24631009 NACE Paper Materials of Construction for Refinery Applications

Paper No.

614CORROSION 96The NACE International Annual Conference and Exposition

MATERIALS OF CONSTRUCTION FOR REFINERY APPLICATIONS

Tom FarraroCITGO Petroleum Corporation

PO BOX 1562Lake Charles, LA 70602

Richard M. Stellina Jr.Unocal San Francisco Refinery

2019 Mira VistaEl Cerrito, CA 94530-1740

ABSTRACT

In today’s modern highly complex oil refineries proper material selection has become one of the most important factors inthe design and repair of refinery processing equipment. Proper material selection will ensure that the expectations of thedesigners for safety, reliability and economy are actually realized. On the other hand, improper materials selection canresult in unexpected equipment failures which can lead to significant losses.

Kevwords: refinery, materials selection, designing, corrosion, corrosion rate, cost, steels ,alloys

Part I - Materials Selection

Designer’s Role in Controlling Corrosion

The designer is in an effective position to lower the tremendous cost of corrosion if he recognizes that there could be aproblem and he has the knowledge to act upon it. The designer should:

e Be aware of technical assistance that is available to him.

● Have a well-defined course of action to follow to determine optimum materials.

● Have the ability to calculate the most economical selection from a number of corrosion control methods thathave been determined to be able to perform well for a given application.

There are many alternatives in solving corrosion problems. Whether the choice is corrosion-resistant materials or lessexpensive materials using electrochemical techniques, coatings, inhibition, etc., for protection, the basis for the selectionrequires the recognition and appraisal of economic factors, as well as an understanding of corrosion technology.

Copyright~1996byNAcEinternational.Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Corrosion control is basically an economic problem. For these reasons, the designer should know how to calculate costs ofvarious alternatives for corrosion control.

A cost appraisal method should be used that will determine the one corrosion control method that offers the greatesteconomic advantage. The appraisal should consider all economic aspects such as safety/mechanical integrity requirements,expected equipment life, operational reliability, maintainability and costs. The cost of an anti-corrosion alternative is notonly the original installed cost, but it is all costs, including operating, maintenance, overhead, and various financia~ costssuch as interest rate, and depreciation. Each of these factors are described in more detail below.

Safety and Mechanical IntegriN is an evaluation of fitness for service and risk based upon the probability of an equipmentfailure occurring, the mode of failure and the consequences of a failure if one should occur. Some of the factors to beconsidered in evaluating safety and mechanical integrity of an equipment design are

● Potential for injury of personnel as a consequence of failure, including both short and long term effects ofexposure to process fluids.

● Potential for damage to the equipment itself as a result of a failure.

● Potential for collateral damage to equipment and the environment in area surrounding the failure

e Toxicity of the process fluids.

● Corrosivity of the process fluids and external environment; specific corrosion mechanisms which may occur.

● Susceptibility of selected materials to specific corrosion mechanisms, such as crevice corrosion, stresscorrosion cracking, general corrosion, etc.

● Sensitivity of the corrosion resistance of the material of construction to changes in process parameters such asprocess fluid composition, temperature, pressure, operational upsets etc.

Expected Equipment Life is the required length of time for which the equipment must perform in a safe and reliable mannerwithout significant maintenance expenditures. It is an estimate of the anticipated life of a specific equipment designalternative. This may be determined through past experience, published data laboratory tests, or pilot plant tests.

Operational Reliability is evaluated on the basis of how long the equipment must operate within operational performancelimits without significant operator intervention and the costs associated with any required intervention.

Maintainability is evaluated on the basis of the ease and cost of making any repairs necessary to maintain operationalreliability,

~ is the cost of an anti-corrosion alternative (not only the original installed cost, but all costs, including operating,maintenance, overhead, and various financial costs such as interest, depreciation, taxes etc.).

Materials Selection Philosophy

The selection of the proper material of construction is an important part of the designer’s job and is the one factor that isgenerally emphasized. However, consider all of the following factors that influence the equipment’s service life:

1, Selection of materials of construction2, Design details3. Specification of materials4. Fabrication and inspection5. Process operation6. Maintenance (cost and frequency)

These six factors which influence the equipment’s service life should always be kept in mind by the designer.

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In other words, for the best equipment or structural design, the materials of construction must be carefully selected from acorrosion-resistance standpoint. The design details should preserve the corrosion resistance of the materials. Concise andclearly written specifications should be provided to the supplier to ensure that the material needed is accurately ordered.The equipment should be fabricated properly and adequately inspected to prove compliance with the specifications. Theequipment must be operated within the specified design parameters (this last item is sometimes overlooked; plants maychange a process without sti]cient regard to the effect of the process change on the construction materials). Lastly, theequipment must be maintained properly. All of these factors must be considered by the designer to ensure the expected lifeof the equipment he designs. When corrosion failures occur, the selection of the materials of construction involved isusually faulted. However, in a large number of cases, failure actually occurred because of other factors. The followingexample may clarify this point:

A materials engineerwas called upon by a plant to determinebetter materialsof constructionfor a critical stainless steel heatexchangerthat corrodedso severelythat accordingto the plant, it had to be replacedfour times. The rejected heat exchangershadbeen disposedof in the scrapyard. Uponexamination,the engineerfoundthat the tubes of the rejected heat exchangerswereseverelycloggedwith corrosionproducts. Atler scrapingthese away,he founda bright stainless steel tube wall underneath,indicatingthat no corrosionhad actuallyoccurredthere. Further investigationdisclosedthat equipmentmade of carbon steelupstream fromthe heat exchangerwas unexpectedlydisintegratingby corrosion. These corrosionproductswere dischargedintothe process stream, The heat exchangeraffordedthe most restrictedarea in the system,and the corrosionproductscollectedthereand cloggedup the tubes. The corrodingcarbonsteel equipmentwas subsequentlyreplacedwith a more resistant material ofconstruction,and the plant operatedon cleanedexchangersfromthe scrapyard until the plant shut down years later. In this case,not onlywas the wrongmaterial faultedfor the failure,but the corrosionproblemhad not been properlydefined. It has been saidthat the definition of a problem is often the solutionto the problem,that axiomcertainlyprovedtrue here. 1

The designer is confronted with three primary concerns regarding materials of construction as he begins the design:

1, Material Properties

* Mechanical Propertv Recmirements - Tensile Strength, Fracture Toughness, Ductility, Fatigue Strength,High/Low Temperature Strength, Hardness, etc.

* Physical/Chemical Prot)erGIRequirements - Melting Point, Thermal Conductivity, ElectricalConductivity, Density, Magnetism, Radiation Resistance, etc.

* Corrosion Resistance to process and atmospheric conditions.

2. Practicality Factors

* AvailabiliW in the Required Product Form

* Ability to be Fabricated, Weldability, Formability, Castability, etc.

3. Economic Factors

* Design Life Expectancy

* Reliability (Mean Time Between Outages)

* Life Cycle Cost (i.e. $/year of life)

The first concern, involves mechanical properties of materials, such as tensile strength, yield strength, ductility, fatiguestrength, wear resistance, etc. Corrosion resistance is as important during the design stage as other operational conditions,such as velocity, temperature, pressure, etc. However, unlike mechanical stress, fatigue life, or temperature resistance, etc.which can be precisely predicted; the prediction of the destructive effect of corrosion is neither precise or reliable,particularly in new processes. Hence the need for an experienced materials engineer to aid in the definition of the potentialcorrosion problems associated with a selected material.

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The second concern deals with the practicality of actual utilization of the selected material. Is the material available in therequired form(s)? Can it be fabricated using conventional methods (welding, forging, casting, etc.)? Can this all beaccomplished within the applicable time and cost constraints? All of these concerns are, of course, very important, and,designers are usually well aware of them. However, designers may not be aware of possible corrosion problems.

And of course, the third and last primary concern is with the costs associated with a given design, including operations.maintenance and financial costs incurred during the entire design life of the equipment. Many times the initial fabricationcost is well defined, but the yearly operation and maintenance costs are either ignored or are so poorly defined as to beuseless. The designer should do everything possible to precisely define all costs associated with a given design to ensurethat the economic analysis used to compare options is accurate.

Optimum Materials Determination

The designer should study a variety of materials through review of past experience, published data, and pertinent tests.Then he should make a selection based on the material’s ability to do the job safely and economically. Candidate materialsrecommended by vendors can be included in the various materials to be evaluated, The designer is responsible for matchinghis unique materials problems with the best materials and design, which requires evaluating pertinent materials on themarket to attain the “best fit,” The best fit is not necessarily the most or the least expensive material. For example, perhapsall corrosion problems for a specific application would disappear if all equipment were made of platinum. Of course, aplant made of platinum would most likely be too expensive to be justified. The designer should use the optimum materialfor a specific application. Optimum simply means the least expensive material that will do an adequate and safe job.

Frequency of Corrosion Failures

During his evaluation the designer must be aware of the most probable corrosion mechanisms attributed to the specificprocess for which the equipment is being designed. The frequency of corrosion failures attributed to the various forms ofcorrosion has been studied by indushy corrosion specialists for many years. One such study was conducted by E. 1. du Pentde Nemours & Co., Materials Engineering group for seven years. and was based on reports from 23 company materialsengineers, Table 1 shows the results of this study.

Table 1- Results of Study on Corrosion Failures in the Process Industries

Forms of Corrosion Failure #of Occurrences 0/0 of Total

General Corrosion 372 31Stress Corrosion Cracking 288 24Pitting 120 10

I1I

Intergranular Corrosion 96 8Erosion-Corrosion 84 7Weld Corrosion 60 5Temperature Related Corrosion 48 4

ICorrosion Fatigue 24 I 2Hydrogen Permeation Related Corrosion 24 2Crevice Corrosion 24 2Galvanic Corrosion 24 2Dealloying 12 1End Grain Attack 12 1Fretting 12 1

Total 1200 100

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The first three forms of corrosion failures shown in Table 1, namely general corrosion, stress corrosion cracking, andpitting, constitute 65% of the over 1200 cases reported. The frequency of failure from general corrosion (3 1%] and pitting(10%) is probably indicative of most process type industries. The high frequency of stress corrosion cracking (24%) istypically due to facilities with corrosive processes where austenitic stainless steels are utilized. Thus, where there is apreponderance of these steels, one might expect a higher frequency of this kind of corrosion. The next four types ofcorrosion failures, namely intergranular, erosion-corrosion, weld corrosion, and high temperature corrosion, constitute 210/o

of the reported cases. The last seven kinds of corrosion failures make up only 110/0of the cases.

Estimated Plant Life

Most plants are designed to function for 10, 15, or, in some cases, 20 years or more. Corrosion allowances, even taking intoaccount some non-uniform penetration, have proven to be very useful in the design of process equipment. However, it mustbe emphasized that the use of corrosion allowances in predicting plant life are only meaningful if general corrosion ispredominate; that is that only substantially uniform corrosion will occur. Based on the results of the study shown in Table 1(30% general corrosion, 70% other corrosion), it is predicted that 70% of corrosion incidents in industry will be of a morelocal nature, such as stress corrosion cracking, pitting, intergranular corrosion, crevice corrosion, dealloying, etc.Corrosion rates are not very useful in predicting the life of equipment subject to these localized type of corrosionmechanisms.

Designingfor General Corrosion

General corrosion is defined as corrosion that attacks the surface of metals evenly and uniformly. General corrosion is themost prevalent form of corrosion in refineries. The designer should deal with this type of corrosion by selecting optimummaterials of construction by review of published data, previous experience or actual corrosion testing and then assigning acorrosion allowance to the equipment that is being designed.

Corrosion Rate Derivation and Calculation

Ever since man became interested in the corrosion of metals, there has been a need for a convenient way of designating justhow much or how little a material has corroded. Older texts on corrosion referred to “loss of milligrams per squaredecimeter per day.” These designations were not very useful except for comparison purposes because it was difficult tovisualize what the effect would be on process equipment. Today, the expected corrosion penetration into the wall of a vesselor tubing caused by corrosion is usually recorded as inches penetration per year(ipy) or roils [0.001 in,] penetration per year(mpy). This corrosion rate designation is used throughout industry. By knowing the weight loss of a corrosion specimenover a number of days and the overall area and density of the specimen, the designer can determine a penetration into thespecimen surfaces by corrosion.

Corrosion Allowance

Different metals and alloys have varying corrosion rates depending, of course, on specific environments. By knowing theexpected general corrosion rate and the expected plant life, the designer can calculate the extra wall thickness required forcorrosion resistance of the process equipment he is designing. After determining a wall thickness that meets mechanicalrequirements. pressure, temperature, weight etc., an extra thickness called a corrosion allowance is added to the wallthickness to compensate for the metal expected to be lost over the equipment’s life.

As an example, suppose a tank wall required a 3/16-in. wall thickness for mechanical considerations. The designer hasdetermined that the corrosion rate will be 15 mpy and the expected life of the equipment will be 10 years. The totalcorrosion allowance is 0.015 in. (corrosion rate per year) x 10 (years) = 0.15 in, The final wall thickness would be 0.15 +0.1875 = 0.3375 in, The designer would then speci~ a 3/8 in wall thickness as the closest standard plate available.

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Table 2- Guidelines for Evaluation of Corrosion Rates5

Quantitative Corrosion Ratempy Qualitative Description and Definition

(roils of penetration per year)

o-1o Insignificant corrosion rate, - materials would suffer no significant dimensionalchange during the life of the process equipment.

10-20 Low corrosion rate - materials with this rate would normally be specified with a1/16”- 1/8” (1,5-3,0 mm) corrosion allowance to ensure adequate equipmentlife; this rate can be considered the “normat” maximum allowed in refine~process equipment.

20-50 Medium corrosion rate - materials with this corrosion rate should only beutilized with a corrosion allowance of 1/4” (6.0 mm) or greater and whenfrequent maintenance and/or low reliability are acceptable.

>50 High corrosion rate - materials with this corrosion rate would require anexcessively high corrosion allowance to be added to ensure an acceptableservice life and therefore, would not normally merit consideration. Generally,these materials would be considered inadequate for general plant construction.

Designing for Other Corrosion Forms

All of the failure types listed in Table 1 can be best controlled by the designer understanding the conditions under whichthey occur and designing to avoid such conditions. Even though certain corrosion forms may indicate a low-failurefrequency rate, this does not mean that the designer should not consider them. For example, there could be 100’s of generalcorrosion failures that gradually leaked and were repaired one at a time without any plant shutdowns resulting, comparedto, one catastrophic hydrogen embrittlement failure that could suddenly shut down an entire plant.

If the designer is unfamiliar with the corrosion mechanisms associated with a given process, then a materials engineeringspecialist familiar with that process should be consulted before beginning the design work.

Using Professional Consultants

Many large companies, particularly in the refining indust~, have materials engineering groups comprising trainedengineers who work directly with designers at company plants to help reduce the costs associated with corrosion.

Materials engineers have intimate knowledge of the processes and corrosion problems in their assigned plants. They arethen available to the designers for consultation on any design problem. The designer, in turn, must have a basic knowledgeof corrosion to recognize when a problem may exist and when to consult a materials engineering expert. The fundamentalobstacle is getting corrosion problems confronted while the process is still at the “drawing board’ stage, rather than afterthe plant has been built.

Many smaller companies do not have the luxury of in-house materials engineering groups. Therefore, designers, in manyinstances, may have to rely on vendors of engineering materials for advice in the selection of materials of construction,design details, specifications, etc. There are many materials suppliers who provide beneficial information on the materialsthey manufacture, having conducted many corrosion and mechanical tests on specific products. They also have knowledgeabout how well their product has performed in the field. However, caution should be exercised by the designer in actingsolely upon the vendor’s recommendations. To be good salesmen, vendors have to be “sold’ on their own products. Forexample, the paint salesman may want to paint over everything, while the stainless steel vendor may want to makeeverything out of stainless steel. As a solution to this problem and to avoid discouraging designers from using vendors (asvendors can render usetld services), it is wise to have the designer follow an established procedure for evaluating differentalternatives so that he will have no doubt about which is the better alternative, in this case, paint or stainless steel.

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Professional corrosion engineering assistance is available from a variety of sources. The designer can inquire through hisown technical society or peruse advertisements in technical journals dealing with metals and corrosion for assistance. Forinstance, in Materials Performance, a monthly journal published by the NACE International, there is a section that listsavailable corrosion engineering assistance.

Specifying Materialsi

To assure that the designer will actually receive the materials which he went to so much trouble to select, he must furnishclear, concise specifications to the supplier, manufacturer, and/or fabricator. If the order is unclear, the supplier mayfurnish wrong or inadequate material.

Materials of construction for process equipment intended for use in corrosive service are generally specified in the followingthree broad categories:

1. Chemical composition and mechanical properties.2. Method of manufacture and heat treatment when required.3. Form, dimensional tolerances, and finish.

Regarding the first category, chemical composition and mechanical properties, many times the notation “killed carbonsteel” or “fire box quality steel plate” have been put on drawings to serve as complete specifications for the steel required.This kind of specification is equivalent to writing down “automobile” on a car order. The buyer may get a Chevette, andthen again, he may get a Cadillac. Killed steel or fire box quality steel could be low, medium, or high carbon steel, alloyedor not.

A exampleof this occurredwhenweldedtowerswere orderedand built foran Americaninstallationin Mexico. During a stoml,the towers fracturedand collapsed. The failurewas causedby brittle welds formedwhen mediumcarbon steel had beenfurnished for the towers instead of the anticipatedlow carbonsteel. At weldedareas, the weldingheat had raised the areasaroundthe welds abovethe lowercritical temperatureof the steeland whenquenchingoccurredin the air, brittle untemperedareas were formedthat fracturedunder the stressof the storm. An adequatematerial specificationhad either not been providedtothe fabricatorof the tower, or the fabricatordid not fullyunderstandthe requirements..

Therefore the designer must be sure that all requirements for the specified material are clearly stated and understood by allconcerned.

The second category, method of manufacture and heat treatment, is, also important. The method of manufacturing, such aswelding, brazing, silver-soldering, bolting, riveting, casting, forging, etc., must be specified because this will directly affectthe corrosion resistance of the equipment ordered. It is also very important that the heat treatment, when required, iscarefidly specified, as improper heat treatment can have very detrimental effects on the corrosion resistance as well as thestrength and ductility of steels.

Within the third category, dimensional tolerance and finish, it is important to ensure that all dimensions are adequatelyspecified. Specifications should include the allowable tolerances for all dimensions. With respect to corrosion, the wallthickness and corrosion allowance are probably the most important dimensions. However, finish can play a significant rolein some failure mechanisms such as fatigue and stress corrosion cracking. When specific finish requirements are specified,acceptable tolerances for the finish should also be included. For example, if a certain finish is required for the corrosionresistance of austenitic or chromium stainless steel equipment, the instructions should be more specific than “a smooth orpolished surface is required’. Specific surface roughness dimensions and acceptable tolerances should be provided.

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National Standards

An excellent way for the designer to assure that he will receive the process equipment from the fabricator as it was designedto reduce corrosion is to use national standards. National standards actually represent an agreement between fabricators orsuppliers and customers about what can and should be furnished. These standards are not permanent since they requireperiodic reviews that may result in an amendment, modification, or other change in a particular standard from year to year.National standards are valuable to the designer because standards:

1. Define what is commercially available together with optional requirements;

2. Provide a convenient reference on company specifications, drawings, and orders;

3. Reduce misunderstandings and minimize disputes;

4. Represent a production standard that result in a more uniform product, fewer varieties, lower inventories,and lower costs.

There are literally hundreds of standards available for use by the designer. A few of the organizations in the United Statesthat publish standards are shown in Table 3.

Table 3- U.S. Standards Organizations

Abbreviation Organization Namet I

AA Aluminum Association

AISI American Iron and Steel Institute

ANSI American National Standards Institute

API American Petroleum Institute

ASME American Society of Mechanical Engineers

ASTM American Society for Testing Materials

AWS American Welding Society

AWWA American Water Works Association

CDA Copper Development Association

CMA American Cast Metal Association

MTI Materials Technology Institute of the Process Industries

NACE NACE International

SAE Society of Automotive Engineers

TEMA Tubular Exchanger Manufacturers Association

Many governments have also developed many standards. For instance, the standards developed by the United StatesDepartment of Commerce acting through the National Institute for Standards and Technology are frequently used byindustry, as are standards issued by the Ordinance and Materials Departments of the US Navy, Army, and Air Force. Theseinclude standard specifications termed QQS-Federal, MIL-S Army-Navy Aeronautical Specs, and Aerospace MaterialSpecifications (AMS).

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Company Standardsl

Because of repetitive demand for certain items or special process specific requirements not completely covered by nationalstandards, many companies have produced their own standards. There also may be certain situations companies have todeal with that are not covered at all by national standards. Special materials specifications, welding, and inspectionprocedures may be required to address process specific corrosion problems such as sulfide stress cracking or hightemperature hydrogen attack. In small companies, national standards are sometimes modified to satisfi this need.However, standards sections are sometimes available in large companies to help the designer. In one large chemicalcompany, one group administers company standards. Thirty-one subcommittees, covering many areas such as welding,insulation, plastics, heat exchangers, and protective coatings, write the specifications and review and update themperiodically. Since these specifications address the unique problems of a company, they can of course be beneficial to thecompany designer.

What the Designer Should Remember When Writing Specifications]

When writing specifications, the designer should remember:

1. Make specifications as short as possible, but they must clearly define what is required and at what qualitylevel. The quality level provides assurance that process equipment will perform reliably and will not failprematurely.

2. Avoid vague statements such as “all equipment and piping after welding must be stress relieved. For instance,when field connections are to be made, stress relieving after welding is difficult, so threaded connections thatrequire no welding maybe substituted. The blanket statement above should not have included the fieldconnections, because costs would have been raised unnecessarily.

3. Do not simply specie that high quality welds are required without defining the level of quality required foracceptance. Failure to filly speci~ acceptance criteria for weld quality can lead to confusion and problems asdemonstrated in the following example.

Miles of 3-in. (76 mm) diameterweldedAISI304Lausteniticstainlesssteel pipes were producedby a singlemanufacturer. The originalpurchasespecificationrequired 100°/0radiographyof all the longitudinalweld seams. Whenlengthsof this pipe were field weldedinto fittings,the fieldwelds were radiographer, which revealednot only the fieldwelds themselves,but short portionsof the longitudinalwelds,which, in manycases,were verypoor. The pipe was cutout, and the pipe fabricatorwas contacted. The fabricatorstoutlymaintainedthat all of his pipe was x-rayquality and hepointed to a cabinet full of radiographs. We read theseradiographsand founda lot of evidencecalling for manyrejectionsand repairs. It was finallydisclosedthat the pipe fabricatorhad not viewedany of the radio~aphs. He thought thatpassingthe x-raysthroughthe weldsmade them x-rayquality! This storyis not a fabrication! Luckily,onlya smallamountof pipe had actuallybeen fieldweldedand all the poorwelds were identifiedand the pipe was rejected.

4. The designer must not only define the acceptance criteria for indications that will be cause for rejection, butmust speci~ the extent of inspection such as 5, 10, 33, or 100°/0of the welds.

5. Consider costs when writing specifications. The specification should not be so restrictive that satisfactoryquality material will be excluded. In addition, specifications should not restrict the manufacturers so muchthat his costs, and hence the price, will be unnecessarily high. On the other hand, the specification must notbe so vague that inferior quality maybe allowed.

6. Make safety paramount in any specification. For instance, if pneumatic testing must be conducted, all welds(that can be) should be inspected before testing. All welds that cannot be inspected by radiography orultrasonics because of geometry should be tested with the liquid penetrant or magnetic particle method alsobefore testing. (If a break occurs during pneumatic testing, catastrophic failure of the equipment tested canoccur.)

7. Whenever possible, to hold down costs, have the equipment made with commercially available materials usingstandard methods of construction.

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8. Before finalizing a specification, have the specification reviewed by potential fabricators. Many times, afabricator can suggest ways to save money because he certainly knows best how to build his product. Speci&primarily how the equipment should perform rather than detailing how the fabricator should build it, Forexample trays or packing in a fractionator tower are normally designed by the tray manufacturer rather thanthe tower designer. This, of course, assumes that the manufacturer is reliable, competent, and hasdemonstrated a capability of fabricating products of consistent quality at competitive cost.

9. Specifically note the tests required to assure quality.

10. Carefully note in the specification the equipment that is to be inspected during its manufacture so that thefabricator can make provisions for the inspector’s visits at the proper time.

11. Do not hesitate to speci~ a trade name or a catalog number for a product if that product will do the requiredjob. Again this, of course, assumes that the manufacturer is reliable, competent, and has demonstrated acapability of fabricating products of consistent quality at competitive cost.

Questions the Designer Should Ask to Control Quality’

A designer who is concerned about corrosion resistance must have some way to control quality; otherwise, he may notreceive from the manufacturer the corrosion performance he expects from the equipment he has designed. His qualitycontrol program should be outlined in the purchase order. Remember that quality can and should be controlled by thedesigner.

There are many inspection methods available to the designer, but before he specifies one or more of these methods, heshould answer the following crucial questions:

1.

2.

3,

4.

5.

6.

7.

8.

9.

10.

How corrosive are the process conditions?

How toxic are the stream components conditions?

How susceptible is the material of construction to a specific corrosion form, such as crevice corrosion or stresscorrosion cracking etc.?

How sensitive is the corrosion resistance of the material of construction to shifts in chemical composition?

What joining method is to be used? How sensitive is the corrosion resistance of the material of construction tothe method ofjoining, such as welding?

How competent is the fabricator? What reputation does he have for self inspection? Does he use codequalified welders? Does he have a formalized and documented QA/QC system?

Is heat treatment required (either for equipment stability or corrosion resistance)?

If heat treatment is required, how sensitive are the materials of construction to the heat treatment?

How sensitive was the material of construction to mill operations when it was originally produced?

If welding is to be the joining method, how important is the tiller metal to corrosion performance?

Based on the answers to these questions, a quality assurance program can be formulated.

As broadly defined by the American Society for Quality Control, quality is the totality of features and characteristics of aproduct or a service that depends on its ability to satisfi a given need. This can be briefly stated as fitness for service.

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Fitness for Service

The designer should decide what inspection or qualification methods are required to assure fitness for service. With this inmind, the designer should confer with his inspection department, his technical people, and the potential fabricators todetermine which inspection methods should be specified to assure quality. Inspection costs money, so care should be takennot to over-inspect, such as on routine jobs being done by proven competent fabricators. However, inspection under otherconditions can be thoroughly justified because of substantial cost savings and elimination of safety hazards.

Part II - Refinery Materials of Construction

Introduction

Pure metals and their alloys are the primary construction materials used in petroleum refinery and chemical plantconstruction. Metals have excellent mechanical urot)erties, that is, they respond well to external loads, Some importantmechanical properties are:

a) &KXEl!l the ability to withstand loads such as needed for refinery equipment pressure containment.b) Ductility the tendeney to bulge or tear rather than to burst or break.c) Tou~hness the ability to absorb impact loads without brittle fracture.d) Hardness an indicator of good wear resistance.e) Elasticity slight deformation is recoverable.

o Creep Stability low flow rate under load.

Metals also have some excellent chemical and ~hysical t)roperties, independent of load, that make them suitable for refineryapplications:

a) Oxidation resistance for scaling resistance at elevated temperatures.b) Corrosion resistance for durability under many adverse refinery environments.c) High melting mints necessary for stability at elevated temperatures.d) Thermal conductivity desirable for good heat transfer.

Metals and alloys also have excellent fabrication capabilities-including:

a) Weldable for ease of joining and alloy overlaying;b) Formable drawing. bending, upsetting, rolling;c) Castable complex shapes can be made;d) Machinable cutting, shearing, grinding;e) Heat treatable permits change and control of mechanical properties.

Low and medium carbon steels are used for at least 80 percent of all refine~ applications and, processes and mechanicaldesigns are often adjusted to permit its use. For example, process temperatures can be lowered, hydrocarbon streams can bedried, inhibitors can be injected. or generous corrosion allowances provided to accommodate the use of carbon steel.

As refining processes have developed and become more complex, so have the demands for suitable materials of constructionto handle more severe conditions of temperature, pressure and corrosivity. The “refine~ steels” have evolved to meet themajority of refinery equipment applications. Some of the refinery steels are listed in Table 6 along with their nominalcompositions. Note that there is an ascending order of alloy additions. Alloying elements improve the mechanical,chemical, and physical properties of steel and enable the handling of corrosive fluids over a wide range of pressures andtemperatures. For example, Cr-Mo steels provide high temperature strength, resistance to high temperature sulfur corrosionand hydrogen attack. Stainless steels are used for fhrnace tubes and to resist high temperature sulfidic corrosion in thepresence of hydrogen. Stainless steels containing molybdenum are used against naphthenic acid attack.

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But be aware, alloying steel does entail additional cost. Also the availability of some alloy steels is less than for plain carbonsteel, Also, alloy steels may have problems specific to the type of alloy such as reduced weldability, susceptibility toenvironmental cracking, and specialized heat treatment requirements etc.

Other metal and alloy systems that are important in refinery construction are copper, nickel, aluminum and titanium basedalloys. Table 7 shows these commonly used metals and alloys along with compositions and principal applications.

As discussed earlier, refinery materials selection is a balance between safety, performance, and cost. Because of thehazardous nature of materials processed by the refining industry, safety considerations demand exceptional equipmentintegrity. This section will describe the principal refinery metals and alloys along with typical applications.

Steels

Steel, iron alloyed with carbon and manganese, is the predominant material for refinery construction. It provides the abovedesired mechanical, chemical, and physical properties at a reasonable cost. Steels are readily available in many forms andhave excellent fabrication capabilities. The weldability of steel is excellent, and this contributes greatly to the reliability andsafety of modern day pressure containing equipment.

“steel” is a general term for iron based alloys containing carbon, manganese and other alloying elements. Table 8 showssome of the common alloying elements, their effects in steel, and principal functions. The carbon content of most refinerysteels is between 0.03°A to 0.30% to assure ductility and wektability.

Steels for refinery applications fall within the following categories:

● Carbon steels● Low-alloy steels● Cr - Mo steels● Stainless steels● Nickel steels

In the United States most are covered by chemistry and/or property specifications of one or more of these organizations: theAmerican Society for Testing Materials (ASTM, the American Society of Mechanical Engineers (ASME), the AmericanPetroleum Institute (API), the American Iron and Steel Institute (AISI), and the American National Standards Institute(ANSI), In other countries other standards organizations maybe utilized. Most specifications embrace a variety ofproducts or grades and these subtypes represent variations in chemistry, method of manufacture, and mechanical properties.Table 11 shows some of the ASTM specifications applicable to tubular products, plates, castings, and forgings.

The code or standard to which a piece of equipment is constructed normally specifies the materials standards to be followedand the design stresses that can be used. In the United States the most common design codes for refinery equipment arethose of ASME, ANSI, and API.

Carbon steel is iron containing controlled amounts of carbon and manganese. The carbon steels are among the mostcommon materials of construction and probably account for 80 percent of all steels used for refinery applications. Sincethey are typically welded, carbon content must be relatively low, between 0.15 and 0.35 percent, and they are commonlytermed low or medium carbon steel. Distillation towers, separators, heat exchangers, storage tanks, most piping, and allstructures are generally fabricated carbon steel. For processes where the expected corrosion rates for carbon steel are <20mpy, economic analysis will normally favor carbon steel at temperatures below 800° F.

When carbon steel is not suitable because of corrosion, it can be lined or coated with other materials that offer bettercorrosion resistance. For large vessels, alloy clad or alloy weld overlay are effective forms of lined construction and moreeconomical than the use of solid corrosion resistant alloys throughout. The use of spray applied coatings both, metallic andnon - metallic. have proven to be a cost effective method of improving corrosion resistance of carbon steel.

C-MO steels, primarily the C-1/2 Mo grade, will exhibit improved high temperature strength and creep resistance overcarbon steels especially at temperatures between 800 and 10000 F. However Mo addition provides no significant increasecorrosion resistance over carbon steels. In the past it had been believed that C-1/2 Mo steel had better resistance thancarbon steel to high temperature hydrogen attack, and it was often specified in hot hydrogen service, However, recently

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questions have been raised regarding the effect of long term exposure to high temperature hydrogen on C-1/2 Mo steel. Asa result, Cr - Mo steels are typically used instead of C-1/2 Mo for most new refinery equipment fabrication.

Low A11oYSteels are steels which contain 1% or less of Cr, Ni, Mo V, Cu, in various ratios. In the U.S. the standardcompositions of these steels are specified in the ANSI or SAE standards. Two of these low alloy steels are commonlyutilized in refineries. These are 4140 and 4340 which are steel with Cr, Ni, and Mo additions. These materials exhibit goodhigh temperature strength and creep resistance. However since these materials normally have relatively high carbonequivalents (>0,4), they can be difficult to weld. Therefore, their use is normally limited to those applications which do notrequire fabrication by welding. Also these materials tend to have high harnesses and are susceptible to sulfide stresscracking if harnesses exceed HRC 22. Common uses of these materials in refineries is for flange bolts, valve parts andshafts or rods in pumps and compressors.

Some specialized grades of these low alloy steels known as the HSLA (High Strength Low Alloy) steels are commonly usedfor high pressure gas transmission pipelines. These steels have their chemistry controlled to allow fabrication by welding.

Cr - Mo steels are alloys containing up to 10 percent chromium, and a few percent or less of molybdenum, copper orvanadium. In refineries early attempts to combat high temperature sulfidic corrosion in refineries involved the use ofstraight chromium steels. Although these steels originally had satisfactory ductility initially, prolonged service producedtemper embrittlement. The addition of 0.5-1 ‘Yo Mo into the straight Cr steels was found to be an effective solution for thisproblem, From a design point of view, the low alloy steels containing up to 9’%Cr and 1’%Mo are generally more costeffective than carbon steel at temperatures above 9000 F. Aside from the stainless steels, Cr - Mo steels are the only steelswhich are rated to 12000 F, in terms of allowable stresses by the ASME Pressure Vessel and ANSI Piping System Codes.

Cr - Mo steels with less than 4% Cr provide only a modest increase in corrosion resistance over plain carbon steels. Thesematerials are normally specified for applications where high temperature strength, creep resistance ancilor resistance to hightemperature/high pressure hydrogen attack are required.

The highest creep strengths are obtained with steels containing 1/2 percent or more molybdenum. It is not surprising tofind, therefore, that 1 1/4 Cr-1/2 Mo and 2 1/4 Cr-1 Mo steel is widely used in refineries for reactor vessels which operate athigh temperatures and pressures. For improved corrosion resistance, these are usually clad or weld overlayed withaustenitic stainless steels.

The $g~o Cr - Mo Steels provide good corrosion resistance to high temperature sulfur corrosion when required as inrefineries processing sour crude oils. These materials have found extensive use in refineries for this application.

Nickel steels contain 1 to 9’%Ni and have significantly greater low temperature toughness compared to plain carbon steel.The 2 1/4 Ni and 3 1/2 Ni steels have been used for low temperature refinery processes such as propane refrigerationsystems. With proper procedures and filler metals these steels can be welded such that the weldment impact propertiesapproach those of the alloyed base metal. The use of Nickel steels in refineries is generally limited to processes operatingbelow -500 F.

Stainless steels are alloyed with at least 11.5% Cr to become “stainless”. Cr promotes formation of passive iron/chromiumoxide films on steel which in turn exhibit excellent corrosion resistance. Many different grades of stainless steels areavailable, and their cost, mechanical properties and corrosion resistance vary considerably. It is important, therefore, thatstainless steels be carefully selected to match the specific service intended.

Various grades of stainless steels used in refineries are listed in Table 9. These are for wrought alloys; cast alloycompositions differ somewhat from the AISI types shown. Stainless steels can be classified into the following categories:

● Martensitic stainless steels* Ferritic stainless steels● Austenitic stainless steels● Duplex stainless steels● Precipitation hardening stainless steels● Specialty stainless steels

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Martensitic stainless steels, such as Type 410, and 440 can be hardened by heat treatment similar to carbon, low alloy andCr - Mo steels. Hardening increases strength and decreases ductility. These stainless steels are less corrosion resistant thanferritic and austenitic stainless steels. Martensitic stainless steels contain 11 to 18 ‘Y. Cr and are relatively high in carboncontent. They are subject to 8850 F embrittlement and must be used with caution above approximately 7000 F. They aremagnetic and difllcult to weld. All martensitic stainless steels will pit in the presence of chlorides. Sulfide stress cracking(SSC) can be a problem with martensitic grades hardened above Rockwell C22 (HRC 22) and welds normally require stressrelieving to meet this hardness requirement. Typical refine~ applications include pump components, fasteners, valve trim,turbine blades, and tray valve and other tray components in distillation towers. Type 410 linings are often used to protecttowers, heat exchanges, and other pressure vessels against high temperature sulfidic corrosion in desulfurization units.

Ferritic stainless steels, such as T~ 405,409, 410S (12% Cr), 430 (177. Cr), and 446 (26’XOCr) are low in carbon but canbe hardened by heat treatment. However, Type 405 contains an aluminum addition that effectively retards its ability toharden during welding. This makes it a better choice than Type410 for vessel linings, especially if clad repairs becomenecessaty during the vessels service life. The ferritic stainless steels are not normally subject to SSC, are resistant tochloride stress corrosion cracking, and have good oxidation and sulfidation resistance. All ferritic stainless steels withchromium content above 11 percent are subject to 8850 F embrittlement which limits their use to applications temperaturesthat do not exceed 7000 F. The high chromium stainless steels, such as Type 430 are also susceptible to pitting from wetsulfides in the presence of air during shutdown conditions.

Austenitic stainless steels, commonly referred to as the “300 series” or 18-8 chromium-nickel alloys, have excellentcorrosion resistance and good high temperature properties. However, they are subject to pitting corrosion and stresscorrosion cracking in the presence of chlorides. This has limited their use in refineries to applications where aqueouscorrosion can be ruled out. Austenitic stainless steels cannot be hardened by heat treatment or during welding. This hasencouraged their use in the place of 5°/0chromium and 9°/0chromium steels to avoid the need for postweld heat treatment.Like the ferritic stainless steels, they can be hardened to some degree by cold working. The most common and readilyavailable grades are Type 304, 304L, 304H, 316 & 316L 316H, 317, 321, 321H, 347, and, 347H.

The low carbon grades (designated by L or ELC) are required for optimum corrosion resistance when welding is to be done.The low carbon content of “L” grades (below 0.03 percent) minimizes the precipitation of chromium carbides at the grainboundaries (called sensitization) which can lead to various forms of intergranular corrosion in certain applications.Sensitization can also be minimized by selecting chemically stabilized grades such as Type 321 and Type 347. In thesegrades the formation of chromium depleting carbides is prevented by alloying with titanium or columbium (niobium)respectively, The high carbon grades (designated by the H suflix) arc normally specified for applications where additionalhigh temperature strength and creep resistance is required such as high temperature (> 1200°F) furnaces tubes. The ‘H’grades are more susceptible to sensitization than the regular or low carbon grades and special welding procedures mayberequired if resistance to intergranular corrosion is required.

Type 316 and Type 317 stainlesssteelare two populargradesthat contain2 to 3 and 3 to 4 percent molybdenum,respectively, and have superior resistance to pitting corrosion and acids. They also contain somewhat greater amounts ofnickel which results in general corrosion resistance superior to Type 304. Type 316 is also available in cast form (CF-8M)These steels are commonly utilized for resistance to napthenic acid corrosion in refineries which process napthenic crudes.

Type 309 (25 Cr- 12 Ni) and Type 310 (25 Cr-20 Ni) are austenitic grades commonly used where high temperatureoxidation resistance is desired. These wrought grades and their cast forms (CH-20, HH 40, CK-20, HK 40) are found infired heaters as tube supports and hangers.

Typical refinery applications for austenitic stainless steels include high temperature processes containing both sulfir andhydrogen, such as dcsulfurizers and hydrocrackers. They are commonly used in heater tubes, heat exchanger tubing,piping, tower trays, reactor internals, and as vessel linings in hydroprocessing units. The austenitics are also used in gastreating units to resist corrosion from HzS and COZ. The molybdenum grades Type 316 and 317 are often specified forheater tubes, transfer lines, and tower internals in units processing naphthenic acid containing crude and gas oils. Type 309and 310, usually in cast form, are found in fired heaters as tube supports and hangers. Caution must always be exercisedwhen considering austenitic stainless steels in aqueous environments and in cooling water systems because of the danger ofpitting and stress corrosion cracking from chlorides.

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Precipitation hardening stainless steels, such as 1’7-4PH, 17-7 PH, and 15-5 PH have application in refinery rotatingmachinery where both corrosion resistance and strength are needed. These stainless steels can be hardened andstrengthened by solution quenching, followed by a precipitation aging treatment at 800-11000 F. They can be easilymachined while in the solution quenched condition and then aged at temperatures that minimize scaling, distortion, andcracking. Tensile strengths as high as 200,000 psi can be obtained. Precipitation hardening stainless steels are used forvalve seats, pump shafts, pump wear rings, and impellers. Their corrosion resistance is somewhat worse than that of Type304 stainless steel. Also due to their high tensile strengths. these materials tend to be highly susceptible to stress corrosioncracking caused by sulfides and/or chlorides.

Duplex stainless steels have a microstructure composed of almost equal amounts of ferrite and austenite. Some alloydesignations are AL6XN, 2205, 3RE60, 2304, and Ferralium 255. The typical composition for the duplex alloys is18-30°Achromium, 3-70/. nickel, and 1-37. molybdenum. The ferrite phase offers high strength and the austenite providesgood corrosion resistance. When welding parameters are carefully controlled the duplex stainless steels have adequateweldability. They have good general corrosion resistance and are also resistant to chloride stress corrosion cracking, sulfidestress cracking, provided proper welding and heat treatment procedures are followed. The duplex stainless steels arenormally proprietary alloy compositions each developed by a specific steel manufacturer. Therefore, the steelmanufacturer should be always consulted to determine the correct forming, welding and heat treatment requirements foreach of these materials.

!%ecialm stainless steels are available to meet severe service conditions and fill the gaps where the corrosion resistance ofcommon stainless steels may be marginal. Some of these materials include austenitic alloys 20 Cb-3, 904L, and 254SM0,ferritic alloys SeaCure, E-Brite 26-4, Monit, and 29-4-2. Often these specialty stainless steels contain significantmolybdenum additions to decrease pitting and crevice corrosion. The specialty stainless steels are normally proprietaryalloy compositions each developed by a specific steel manufacturer. Therefore, the steel manufacturer should always beconsulted to determine if the corrosion resistance to the specific process is adequate and to determine the correct forming,welding and heat treatment requirements for each of these materials.

Cast Irons

Gray cast iron contains 3% carbon and 1.5’%.silicon with most of the carbon in flake form. Because of its inherentbrittleness and low strength, gray cast iron is susceptible to damage by thermal and mechanical shock. Although oncecommonly used for many refinery applications, it is no longer specified for hydrocarbon services within unit boundaries.Exceptions are pump and valve components, ejectors, strainers, and some fittings where the high hardness is needed toreduce the velocity effects of corrosion, such as impingement, erosion, and cavitation. The excellent damping properties ofgray cast iron leads to its continued use in machine~ bases. Although somewhat repairable by special welding techmques,gray cast iron is generally considered non weldable for pressure containing component repairs.

Ductile Iron, also called nodular cast iron, has replaced gray cast iron in valve, pump, and compressor pressure containingcomponents. The carbon is present as nodules which promote ductility. It has substantially better toughness than gray castiron but is not usually repaired by welding.

High silicon cast iron are gray cast irons containing at least 14 percent silicon. These cast irons are extremely corrosionresistant due to a passive SiOz surface layer which forms during exposure to many chemical environments. Duriron is astraight high silicon cast iron containing about 14.5 percent silicon, 1 percent carbon, and up to 15 percent manganese.Durichlor51 also contains 4 to 5 percent chromium for increased resistance to hydrochloric acid in the presence ofoxidizing compounds. Superchlor is vacuum melted Durichlor 51 and possesses twice its tensile strength. High silicon castirons are not machinable and can be shaped only by grinding. These materials are commonly considered as non-weldable.

Nickel cast irons typically contain 13 to 36 percent nickel and up to 6 percent chromium. Known as Ni-Resist, theseaustenitic alloys are the toughest of the cast irons, They are also produced as ductile irons, with high strength and ductilityover a wide temperature range. All have excellent corrosion, wear and high temperature resistance due to the relativelyhigh alloy content. Ni-Resist alloys can be machined to close tolerances. Typical refinery uses are valve components, pumpcomponents, dampers, diffusers, tray components, and engine and compressor parts.

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Other Metals and Alloys

Comer and its alloys combine excellent corrosion resistance with good thermal conductivity, ease of machinability andgood strength, especially when alloyed. Copper is a relatively noble metal and is usually not corroded unless oxygen orother oxidizing agents are present. Copper alloys are especially resistant to aqueous corrosion, in both fresh and saltwater,and are commonly used for heat exchanger tubes.. Copper alloys experience significant loss of strength above 4000 F andalso have poor resistance to sulfidic corrosion above this temperature.

One of the more common copper alloys used in refineries is admiralty brass, a copper alloy containing 28% zinc and 17.tin, with trace amounts of antimony, arsenic, or phosphorous added for improved resistance to corrosion. It has goodresistance to brackish and salt water corrosion and wet HzS corrosion. Admiralty tubes have been used extensively in watercooled condensers and coolers. Like most copper alloys admiralty brass is susceptible to dealloying and has been shown tostress corrosion crack when exposed to aqueous ammonia solutions.

Aluminum bronze, 90-10 cupro-nickel, and 70-30 cupro-nickel are other copper alloys often used in refine~ applications.

Nickel A11OYSNickel is an important alloy constituent of many corrosion resistant materials, including, the austeniticstainless steels. The stress corrosion cracking resistance of austenitic stainless steels rapidly increases as the nickel contentis increased above 20 percent. For example, Inconel 600 (a 700/. Ni Cr Fe alloy) shows excellent stress corrosion crackingresistance and is used for this reason in many refinery applications. Nickel also forms the basis for many high temperaturealloys, but nickel alloys can be attacked and embrittled by sulfbr bearing gases at elevated temperatures.

Various nickel alloys used in refineries are listed in Table 10. Monel 400 (a Ni-Cu alloy) is used extensively as a lining inthe top of crude oil distillation towers and as the upper 4 or 5 trays to resist hydrochloric acid. It is also used for crudetower overhead condenser tubes and components. Monel 400 is also used to combat corrosion by hydrofluoric acid inalkylation units and in hydrodesulfurization and reforming unit overhead systems. High nickel alloys, including Inconel625 and Incoloy 825, are used to prevent polythionic acid corrosion of flare stack tips and in hydroprocessing effluentpiping. Hastelloy B-2 is particularly well suited for handling hydrochloric acid at all concentrations and temperaturesincluding the boiling point. It is, however, attacked in the presence of oxidizing salts. Alloys B-2 and C-276 have excellentresistance to all concentrations of sulfuric acid up to at least 2000 F. The high nickel alloys are expensive and their userestricted to applications having unusually severe corrosion problems.

Aluminum is a highly reactive metal which develops oxide films which protect it against corrosion. These oxide films canbe improved by anodizing. They tend to break down, however, at pH values below 5 and above 8 and this limits the use ofaluminum and its alloys in many environments. Another limitation of aluminum is its relatively low strength at elevatedtemperatures. Two alloys of aluminum are commonly used in refinery applications. Alloy 3003, alloyed with manganesehas been successfully used in tower overhead condensers cooled by water on the condenser tubeside. Resistance to shell sideaqueous sulfide corrosion has been good but, water side pitting and fouling has detracted from the use of aluminum tubes.Aluminum alloy 3003 can be successfully used in sour water overhead condensers if the process fluid velocity is kept low toavoid erosion corrosion. Alloy 606 1-T6 is a magnesium and silicon aluminum alloy that is precipitation hardenable. It hasbeen used for pressure containing components, such as exchanger shells, because of its relatively high strength.

Aluminum and its alloys have been used for distillation tower tray components subject to naphthenic acid corrosion and hasbeen applied in various forms of aluminizing to protect furnace tubes and piping in high temperature hydrogensuIfide/hydrogen services.

Titanium and its alloys Titanium is a highly reactive metal which depends on a protective oxide film for corrosionprotection. Titanium is not suitable for high temperature service and because of its reactivity must be welded and cut underinert gas conditions to prevent contamination and embrittlement. From a practical standpoint, use of titanium in refine~service is limited to temperatures below 5000 F. If hydrogen is present, temperatures should not exceed 3500 F to preventembrittlement by hydride formation.

Titanium exhibits high corrosion resistance to most refinery streams. Tubes made from pure titanium (Grade 2) are usedextensively in overhead coolers and condensers on a number of units to prevent corrosion by chlorides, sulfides, and

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aqueous sulfur dioxide. These tubes can corrode, however, underneath acidic deposits. Titanium tubes are very useful atlocations where seawater or brackish water is used for cooling. They are also good in sour water stripper overhead service,Titanium alloyed with nickel and molybdenum (Grade 12) is generally better than Grade 2 and can be use in under depositcorrosion and higher temperature services where the pure grade is unsuitable. Anodizing and high temperature airoxidation of pure titanium can also improve the corrosion performance of titanium.

Non Metallic Materials

Refractories are inorganic ceramic materials which are normally utilized either for thermal insulation, corrosion resistance,erosion resistance or any combination of these. Refractories are available in several product forms, including, ceramic fiberblankets, bricks, castable mixes (similar to concrete) or plastic ramming mixes, Refractories generally have very hightemperature resistance (3000 T +), are chemically inert to most chemicals and solvents, and when in cast form have verygood erosion resistance. Typical retinery uses for refractories areas thermal insulation on the inside of fired heater andboilers, for insulation and erosion resistance in catalyst handling systems such as in fluid catalytic cracking units, and ascorrosion resistant lining in sulfuric acid production and sulfur recovery units.

Plastics An engineering plastic maybe defined as a synthetic organic polymer resin capable of being formed into loadbearing shapes that enable it to be utilized in the same manner as metallic materials. Plastics are man made materials andeach type of plastic was originally developed with a specific application in mind. For this reason, there exist a large numberof plastic materials available for use in equipment design and new plastics are being developed on a regular basis. Eachparticular polymer has its own unique properties. This vast diversity in material types and properties is one the majordifferences between metals and plastics. Plastics are divided into 2 groups thermoplastic materials and thermosettingmaterials. Thermoplastics are capable of being repeatedly softened by increase in temperature and hardened by a decreasein temperature, Thermoses on the other hand undergo a cure in the molding or forming process and as a result of chemicalreactions (produced by heat and/or added chemical catalysts) become substantially infusible.

Plastics generally exhibit excellent corrosion resistance in the type of environments for which they were originallydeveloped. However like metals, plastics do suffer from corrosion when exposed to some environments. Corrosionmechanisms in plastics are generally completely different than those which occur in metallic components. Corrosion inplastics is best defined as any reaction with an environment which significantly changes the physical and chemicalproperties of the plastic. The term corrosion rate is not normally applicable to plastics. Typical corrosion mechanismsfound in plastics include polymer chain scission (cutting), liquid oxidation degradation, melting, swelling, chemicalembrittlement, and stress cracking just to name a few, there are as many different failure modes for plastics as there aretypes of plastic materials

In recent years some thermoplastics have found their way into a limited number of refinery applications. Some of the morecommon materials include Polyvinyl chloride (PVC), Chlorinated polyvinyl chloride (CPVP), polyethylene (PE),polypropylene (PP), Polyvinylidene Fluoride (PVDF), and Polytetrafluoroethylene (PTFE).

~ is the most widely used thermoplastic in the manufacture of plastic pipe, fittings, and valves because of its economy,versatility, excellent chemical resistance, high tensile strength, good impact resistance and the ability to withstand longterm exposure to pressures.

CPVC has all of the properties of PVC plus the ability to handle temperaturesupto210T. This makes CPVC pipe,fittings, and valves suitable not only for hot corrosive service but also for hot water distribution systems.

~ is the lightest thermoplastic and is widely used due to its low cost and good chemical resistance and temperatureresistance up to 140”F. There are 2 commercial forms of PE, high density(HDPE) and low density (LDPE). Each has itsown specific strengths and weaknesses.

~ is another widely used thermoplastics, PP is suitable for corrosive waste as well as pressure applications because of itsinertness to a wide range of chemicals including most solvents and because of its ability to withstand temperatures up to200T.

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PVDF features remarkable high temperature performance . PVDF pipe and fittings can handle corrosive fluids at workingtemperatures up to 280”F. Other PVDF qualities are excellent chemical resistance, to such chemicals as halogens, andresistance to weathering (UV resistant). It is also highly resistant to Gamma radiation,

m is literally the “superman” of thermoplastic materials with excellent corrosion resistance to most chemicals andsolvents, and the ability to withstand long term exposure at temperatures up to 450°F.

Thermoplastic materials are available in a number of product forms, piping, valves, fittings, sheets, etc. Thermoplastics areavailable either alone or as a corrosion resistant lining on carbon steel components.

ThermosettinE resins such as polyesters, epoxies, urethanes and vinyl esters have excellent chemical resistance, and arenormally utilized either as a spray on type coating (paints) or in combination with some type of inorganic reinforcingmaterial such as glass or carbon fibers. The most common form of this material used in refineries is fiberglass, Fiberglasswhich is a thermosetting plastic resin reinforced with glass or carbon fibers. The properties of a fiber glass material aredetermined by the type of resin utilized to produce the material and the type of material utilized for reinforcement, Typicalresins utilized for refinery applications are polyester, epoxy, and vinyl ester. Fiberglass materials are commonly used forchemical storage tanks and drums. Fiberglass is also commonly used as a lining material to protect the internal surface ofstorage tank bottoms from corrosion in refineries.

Part Ill Heat Treatment

History of Heat Treatment 1

Many years ago, heat treating specifications were very important. A man’s life sometimes depended on his sword. A swordhad to maintain its sharpness and be tough at the same time, Damascus Steel was the prized material that satisfied bothrequirements. However, it was very scarce. Because the production of Damascus swords was a closely guarded secret, agreat search was conducted to determine a substitute steel that would perform like the Damascus steel. The search failedbecause the swords made from the selected steels were either tough and became dull quickly or stayed sharp but brokebecause of brittleness during the first blow. Years later, it was learned that this marvelous Damascus steel was not a newalloy at all but was simply produced by a new method of heat treatment. The specification of a slower quench from thehardening heat was what made the sword so effective. According to one historian, the heat treatment specification calledfor a quench in a “bath of urine from a calf collected in the light of the new moon.”

What the Designer Should Know About Heat Treatments’

The designer should know about the various heat treatments available for the particular metal or alloy he is planning to use.It is best to consult with a metallurgist to determine the actual need for heat treatment and, if required, what the scheduleshould be. The designer should speci~ the full heat treatment schedule required. A notation of just “anneal, stress relieve,or solution heat treatment,” etc., is not adequate. An example of a proper notation is a stress relief schedule specified forlarge waste storage tanks, 60 ft ( 18m) in diameter and 35 ft (1 lm) high, to obviate stress corrosion cracking and, at thesame time, to avoid warping the large tank. The following is an example of a heat treatment schedule:

1. Heat to 600°(315 C)

2. Above 600° (315 C), heating is not to exceed 100° (38 C)/hour. During this pericd, the temperature gradient is not to exceed 125° (52 C) in any

15-tl (5-m) interval and then there should not be a ~eater variation than 200° (93 C) between the lowest and highest temperature points in thevessel.

3. The temperature is to be held at a minimum of 1100° (593 C) for a period of at least 1 hour

4. The rate of cooling should not be greater than 125° (52 C) per hour. During this period, the greatest variation between the highest and the lowest

temperature in the vessel is not to exceed 200° (93 C).

5. Below 600° (315 C), no restriction on the cooling rate is required.

The above heat treatment schedule was successful since very little, if any, warping occurred. More importantly, no stresscracking has occurred since in any of these heat treated vessels. The heat treatment schedule is not usually as detailed asthe schedule described above. For instance, “Heat slowly to a temperature of 1100 to 1200° (598 to 648 C) and hold for 1

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hour per in. of thickness, then furnace cool to ambient temperature” could be used for stress relieving a piece of productionequipment not susceptible to warping.

No matter which alloy is used, the complete heat treatment should be clearly defined in the designer’s specification,However, some engineers argue that such a treatment is up to the heat treater’s discretion, and all the designer needs to dois to speci~ the end result, such as the desired hardness of the part or equipment, Consulting with the heat treater first is aprudent step, but the designer should speci~ the heat treatment schedule agreed upon.

For example, tool steel blocks that were to be used as important parts in a piece of equipment were sent to a heat treaterwith the specification that the blocks “are to be heat treated to a Rockwell C hardness of 63.’ After the parts were quenchedfrom the hardening temperature, the heat treater found that the parts were already at the required hardness so he did notbother to temper the parts (like he should have) because of fear that the parts would become too soft. Consequently, theblocks were so brittle that they failed immediately when used, No recourse was expected from the heat treater becausetempering had not been specified. The designer should always speci& the complete heat treatment schedule, includingtemperature. time at heat, quenching medium, quench temperature, and the tempering temperature (when a temper isrequired). Such a specification can also be used by the inspector later to assure that the required heat treatment has beenaccomplished.

Heat Treatment Verification

Because the heat treatment of metals and alloys otlen affect corrosion resistance, it is essential that the designer imposesome manner of quality control on heat treatment operations. The importance of assuring that the proper austenitizingtemperature and time at heat, the type of quenching media, the temperature, the tempering temperature, and the time at heatmaintained cannot be overemphasized. However, it is also essential to assure that the heat-treating equipment is in goodoperating condition. For instance, standard temperature thermocouples can be used to determine if a furnace is actuallyoperating at the set temperature. Furnaces have been found by this procedure to be operating at temperatures hundreds ofdegrees off the designated set temperature. Competent heat treaters routinely check their furnace temperatures andtherefore, their records may be used by the inspectors as verification.

On critical jobs, the designer can specifi that specimens of the same material involved are heat treated along with the actualprocess equipment or part. In this way, the specimen after heat treatment maybe sectioned, polished, etched, and observedunder the microscope to verify that the required microstructure has been obtained. When appropriate, the hardness of theprocess equipment or part maybe determined and compared with the specified hardness.

Normalization

Normalizing consists of heating a steel to a temperature 50-100 “F above its specific upper transformation temperature. Thisis followed by cooling in still air to a temperature which is well below the transformation range. Normalizing is usuallyused as a conditioning treatment, notably for refining the grains of steels that have been subjected to high temperatures forforging or other hot working operations. Normalizing is normally followed by another heat treating operation such astempering, or hardening.

Annealing

Annealing may be described as heating metals above a critical temperature range, holding for a certain period of time, andslowly cooling. The process of annealing consists of three stages, recovery, recrystallization, and grain growth. Theannealing temperature will vary with the composition of the metal involved. For instance, the annealing temperature forlow carbon steels will vary with carbon content from 1600 to 1700° (871 to 927 C), while that for high carbon steels willvary from 1450 to 1500° (788 to 816 C). The time required to homogenize metals will vary with the specific metal fromhours to several days. Cooling is always slow to ensure a homogeneous structure and obtain maximum softness.

The purposes for annealing a ferrous metal maybe to improve machinability, facilitation of cold work, improvement ofmechanical properties, or to increase dimensional stability. When it is desired to preserve most of the mechanicalproperties imparted by cold work, but at the same time (to an extent) maintaining corrosion resistance, a stress relief heattreatment may be more suitable than annealing.

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Non-ferrous alloys are usually heated to temperatures Just below the solidus temperature (just below melting) for annealing.For non ferrous materials annealing is performed to remove the effects of cold work, cause coalescence of precipitates fromsolid solution, or both.

Quenching 2

Quenching is the rapid cooling of a steel or alloy from the austenitizing temperature by immersing the work piece in aliquid or gaseous medium. The quenching medium maybe water, brine, caustic, oil, polymer, air, or nitrogen. The purposeof quenching is to obtain maximum possible hardness and strength forma steel. Quenching is almost always followed byeither tempering or stress relieving.

Temperin<

For ferrous materials tempering consists of reheating an austenitized and quench hardened steel or iron to some preselectedtemperature that is below the lower transformation temperature. Tempering offers a means of obtaining variouscombinations of mechanical properties from the same steel. The term tempering should not be confused with stressrelieving Even though time and temperature cycles maybe the same, the conditions of materials treated and the objectivesmay be different. The purpose of tempering is usually to improve the ductility and fracture toughness of a quenched ornormalized material,

Stress Relievin~

Stress relieving like tempering is always done by heating the work piece to some temperature below the lowertransformation temperature for steels and alloys. The primary purpose of stress relieving is to relieve stresses that havebeen imparted to the work piece from processes such as forming, rolling, machining, or welding. The usual procedure is toheat the work piece to a pre-established temperature long enough to reduce residual stresses to an acceptable level (this is atime and temperature dependent operation) this is normally followed by cooling at a relatively slow rate to avoid thecreation of new stresses.

The amount of residual stress in a material plays a critical role in determining its susceptibility to many forms of stresscorrosion cracking stress. Therefore, stress relieving can be specified to improve a materials resistance to this corrosionmechanism, This is one of the reasons why carbon steel weldments are often stress relieved (another reason is to maintaindimensional stability).

An example of the use of stress relief to prevent stress corrosion cracking to reduce material costs would be for equipmentin caustic service. The concentration and temperature of a sodium hydroxide solution (caustic soda) determines whether ornot carbon steel will suffer stress corrosion cracking. When there is an indication that cracking will occur, specification ofa stress relief heat treatment would permit usage of carbon steel without cracking.

Solution Heat Treatmenti

It is sometimes necessary to put certain precipitates back into a solid solution to improve corrosion resistance, For instance,unstabilized austenitic stainless steels, when sensitized, either in service or by welding, may have their corrosion resistancerestored if this heat treatment, called solution heat treatment, is specified; this treatment involves heating at 1650 to 2000°F(899 to 1093 C) (actual temperature depends on type of stainless steel) for one hour per in. (25mm) of maximum thickness(one hour minimum) and quenching in water to black heat within 3 minutes. Solution heat treatment places the chromiumcarbides back into solution.

When either stress relieving or annealing of austenitic stainless steel is thought to be required, the designer should specifj’only the solution heat treatment. If the equipment involved has a geometry that will not allow it to take the water quenchrequired by this heat treatment without warping, the designer has two options; he can

1. Consult a metallurgist to determine whether the heat treatment is really necessary, or

2. Change a material that does not require a heat treatment to preserve corrosion resistance

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Specialized Heat Treatments

Several specialized heat treatments are applied to refinery equipment either to enhance corrosion resistance in certainenvironments, facilitate in service repair, or restore mechanical properties that have deteriorated during long term service.Some of these are now briefly described:

Deembrittlement heat treatment is applied prior to weld repair of C- 1/2 Mo and other Cr-Mo alloy steels, such as 1 1/4 Cr-1/2 Mo after long term exposure, in high temperature in service. Its purpose is to restore ductility to the material such thatrepairs by welding can be successtldly made free of cracking. The treatment involves heating the weld zone to 1300”F,holding for 4-8 hours, and cooling at 400T per hour per inch of thickness.

DehYdrogenation heat treatment is normally applied to steels prior to repair welding of refinery equipment which has beenexposed to processes which can cause hydrogen induced cracking these services include wet hydrogen sulfide service, highpressure/temperature hydrogen service, caustic service or amine service etc. The typical procedure is to “bake out” anyresidual atomic hydrogen in the steel by heating it to 400-600”F and holding for 2-4 hours depending on the thickness ofthe material and severity of the exposure. The procedure is intended to help avoid delayed hydrogen cracking fromoccurring during or after repair welding.

Stabilization heat treatment Chemically stabilized grades of stainless steel (321 & 347) may become “sensitized’ afterprolonged exposure in the sensitization temperature range (700° F -1500° F), Sensitization is the terminology used todescribe the phenomenon of intergranular carbide precipitation which occurs in austenitic steels when subjected totemperatures in sensitization temperature range. The resistance of these stainless steels to polythionic acid stress corrosioncracking may be significantly improved by a stabilization heat treatment performed prior to placing the equipment inservice. Typically stabilization heat treatments consist of heating the material to 1650° F and holding at that temperaturefor 2-4 hours. The material is then cooled to ambient temperature. The rate of cooling is controlled to minimize distortion.

Heat Treatments for Welds

Preheat

Preheat is heating the weldment to a prescribed temperature above ambient temperature prior to welding and maintainingthis minimum temperature for the duration of welding. Preheating maybe conducted to reduce residual stress, reducedistortion, lower heat tiected zone hardness, and prevent under bead cracking. Typical preheat treatments for the refinerysteels is as follows:

Table 4- Preheat Temperatures for Refinery Steels

Steel Preheat ‘F

Carbon 50Carbon-1/2 Mo 501 1/4 Cr-1/2 Mo 3002 1/4 Cr-1 Mo 3505 Cr-1/2 Mo 3507 Cr-1 Mo 3509 Cr-1 Mo 35012 Cr 300

17 Cr 50300 series stainless 50Nickel alloy steels 200

Preheat requirements are usually specified by the code or standard under which the equipment is built. Some are mandatoryrequirements and others are recommended.

614/21

Postweld Heat Treatment

Postweld heat treatment “conditions” the weldment following welding. Its application, or misapplication, can dramaticallyatlect in service mechanical and corrosion performance. PWHT is conducted at an elevated temperature, slightly below thetransformation temperature for the alloy involved. The PWHT temperature is high enough for stress to flow and hardmicrostructure to temper. This results in reduced residual stress and a softer weld and heat affected zone (HAZ), Ingeneral, PWH’Timproves corrosion resistance, reduces the chances of stress corrosion cracking, increases ductility, andimproves toughness of the material, especially in the heat affected zones next to the weld. The list below contains typicaltemperature ranges commonly used for postweld heat treatment of the refinery steels and, where appropriate, the hardnesslimit acceptable.

Table 5- PWHT Temperatures for Refinery Steels

Steel PWHT Range 0 F Hardness,BHN

Carbon 1100-1200 <200”

Carbon- 1/2 Mo 1100-1325 <225

1 1/4 Cr-1/2 Mo 1300-1375 <225

2 1/4 Cr-1 Mo 1300-1400 <241

5 Cr-1 Mo 1300-1400 <241

7 Cr- 1 Mo 1300-1400 <241

9 Cr-1 Mo 1300-1400 <241

12 Cr 1350-1450 <241

17 Cr None300 series stainless NoneDuplex Stainless SteelsNickel alloy steels 1100-1175

The holding time at temperature is typically 1 hour per inch of weld thickness. As with preheat, PWHT requirements arefound in the code or standard to which the equipment is fabricated. Many times PWHT is specified solely for the purpose ofpreventing stress corrosion cracking even if PWHT is not specified by the fabrication code being utilized. Some typicalrefinery processes where PWHT is specified to prevent stress corrosion cracking are for amine and or caustic service.

Austenitic stainless steels, such as Type 304 and 316, remain austenitic throughout the welding process and do not harden.They are generally not preheated or postweld heat treated and the interpass temperature for the 300 series alloys is oftenrestricted to 300° F to preseme corrosion resistance. When residual stress is judged unacceptable, a significant stressreduction can be accomplished by heating in the range of 1550-1650° F for 15-60 minutes and cooling rapidly to roomtemperature.

Since welding also results is the sensitization of the regular grades (Type 304, 316. 317), the use of low carbon (Type 304L.316L, 3 17L) or chemically stabilized grades (Type 321, 347) are very often used to minimize sensitization in weldedfabrications. If sensitization has occurred, the regular grades can be solution annealed by heating to about 2000° F followedby water quenching. Although this will redissolve the precipitated chromium carbides, accomplishing the process onwelded assemblies in the field may not be practical.

Normalizing

Postweld heat treatment of the refinery steels tempers the welds and reduces residual stresses. The weld metal, however,retains a microstructure considerably different than the adjacent base metal. In most services this difference is of noconsequence. In some situations, however, such as acidic aqueous environments, preferential weld corrosion can occur.This selective attack of the weld can often be reduced by normalizing the weldment in the temperature range of 1500°-1600”F and then air cooling. This elevated temperature treatment results in a weld microstructure having corrosionbehavior nearly identical to the base metal. Since normalization is done at relatively high temperature, distortion of thewelded assembly can easily result. Special fixturing and handling may be needed to prevent distortion.

61 4/22

Part IV - Welding

The Nature of Welding

Weldingis the processofjoining materialstogetherby fusion. Most refineryequipmentis fabricatedby weldingand mostmetalsused in the refinery can be joined by one or more of the many welding processes available. In addition to newconstruction, welding is extensively used to repair, modify, and line refinery equipment during shutdowns. Safe operationof pressure containing equipment depends on welded joints of acceptable quality that meets or exceeds the requirements andprocedures of applicable codes and standards, Welds must be clean and free from defects, including porosity, slag,inclusions, cracks, incomplete penetration, and lack of tision. Welding alters the base material and the changes that occurcan result is degraded mechanical, metallurgical, and corrosion performance in and near the weld

Fatigue cracking, stress corrosion cracking, hydrogen embrittlement, sulfide stress cracking, accelerated corrosion, andpreferential weld zone corrosion are all failure mechanisms that are associated with welding, This section will brieflydescribe some of the principal welding processes used on refinery equipment, discuss welding procedures, and describevarious heat treatments used to enhance the properties and performance of weldments.

Welding is used almost universally in the fabrication of process equipment. Over 90% of all permanent closures are madeby fusion welding or brazing. For all its utility, welding has inherent characteristics that can foster corrosion.

For instance: *

1. The cast structure of a weld can be quite different from the usual wrought structure of the parent materials.

2. Weld spatter can create obstructions that can result in localized corrosion.

3. Many weld joints can contain crevices if not welded properly.

4. The weld surface is generally rougher than the parent material’s surface.

5. Shielded metal arc and submerged arc welding processes generate slag, which can setup corrosion cells

6. Welding entails intense localized heat, which creates heat affected zones in the parent metal where phasetransformations and precipitation can occur.

7. Welds contain internal shrinkage stresses.

8. Residue not removed from welding and brazing fluxes can be corrosive

Although welding has some drawbacks, as noted above, it is still the best and the soundest method of closure available.Rivets, for instance, have built-in crevices and are ditlcult to maintain. These problems are also experienced with bolting.

Welding Decisionsl

Because of the problems of welding, the designer must assure that the welds in his structures are properly designed andspecified. A mistake a designer can make is to simply note on his drawing that the structure “is to be welded.”Unfortunately. this is standard procedure with some designers. Such a spectilcation leaves the welding decisions up to thewelder or the welding foreman who probably does not know what process conditions are involved. The end-use of theequipment must be carefully considered in advance and the appropriate weld design must be specified while the equipmentis still in the design stage, not when it is already in the weld shop.

614/23

Welding Processes’

The following are various welding processes a designer can specifju

1. Shielded Metal Arc Welding (SMAW), commonly called stick electrode welding.

2. Gas Tungsten Arc Welding (GTAW), commonly called TIG or heliarc welding

3. Gas Metal Arc Welding (GMAW), commonly called MIG welding, This process can be manual, semi-automaticor automatic welding.

4. Submerged Arc Welding (SMAW), commonly called subarc welding. This process can be semi-automatic orautomatic welding.

5. Gas Welding. Oxyacetylene is the most common gas welding, although there are other gas combinations.

6. Brazing,

Each process has its individual characteristics; therefore, it is important that the designer select the welding carefidly so hepreserves the material’s corrosion resistance. For instance, although the deposit left by the shielded metal arc process isinherently no less corrosion resistant than the inert gas process, the metal arc process may result in slag inclusions whichresult from sloppy welding techniques, while the inert gas process does not produce any slag inclusions. As an example, theadverse effect of slag inclusions was pointedly observed in a shielded metal-arc welded stainless steel tank containing anacid. When the tank was emptied, cleaned out, and given a routine inspection, it was noted that the double-butt weldedgirth weld inside bead had been aggressively attacked, while the adjacent tank wall was relatively unaffected. The cause ofthe failure was concluded to be very poor workmanship because the slag had not been adequately removed. The remainderof the inside weld was gouged out and rewelded with the inert gas process. After that, no more corrosion problems werereported.

When the fabricator is equally familiar with both metal arc welding and inert gas welding and when practical, inert gaswelding should be specified by the designer for corrosive service. Brazing, silver soldering, and soldering should not bespecified for corrosive conditions. Exceptions to this rule may exist; however, these joining processes usually entail adifferent material than the parent metal, which can lead to galvanic corrosion.

Shielded Metal Arc Welding (SMAW) is the most commonly used and most versatile welding process applicable to shopand field work on refinery equipment. SMAW uses an electrode consisting of a straight piece of filler metal coated with aflux covering. The flux melts with the wire and provides a gaseous shield to protect the molten weld puddle from oxidation.The flux also acts as a deoxidizing agent to improve cleanliness in the weld deposit. Commonly called “stick welding”, theprocess requires a relatively high degree of welder skill but can be successfidly used in all positions and under a widevariety of welding parameters. Hydrogen pickup during welding can cause porosity and cracking problems. A commonsource of hydrogen is moisture in the electrode coating. To control hydrogen problems, low hydrogen electrodes maybeused. The bulk of carbon steel welding done in the refinery is done with low hydrogen electrodes, E7018 being the mostcommon.

Gas Metal Arc Welding (GMAW) uses a filler metal wire fed continuously through a gun or torch. Shielding is provided bya gas or mixture of gasses that pass through the torch. A trigger on the gun starts or stops wire movement along with gasflow. Wire speed is controlled at the feeder which holds the filler wire coil. Only a small amount of glassy slag is normallyproduces and the absence of flux decreases the amount of hydrogen in the weldment. Compared to shielded metal arcwelding, GMAW permits higher rates of weld metal deposition with fewer stop/starts, does not require slag removal, andavoids the possibility of slag entrapment in the weld. A relative low degree of welding skill is required for GMAW but caremust be taken to assure that sidewall fusion takes place between the weld and base metal.

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In gas metal arc welding, electrical parameters can be varied to provide several different modes of metal transfer across thearc from the consumable wire electrode. Modes of transfer include spray transfer (high current, high deposition rate), shortcircuiting arc (the short-arc, low heat input process ideal for welding light gage tubing and sheet metal), globular transfer(in which a relatively low current to filler metal diameter is used that produces transfer by droplets), and pulsed arc (similarto spray transfer, except that the electrical waveform is cycled to produce short spurts of metal spray with a lower total heatinput).

Gas tunmten arc welding (GTAW) uses a non-consumable electrode (most commonly tungsten), an inert gas shielddelivered through the gun, and filler metal that is manually fed into the weld. Welds can also be made without theintroduction of tiller metal. These are termed autogenous welds. The filler is melted by the heat of the electric arc andmolten puddle, GTAW produces welds of high quality, with low hydrogen content, but at relatively slow welding rates. Itrequires a relatively high degree of welder skill and is commonly used to make root passes in low and high alloy steel welds.

Submerged arc weldirw (SAW) is similar to GMAW except the protective gas shield is replaced by granular flux which issimilar to the flux on coated electrodes. There are two primary flux types, neutral and active. The neutral fluxes do not addmetallic elements to the weld deposit and are preferred over active fluxes because the deposit chemistry in more easilycontrolled. With active fluxes, variations in heat input during welding can alter the chemistry of weld deposits. SAW isusually done in the flat (welding torch pointing down) position. With special set up, welds can also be made with the torchin the horizontal position. SAW is normally used during the shop fabrication of refinery equipment and offers theadvantage of high deposition rates combined with good weld quality. The process is often used for the deposition of cladoverlays and through the use of ribbon electrcxies, high deposition rates alloy overlays are possible.

Welding Procedures and Welder Qualification

Most codes and standards under which petroleum refinery equipment is fabricated and maintained require that weldingconform to Section IX of the ASME Boiler and Pressure Vessel Code. It requires that procedures, used for welding, betested prior to use to insure that they are capable of producing joints having adequate mechanical properties. The details ofthe welding procedure are first written as a Welding Procedure Specification (WPS). The WPS is then used to weld testpieces which are used to evaluate the mechanical properties using destructive tests such as tensile testing, bent tests, and,when required, impact toughness tests.

The actual parameters which were used to weld the test samples are recorded on the Procedure Qualification Record (PQR)along with the results of the mechanical test. A welder who performs the welding on the procedure qualification samples isautomatically qualified to use the qualified procedure for production welding. Other welders who wish to use the qualifiedprocedure must produce performance test welds of acceptable quality and be evaluated using either destructive tests orradiographic examination. This performance qualification test is to insure that the welder can produce a weld withoutdefects using the qualified procedure. The record of the welder’s performance qualification testis the Welder PerformanceQualification (WPQ).

Welders and procedures must be requalified if any of the “essential variables” in the welding process are changed. Essentialvariables are described in the applicable code or standard for each welding process. For example, a change in base metal ora change in tiller metal can require requalification. Other essential variables pertain to the type of joint, electricalcharacteristics, welding technique, preheat, postweld heat treatment, and shielding gas. Codes and standards differ in whatare considered essential variables.

Inspection of Welding Electrodes and Filler Metall

Many corrosion failures have been caused by the mix-up of electrodes or tiller rods in the fabricators’ bins. Themanufacturer of welding electrodes and filler rods is required to make many tests prescribed by the AWS on his productbefore it is sealed into cartons. Consequently, the integrity of the welding electrodes can be relied upon but not after thecartons have been opened. For this reason, the designer should not only speci~ the types of electrodes required, but alsothat the company’s inspector allow only unopened cartons of the electrodes to be used on the job and then those electrodesmust be kept not only isolated from other welding jobs, but carefully marked so no mix-ups can occur.

614125

Table 6- The Refinery Steelsd

ALLOY NOMINAL COMPOSITION

Carbon Steel O.1O-O.3O’XOC, 0.30 -1.0% Mn, bal. FeCarbon - 1/2 Mo O.10-0.2O% C, 0.50?4.Mo, bal, Fe1-1/4 Cr - 1/2 Mo O.15% C max., 1.2S?/. Cr, 0.5% Mo, bal. Fe2-1/4 Cr - 1/2 Mo O.15’%C max., 2.257. Cr, 1.0% Mo, bal. Fe5 Cr - 1/2 Mo O.15% C max., 4-67. Cr, 0.57. Mo, bal. Fe9Cr-l Mo O.15% C max., 8-10% Cr, 1’?40Mo, bal. Fe12 Cr (Type 410) O.15’%C max., 11-137. Cr, bal. Fe17 Cr (Type 430) O.12% C max., 14-18% Cr, bal. Fe26 Cr (Type 446) 0.20’%C max., 23-307. Cr, bal. FeType 304 Stainless 0.08% C max., 18-20% Cr, 8-1 l% Ni, bal. FeType 304L Stainless 0.03’XOC max., 18-20% Cr, 8-12’%Ni, bal. FeType 316 Stainless 0.08’70C max., 16-18% Cr, 1O-14’XONi, 2-3% Mo bal. FeType 309 Stainless O.15% C max., 22-24?40Cr, 12-15% Ni, bal. FeType 310 Stainless O.15’%.C max., 24-26% Cr, 19-22% Ni, bal. Fe2-1/4VoNickel Steel O.19% C max., 2.03-2.57?4.Ni, bal. Fe3-1/2?Z0Nickel Steel O.19’XOC max., 3.18-3.82% Ni, bal. Fe

Table 7- Other Metals And Alloys4

ALLOY GROUP NOMINAL COMPOSITION APPLICATION IIAluminum Alloys

Alloy 1100 99% Al Light StructuralAlloy3003 1,0-1.5V0Mg, bal. Al Exchanger Tubing

AlloyAl~l~rl

6061 ] 0.8-1.2%Mg, 0.4-0.8 Si, bal. Al I HeatTreatable,Plate, Rod II Rlre Al annlid mwr nther material I (_’athoriicProtection I

! . ..W.UU .-. s...” ---- ., . -. ./ . ..-. . --------- --------- -----------

Copper Alloys

Copper 99’%0pure Tubing

Inhibited Admiralty 28% Zn, 1% Sn, 71’?LoCu, (Sb, P, As) Condenser Tubing

Naval Brass 39’%.Zn, 1’%Sn, 60’%0Cu Tubesheets

Aluminum Brass 22% Zn, 29’.Al, 76% Cu Condenser Tubing

70-30 Copper-Nickel Toy. Cu, qo~o Ni Tubing--Plate

90-10 Copper-Nickel 90% Cu, 10”ANi Tubing1

Other Materials

Titanium 99’%o Pure Titanium Tubing

Monel 70°ANi, 30V0Cu Tubing, Plate

Alloy 800 30-35% Ni, 19-23% Cr, 40% Fe, O.10% C Pipe , Tubing, Plate

Alloy 20 28-30% Ni, 19-217. Cr, 494.Cu, 3’%Mo, bal. Fe Pipe, Tubing, Plate

HF Modified A297-67 (cast) O.15-O.20”AC, 21-25°A Cr, 6.5-10% Ni Heater Tubes, Piping

Supertherm 0.5% C, 26’%Cr. 35’?40Ni, 15% Co, 5’%W Heater Tubes

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Table 8- Some Specific Effects of Alloys in Steel

Element Influence upon Influence Upon Carbide forming PrincipalFerrite Austenite Tendency Functions

(Hardenability)

Chromium(Cr) Hardensmarkedly; Increaseshardenability Greaterthan Mn; Increases corrosionincreases corrosion moderately, similarly to Less than W and oxidationresistance manganese resistance and

hardenability; addssome strength athigh temperatures

Manganese (Mn) Hardens markedly; Increases hardenability Greater than Fe; Counteractsreduces plasticity moderately, similarly to Less than Cr brittleness from thesomewhat chromium sulfur; increases

hardenabilityinexpensively,

Molybdenum (Me) Provides age- Increases hardenability Strong; Greater Deepenshardening system in strongly (Mo-Cr) than Cr hardening;high Me-Fe alloys improves hot and

creep strength andcorrosionresistance instainless.

Nickel (Ni) Strengthens and Increases hardenability Less than Fe Strengthenstoughens by solid mildly, but tends to retain (Graphitizes) unquenched orsolution austenite with higher annealed steels;

carbon toughens pearlitic-ferritic steels(especially at lowtemperature) andrenders highchromium- ironalloys austenitic,

Phosphorus (P) Hardens strongly by Increases hardenability Nil Strengthens lowsolid solution similarly to manganese carbons steel;

increases resistanceto corrosion

Silicon (Si) Hardens with loss in Increases hardenability Negative Used as generalplasticity (Mn-Si-P) more than Nickel (Ni-Si- (Graphitizes) purpose deoxidizer;

Mn) improves oxidationresistance andstrengthens lowalloy steels.

ritanium (Ti) Provides age- Probably increases Greatest known Fixes carbon inhardening system in hardenability very (2!!4.Ti renders inert particles andhigh Ti-Fe alloys strongly, as dissolved. Its 0.50’%carbon steel prevents localized

carbide effects reduce unhardenable) depletion ofhardenability chromium in

stainless duringlong heating.

614127

Table 9- Chemical Composition of Principal Stainless Stee1s4

I AISIType I OAC I %c, I OANi I OAOtherElemmls I Equivalent IICast Alloy

I

Steels

410 0.15 max. 11.5 -13.5 -- -- CA-15MwtensiticStainless 416 0.15max. 12-14 Se,Mo,orZr --

431 0.2max. 15-17 1.25-2.5 -- CB-30440A n 6-(I75 16.1X .. .- . .

405 0.08 max. 11.5 -14.5 0.5 max. 0.1-0.3 AI .-

FerriticStainlessSteels 430 0.12max. 14-18 0.5max. .- -.442 0.25max. 18-23 0.5max. .. -.446 0.20max. 23-27 0.5max. 0.25Nmax. CC-50,HC,

I 301 0.15 max. 16-18 6-8 2 Mn max. --

302 0.15 max. 17-19 8-10 2 Mn max. CF-20

304 0.08 max. 18-20 8-12 1 Si max. CF-8

3041. 0.03 max. 18-20 8-12 1 Si max. CF-3

308 0.08 max. 19-21 10-12 1 Si max. . .

309 0.2 max. 22-24 12-15 1 Si max. CH-20, 1{11

309s 0.08 max. 22-24 12-15 1 Si max. -.

IAusteniticStainlessSteels 310 0.25max. I 24-26 19-22 I 1.5Simax. I CK20,HK310s 0.08max. 24-26 19-22 1.5Simax. -. I316 0.08 max. 16-18 10-14 2-3 Mo CF-8M,

CF-12M

316L 0.03 max. 16-18 10-14 2-3 Mo CF-3L4

317 0.08 max. 18-20 11-14 3-4 Mo CG-8h4317L 0.03 max. 18-20 11-14 3-4 Mo -.

321 0.08 max. 17-19 8-11 Ti:4x Cmin. . .

I 347 0.08 max. 17-19 9-13 Cb+Ta: 10x Cmin. CF-8C !

AgeHanlenableStainless 17-7PH 0.07 17 7 1Al ..Steels

17-4 PH 0,05 16.5 4.2 4 Cu -.

I I1

E-Brite 0.002 26 0,1 I Mo, 0.1 Cb -.

Al 29-4-2 0.005 29 2 4 hfo, 0.013N --

SpecialtyStainlessSteels 329 0.08max. 25 3.5 1.5MO --

3RE60 0.03 max. 18.5 4.5 2.7 Mo, 1.7 Si --

SAF-2205 0.03 max. 22 5.5 3 Mo, 0.8 Si, 0.14N ..

904L 0.02 20 25 4M0,1.5CU -.

Table 10- Chemical Composition of Principal Nickel Alloys4

Alloy ‘XOc 0/0Cr Yo 9’0 % % %W ‘%.Other ElementsNi Mo Fe co

UNS N04400 0,15 -- 66 -- 1.4 -- -- 31 Cu

Alloy K500 0,15 -- 66 -- 0.9 -- -- 29 Cu, 3 Al

UNs N06600 0,08 16 76 -- 8 -- -- 0.2 Cu

UNS N06625 0,1 21 60 9 5 -- . . 3.6 Cb+Ta

UNs N08800 0.04 20 32 -- 47 -- -- 0.3 Cu

UNS N08825 0.03 21 42 3 30 -- -- 1.8 Cu, 0,15 Al, 0.9 Ti

UNS N10665 0.02 1 67 28 2 1 -- . .

UNS N10276 0.02 15 54 16 5 2.5 4 0.4V

UNS N06455 0.015 16 61 16 3 2 -- 0.7 Ti

Alloy 20 0.05 20 29 2 44 -- -- 3 Cu

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Table 11- ASTM Standard Specifications for Refinery Steelsd

Material Pipes and Tubes Plates Castings Forgings

Carbon Steel A53, A106, A120, A134, A283, A285, A299, A27*, A2 16, A105, A181,A135, A139, A178, A179, A442, A455, A515, A352 A234, A268,A192, A21O, A211, A214, A5 16, A537, A570, A350, A372,A226, A333, A334, A369, A573*A381*, A524, A587, A671,

A420, A508,A541

A672, A691C-1/2 Mo Steel A161*, A209, A250, A335, A204, A302, A517, A217, A352, A182, A234,

A369, A426, A672, A691 A533 A487 A336, A508,A541

1 Cr- 1/2 Mo Steel A213, A334, A369, A426 A387, A517 -- A182, A234,A336

1 114Cr-lt2 Mo A199, A200*, A213, A335, A387, A389*, A517 A217, A182, A234,Steel A369, A426, A691 A389* A336, A541

2 Cr- 1/2 Mo Steel A199, A200*, A213, A369 -- .- --

2 1/4 Cr-1 Mo Steel A199, A213, A335, A369, A387, A542 A217, A487 A182, A234,A426, A691 A336, A541,

A5423 Cr-1 Mo Steel A199, A200*, A213, A335, A387 . . A182, A336

A369, A426, A6915 Cr-1/2 Mo Steel A199. A200*, A213, A335, A387 A217 A182, A234.

A369, A426, A691 A336

7 Cr- 1/2 Mo Steel A199, A20W, A213, A335, A387 -- A182, A234A369, A426

9 Cr- 1 Mo Steel A199, A200”, A213, A335, A387 A217 A182, A234,A369, A426 A336

Ferritic. Martensitic, A2 13, A249, A268, A269, A167, A176*, A297*, A182, A336,and Austenitic A271*, A3 12, A358, A376, A240, A412, A457 A351, A403,Stainless Steel A409, A430, A451, A452, A447* A473*

A511*

* Carbon and alloy steel bolts and nuts covered by Specifications A193, Al 94, A320, A354, A449, A453, A540, A563 *.

* These specifications are not approved by the ANSI/ASME Boiler and Pressure Vessel Code

References

1, R. J. Landrum, Fundamentals of Designing for Corrosion Control (Houston, Tx National association of corrosionEngineers, 1989)

2. Howard .E. Boyer, Practical Heat Treating (Metals Park, Ohio, American Society for Metals, 1984)

3. Engineered Materials Handbook Vol. 2 Engineered Plastics (Metals Park, Ohio American Society for Metals, 1988)

4. Robert B. Ross, Metallic Materials Specification Handbook 4th Edition (New York, NY Chapman& Hall, 1992)

5. Norman E. Hamner, Corrosion Data Sumey Metals Section, (Houston, Tx, National Association of CorrosionEngineers 1974)

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