Paper No.
614
CORROSION
96
The NACE International Annual Conference and Exposition
MATERIALS OF CONSTRUCTION FOR REFINERY APPLICATIONS
Tom Farraro
CITGO Petroleum Corporation PO BOX 1562
Lake Charles, LA 70602
Richard M. Stellina Jr. Unocal San Francisco Refinery
2019 Mira Vista El Cerrito, CA 94530-1740
ABSTRACT
In today’s modern highly complex oil refineries proper material selection has become one of the most important factors in the design and repair of refinery processing equipment. Proper material selection will ensure that the expectations of the designers for safety, reliability and economy are actually realized. On the other hand, improper materials selection can result 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 a problem 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 that
have 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 less expensive materials using electrochemical techniques, coatings, inhibition, etc., for protection, the basis for the selection requires the recognition and appraisal of economic factors, as well as an understanding of corrosion technology.
Copyright
~1996byNAcEinternational.Requests for permissionto publishthis manuscript in any form, in part or in whole must be made in writing to NACE
International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper 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 of various alternatives for corrosion control.
A cost appraisal method should be used that will determine the one corrosion control method that offers the greatest economic 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 not only the original installed cost, but it is all costs, including operating, maintenance, overhead, and various financia~ costs such 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 equipment failure occurring, the mode of failure and the consequences of a failure if one should occur. Some of the factors to be considered 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 of
exposure 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, stress
corrosion cracking, general corrosion, etc.
● Sensitivity of the corrosion resistance of the material of construction to changes in process parameters such as
process 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 manner without significant maintenance expenditures. It is an estimate of the anticipated life of a specific equipment design alternative. 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 performance limits 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 necessaryto maintain operational
reliability,
~ 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 is generally emphasized. However, consider all of the following factors that influence the equipment’s service life:
1, Selection of materials of construction
2, Design details
3. Specification of materials
4. Fabrication and inspection
5. Process operation
6. Maintenance (cost and frequency)
These six factors which influence the equipment’s service life should always be kept in mind by the designer. 61 4/2
In other words, for the best equipment or structural design, the materials of construction must be carefully selected from a corrosion-resistance standpoint. The design details should preserve the corrosion resistance of the materials. Concise and clearly 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. The equipment must be operated within the specified design parameters (this last item is sometimes overlooked; plants may
change a process without sti]cient regard to the effect of the process change on the construction materials). Lastly, the
equipment must be maintained properly. All of these factors must be considered by the designer to ensure the expected life of the equipment he designs. When corrosion failures occur, the selection of the materials of construction involved is usually faulted. However, in a large number of cases, failure actually occurred because of other factors. The following example may clarify this point:
Amaterials engineer was called upon by a plant to determinebetter materialsof constructionfor a critical stainless steel heat exchangerthat corrodedso severelythat accordingto the plant, it had to be replaced four times. The rejected heat exchangershad been disposedof in the scrap yard. Upon examination,the engineerfoundthat the tubes of the rejected heat exchangerswere severelycloggedwith corrosionproducts. Atler scrapingthese away,he founda bright stainless steel tube wall underneath, indicatingthat no corrosionhad actuallyoccurredthere. Further investigationdisclosedthat equipment made of carbon steel upstream from the heat exchangerwas unexpectedlydisintegratingby corrosion. These corrosionproductswere dischargedinto the process stream, The heat exchangeraffordedthe most restrictedarea in the system,and the corrosionproducts collectedthere and cloggedup the tubes. The corrodingcarbonsteel equipmentwas subsequentlyreplacedwith a more resistant material of construction,and the plant operatedon cleanedexchangersfromthe scrapyard until the plant shut down years later. In this case, not only was the wrong material faulted for the failure, but the corrosionproblemhad not been properly defined. It has been said that the definition of a problem is often the solutionto the problem,that axiom certainlyprovedtrue 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)erGI Requirements - Melting Point, Thermal Conductivity, Electrical
Conductivity, 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, fatigue strength, 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 potential corrosion problems associated with a selected material.
The second concern deals with the practicality of actual utilization of the selected material. Is the material available in the required form(s)? Can it be fabricated using conventional methods (welding, forging, casting, etc.)? Can this all be accomplished 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 fabrication cost is well defined, but the yearly operation and maintenance costs are either ignored or are so poorly defined as to be useless. The designer should do everything possible to precisely define all costs associated with a given design to ensure that 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 materials recommended by vendors can be included in the various materials to be evaluated, The designer is responsible for matching his unique materials problems with the best materials and design, which requires evaluating pertinent materials on the market to attain the “best fit, ” The best fit is not necessarily the most or the least expensive material. For example, perhaps all corrosion problems for a specific application would disappear if all equipment were made of platinum. Of course, a plant made of platinum would most likely be too expensive to be justified. The designer should use the optimum material for 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 specific process for which the equipment is being designed. The frequency of corrosion failures attributed to the various forms of corrosion has been studied by indushy corrosion specialists for many years. One such study was conducted by E. 1. du Pent de Nemours & Co., Materials Engineering group for seven years. and was based on reports from 23 company materials engineers, 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 31
Stress Corrosion Cracking 288 24
Pitting 120 10 I 1I Intergranular Corrosion 96 8 Erosion-Corrosion 84 7 Weld Corrosion 60 5
Temperature Related Corrosion 48 4
I
Corrosion Fatigue 24 I 2
Hydrogen Permeation Related Corrosion 24 2
Crevice Corrosion 24 2
Galvanic Corrosion 24 2
Dealloying 12 1
End Grain Attack 12 1
Fretting 12 1
Total 1200 100
The first three forms of corrosion failures shown in Table 1, namely general corrosion, stress corrosion cracking, and pitting, 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%) is typically due to facilities with corrosive processes where austenitic stainless steels are utilized. Thus, where there is a preponderance of these steels, one might expect a higher frequency of this kind of corrosion. The next four types of
corrosion 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 into account some non-uniform penetration, have proven to be very useful in the design of process equipment. However, it must be emphasized that the use of corrosion allowances in predicting plant life are only meaningful if general corrosion is predominate; 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 more local 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 corrosion mechanisms.
Designing for General Corrosion
General corrosion is defined as corrosion that attacks the surface of metals evenly and uniformly. General corrosion is the most prevalent form of corrosion in refineries. The designer should deal with this type of corrosion by selecting optimum materials of construction by review of published data, previous experience or actual corrosion testing and then assigning a corrosion 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 just how much or how little a material has corroded. Older texts on corrosion referred to “loss of milligrams per square decimeter per day.” These designations were not very useful except for comparison purposes because it was difficult to visualize what the effect would be on process equipment. Today, the expected corrosion penetration into the wall of a vessel or 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 specimen over a number of days and the overall area and density of the specimen, the designer can determine a penetration into the specimen surfaces by corrosion.
Corrosion Allowance
Different metals and alloys have varying corrosion rates depending, of course, on specific environments. By knowing the
expected general corrosion rate and the expected plant life, the designer can calculate the extra wall thickness required for corrosion resistance of the process equipment he is designing. After determining a wall thickness that meets mechanical requirements. pressure, temperature, weight etc., an extra thickness called a corrosion allowance is added to the wall thickness 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 has determined that the corrosion rate will be 15 mpy and the expected life of the equipment will be 10 years. The total corrosion 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.
Table 2- Guidelines for Evaluation of Corrosion Rates5 QuantitativeCorrosion Rate
mpy Qualitative Description and Definition
(roils of penetration per year)
o-1o Insignificant corrosion rate, - materials would suffer no significant dimensional
change during the life of the process equipment.
10-20 Low corrosion rate - materials with this rate would normally be specified with a
1/16”- 1/8” (1,5-3,0 mm) corrosion allowance to ensure adequate equipment life; 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 be
utilized with a corrosion allowance of 1/4” (6.0 mm) or greater and when frequent maintenance and/or low reliability are acceptable.
>50 High corrosion rate - materials with this corrosion rate would require an
excessively high corrosion allowance to be added to ensure an acceptable
service 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 which they occur and designing to avoid such conditions. Even though certain corrosion forms may indicate a low-failure
frequency rate, this does not mean that the designer should not consider them. For example, there could be 100’s of general corrosion failures that gradually leaked and were repaired one at a time without any plant shutdowns resulting, compared to, 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 engineering specialist 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 trained engineers 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 are then available to the designers for consultation on any design problem. The designer, in turn, must have a basic knowledge of corrosion to recognize when a problem may exist and when to consult a materials engineering expert. The fundamental obstacle is getting corrosion problems confronted while the process is still at the “drawing board’ stage, rather than after the plant has been built.
Many smaller companies do not have the luxury of in-house materials engineering groups. Therefore, designers, in many instances, 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 materials they manufacture, having conducted many corrosion and mechanical tests on specific products. They also have knowledge about how well their product has performed in the field. However, caution should be exercised by the designer in acting
solely upon the vendor’s recommendations. To be good salesmen, vendors have to be “sold’ on their own products. For
example, the paint salesman may want to paint over everything, while the stainless steel vendor may want to make everything out of stainless steel. As a solution to this problem and to avoid discouraging designers from using vendors (as vendors can render usetld services), it is wise to have the designer follow an established procedure for evaluating different alternatives so that he will have no doubt about which is the better alternative, in this case, paint or stainless steel.
Professional corrosion engineering assistance is available from a variety of sources. The designer can inquire through his own technical society or peruse advertisements in technical journals dealing with metals and corrosion for assistance. For instance, in Materials Performance, a monthly journal published by the NACE International, there is a section that lists available 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 furnish clear, concise specifications to the supplier, manufacturer, and/or fabricator. If the order is unclear, the supplier may furnish wrong or inadequate material.
Materials of construction for process equipment intended for use in corrosive service are generally specified in the following three 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 carbon steel” 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, and then again, he may get a Cadillac. Killed steel or fire box quality steel could be low, medium, or high carbon steel, alloyed or not.
A example of this occurredwhen welded towerswere orderedand built for an Americaninstallation in Mexico. During a stoml, the towers fractured and collapsed. The failure was causedby brittle welds formedwhen medium carbon steel had been
furnished for the towers instead of the anticipatedlow carbonsteel. At welded areas, the weldingheat had raised the areas around the welds abovethe lowercritical temperatureof the steel and when quenchingoccurredin the air, brittle untempered areas were formedthat fracturedunder the stress of the storm. An adequatematerial specificationhad either not been providedto the fabricator of 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 all concerned.
The second category, method of manufacture and heat treatment, is, also important. The method of manufacturing, such as
welding, brazing, silver-soldering, bolting, riveting, casting, forging, etc., must be specified because this will directly affect
the corrosion resistance of the equipment ordered. It is also very important that the heat treatment, when required, is carefidly specified, as improper heat treatment can have very detrimental effects on the corrosion resistance as well as the strength and ductility of steels.
Within the third category, dimensional tolerance and finish, it is important to ensure that all dimensions are adequately specified. Specifications should include the allowable tolerances for all dimensions. With respect to corrosion, the wall thickness and corrosion allowance are probably the most important dimensions. However, finish can play a significant role in 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 corrosion resistance of austenitic or chromium stainless steel equipment, the instructions should be more specific than “a smooth or
polished surface is required’. Specific surface roughness dimensions and acceptable tolerances should be provided.
National Standards
An excellent way for the designer to assure that he will receive the process equipment from the fabricator as it was designed to reduce corrosion is to use national standards. National standards actually represent an agreement between fabricators or suppliers and customers about what can and should be furnished. These standards are not permanent since they require periodic 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 States that publish standards are shown in Table 3.
Table 3- U.S. Standards Organizations
Abbreviation Organization Name
t
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 States Department of Commerce acting through the National Institute for Standards and Technology are frequently used by industry, as are standards issued by the Ordinance and Materials Departments of the US Navy, Army, and Air Force. These include standard specifications termed QQS-Federal, MIL-S Army-Navy Aeronautical Specs, and Aerospace Material Specifications (AMS).
Company Standardsl
Because of repetitive demand for certain items or special process specific requirements not completely covered by national standards, many companies have produced their own standards. There also may be certain situations companies have to deal with that are not covered at all by national standards. Special materials specifications, welding, and inspection procedures may be required to address process specific corrosion problems such as sulfide stress cracking or high temperature 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 chemical company, 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 them periodically. Since these specifications address the unique problems of a company, they can of course be beneficial to the company 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 quality
level. The quality level provides assurance that process equipment will perform reliably and will not fail prematurely.
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 that require no welding maybe substituted. The blanket statement above should not have included the field connections, 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 for
acceptance. Failure to filly speci~ acceptance criteria for weld quality can lead to confusion and problems as demonstrated in the following example.
Miles of 3-in. (76 mm) diameter weldedAISI 304Lausteniticstainlesssteel pipes were producedby a single
manufacturer. The originalpurchasespecificationrequired 100°/0radiographyof all the longitudinalweld seams. When lengths of this pipe were field welded into fittings,the field welds were radiographer, which revealed not only the field welds themselves,but short portionsof the longitudinalwelds, which, in manycases, were very poor. The pipe was cut out, and the pipe fabricatorwas contacted. The fabricatorstoutlymaintainedthat all of his pipe was x-ray quality and he pointed to a cabinet full of radiographs. We read these radiographsand founda lot of evidencecalling for many rejections and repairs. It was finally disclosedthat the pipe fabricatorhad not viewedany of the radio~aphs. He thought that passing the x-rays throughthe welds made them x-rayquality! This story is not a fabrication! Luckily,only a small amount of pipe had actuallybeen field weldedand all the poor welds were identified and the pipe was rejected.
4. The designer must not only define the acceptance criteria for indications that will be cause for rejection, but
must 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 satisfactory
quality material will be excluded. In addition, specifications should not restrict the manufacturers so much that his costs, and hence the price, will be unnecessarily high. On the other hand, the specification must not be 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 or ultrasonics because of geometry should be tested with the liquid penetrant or magnetic particle method also before testing. (If a break occurs during pneumatic testing, catastrophic failure of the equipment tested can occur.)
7. Whenever possible, to hold down costs, have the equipment made with commercially available materials using
standard methods of construction.
8. Before finalizing a specification, have the specification reviewed by potential fabricators. Many times, a fabricator 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, For example trays or packing in a fractionator tower are normally designed by the tray manufacturer rather than the tower designer. This, of course, assumes that the manufacturer is reliable, competent, and has
demonstrated 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 the
fabricator 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 required
job. Again this, of course, assumes that the manufacturer is reliable, competent, and has demonstrated a capability 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 not receive from the manufacturer the corrosion performance he expects from the equipment he has designed. His quality control program should be outlined in the purchase order. Remember that quality can and should be controlled by the designer.
There are many inspection methods available to the designer, but before he specifies one or more of these methods, he should 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 stress corrosion 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 to the method ofjoining, such as welding?
How competent is the fabricator? What reputation does he have for self inspection? Does he use code qualified 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 a product or a service that depends on its ability to satisfi a given need. This can be briefly stated as fitness for service.
Fitness for Service
The designer should decide what inspection or qualification methods are required to assure fitness for service. With this in mind, the designer should confer with his inspection department, his technical people, and the potential fabricators to determine which inspection methods should be specified to assure quality. Inspection costs money, so care should be taken not to over-inspect, such as on routine jobs being done by proven competent fabricators. However, inspection under other conditions 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 plant construction. Metals have excellent mechanical urot)erties, that is, they respond well to external loads, Some important mechanical 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 refinery applications:
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 mechanical designs are often adjusted to permit its use. For example, process temperatures can be lowered, hydrocarbon streams can be dried, 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 construction to handle more severe conditions of temperature, pressure and corrosivity. The “refine~ steels” have evolved to meet the majority of refinery equipment applications. Some of the refinery steels are listed in Table 6 along with their nominal compositions. 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 and
temperatures. For example, Cr-Mo steels provide high temperature strength, resistance to high temperature sulfur corrosion
and hydrogen attack. Stainless steels are used for fhrnace tubes and to resist high temperature sulfidic corrosion in the presence of hydrogen. Stainless steels containing molybdenum are used against naphthenic acid attack.
But be aware, alloying steel does entail additional cost. Also the availability of some alloy steels is less than for plain carbon steel, Also, alloy steels may have problems specific to the type of alloy such as reduced weldability, susceptibility to environmental cracking, and specialized heat treatment requirements etc.
Other metal and alloy systems that are important in refinery construction are copper, nickel, aluminum and titanium based alloys. 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 the hazardous nature of materials processed by the refining industry, safety considerations demand exceptional equipment integrity. 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 above
desired mechanical, chemical, and physical properties at a reasonable cost. Steels are readily available in many forms and have excellent fabrication capabilities. The weldability of steel is excellent, and this contributes greatly to the reliability and safety of modern day pressure containing equipment.
“steel” is a general term for iron based alloys containing carbon, manganese and other alloying elements. Table 8 shows some of the common alloying elements, their effects in steel, and principal functions. The carbon content of most refinery steels 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: the American Society for Testing Materials (ASTM, the American Society of Mechanical Engineers (ASME), the American Petroleum 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 of products 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 followed and the design stresses that can be used. In the United States the most common design codes for refinery equipment are those of ASME, ANSI, and API.
Carbon steel is iron containing controlled amounts of carbon and manganese. The carbon steels are among the most
common materials of construction and probably account for 80 percent of all steels used for refinery applications. Since
they are typically welded, carbon content must be relatively low, between 0.15 and 0.35 percent, and they are commonly termed low or medium carbon steel. Distillation towers, separators, heat exchangers, storage tanks, most piping, and all structures are generally fabricated carbon steel. For processes where the expected corrosion rates for carbon steel are <20 mpy, 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 better corrosion resistance. For large vessels, alloy clad or alloy weld overlay are effective forms of lined construction and more economical than the use of solid corrosion resistant alloys throughout. The use of spray applied coatings both, metallic and non - 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 over carbon steels especially at temperatures between 800 and 10000 F. However Mo addition provides no significant increase corrosion resistance over carbon steels. In the past it had been believed that C-1/2 Mo steel had better resistance than carbon steel to high temperature hydrogen attack, and it was often specified in hot hydrogen service, However, recently
questions have been raised regarding the effect of long term exposure to high temperature hydrogen on C-1/2 Mo steel. As a 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 standard compositions of these steels are specified in the ANSI or SAE standards. Two of these low alloy steels are commonly utilized in refineries. These are 4140 and 4340 which are steel with Cr, Ni, and Mo additions. These materials exhibit good high temperature strength and creep resistance. However since these materials normally have relatively high carbon equivalents (>0, 4), they can be difficult to weld. Therefore, their use is normally limited to those applications which do not
require fabrication by welding. Also these materials tend to have high harnesses and are susceptible to sulfide stress
cracking if harnesses exceed HRC 22. Common uses of these materials in refineries is for flange bolts, valve parts and
shafts 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 used for 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 or
vanadium. In refineries early attempts to combat high temperature sulfidic corrosion in refineries involved the use of
straight chromium steels. Although these steels originally had satisfactory ductility initially, prolonged service produced
temper embrittlement. The addition of 0.5-1‘Yo Mo into the straight Cr steels was found to be an effective solution for this
problem, From a design point of view, the low alloy steels containing up to 9’%Cr and 1’%Mo are generally more cost effective than carbon steel at temperatures above 9000 F. Aside from the stainless steels, Cr - Mo steels are the only steels which 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. These materials are normally specified for applications where high temperature strength, creep resistance ancilor resistance to high temperature/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 to find, 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 at high temperatures and pressures. For improved corrosion resistance, these are usually clad or weld overlayed with
austenitic stainless steels.
The $g~o Cr - Mo Steels provide good corrosion resistance to high temperature sulfur corrosion when required as in
refineries 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 refrigeration systems. With proper procedures and filler metals these steels can be welded such that the weldment impact properties approach those of the alloyed base metal. The use of Nickel steels in refineries is generally limited to processes operating below -500 F.
Stainless steels are alloyed with at least 11.5% Cr to become “stainless”. Cr promotes formation of passive iron/chromium oxide films on steel which in turn exhibit excellent corrosion resistance. Many different grades of stainless steels are available, and their cost, mechanical properties and corrosion resistance vary considerably. It is important, therefore, that stainless 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 alloy compositions 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
Martensitic stainless steels, such as Type 410, and 440 can be hardened by heat treatment similar to carbon, low alloy and Cr - Mo steels. Hardening increases strength and decreases ductility. These stainless steels are less corrosion resistant than
ferritic and austenitic stainless steels. Martensitic stainless steels contain 11 to 18‘Y. Cr and are relatively high in carbon
content. They are subject to 8850 F embrittlement and must be used with caution above approximately 7000 F. They are magnetic 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 stress relieving 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 protect towers, 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 can
be hardened by heat treatment. However, Type 405 contains an aluminum addition that effectively retards its ability to harden during welding. This makes it a better choice than Type410 for vessel linings, especially if clad repairs become necessaty during the vessels service life. The ferritic stainless steels are not normally subject to SSC, are resistant to chloride stress corrosion cracking, and have good oxidation and sulfidation resistance. All ferritic stainless steels with chromium content above 11 percent are subject to 8850 F embrittlement which limits their use to applications temperatures that do not exceed 7000 F. The high chromium stainless steels, such as Type 430 are also susceptible to pitting from wet sulfides 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 excellent corrosion resistance and good high temperature properties. However, they are subject to pitting corrosion and stress corrosion cracking in the presence of chlorides. This has limited their use in refineries to applications where aqueous corrosion can be ruled out. Austenitic stainless steels cannot be hardened by heat treatment or during welding. This has encouraged 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 readily available 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 grain boundaries (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 these grades 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 additional high 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 maybe required if resistance to intergranular corrosion is required.
Type 316and Type 317 stainless steel are two populargrades that contain 2 to 3 and 3 to 4percent molybdenum,
respectively, and have superior resistance to pitting corrosion and acids. They also contain somewhat greater amounts of nickel 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 temperature
oxidation resistance is desired. These wrought grades and their cast forms (CH-20, HH 40, CK-20, HK 40) are found in fired heaters as tube supports and hangers.
Typical refinery applications for austenitic stainless steels include high temperature processes containing both sulfir and hydrogen, 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 gas treating units to resist corrosion from HzS and COZ. The molybdenum grades Type 316 and 317 are often specified for heater tubes, transfer lines, and tower internals in units processing naphthenic acid containing crude and gas oils. Type 309 and 310, usually in cast form, are found in fired heaters as tube supports and hangers. Caution must always be exercised when considering austenitic stainless steels in aqueous environments and in cooling water systems because of the danger of pitting and stress corrosion cracking from chlorides.
Precipitation hardening stainless steels, such as 1’7-4PH, 17-7 PH, and 15-5 PH have application in refinery rotating machinery where both corrosion resistance and strength are needed. These stainless steels can be hardened and
strengthened by solution quenching, followed by a precipitation aging treatment at 800-11000 F. They can be easily
machined while in the solution quenched condition and then aged at temperatures that minimize scaling, distortion, and cracking. Tensile strengths as high as 200,000 psi can be obtained. Precipitation hardening stainless steels are used for valve seats, pump shafts, pump wear rings, and impellers. Their corrosion resistance is somewhat worse than that of Type 304 stainless steel. Also due to their high tensile strengths. these materials tend to be highly susceptible to stress corrosion cracking caused by sulfides and/or chlorides.
Duplex stainless steelshave amicrostructure composed of almost equal amounts of ferrite and austenite. Some alloy
designations are AL6XN, 2205, 3RE60, 2304, and Ferralium 255. The typical composition for the duplex alloys is
18-30°Achromium, 3-70/. nickel, and 1-37. molybdenum. The ferrite phase offers high strength and the austenite provides
good corrosion resistance. When welding parameters are carefully controlled the duplex stainless steels have adequate weldability. They have good general corrosion resistance and are also resistant to chloride stress corrosion cracking, sulfide stress cracking, provided proper welding and heat treatment procedures are followed. The duplex stainless steels are normally proprietary alloy compositions each developed by a specific steel manufacturer. Therefore, the steel
manufacturer should be always consulted to determine the correct forming, welding and heat treatment requirements for each of these materials.
!%ecialm stainless steels are available to meet severe service conditions and fill the gaps where the corrosion resistance of common 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 significant
molybdenum additions to decrease pitting and crevice corrosion. The specialty stainless steels are normally proprietary alloy compositions each developed by a specific steel manufacturer. Therefore, the steel manufacturer should always be consulted 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 inherent brittleness and low strength, gray cast iron is susceptible to damage by thermal and mechanical shock. Although once commonly 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 to reduce the velocity effects of corrosion, such as impingement, erosion, and cavitation. The excellent damping properties of gray 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 containing components. The carbon is present as nodules which promote ductility. It has substantially better toughness than gray cast iron 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 corrosion
resistant due to a passive SiOz surface layer which forms during exposure to many chemical environments. Duriron is a
straight 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 of
oxidizing compounds. Superchlor is vacuum melted Durichlor 51 and possesses twice its tensile strength. High silicon cast irons 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, these austenitic alloys are the toughest of the cast irons, They are also produced as ductile irons, with high strength and ductility over a wide temperature range. All have excellent corrosion, wear and high temperature resistance due to the relatively high alloy content. Ni-Resist alloys can be machined to close tolerances. Typical refinery uses are valve components, pump components, dampers, diffusers, tray components, and engine and compressor parts.
Other Metals and Alloys
Comer and its alloys combine excellent corrosion resistance with good thermal conductivity, ease of machinability and
good strength, especially when alloyed. Copper is a relatively noble metal and is usually not corroded unless oxygen or other 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 and also 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 good resistance to brackish and salt water corrosion and wet HzS corrosion. Admiralty tubes have been used extensively in water cooled condensers and coolers. Like most copper alloys admiralty brass is susceptible to dealloying and has been shown to stress 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 austenitic
stainless steels. The stress corrosion cracking resistance of austenitic stainless steels rapidly increases as the nickel content is increased above 20 percent. For example, Inconel 600 (a 700/. Ni Cr Fe alloy) shows excellent stress corrosion cracking resistance and is used for this reason in many refinery applications. Nickel also forms the basis for many high temperature alloys, 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 in the top of crude oil distillation towers and as the upper 4 or 5 trays to resist hydrochloric acid. It is also used for crude tower overhead condenser tubes and components. Monel 400 is also used to combat corrosion by hydrofluoric acid in alkylation units and in hydrodesulfurization and reforming unit overhead systems. High nickel alloys, including Inconel 625 and Incoloy 825, are used to prevent polythionic acid corrosion of flare stack tips and in hydroprocessing effluent piping. Hastelloy B-2 is particularly well suited for handling hydrochloric acid at all concentrations and temperatures including the boiling point. It is, however, attacked in the presence of oxidizing salts. Alloys B-2 and C-276 have excellent resistance to all concentrations of sulfuric acid up to at least 2000 F. The high nickel alloys are expensive and their use restricted 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 can be improved by anodizing. They tend to break down, however, at pH values below 5 and above 8 and this limits the use of aluminum and its alloys in many environments. Another limitation of aluminum is its relatively low strength at elevated
temperatures. Two alloys of aluminum are commonly used in refinery applications. Alloy 3003, alloyed with manganese
has been successfully used in tower overhead condensers cooled by water on the condenser tubeside. Resistance to shell side aqueous 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 to avoid erosion corrosion. Alloy 606 1-T6 is a magnesium and silicon aluminum alloy that is precipitation hardenable. It has been 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 has been applied in various forms of aluminizing to protect furnace tubes and piping in high temperature hydrogen
suIfide/hydrogen services.
Titanium and its alloys Titanium is a highly reactive metal which depends on a protective oxide film for corrosion
protection. Titanium is not suitable for high temperature service and because of its reactivity must be welded and cut under inert 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 prevent embrittlement by hydride formation.
Titanium exhibits high corrosion resistance to most refinery streams. Tubes made from pure titanium (Grade 2) are used extensively in overhead coolers and condensers on a number of units to prevent corrosion by chlorides, sulfides, and
aqueous sulfur dioxide. These tubes can corrode, however, underneath acidic deposits. Titanium tubes are very useful at locations 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 deposit corrosion and higher temperature services where the pure grade is unsuitable. Anodizing and high temperature air oxidation 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 fiber blankets, bricks, castable mixes (similar to concrete) or plastic ramming mixes, Refractories generally have very high temperature resistance (3000 T +), are chemically inert to most chemicals and solvents, and when in cast form have very good erosion resistance. Typical retinery uses for refractories areas thermal insulation on the inside of fired heater and boilers, for insulation and erosion resistance in catalyst handling systems such as in fluid catalytic cracking units, and as corrosion 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 load
bearing shapes that enable it to be utilized in the same manner as metallic materials. Plastics are man made materials and
each type of plastic was originally developed with a specific application in mind. For this reason, there exist a large number of plastic materials available for use in equipment design and new plastics are being developed on a regular basis. Each
particular polymer has its own unique properties. This vast diversity in material types and properties is one the major
differences between metals and plastics. Plastics are divided into 2 groups thermoplastic materials and thermosetting materials. Thermoplastics are capable of being repeatedly softened by increase in temperature and hardened by a decrease
in temperature, Thermoses on the other hand undergo a cure in the molding or forming process and as a result of chemical
reactions (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 originally
developed. However like metals, plastics do suffer from corrosion when exposed to some environments. Corrosion
mechanisms in plastics are generally completely different than those which occur in metallic components. Corrosion in plastics is best defined as any reaction with an environment which significantly changes the physical and chemical properties of the plastic. The term corrosion rate is not normally applicable to plastics. Typical corrosion mechanisms found in plastics include polymer chain scission (cutting), liquid oxidation degradation, melting, swelling, chemical embrittlement, and stress cracking just to name a few, there are as many different failure modes for plastics as there are types of plastic materials
In recent years some thermoplastics have found their way into a limited number of refinery applications. Some of the more
common 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 long term 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 temperature
resistance up to 140”F. There are 2 commercial forms of PE, high density(HDPE) and low density (LDPE). Each has its own specific strengths and weaknesses.
~ is another widely used thermoplastics, PP is suitable for corrosive waste as well as pressure applications because of its
inertness to a wide range of chemicals including most solvents and because of its ability to withstand temperatures up to 200T.
PVDF features remarkable high temperature performance . PVDF pipe and fittings can handle corrosive fluids at working temperatures up to 280”F. Other PVDF qualities are excellent chemical resistance, to such chemicals as halogens, and resistance 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 and
solvents, 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 are available 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 are normally utilized either as a spray on type coating (paints) or in combination with some type of inorganic reinforcing material such as glass or carbon fibers. The most common form of this material used in refineries is fiberglass, Fiberglass which is a thermosetting plastic resin reinforced with glass or carbon fibers. The properties of a fiber glass material are
determined by the type of resin utilized to produce the material and the type of material utilized for reinforcement, Typical
resins utilized for refinery applications are polyester, epoxy, and vinyl ester. Fiberglass materials are commonly used for chemical storage tanks and drums. Fiberglass is also commonly used as a lining material to protect the internal surface of storage 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 sword had to maintain its sharpness and be tough at the same time, Damascus Steel was the prized material that satisfied both
requirements. However, it was very scarce. Because the production of Damascus swords was a closely guarded secret, a
great search was conducted to determine a substitute steel that would perform like the Damascus steel. The search failed because the swords made from the selected steels were either tough and became dull quickly or stayed sharp but broke because of brittleness during the first blow. Years later, it was learned that this marvelous Damascus steel was not a new alloy at all but was simply produced by a new method of heat treatment. The specification of a slower quench from the hardening heat was what made the sword so effective. According to one historian, the heat treatment specification called for 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 schedule should 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 for large waste storage tanks, 60 ft ( 18m) in diameter and 35 ft (1 lm) high, to obviate stress corrosion cracking and, at the same 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 the vessel.
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 stress cracking has occurred since in any of these heat treated vessels. The heat treatment schedule is not usually as detailed as the schedule described above. For instance, “Heat slowly to a temperature of 1100 to 1200° (598 to 648 C) and hold for 1
