior and

workhours for site supervision shall be considered starting from the design and tender phase.

At least the last two of the above points (i.e. Quality Control and Installation Commissioning) are fully applicable also to the


A pump is properly selected when the operating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e. lower vibration levels, longer life for bearings and mechanical seals etc.).

Pumps shall be selected with a proper margin between NPSHa and NPSHr.





Not only the final tests (i.e. mechanical running test, performance test etc.) shall be considered. Inspections and tests on the raw materials (i.e. castings, forgings etc.) and during the manufacturing process (i.e. hydrostatic test, spin test, balancing test

installation and commissioning is essential in order to achieve the best possible mechanical behavior and

for site supervision shall be considered starting

At least the last two of the above points (i.e. Quality Control and Installation and Commissioning) are fully applicable also to the


when the operating point will fall into the Preferred Operating Region, in other words when it is close to Best Efficiency Point (BEP). This is important not only for the efficiency but also for the mechanical behavior (i.e.

r life for

Pumps shall be selected with a proper




•

Mechanical Seal SelectionThe seal selection shall be done during the design phase considering the properties of the processfor each specific service shall be also taken into account.

For some services the installation of double pressurized seals with flushing from an external source is required (i.e. HP Ammonia Pump of seal arrangement is the one that guarantee the maximum level of reliability, safety and environmental protection. For those services having fluids that can crystallize butseals (Fig. 7) urea solutionflushing plan is necessary. For these service Urea CASALEinternal recirculation (API Plan 01) in conjunction with steam (or hot water) quench (API Plan 62) to the aside of the seal (

For melt urea required.

Mechanical Seal SelectionThe seal selection shall be done during the design phase considering the properties of the process fluid. The previous experience for each specific service shall be also taken into account.

Fig. 7. Mechanical seal

For some services the installation of double pressurized seals with flushing from an external source is required (i.e. HP Ammonia Pump of seal arrangement is the one that guarantee the maximum level of reliability, safety and environmental protection.

For those services having fluids that can crystallize but

(Fig. 7) can be appliurea solution, flushing plan is necessary. For these service

CASALEinternal recirculation (API Plan 01) in conjunction with steam (or hot water) quench (API Plan 62) to the aside of the seal (

For melt urea required.

Mechanical Seal SelectionThe seal selection shall be done during the design phase considering the properties of

fluid. The previous experience for each specific service shall be also taken


For some services the installation of double pressurized seals with flushing from an external source is required (i.e. HP Ammonia Pump for urea plants). This type of seal arrangement is the one that guarantee the maximum level of reliability, safety and environmental protection.

For those services having fluids that can crystallize but, where single mechanical

can be appli etc.), a proper selection of

flushing plan is necessary. For these service CASALE requirement is to apply an

internal recirculation (API Plan 01) in conjunction with steam (or hot water) quench (API Plan 62) to the aside of the seal (Fig. 8).

For melt urea a steam jacket is also

Page

Mechanical Seal Selection The seal selection shall be done during the design phase considering the properties of



For some services the installation of double pressurized seals with flushing from an external source is required (i.e. HP

for urea plants). This type of seal arrangement is the one that guarantee the maximum level of reliability, safety and environmental protection.

For those services having fluids that can where single mechanical

can be applied (i.e. melt urea, etc.), a proper selection of

flushing plan is necessary. For these service requirement is to apply an

internal recirculation (API Plan 01) in conjunction with steam (or hot water) quench (API Plan 62) to the a

).

steam jacket is also

Page 13 of

The seal selection shall be done during the design phase considering the properties of




for urea plants). This type of seal arrangement is the one that guarantee the maximum level of reliability, safety and environmental protection.


ed (i.e. melt urea, etc.), a proper selection of


internal recirculation (API Plan 01) in conjunction with steam (or hot water) quench (API Plan 62) to the atmospheric


of 16

The seal selection shall be done during the design phase considering the properties of



for urea plants). This type of seal arrangement is the one that guarantee the maximum level of reliability,


ed (i.e. melt urea, etc.), a proper selection of


internal recirculation (API Plan 01) in conjunction with steam (or hot water)

tmospheric





4. The reliability of ammonia and urea plants is a master to meet during the development ofengineering. The good and friendly control philosophy is the first issue to reach. The operator interface is a very important issue to take in consideration. It shall be easy to read and simple to operate in order to minimize the intervention in case of problems. The redundancy of the control and safety systems such as DCS or ESD in vais the way to balance the availability and the safety of the plants. The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is typically redundant but final needs are defined with dedicated SIL study. For field instrumentation attention shall be paid to critical High Pressure Control Valves for Urea service where type of material, particular design of the trim and body, jackets, heating and exto guarantee a good result. Those valves are the result of years of operation and modification to meet good a reliability


Fig. 8. Flushing plans ( 4. Instrumentation Design The reliability of ammonia and urea plants is a master to meet during the development ofengineering. The good and friendly control philosophy is the first issue to reach. The operator interface is a very important issue to take in consideration. It shall be easy to read and simple to operate in order to minimize the intervention in case of problems. The redundancy of the control and safety systems such as DCS or ESD in vais the way to balance the availability and the safety of the plants. The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is typically redundant but final needs are defined with dedicated SIL study. For field instrumentation attention shall be paid to critical High Pressure Control Valves for Urea service where type of material, particular design of the trim and body, jackets, heating and experiences of specialized vendors are the basis to guarantee a good result. Those valves are the result of years of operation and modification to meet good a reliability


Fig. 8. Flushing plans (

Instrumentation Design

The reliability of ammonia and urea plants is a master to meet during the development ofengineering. The good and friendly control philosophy is the first issue to reach.

The operator interface is a very important issue to take in consideration. It shall be easy to read and simple to operate in order to minimize the intervention in case of problems.

The redundancy of the control and safety systems such as DCS or ESD in vais the way to balance the availability and the safety of the plants.

The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is typically redundant but final needs are defined with dedicated SIL study.

For field instrumentation attention shall be paid to critical High Pressure Control Valves for Urea service where type of material, particular design of the trim and body, jackets, heating and

periences of specialized vendors are the basis to guarantee a good result. Those valves are the result of years of operation and modification to meet good a reliability


Fig. 8. Flushing plans (




The redundancy of the control and safety systems such as DCS or ESD in vais the way to balance the availability and the safety of the plants.

The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is typically redundant but final needs are defined with dedicated SIL study.




Fig. 8. Flushing plans (Source: John Crane




The redundancy of the control and safety systems such as DCS or ESD in vais the way to balance the availability and the

The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is typically redundant but final safety strategy needs are defined with dedicated SIL study.




Source: John Crane


The operator interface is a very important issue to take in consideration. It shall be easy to read and simple to operate in order to minimize the

The redundancy of the control and safety systems such as DCS or ESD in various aspects is the way to balance the availability and the

The minimum requirements of redundancy DCS are concentrated in the common parts plus those parts related to critical controls. ESD is

safety strategy needs are defined with dedicated SIL study.


periences of specialized vendors are the basis

Those valves are the result of years of operation and modification to meet good a reliability


Source: John Crane)

The reliability of ammonia and urea plants is a master to meet during the development of the engineering. The good and friendly control

The operator interface is a very important issue to take in consideration. It shall be easy to read and simple to operate in order to minimize the

The redundancy of the control and safety rious aspects

is the way to balance the availability and the

The minimum requirements of redundancy on DCS are concentrated in the common parts plus those parts related to critical controls. ESD is

safety strategy needs are defined with dedicated SIL study.


periences of specialized vendors are the basis

Those valves are the result of years of operation and modification to meet good a reliability

required on those type of plants.Other critical devices are the Safety Relief Valves flushingand safety of the plant. A steam injection is necessary crystallization in the seat discharge piping (Fig. 9)

In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and teststo verify the real conformity with the requirements. Critical instruments are on the area of urea reactor, stripper, compressor and pumps discharge. The engineering experience on these type of plants is a must to develop a good detail and a choice of the dedicated valves and instruments. 5. Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment. One of most importantsystem in the necessity of installing very large motors

required on those type of plants.Other critical devices are the Safety Relief Valves on urea service where heating and flushing is a must to obtain an high reliability and safety of the plant.

A steam injection is necessary crystallization in the seat discharge piping (Fig. 9)

Fig. 9. Relief valve for urea application

In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and teststo verify the real conformity with the requirements.

Critical instruments are on the area of urea reactor, stripper, compressor and pumps discharge.

The engineering experience on these type of plants is a must to develop a good detail and a choice of the dedicated valves and instruments.

Electrical Design

Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment. One of most importantsystem in ammonia and urea plantsthe necessity of installing very large motors

required on those type of plants.Other critical devices are the Safety Relief

on urea service where heating and is a must to obtain an high reliability

and safety of the plant.

A steam injection is necessary crystallization in the seat discharge piping (Fig. 9)

9. Relief valve for urea application

In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and teststo verify the real conformity with the requirements.

Critical instruments are on the area of urea reactor, stripper, compressor and pumps discharge.


Electrical Design

Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment. One of most important

ammonia and urea plantsthe necessity of installing very large motors

required on those type of plants.Other critical devices are the Safety Relief


and safety of the plant.

A steam injection is necessary crystallization in the seat discharge piping (Fig. 9).


In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and teststo verify the real conformity with the

Critical instruments are on the area of urea reactor, stripper, compressor and pumps


Electrical Design

Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment. One of most important issues of the electrical

ammonia and urea plantsthe necessity of installing very large motors

Page

required on those type of plants. Other critical devices are the Safety Relief


A steam injection is necessary for preventing crystallization in the seat of the valve


In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and teststo verify the real conformity with the



Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment.

issues of the electrical ammonia and urea plants

the necessity of installing very large motors

Page 14 of

Other critical devices are the Safety Relief on urea service where heating and is a must to obtain an high reliability

for preventing of the valve


In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care required during construction and tests necessary to verify the real conformity with the



Ammonia and Urea pants operation and reliability depend on safe and efficient operation on their electrical supply equipment.

issues of the electrical ammonia and urea plants is related to


of 16

Other critical devices are the Safety Relief on urea service where heating and is a must to obtain an high reliability

for preventing of the valve and


In general for transmitters the attention shall be made for instruments with diaphragms and capillary as they are not simply to bypass in case malfunctioning. The special care is

necessary to verify the real conformity with the



Ammonia and Urea pants operation and reliability depend on safe and efficient operation

issues of the electrical is related to


Steam injection

Steam injection




which may require direct-on-line starting. In particular, when compressors are not driven by steam turbine, for example CO2 compressor in Urea plant and Natural Gas and Syngas compressors in Ammonia plant, they are normally driven by very large induction or synchronous electrical motors. In that case, the network study is the best solution to analyse all the aspects related to the behaviour of the electrical distribution system; the results of load flow study, short circuit calculation, power system dynamic calculation, harmonic analysis, protection coordination and discrimination studies shall be carefully examined during engineering stage and appropriate solutions shall be adopted to anticipate any motor starting complications or undesired disturbing of external network which can be revealed during commissioning phase or when the plant is in operation. The arrangement of the power distribution and control system shall be defined matching the process and operating requirements with the most modern electrical equipment and control philosophy. For the power distribution, redundant feeding line is the typical supply concept adopted for the Urea and Ammonia plants. In case of power failure of both incoming feeders, an emergency feeding line (coming from external source or from a dedicated Emergency Diesel Generator) shall automatically feed the critical loads necessary to shut-down the plant and keep it in safe conditions. Automatic restarting of essential motors is usually applied to make operators task easier. On the other hand, high performance automatic transfer systems with suitable interlock/inter-trip and Power Distribution control System (PDCS), based upon one programmable logic controller,

represent the basis of the control system. 6. Piping Design Regarding plot plan development, engineers have today all the necessary instructions from dedicated and complete procedures and specifications. Moreover using modern technologies like 3D software and internet, all the aspects of safety, ergonomic, constructability, cost saving can be analyzed and solved by dedicated teams of engineers. Then we can say that almost everything is already written in the literature, we have just to apply it. We highlight just one of the aspects that most of the times are not sufficiently considered. We are speaking of developing and hand over a plant where the maintenance can be done using the staff normally dedicated to this activity in the modern plants. We are speaking of really few people that have the necessity to close activities more or less immediately after their assignments. Since technologies were not substantially modified in the years and Companies developed carbon copy plants, layouts for fertilizer plants have been consolidated, compacted and extremely rationalized during the years, using or cutting any available room in order to reduce construction costs. Most of the times, for Clients this means tight access to equipment, valves or instruments and additional manpower and devices are always used for the purpose, increasing costs and time. Also equipment, that is known to be replaced after a limited number of years, is sometimes placed where their removal is critical. Then in the ideal plant any heavy removable part, such as equipment or part of equipment, instruments, valves, devices, should be studied and places in such a way to be accessible and dismantled using always a crane.




This method allow to a small team to maintain easily and safely their plants. 7. Conclusions[D1] This paper has described all the aspects relevant to reliability that are always taken in consideration by CASALE during the design stages of ammonia and urea plants in addition to process performance. The design of a new plant requires deep knowledge on all engineering discipline, about possible problem as well as a specific experience in the relevant process. CASALE as engineering company has in-house specialist able to manage in efficient and professional way all the issues related to plant reliability. Moreover, CASALE, as a supplier of a wide range of proprietary equipment, has developed several solutions that best suits all needs required both for plant retrofitting to new plants design, which have been proven their reliability in the industry in several pants.



Reliable Design of Ammonia and Urea Plants

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