CHP – A Guide to Steam Conditioning/media/Files/C/CCI/pdf/539-CHP-brochure-2011.pdf · CHP – A Guide to Steam Conditioning Throughout the world, companies rely on CCI to solve

CHP – A Guide to Steam Conditioning

Throughout the world, companies rely on CCI to solve their severe service control valve problems. CCI has provided custom solutions for these and other industry applications for more than 80 years.

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2 15Overview of CHP Desuperheater Technology

CHP (Combined Heat and Power) is an Efficient Technology for Generating Electricity and Heat Together

A CHP plant is an installation where there is simultaneous generation of

usable heat (usually steam and sometimes hot water) and power (usually

electricity) in a single process. CHP is sometimes referred to as cogeneration ,

energy centres and total energy. The basic elements of a CHP plant comprise

one or more prime movers usually driving electrical generators, where the

heat generated in the process is utilized via suitable heat recovery equipment

for a variety of purposes including: industrial processes, district heating and

space heating. Figure 1 shows a possible configuration for a CHP plant. For the

purposes of this document we will cover Large Industrial Users.

The heat source can be established from many different sources. Waste heat

from process (e.g. ethylene, ammonia plants), incineration of waste, and waste

heat from gas turbine (also electricity generator) by a heat recovery steam

generator (HRSG) and from fossil fired boilers.

Once the industry has established its need for heat, it then has to determine

if the investment for power generation is economically viable. A study of the

economical benefits typically includes:

• Cost of the added investment

• Cost of added maintenance and man power

• Economical benefits to secure supply of power in case of external supply

failure. (key benefit)

• Cost of produced power compared with purchased power

Figure 1: Typical simple CHP scheme with gas turbine, heat recovery steam

generator (HRSG) and steam turbine

Figure 26:DA-O variable area nozzle desuperheater

Application: Desuperheating for Extraction and Exhaust Steam

As mentioned earlier control of the extraction and exhaust can be difficult

owing to the following.

Figure 25: Multi-nozzle DAM desuperheater

• Low velocity/ water fall out

• Insufficient cross sectional coverage

• Large piping diameters don’t encourage mixing

• Set temperature close to saturation is required

• Desuperheaters subject to transient conditions

• Excess water fall out creating inef-

ficiency, erosion, water hammer etc.

Key Components for Desuperheating

• Small inside Diameter + High Velocity = Good Mixing

• Quality of atomization proportional velocity2 (steam)

• Hotter Water = smaller water droplet dia(function of surface tension forces)

• More DP = smaller water droplet diameter

• Smaller water droplet diameter = quicker atomization

• Even distribution (across the area of the steam) of the spraywater regardless of steam flow

• Good Control of downstream temperature

• Installation

Solutions

CCI have several innovative styles of desuperheaters, but for extraction &

exhaust solutions, review and advice of the system is necessary. Aspects such

as liners, control, reduced sections of piping, location of instrumentation and

installation are all aspects necessary to meet performance requirements.

There are 3 stages to desuperheating:

• Primary. The spraywater is admitted into the steam via the nozzle.

The desuperheating nozzle can be either of the mechanical type or the

pneumatic type. The pneumatic type in this instance refers to steam

atomising. Mechanical relies on DP to provide spray pattern through nozzle.

• Secondary. This is where the momentum of the steam accelerates the water

droplets and this action breaks up the water droplets. The higher the velocity

of the steam the better the secondary atomization.

• Tertiary. This is where the water droplets evaporate in the steam when being

transported. If the velocity is to low or the size of the water droplets too

large, there will be water fall out. Time is required to complete this process.

To achieve excellent primary desuperheating:

• Variable area nozzles are used

which maintain excellent spray

pattern and fine constant droplet

size regardless of water flow.

• A swirl chamber to improves the

coverage of the spray pattern.

• Even distribution over the total

cross section.

• Avoid multiple spray patterns

recombining to form larger droplets.

• Accurate control of spray water with

well selected water control valve.

• High water turndown capability

(steam turndown is a function of

several other factors.)

CCI will provide the correct total system solution for the application.

CHP – A Guide to Steam Conditioning

Overview of CHP 2

Benefits of CCI CHP Technology 3

The Critical Role of Steam 4

Unique Requirements for CHP Steam 5

CHP Application Examples

Extraction/Exhaust 6

Bypass to Exhaust and Others 7

Condensing 8

Desuperheater 9

Vents, Startups & Silencers 10

Key Products for Severe Service Applications

VST-SE 11

VLB 12

DRAG® 13

Desuperheating 15

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14 3Benefits of CCI CHP Technology

Figure 2: Conventional fossil fuelled power station

Figure 3: For combined power plant CCPP

Figure 4: Conventional power plant with CHP

Figure 5: CCPP with CHP

Benefits and Examples of Industry Utilizing CHP Technology

CHP provides a secure and highly efficient method of generating electricity

and heat at the point of use. Due to the utilization of heat from electricity

generation and the avoidance of transmission losses because electricity

is generated on site, CHP typically achieves a 35% increase in efficiency

compared with power stations and heat only boilers. This can allow economic

savings where there is a suitable balance between the heat and power loads.

Figure 2 shows typical percentage gains and losses for conventional fossil

fuelled power station. Figure 3 similarly indicates typical figures for combined

cycle power plant (CCPP) incorporating electricity generated from the gas

turbine and a steam turbine. Figure 4 shows the immediate benefits in useful

energy in CHP when the steam turbine exhaust/extraction steam is utilised

as heat energy. Figure 5 indicates CHP plant with CCPP where electricity is

generated from the gas turbine and steam turbine and the exhaust heat energy

from the steam turbine is used for the process. Note that owing to the gas

turbine the proportion of useful electrical energy on Figure 5 is higher than

that in Figure 4.

The current mix of CHP installations achieves a reduction of over 30 percent in

CO2 emissions in comparison with generation from coal-fired power stations,

and over 10 percent in comparison with gas fired combined cycle gas turbines.

The newest installations achieve a reduction of over 50 percent compared with

generation from coal-fired power stations.

With this in mind both the EU and US have optimistic goals of increasing the

percentage of electrical generation by 2010 to approximately double the current

level. The USCHPA mission is to double the contribution of CHP to the nation’s

power supply (46GW in 1998 to 92GW by 2010.)

Examples of industry utilizing the CHP technology:

• Ammonia/fertilizer plants

• Incineration plants

• Chemical plants

• Pharmaceutical plants

• Pulp & Paper

• Sugar/food

• Power and desalination

• District heating

• Universities/hospitals

Typical industries that require hot water:

• District/Community heating

• Fish farming

• HVAC, heating, ventilating, and air conditioning

• Universities/hospitals

Velocity Control Technology

Applications for DRAG® Valves on CHP

The DRAG® trim can be installed in several body styles and can even

incorporate steam conditioning as a total system solution. Applications for

DRAG® in CHP in general are where service is particularly severe, for example

very high differential pressure, high risk of cavitation and especially when

there are strict low noise requirements. The DRAG® valve can be utilized for the

following example applications.

• Bypass to condenser (low noise)

• Vent valves

• Vent resistor

• Dump tube (low noise to air cooled condenser)

• Combined startup and feedwater control valves

• Boiler feedpump minimum flow recirculation control valves

• Startup valves

• Spraywater control valves

CCI DRAG® Benefits

• Low noise: depending on application, noise levels of 85 dBA or lower at

1 m are possible even with large flow and high DP. Working with CCI can

provide reduced total system noise rather than just individual product.

• Reliable operation: by controlling velocity.

• Longer valve life: controlling velocity and pressure head, preventing

damaging conditions such as cavitation.

• High performance: disk stack can be custom characterized to suit

particular application, such as boiler level control valve (feedwater control

valve.)

• Reduced maintenance cost & downtime: provide repeatable tight shutoff

utilizing high shutoff capability MSS-SP61 shutoff with 1000 pounds per

linear inch utilizing pressurized seat design.

• Reduced installation cost: custom designed including inlet/outlet

connections to suit application.

Figure 23: Steamjet® for high pressure drop/low noise applications

Figure 22: Low noise DRAG® for turbine bypass

Figure 24: Low noise DRAG® dump

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4The Critical Role of Steam

Why is Steam Required and at What Degree of Superheat?

Steam is normally required at a condition close to saturation owing to the

excellent heat transfer properties of saturated steam. The industrial process

will require heat at a particular temperature and the steam conditions will

reflect this. Examples of the use of steam are as follows:

• Paper making. Typically 3.5 bar a at 145 C

• Steam used for evaporating the juices in sugar making process are

normally at about 3.5 bar a at saturation.

• Steam used at varying pressure and temperatures for distillation and

cracking in a refinery.

For heat transfer purposes, it is important to provide the steam to the

process at a temperature as close to saturation as possible. If there is

too much superheat in the steam, then there the heat transfer at the

process will be inefficient. If the temperature is too high, then the steam

consumer’s equipment can be damaged or the paper run can be ruined.

Steam is therefore normally available from the steam turbine or its bypass

valve or a combination of both. It should be noted, the requirement of

the power plant is primarily to provide steam for the process (industry)

and generating electricity is merely a benefit as the electrical needs can be

imported if necessary, but if there is no steam then there may be not be

any production. Steam supply at the correct pressure and temperature is

therefore of the utmost importance to the relevant industry.

Steam Provided by Steam Turbine

Steam from the steam generator (HRSG), normally high pressure and

superheated, will pass through the steam turbine. The steam to the

relevant process can be taken from extraction or the exhaust of a steam

turbine if the turbine is of the backpressure design (see Figure 1.) The

temperature of this steam will vary depending on flow and will therefore

require temperature reduction (desuperheating) before being supplied to

the process.

Steam Provided by the Bypass Valve

If the steam turbine is not available, then the bypass valves is utilized to

condition the steam to the exact conditions required for the process. In

some cases, the steam flow through the turbine does not meet the process

demand and in this case the bypass valve must respond to make up the

difference between the process demand and that being supplied by the

steam turbine. Availability of turbine bypass valve is therefore critical to

overall profit of the associated industry.

13

Figure 6: CHP in pulp and paper application using VST-SE steam conditioning valve.

Figure 7: VST-SE providing low pressure steam for paper making.

DRAG® — Velocity Control Technology

How to Solve Severe Service Valve Problems

Uncontrolled flowing velocity—erosion—a control valve’s worst enemy. High

velocity fluid or gases as a result of high pressure drop or large change in

pressure ratio creates velocity, which if to high causes cavitation and or erosion

resulting in valve failure (refer Figure 19.)

Even today, despite widespread attempts to copy the CCI DRAG® solution is

unique in solving this, utilizing multi flow paths and introducing the required

number of pressure reducing stages. Refer to CCI DRAG® brochure.

Taming Velocity

Fortunately, the solution is found in basic engineering principles.

The fluid in a valve reaches its maximum velocity just slightly downstream of

the valve trim’s vena contracta or minimum flow area. This high velocity in a

single path or multi-path design can produce cavitation, erosion and abrasion

— all of which can quickly destroy the valve. Even before damaging the valve,

the symptoms of excessive noise, severe vibration, poor process control and

product degradation may be observed.

DRAG® velocity control valves from CCI solved the problem a generation ago.

DRAG® valves prevent the development of high fluid velocities at all valve

settings. At the same time, they satisfy the true purpose of a final control

element: to effectively control system pressure over the valve’s full stroke.

Here’s how the DRAG® valve accomplishes what the others can only approach:

• The DRAG® trim divides flow into many parallel multi-path streams

(Figure 21.) Each flow passage consists of a specific number of right angle

turns—a tortuous path where each turn reduces the pressure of the flowing

medium. By increasing the number of turns, damaging velocity can be

controlled while an increased pressure drop across the control valve can be

successfully handled.

• The number of turns, N, needed to dissipate the maximum expected

differential pressure across the trim is determined by limiting the velocity

to an acceptable level, then changing element = 2gh/N and solving for

N. Applying this principle to the DRAG® valve’s disk stack and plug as

shown in Figure 20 means that velocity is fully controlled in each passage

on every disk in the stack and that the valve can operate at a controlled,

predetermined velocity over its full service range.

• In the DRAG® trim, the resistance, number and area of the individual flow

passages is custom matched to the specific application and exit velocities

are kept low to eliminate cavitation of liquids and erosion, vibration and

noise in gas service.

Velocity Control Technology

V2

V1

V2

V2 V1

= 2gh

>

VenaContracta

Figure 19: Uncontrolled velocity – a control valve’s worst enemy.

Figure 20: Single-stage and single-path pressure reduction.

Figure 21: DRAG® disk multi-trim multi flow path

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Unique Requirements for CHP Steam512

What are the differences in the requirements for steam conditioning equipment in a CHP plant compared with a conventional fossil fuel power station?

In a power station the reason for using steam conditioning (steam turbine

bypass) equipment is to allow quick and easy start and stop and to protect the

equipment in case of turbine trip etc. These demands are also applicable to a

CHP plant, with the addition that there is a requirement for tightly controlled

Parameters to meet the downstream process requirements. Here are some

examples of the erroneous conditions that steam conditioning Equipment has

to handle on a CHP plant.

• While the steam conditioning valve (bypass) on a power plant must

open sufficiently quick to prevent safety valves from opening, the steam

conditioning valve on a CHP must additionally open quickly enough to

prevent pressure fluctuations in the process header, this can be sometimes

less than 1 second.

• Downstream temperature control on a CHP plant is far more critical

than on conventional power plants. Typically the temperature should be

within parameters acceptable for the condenser or reheater, while in a CHP

application it has to be close to the set point.

• The CHP steam conditioning valve will operate more frequently and can

depend on several factors.

a) Steam turbine not in operation.

b) Export of electricity to the grid may or may not be required and the

bypass will provide the required flexibility in operation.

c) It has to make up the shortfall of steam supply from the steam

turbine compared to system demand.

d) Sometimes the steam conditioning valve can operate almost

constantly.

• Extreme turndown with respect to control of steam flow to the correct

temperature at can be expected. If for example the steam turbine available

supplies to a process is 39T/hr and demand is 40 T/hr, the bypass will

have to supplement with 1T/hr, thereby requiring 40 to 1 turndown.

If the bypass valve can only achieve 5 to 1 turndown, it would have to

supplement a minimum of 8T/hr and the steam turbine would have to

back down to 32T/hr, meaning that there is 7T/hr not going through

the steam turbine resulting in lost revenue, due to decreased electrical

production. Assuming inlet steam conditions to be 80 bar a at 520 C and a

back pressure of 4 bar a, this 7T/hr would equate to a power loss of 1.4MW.

Every CHP plant is unique and requires system understanding to provide not

only the correct equipment, but also knowledge and experience regarding

aspects such as installation and control. Consult with CCI, who have more

than 80 years of expert knowledge and experience, to establish best practice

and operational performance for your CHP plant.

Steam Conditioning Technology

Variable area nozzles

Yes qty as required

In-Line Repair YesMaterial of

ConstructionTo suit application

Shut-off Class III, IV or V, MSS SP 61Plug Size 28-400 mm/1.1” – 16”

Characteristic Modified LinearStem Guiding, 2

PositionsYes

Equivalent Rating

To Cl 2500 (PN420)

Max Temperature

Up to 600 C

Pressure Reducing Stages

Up to 8

Valve Specifications and FeaturesApplication: Bypass to Condensor

The VLB was designed as a steam turbine bypass valve and is widely used for

bypass or dump to condenser. The requirements are:

• Allow independent operation of

the steam turbine and the

H.R.S.G. during startup

• Bypass the turbine in the event

of a turbine trip (<2 seconds)

• Stabilize steam header in the

event of island operation

• Allow flexible plant operation

• In the event of short-term

process trip, The bypass valve

will stabilize system.

The bypass system with VLB will benefit from:

• Reliable operation: suitable for

up to 300 C thermal shock.

• High performance and stable

control: system stability despite

pressure, flow and temperature

transients with CCI total system

understanding implemented.

• Reduced maintenance cost &

downtime: provide repeatable

tight shutoff despite exposure to

thermal shock.

• Accurate control of final steam

conditions to condenser,

preventing condenser damage

owing to overspray and

vibration.

• Low noise (DRAG® dump tube

used if noise requirements are

onerous.)

• Custom design of bypass valve

inlet/outlet connections to suit

application.

The Solution

The VLB design in conjunction with a CCI dump tube can simply and easily

cater for these requirements as the valve was designed to meet these arduous

requirements.

• Thermal shock: forged fully machined valve body both outside and inside

to handle thermal fatigue which is critical for reliable service.

• Flexible seat and excellent guiding. Thermal change can cause crushing

of the seat as the body contracts. The special two piece seat prevents

crushing of the seat. Good guiding ensure that the valve can be installed

horizontally or vertically without risk of sticking.

• Multi nozzle variable area desuperheating. Variable area nozzles provide

excellent atomization regardless of water flow and are easily exchangeable.

The nozzles are positioned around the circumference to provide an even

distribution and to best utilize the energy of the high velocity steam

exiting the diffusers which further atomize the desuperheating spraywater.

The nozzles are after pressure reduction and situated so that cool

spraywater will not impinge on components or piping. The combination

results in reliable and accurate control of outlet conditions.

• Pressure sealed bonnet: maintains tightness regardless of temperature

transients.

Figure 17: Bypass to water cooled condensor

Figure 18: VLB desuperheating features

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6 11CHP Application Examples – Extraction/Exhaust

Turbine Extraction/Exhaust

The outlet steam temperature from extraction or exhaust varies depending

on the steam going through the steam turbine. For example, considering

exhaust steam only, as the steam flow through the turbine decreases, the

outlet temperature increases. Depending on the exhaust flow in general as

the extraction flow reduces, the extraction steam temperature increases. This

means to obtain a constant set temperature downstream, the proportion of

spraywater required at low flow is higher than compared to at full flow where

the requirement will be small if any at all.

On most CHP plants, the exhaust line can be of a large diameter and in

view of the conditions detailed above combined with the large diameter and

potentially low flow, providing good temperature control to the process close to

saturation can be extremely difficult and needs special consideration.

CCI with extensive experience and knowledge can provide installation

guidelines and recommendations in conjunction with the correct product

selection for the optimum system solution.Figure 8: Turbine extraction/exhaust desuperheating

Why Severe Service Solutions Recommended Input Data for Selection

Large diameter process piping

Desuperheater should provide good cross sectional coverage

Steam flow rate, max, min normal

Steam pressure

Upstream steam temperature

Required downstream temperature

Pipe diameter and schedule

Water pressure and temperature available

Atomizing steam if applicable

Design pressure of steam

Design temperature of steam

Design pressure and temperature of water

Type of actuation pneumatic, electric or hydraulic

Failure mode

Use multi-nozzle configuration

High rangeability of steam flow tending to water fall out at low flow

Installation should incorporate smaller diameter piping

Partial or full steam atomization

Control set point temperature close to saturation

Use hottest water available for desuperheating

Utilize enthalpy control

Consider steam atomization

Increase velocity at point of desuperheating

Application: Exhaust or Extraction Steam to Process Desuperheating

Steam Conditioning Technology

Application: Bypass to Process

The VST-SE was designed as a steam turbine bypass to process conditioning

valve. The requirements are to open and close very quickly (refer to application

examples) in response to a turbine trip, startup or to provide additional steam

flow to the process. This means that the system will benefit from:

Benefits

• Reliable operation: suitable for up to 300 C thermal shock

• More revenue owing to higher electrical production. This is achieved by

providing high turndown capability with regard to steam flow.

• High performance and stable control: system stability despite severe

transients with respect to pressure and flow. Solved by integral

water proportioning.

• Reduced maintenance cost & downtime: provide repeatable tight

shutoff despite exposure to thermal shock.

• Maximize plant flexibility: the VST-SE provides modulating steam

atomization. Generally standard systems provide on/off atomization

requiring at least 5% steam for atomization.

The Solution

The VST-SE design is unique as it can simply and easily cater for these

requirements as the valve was designed to provide solutions to these

requirements.

• Thermal shock: forged fully machined valve body both inside and

outside to handle thermal fatigue, critical for reliable service.

• Steam atomized desuperheating: Steam is bled through the central stem

to atomize the spraywater. From 0-15% of stroke, (0-5% of steam flow)

the control of steam is only through this channel and is controlled by

the positioning of the main plug and which uncovers sequential holes

leading to the atomizing channel. Above 15% (5% flow), then the main

cage proper opens and the steam flow modulates normally through

control section providing a linear characteristic. The total characteristic

will therefore be modified linear providing excellent control at low flow.

With steam atomization the VST-SE will achieve turndown with respect to

desuperheated steam flow of greater than 50 to 1.

• Water proportioning: As steam flow modulates, the spraywater flow is

proportioned mechanically by a unique system linked to the main steam

plug. This minimizes temperature spikes and enhances system stability

regarding temperature control.

• Flexible seat and excellent guiding. Thermal change can cause crushing

of the seat as the body contracts. The special two piece seat prevents

crushing of the seat. Good guiding ensure that the valve can be installed

horizontally or vertically without risk of sticking.

Figure 14: VST-SE valve

Figure 15: VST-SE features

Figure 16: VST-SE mini valve

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CHP Application Examples – Vents, Startups and Silencers

Steam Turbine Bypass to Extraction/Exhaust for Back Pressure Turbines

The steam turbine bypass is used to reduce the pressure and temperature of the

steam to match the appropriate extraction/exhaust conditions. These valves are

used during startup, in the event of a turbine trip, non availability of the steam

turbine or to supplement steam to process that may not be available from the

extraction or exhaust from the steam turbine.

The bypass valve should:

• Be suitable for severe thermal shock (up to 300 C)

• Modulate in 2-3 seconds or less. Snap action in this time is not

acceptable as the boiler will trip and the system will be unstable.

• Have high range ability to maximize turndown

• Provide repeatable tight shutoff

• Inline repairability

• Be of low noise design

Reliability of this valve is of the utmost importance. Non availability of this

valve can often mean loss of production. CCI with extensive experience

and knowledge can provide installation guidelines and recommendations

in conjunction with the correct product selection for the optimum system

solution.

Figure 9: Steam turbine bypass to extraction/exhaust for back pressure turbine

Exhaust

Extraction


Noise and vibration

Control of inlet and outlet velocity by providing connections to suit application/piping Steam flow rate, max, min normal

Upstream steam pressure

Upstream steam temperature at the applicable steam flow rate

Required downstream pressure


Pipe diameter and schedule, inlet and outlet

Water pressure available

Water temperature available

Design pressure of upstream and downstream steam.

Design temperature of upstream steam

Design pressure of water

Design temperature of water

Actuating speed

Type of actuation (pneumatic hydraulic)

Noise requirements

Failure mode

Multiple pressure reduction stages

Thermal shock, up to 300 C in less than 2-3 seconds

Forged circular section body machined on inside and outside to provide even material distribution

Pressure seal bonnet

High rangeability of steam flow

Steam atomizing to avoid water fall out at low flow

Modified linear characteristic, typically from opening, 15% stroke = 5% capacity

Piston double acting pneumatic actuators or hydraulic actuators


Use hottest water available for desuperheating, typically above 100 C if possible

Proportion water flow with steam flow to prevent temperature spikes and water fall out

Steam atomization


Increase velocity by reducing pipe diameter at point of desuperheating

Application: Bypass to Exhaust or Extraction Process Line

CHP Application Examples – Bypass to Exhaust

CHP Vent Valves, Startup valves and Silencers

Startup vents are used to warm up piping in the various header. In the case

of the HP header, the steam should be superheated to preset pressure and

temperatures before steam can be admitted to the turbine. The vent can also

be used in the process headers for warming up the long length of piping.

Furthermore if for example the process shuts down for a short time and there is

a need to keep the gas turbine generating electricity, then it may be necessary to

vent the steam (assuming there is no dump condenser.)

Requirements of vent valves and silencers:

Figure 12: CHP venting and startup systems

Figure 13:DRAG® vent resistor with shroud

Other severe service valves and desuperheaters which are covered in other CCI

literature. They include:


Noise and vibration

Control of inlet and outlet velocity by providing connections to suit application/piping Steam flow rate, max, min normal



Pipe diameter and schedule inlet



Actuating speed

Type of actuation (pneumatic electric or hydraulic)

Failure mode (normally closed)

Noise requirements

Multiple pressure reduction stages

Consideration of silencer, or resistor



Angle pattern body to provide integrity against thermal transients

Provide adequate drains and preheating


High rangeability of steam flow Modified linear characteristic

Double acting pneumatic or hydraulic actuators

Leakage Class V repeatable tight shutoff

• If required to quick open

should be capable of handling

severe thermal shock.

• If required for operational

purposes, then valve and

silencer should be designed

for low noise.

• Be suitable for severe thermal

shock (up to 300 C)

• Modulate in 2-3 seconds or less



• Main and booster feed-pump

recirculation

• Startup and main feedwater

regulation

• Deaerator level control

• Spraywater control

• High level heater drains

• Auxiliary steam PRDS

• Blow-down, continuous &

intermittent

• Boiler attemperators, final and

inter-stage

• HP/LP heater bypass

• Economizer mixer valve

(3 way valve)

Application: CHP Venting and Startup Systems

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CHP Application Examples – Desuperheater

Desuperheaters for Controlling Final Temperature

On some CHP plants, the process that requires the steam can be quite a

distance from the CHP plant and to prevent excessive water fall out and

wetness in the steam close to the plant, the steam exiting the CHP plant can

have a higher than required temperature to allow for the temperature drop

owing to the transportation time. The drop in temperature is a function of

distance, ambient conditions and flow rate at the time.

As the inlet temperature will fluctuate to the process (higher the flow, hotter the

steam owing to less time for transportation loss) then there is a need to have

a final control of the temperature close to process using the steam. It should

also be noted that there may be more than one process demanding the steam

and each one may require a separate desuperheating station to control the

temperature.

As the process will benefit from a temperature close to saturation, and with the

little superheat available in the steam, the energy available for atomising the

spraywater is low and this application sometimes requires special consideration

particularly in large diameter piping and high turndown requirements.

Figure 11: Desuperheater for trimming process temperature


Large diameter process piping

Desuperheater should provide good cross sectional coverage in steam flow

Steam flow rate, max, min normal

Steam pressure



Pipe diameter and schedule



Atomizing steam if applicable design pressure of steam

Design pressure and temperature of steam

Design pressure and temperature of water

Type of actuation (pneumatic electric or hydraulic)

Failure mode (desuperheater to close or open)

Variable area nozzle to provide good evaporation of water regardless of flow

High range ability of steam flow tending to water fall out at low flow

Installation should incorporate smaller diameter piping to increase velocity at point of desuperheating

Partial or full steam atomization


Use hottest water available for deuperheating

Utilize enthalpy control with and trim with temperature feedback & CCI guidance


Increase velocity by reducing pipe diameter at point of desuperheating

Application: Desuperheaters for Trimming Process Temperature

CHP Application Examples – Condensing

Condensing Steam Turbine with Extraction

Depending on the proportion of output energy with respect to electrical versus

heat, sometimes a CHP plant, will have a higher proportion of electricity

output. To facilitate this, the turbine will exhaust to condenser and extract

steam to process. Sometimes when electricity price is at a premium, the gas

turbine will continue to generate electricity even in the event of non availibility

of steam turbine, as the waste heat needs to be removed from the heat recovery

steam generator (HRSG.) On some occassions, the process may be stopped for

short periods and excess steam can be dumped to the condenser to keep the

system stable and when the process start again the condenser bypass will close

and the steam will continue to process.

The bypass valve to condenser should:

• Be suitable for severe thermal shock (up to 300 C)

• Modulate in 2-3 seconds or less. Snap action in this time is not

acceptable as the boiler will trip and the system will be unstable.

• Have high rangeability to maximize turndown



• Be of low noise design

Reliability of this valve provides optimum plant flexibility. CCI with

extensive experience and knowledge can provide installation guidelines and

recommendations in conjunction with the correct product selection for the

optimum system solution.

Figure 10: Condensing steam turbine with extraction


Noise and vibration

Control of inlet and outlet velocity by providing connections to suit application/piping

Steam fow rate, max, min normal



Condenser pressure

Required enthalpy of steam to condenser

Pipe diameter and schedule inlet





Design pressure of water

Design temperature of water

Actuating speed

Type of actuation (pneumatic or hydraulic)

Failure mode (normally closed)

Noise requirements

Multiple pressure reduction stages, introduce sufficient stages to meet noise and vibration requirements

Consideration of dump tube, single stage or resistor



Provide adequate drains and preheating


High rangeability of steam flow

Multiple variable area orifice desuperheaters circumferentially mounted

Modified linear characteristic

Double acting pneumatic actuators or hydraulic actuators

Control of >30% of water to steam without vibration and damage to condenser

Utilizing CCI enthalpy control algorythm

Proportion water flow

Water cooled condenser use single stage dump tube

Air cooled condenser < 90 dBA use resistor type dump to condenser device

Careful installation to ducting for air cooled condenser

Application: Turbine Bypass to Condensor

CHP – A Guide to Steam Conditioning/media/Files/C/CCI/pdf/539-CHP-brochure-2011.pdf · CHP – A Guide to Steam Conditioning Throughout the world, companies rely on CCI to solve

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