Materials of Construction for Pressure Vessels

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is not alreadyin the public domain may not be copied, reproduced, sold, given, ordisclosed to third parties, or otherwise used in whole, or in part, withoutthe written permission of the Vice President, Engineering Services, SaudiAramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX20202 J.H. Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Materials of Construction for Pressure Vessels

Engineering Encyclopedia Vessels


Saudi Aramco DeskTop Standards

CONTENT PAGE

HOW MATERIAL SELECTION FACTORS INFLUENCEMATERIAL SELECTION....................................................................................... 1

Strength, Including Creep............................................................................. 2

Resistance to Corrosion................................................................................ 2

Increasing Resistance to Corrosion................................................... 4

Resistance to Hydrogen Attack......................................................... 5

Fracture Toughness ...................................................................................... 7

Material Fractures ............................................................................. 7

Fracture Toughness Determination................................................... 8

Factors That Influence Fracture Toughness...................................... 9

Control of Fracture Toughness ....................................................... 10

ASME Code and Brittle-Fracture Evaluation ................................. 12

Fabricability................................................................................................ 16

Requirements for Fabricability ....................................................... 16

Postweld Heat Treatment ................................................................ 17

DETERMINING MAXIMUM ALLOWABLE STRESSES ................................. 21

ASME Criteria for Determining Maximum Allowable Stress.................... 21

Division 1 Criteria........................................................................... 22

Division 2 Criteria........................................................................... 25

ASME Maximum Allowable Stress Tables................................................ 25

Maximum Allowable Compressive Stress.................................................. 30

DETERMINING WHETHER PRESSURE VESSEL MATERIALSMEET SAUDI ARAMCO MATERIAL SELECTIONREQUIREMENTS................................................................................................. 31

SAES-D-001............................................................................................... 31



Saudi Aramco DeskTop Standards

Rimmed Steels ................................................................................ 32

Nozzle Reinforcing Plates and Shell Stiffener Rings...................... 32

Corrosion Allowance ...................................................................... 32

32-SAMSS-004 .......................................................................................... 34

Contractor Design Package ........................................................................ 37

GENERIC MATERIAL TYPE .............................................................................. 40

MAXIMUM ALLOWABLE-STRESS TABLE .................................................... 40

Legend and Notes for Figure 11................................................................. 44

GLOSSARY .......................................................................................................... 48



Saudi Aramco DeskTop Standards 1

HOW MATERIAL SELECTION FACTORS INFLUENCE MATERIAL SELECTION

Materials that are used to construct ASME Code pressure vessels must be selected frommaterial specifications that are approved under the Code. A materials engineer normallymakes material selections for specific applications after the process environment and therequired design conditions have been defined. However, a mechanical engineer must also befamiliar with the factors that influence material selection.

The main factors that influence material selection are:

Strength, including creep

Resistance to corrosion

Fracture toughness

Fabricability

These material selection factors were discussed in COE 105 and will be briefly reviewed andexpanded upon here. Other factors that influence material selection are cost, availability ofmaterials, and ease of maintenance.

Alloys of carbon steel may be used to construct pressure vessels because of the suitability ofthese alloys in terms of the first three material selection factors. Fabricability considerationsmust also be evaluated, based on the particular alloy used. Alloys have the followingcharacteristics:

Increase the steel's resistance to corrosion and hydrogen attack. This resistanceimproves the reliability of the pressure vessel.

Increase the steel's fracture toughness.

May allow components to be fabricated from thinner material, which reducesweight and cost.

May allow the steel to withstand extremes in operating pressure or temperaturethat may be encountered during normal use of a pressure vessel, withoutcomponent failure.

The primary alloying elements that are used in carbon and low-alloy steels are chromium,magnesium, silicon, molybdenum, vanadium, nickel, copper, and columbium (also calledniobium). The specific alloying elements that are used and their quantities directly influencematerial properties.




Strength, Including Creep

Strength is a material's ability to withstand an imposed force or stress. Strength is asignificant factor in the selection of a material for a particular application inasmuch asstrength determines how thick a pressure vessel component must be in order to withstand theimposed loads. Inasmuch as the yield and ultimate tensile strengths of materials are relativelylow at elevated temperatures, creep and rupture strengths of materials may determineallowable stress values. At elevated temperatures in the creep range (above about 427C[800F]), a material will continue to deform without an increase in the applied load andresultant stress. Creep resistance is increased by the addition of alloying elements such aschromium, molybdenum, and/or nickel to carbon steel. Therefore, in elevated temperatureapplications, alloy materials are often employed for the sole purpose of increasing creepresistance.

The overall strength of a material is determined by its yield strength, ultimate tensile strength,creep and rupture strengths. These strength properties depend on the chemical composition ofthe material. Material strength determines the allowable stresses that are used in the ASMECode for detailed component design. Allowable stress values are discussed later in thismodule.

Resistance to Corrosion

Corrosion is the deterioration of metals by chemical action. A material's resistance tocorrosion is probably the most important factor that influences its selection for a specificapplication. The corrosion rates of various metals in established processes are determined onthe basis of experience, while laboratory tests are used to determine the corrosion rates fornew processes. The corrosion resistance of a particular metal can be significantly changed bya slight change in environmental chemistry. Since corrosion rates increase with temperature,temperature also plays a major role in corrosion resistance.

The most common method that is used to address corrosion in pressure vessels is to specify acorrosion allowance. A corrosion allowance is supplemental metal thickness which is addedto the minimum thickness that is required for the component to resist applied loads. Thisadded thickness compensates for thinning (corrosion) that will take place during service.Saudi Aramco requirements for corrosion allowance are discussed later in this module. Theconcept of corrosion allowance is illustrated in Figure 1.




Where:

Tmin = Minimum thickness of component that is required to resist applied loads

c = Corrosion allowance

T = Total required component thickness

Corrosion AllowanceFigure 1




Increasing Resistance to Corrosion

The corrosion resistance of carbon steel is increased through the addition of alloying elementssuch as chromium, molybdenum, or nickel. To determine whether the use of an alloy isappropriate, and to determine the particular alloy material to use, both cost and an acceptablecorrosion rate must be considered. Before a final material selection is made, the cost increasein going from plain carbon steel to alloy steel must be compared with the corrosion rate andhigher required corrosion allowance for carbon steel.

Stainless steels are the most common, readily available, and among the more expensivecorrosion-resistant materials. Stainless steel can be used either as a solid plate or as a liningthat is bonded to a carbon or low-alloy steel baseplate. The use of solid stainless-steel plate isthe more economical approach for relatively thin-walled pressure vessels (up to about 19 mm[3/4 in.] thick). The exact thickness where one approach becomes more economical thananother depends on current cost and material availability, and these factors vary based onmarket conditions and location. If a stainless-steel lining is used, the following three choicesare available:

Integral cladding

Strip or sheet lining

Weld overlay

Integral Cladding is a lined plate that is made by hot rolling a carbon or low-alloy steel backingplate together with a corrosion-resistant sheet. The two layers that form this lined plate arethen welded at the edges.

Strip or Sheet Lining is fabricated by welding alloy strip or sheet to the vessel shell. This liningmethod is normally used in retrofit applications rather than in new vessel construction.

Weld Overlay is a lining method in which corrosion-resistant weld metal is added directly overa carbon or low-alloy steel backing material. Weld overlay is frequently more economicalthan cladding, based on the choice of vendor and the availability of material. Weld overlayalso often supplements other lining methods. For example, when the cladding on clad plate islocally removed to make an attachment directly to the base material (such as for a nozzle), thecorrosion-resistant layer is restored by weld overlay. Standard Drawing No. AB0-036367,Joint Preparation and Welding Details, Alloy and Clad Pressure Vessels and HeatExchangers, provides standard weld overlay details for nozzle attachments to clad pressurevessels.




Corrosion resistance in pressure vessels may also be increased through the use of anonmetallic, internal coating. In this application, the coating is bonded to the metallic surfaceand protects the metal from the corrosive process environment. Maximum temperaturelimitations of internal coatings prohibit their wide use in pressure vessels. However, SaudiAramco does use internal coatings in production applications where pressure vesseltemperatures are within the limitations of the coating material.

SAES-H-001, Selection Requirements for Industrial Coatings, specifies the requirements forthe following:

Acceptable coating systems based on the type of structure or equipment to becoated and on whether new construction or maintenance is involved.

Coating selection for onshore and offshore applications.

Special surface preparation or coating requirements.

The extent of coating required, if not the entire surface.

Resistance to Hydrogen Attack

Hydrogen attack is sometimes discussed here because it is thought of as a form of corrosion.However, hydrogen attack differs from corrosion in that damage occurs throughout thethickness of the component, rather than just at its surface, and this damage occurs without anymetal loss. Thus, it is not practical to provide a corrosion allowance to allow for hydrogenattack. In addition, once hydrogen attack has occurred, the metal cannot be repaired and mustbe replaced.

Monatomic hydrogen easily diffuses through steel. However, molecular hydrogen does notdiffuse through steel. The diffusion of monatomic hydrogen through steel depends on thefollowing factors:

Hydrogen partial pressure in the process environment

Material composition

Temperature

Time




At intermediate temperatures, from approximately 150C to 200C (300F to 400F),monatomic hydrogen diffuses into voids that are normally present in steel. In these voids themonatomic hydrogen forms molecular hydrogen, which cannot diffuse out of the steel. If thishydrogen diffusion continues, pressure can build to high levels within the steel, and the steelcan crack.

At elevated temperatures, over approximately 315C (600F), monatomic hydrogen not onlycauses cracks to form but also attacks the steel. Plain carbon steel is especially susceptible tohydrogen attack. At these elevated temperatures, monatomic hydrogen forms and combineswith the carbon in the steel to form methane gas. Methane gas cannot diffuse out of the steel.When the pressure of the methane gas becomes high enough, intergranular cracks occur. Thesteel then becomes spongy and embrittled, and permanent damage results. Over time, thesteel loses tensile strength, hardness, notch toughness, and ductility. Hydrogen attack, plusthe stresses in the steel that are caused by operating conditions and residual fabricationstresses, have caused catastrophic failure of pressure vessels.

Protection Against Hydrogen Attack - Plain carbon steels are satisfactory materials for hydrogenservice at low operating temperatures and high hydrogen partial pressures or at high operatingtemperatures and low hydrogen partial pressures. The addition of carbide stabilizingelements, such as chromium and molybdenum, decreases the reaction of hydrogen with thecarbides in steels. Therefore, engineers must often use low-alloy steel that containschromium, molybdenum, or both elements to provide adequate protection against hydrogenattack in refinery and petrochemical services.

When hydrogen attack is a factor, API Publication 941, Steels for Hydrogen Service atElevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants, isused for material selection. This document contains a graph known as the Nelson Curves.Figure 10 in Work Aid 2 depicts the Nelson Curves that are excerpted from API 941.

The Nelson Curves were developed from reported experience with steels in hydrogen service.If the combination of maximum design temperature and hydrogen partial pressure falls on,below, or to the left of the curve for the type of steel being used as pressure vessel material,the material is not subject to hydrogen attack. The acceptable maximum design temperatureincreases as the alloy content (chromium, molybdenum, or both elements) increases, given aspecific hydrogen partial pressure. Figure 10 shows why it is common to use low-alloy steelsfor pressure vessels that are used in hydrogen service and that operate at elevatedtemperatures and pressure.




Note that Figure 10 shows 1.0 Cr-0.5 Mo steel as equivalent to 1.25 Cr-0.5 Mo steel athydrogen partial pressures above about 8.28 MPa(a) (1200 psia). The 1.0 Cr-0.5 Mo steel isalso adequate for somewhat lower temperatures at lower hydrogen partial pressures. From apractical standpoint, 1.0 Cr-0.5 Mo material is rarely used in pressure vessel construction.1.25 Cr-0.5 Mo is normally the lowest alloy that would be considered for use in situationswhere carbon steel is not adequate due to hydrogen attack considerations.

SAES-D-001 and 32-SAMSS-004 do not directly address hydrogen attack considerations.However, 1.25 Cr-0.5 Mo material selection options are provided in 32-SAMSS-004 for hightemperature applications. Concern over hydrogen attack is one reason why the 1.25 Cr-0.5Mo material might be used rather than the carbon steel alternatives. Specific materialselection requirements are discussed later in this module.

Fracture Toughness

Fracture toughness refers to the ability of a material to withstand conditions that could causebrittle fracture. Pressure vessel components that are constructed of ferrous material haveoccasionally fractured at a pressure that was well below the design value. Such fracturesgenerally occurred at low temperatures and were brittle rather than ductile in nature. Brittlefractures are characterized by the lack of deformation or yielding before the component failscompletely. In a ductile fracture, the component yields and deforms before it breaks.

Material Fractures

For a brittle fracture to occur, three conditions must exist simultaneously at a particularlocation in a pressure vessel:

Enough stress must exist in the component to cause a crack to initiate andgrow.

The material must have a sufficiently low fracture toughness at the temperature.

There must be a critical size defect in the component, such as at a weld, to actas a local stress concentration point and as a site for crack initiation.

A brittle fracture will occur without warning the first time the component is exposed to thenecessary combination of low temperature, high stress, and critical size defect.




Fracture Toughness Determination

As discussed in COE 105, the Charpy V-notch test (Cv) is commonly used to qualitativelydetermine the fracture toughness of steel. In this test, an impact test is performed on anotched specimen that is taken from a specific location in the material; and the impactenergies that are required to fracture the specimen at various temperatures are recorded.

The fracture toughness of the material can be determined by the magnitude of the impactenergy that is required to fracture the specimen. Figure 2 shows the typical shape of impactenergy transition curves for low- and high-strength steels.

Typical Impact Energy Transition Curves

Figure 2




Factors That Influence Fracture Toughness

The fracture toughness at a given temperature varies with different steels and with differentmanufacturing and fabrication processes. Additional factors that affect brittle fracturebehavior are as follows:

Arc strikes can cause brittle fracture, especially if the arc strike is made over arepaired area.

Cold forming of thick plates may cause brittle fractures in areas with stressraisers or plate scratches.

Torch cutting (or beveling) of plate edges may produce hard and brittle areas,which make the edges more prone to cracking.

The slope of the impact energy curve in Figure 2 indicates the rate of change of fracturetoughness with temperature. The "lower shelf" is the lower section of the impact energycurve, and the "upper shelf" is the upper section. A material is very brittle at lower shelfenergy temperatures and can behave like a piece of glass. Fracture at lower shelf energytemperatures is very abrupt, as when a piece of glass is dropped. A material is ductile atupper shelf energy temperatures. Fracture at upper shelf energy occurs after a small amountof yielding has taken place.

Low-strength steels have a significant increase in fracture toughness as the temperatureincreases (see curve A of Figure 2). High-strength steels show only a slight increase infracture toughness as temperature increases (see curve B of Figure 2).

The dotted lines in Figure 2 show the nil ductility transition (NDT) temperatures for bothhigh- and low-strength steels. The NDT temperatures are the starting points of the transitionsbetween brittle and ductile fractures. Below the NDT temperatures, material fracture is brittlein nature. Above the NDT temperatures, material fracture is ductile in nature. The rate ofchange of fracture toughness is significantly different between high- and low-strength steels.The NDT is more important for low-strength steel due to the much greater increase in fracturetoughness when going from low to high temperature.




Material selection must confirm that the material has adequate fracture toughness at thelowest expected metal temperature. The lowest One-Day Mean Temperature for the site andthe lowest temperature to which the vessel may be exposed during any phase of its operationdetermine the lowest expected temperature that the vessel must be designed for. This lowesttemperature identification must also consider temperatures that will occur during pre-commissioning, startup, shutdown, or upsets.

The mechanical design of a pressure vessel must avoid either a brittle or a ductile fracture.However since a brittle fracture will occur without warning and can be catastrophic in nature,it is especially important for material selection to eliminate the risk of brittle fracture.

Control of Fracture Toughness

Saudi Aramco imposes additional requirements in SAES-D-001 and 32-SAMSS-004 toensure that pressure vessels have adequate fracture toughness.

SAES-D-001 defines the basis for the Critical Exposure Temperature (CET) or the minimumdesign temperature of a pressure vessel. The first step in the specification of material withadequate fracture toughness is to set the minimum design temperature. The minimum designtemperature is defined as the minimum metal temperature that is coincident with a pressurethat is greater than 25% of the design pressure. Pressures that are below this level producetoo little stress to cause a brittle fracture. To determine the minimum design temperature, allpossible scenarios to which the vessel may be exposed in addition to normal operation mustbe considered. For example, autorefrigeration must be considered, where this is possible.Identification of various possible operating scenarios and the minimum design temperaturesassociated with them is the responsibility of the process design engineer.




SAES-D-001 also requires that the materials that are used for internal and externalattachments to pressure vessels that have a minimum design temperature less than 0C (32F)must be the same or equivalent to the vessel component to which the attachment is welded.The attachment weld and the portion of the attachment that is nearest to the vessel will be atnearly the same low temperature as the vessel. Thus, use of the same material for theseattachments and the vessel shell ensures that they will have comparable fracture toughness.Use of a material with less fracture toughness than the vessel component to which it isattached increases the potential for the initiation of a brittle fracture at the junction betweenthe attachment and the vessel component. Such a brittle fracture could progress into thevessel shell itself. Postweld heat treatment (PWHT) is also required for all carbon and low-alloy steel vessels that have a minimum design temperature below 0C (32F). PWHTenhances the low-temperature fracture toughness of the material.




32-SAMSS-004 requires that all material for pressure-containing components be impact tested,if required by the ASME Code Section VIII, Division 2. The Division 2 impact-testingcriteria must be used even for pressure vessels that are designed in accordance with theASME Code Section VIII, Division 1 in all other respects. When required, impact testingmust meet Division 2 procedures and acceptance criteria.

32-SAMSS-004 specifies the following additional requirements, based on material fracturetoughness considerations:

If the minimum design temperature of the pressure vessel is below 0C (32F):

- The material for all internal and external attachments to the shell orheads must be the same or equal to the vessel component to which theyare attached. This repeats the requirement that was stated in SAES-D-001. Acceptable material alternatives are specified for plate, pipe,flanges and forgings, fittings, bolts and nuts, and supports andattachments for low-temperature service. These material alternatives areexpected to have adequate fracture toughness for low temperatures.

- All butt welds are to be 100% radiographed. This procedure willprovide greater assurance that defects that may serve as locations toinitiate cracks or brittle fracture are not present.

- Postweld Heat Treatment (PWHT) is required. Heat treatment relievesweld shrinkage stresses, improves ductility, and reduces brittle-fracturerisk. This also repeats the requirement that was stated in SAES-D-001.PWHT is discussed further in the section that follows.

The metal temperature must not be below 16C (60F) during hydrotest. Thepressure vessel has its highest membrane stresses during hydrotest. Theprimary reason for the 16C (60F) minimum metal temperature duringhydrotest is that some plain carbon steels (such as ASTM A283 Gr. A) have anNDT just below this value.

ASME Code and Brittle-Fracture Evaluation

The ASME Code, Section VIII, contains a simplified approach to evaluate brittle fracture incarbon and low-alloy steel. Material specifications are classified within Groups I through V,for the purpose of brittle fracture evaluation (Figure AM-218.1 of the ASME Code, SectionVIII, Division 2). The Code contains exemption curves for those Material Groups thatidentify the acceptable minimum design metal temperature versus thickness, 0 mm through 75mm (0 in. through 3 in.), where impact testing (Charpy V-notch) is not required. The curvesthat are shown in Figure 3 are taken from Figure AM-218.1, ASME Code, Section VIII,Division 2. The curves are based on both experience and test data. If the design conditionsdo not permit exemption in accordance with this basis, then material impact testing at thespecified minimum design temperature is required to permit its use.




Source: ASME Boiler and Pressure Vessel Code , with permission from ASME.

Impact Test Exemption Curves for Carbon Steels

Figure 3




A Roman numeral that designates the corresponding Material Group appears above eachcurve in Figure 3. If the minimum design metal temperature of a pressure vessel is equal to orabove that shown by the intersection of the Material Group curve and of the componentthickness, then impact testing is not required. For example, a Group III material that is 38mm (1.5 in.) thick and operates at 16C (60F) does not require impact testing. It should benoted that the exemption of a material from impact testing through the use of this basis doesnot mean that the ASME Code ignores brittle fracture. Impact-test exemption means thatthere is sufficient data to conclude that the combinations of material, temperature, andthickness defined by the exemption curves result in material that has sufficient fracturetoughness, without the need for additional impact testing.

The minimum design temperature at which impact testing is not required increases with thematerial thickness. Thick material is more prone to brittle fracture than thin material, and ahigher temperature is required to prevent brittle fracture in thicker material. For all weldedconstruction over 75 mm (3 in.) thick and with a minimum design temperature below 49C(120F), impact testing is required. The ASME Code also contains impact-testing proceduresand impact-energy requirements for cases that are subject to impact testing. Participantsshould refer to the ASME Code for details.

Paragraph AM-218.2 of the ASME Code, Section VIII, Division 2 provides an additionalexemption from impact testing. The paragraph states that impact testing is not required if theactual design stress does not exceed 41 364 kPa (6 000 psi) and if the minimum designtemperature is -46C (-50F) or above. This exemption is valid even if the minimum designmetal temperature is below the Figure 3 exemption curves. This additional exemption isbased on experience which indicates that the membrane stress must exceed 41 364 kPa (6 000psi) for a brittle fracture to occur.




Figure 3 (based on Figure AM-218.1) covers only carbon steel materials. The addition ofalloying elements will typically improve the toughness of steels. In general, the addition ofmanganese, silicon, and/or nickel to carbon steel will improve its fracture toughness. Thefollowing list highlights some of the ASME Code, Section VIII, Division 2 impact-testrequirements for alloy steels:

Base metal impact tests are not required for certain low-allow steels andproduct forms when the minimum design temperature is not below the GroupIV curve of Figure 3. 2 1/4 Cr-1 Mo material that conforms to SA-387, Grade22, Classes 1 and 2 (plate), and SA-182, Grades F21 and F22 (forgings) areincluded in this category. Weld and heat-affected zone (HAZ) impact tests arerequired.

For high-alloy steels:

- Types 304 (18 Cr-8 Ni), 304L (18 Cr-8 Ni-Low Carbon), and 347 (18Cr-10 Ni-Cb) require impact testing at minimum design temperaturesbelow -254C (-425F). All other materials must be impact tested atminimum design temperatures below -198C (-325F).

- Impact testing is required for minimum design temperatures below -29C (-20F) for the following:

+ Chromium stainless steels, P-Nos. 6 and 7 (11-17% Cr).

+ Austenitic chromium nickel stainless steels with carbon contentover 0.10%, or with a chrome or nickel content over the requiredAISI analysis range.

+ High-alloy steel castings.

- Impact testing is required for the following materials at all minimumdesign temperatures if PWHT has been done below 899C (1 650F):Type 309 (23 Cr-12 Ni), Type 310 (25 Cr-20 Ni), Type 316 (16 Cr-12Ni-2 Mo), Type 309 Cb, Type 310 Cb, and Type 316 Cb.

It should be noted from the above summary that the impact test exemptions for high-alloysteels are not a function of material thickness. Further, high-alloy steels exhibit ductilebehavior to much lower temperatures than carbon and low-alloy steels.




Fabricability

Fabricability is the last major consideration in the selection of pressure-vessel material.Fabricability refers to the ease of construction and to any special fabrication practices that arerequired in the use of the material. Fabricability includes the following considerations:

Plate material intended for pressure vessel construction must have sufficientductility to permit it to be rolled into the required geometric shapes (such ascylinders, cones, or spheres).

Plate material must be weldable so that individual plate segments can beassembled into the required shapes. The effects of welding on materialproperties must be considered. Weldable materials, fabrication methods, andwelding procedures have been known and used for years.

Specific fabrication requirements vary among material types. The sections that follow discussthese considerations.

Requirements for Fabricability

Pressure vessels that are of interest to the Participants use welded construction. Weldingprocedures are used to ensure that welded joints are of acceptable strength and quality. Thematerial chemistry of the weld area must be equivalent to the material chemistry of the basematerial so that the material properties and corrosion resistance of the weld area will be thesame as those of the base material. Special concerns arise where a ferritic material is weldedto an austenitic material, resulting in a bimetallic weld. In the case of a bimetallic weld, thedifference in the thermal expansion coefficient between the two materials causes high localstresses at elevated temperatures. These local stresses must be considered in the detailedmechanical design. Sometimes in the case of bimetallic welds, a welding electrode material isselected that has a thermal expansion coefficient that is between the coefficient of the twobase materials that are to be welded. This welding electrode selection reduces the localizedthermal stresses. In all cases, the ASME Code requires that written and tested weldingprocedures be followed.

All welders must be tested to verify their capabilities. In order to achieve the requiredfinished quality, only qualified welding procedures and welders are used to fabricate ASMECode equipment. Welding, welding procedures, and welder qualification are discussedfurther in MEX 202.04.




Postweld Heat Treatment

Postweld Heat Treatment (PWHT), because it adds to total cost, is another consideration inthe fabricability of pressure vessels. In PWHT, the pressure vessel is heated to a hightemperature after the completion of all welding, and the high temperature is maintained for aspecified period of time. PWHT is required for the following:

Residual stress relief

Hardness reduction

Process considerations

During welding, the weld and the adjacent base material both become very hot and thencontract as they cool. This contraction causes stresses due to the uneven cooling andconstraint of the overall structure. PWHT is used to relieve these stresses so that a vesselfailure does not occur. The ASME Code contains rules which determine when PWHT isnecessary. These rules are based on the material type and wall thickness. Figure 4 is anexcerpt from the ASME Code, Section VIII, Division 1, Table UCS-56, and gives PWHTrequirements for a particular material class. While Figure 4 and the discussion that followsfocus on a specific material class, similar considerations apply to the other material classes aswell.

MATERIAL

NORMALHOLDING

TEMPERATURE, F, MINIMUM

MINIMUM HOLDING TIME AT NORMALTEMPERATURES FOR NOMINAL THICKNESS

Up to 2 in. Over 2 in. to 5 in. Over 5 in.

P-No. 1Gr. Nos.1, 2, 3

1100 1 hr./in., 15 min.minimum

2 hr. plus 15 min. foreach additional inchover 2 in.

2 hr. plus 15 min.for eachadditional inchover 2 in.

Gr. No. 4 Not applicable None None None

PWHT Requirements for Carbon and Low-Alloy Steels

Figure 4




The material in Figure 4 is identified by "P-No." ("P" Number) and "Gr. No." (GroupNumber). The allowable stress tables in the ASME Code provide these numbers for everymaterial. The P-No. and Gr. No. are ASME designations for materials that have commonwelding and heat-treating characteristics. The P-No. 1 material that is shown in Figure 4corresponds to the carbon steel material specifications. The ASME Code contains similartables for all materials that may be used. Figure 4 specifies the minimum PWHT temperatureand the minimum holding time at temperature, based on wall thickness. When the vessel isheated to this elevated temperature, the residual welding stresses relax and the vessel reachesan initial stress-free state. The minimum holding time at the PWHT temperature increaseswith wall thickness. More time is needed to relax the welding stresses in large volumes ofweld metal since more weld shrinkage occurs. As previously noted, PWHT to relax residualwelding stresses also is required for pressure vessels that are in low-temperature service.

Another reason for using PWHT is to reduce the weld hardness for particular materials. Thewelding process produces locally hard regions in the weld and in adjacent areas of certainmaterials (for example, low chrome-alloy materials).

The locally hard areas are less ductile and more prone to the formation of cracks. The PWHTsoftens the hard areas and restores ductility. The ASME Code does not have specificrequirements for weld hardness, and it does not require PWHT for the purpose of hardnessreduction. Therefore, the pressure vessel user must specify weld hardness limitationsseparately. Weld hardness is discussed further in MEX 202.04.

Process considerations are the last reason for the use of PWHT. Some process environments,such as those with high caustic concentrations, may cause cracks to occur at highly stressedwelds in carbon steel material. Residual stresses that remain after welding may cause crackformation in this environment. As noted above, the ASME Code does not require PWHT forthis purpose, and the user must specify PWHT.




Notes that accompany Table UCS-56 provide several exemptions to the PWHT requirementsthat are specified. These exemptions are only valid if the PWHT is required for relief ofresidual stress. The exemptions do not apply if the PWHT is required for reduction of weldhardness or because of process considerations. Exemptions from the PWHT requirements ofTable UCS-56 are based on material, weld type, and weld size. For example, PWHT is notmandatory for groove or fillet welds in P-No. 1 material not over 13 mm (0.5 in.) size thatattach nonpressure parts to pressure parts provided a minimum preheat temperature of 93C(200F) is used and the pressure part is no more than 32 mm (1.25 in.) thick. Refer to TableUCS-56 for the details of the PWHT exemptions.

The requirements in the paragraphs that follow expand on the discussion of requirementsshown in Figure 4. Refer to Table UCS-56.1 and associated notes in the ASME Code,Section VIII, Division 1, for the complete text of PWHT-associated requirements.

It may be possible to weld something to a vessel that has been postweld heat-treated after it has been in service without doing another PWHT. For example,if a new structural attachment or a new nozzle must be added, a PWHT is notnecessary, provided that the new welds are within the PWHT exemptions ofTable UCS-56, and provided that the original PWHT was not necessary forhardness reduction or process considerations or due to low-temperature service.

Repairs can be made to P-No. 1, Group No. 1, 2, and 3, as well as P-No. 3,Group No. 1, 2, and 3 materials and weld metal after PWHT but before thefinal hydrotest without the need for another PWHT. The ASME Code limitsthe size of the repair that is permissible without subsequent PWHT. The totalrepair depth should not exceed 38 mm (1.5 in.) for P-No. 1, Group No. 1, 2,and 3 materials, and 16 mm (0.63 in.) for P-No. 3, Group No. 1, 2, and 3materials. Additional weld procedure and inspection requirements must also bemet.




Table UCS-56.1 of the ASME Code specifies permissible PWHT temperaturereductions versus increased holding times. The PWHT temperature cannot bereduced more than 93C (200F) below the temperature specified in TableUCS-56. Temperature reduction cannot be employed if PWHT is required toreduce weld hardness or for process considerations.

The 93C (200F) preheat reduces weld shrinkage stresses sufficiently inmaterials from 32 mm to 38 mm (1.25 in. to 1.5 in.) thick and eliminates theneed for PWHT. PWHT is required for stress relief in materials withthicknesses above 38 mm (1.5 in.) regardless of the amount of preheat. PWHTis not required in materials with thicknesses that are below 32 mm (1.25 in.).




DETERMINING MAXIMUM ALLOWABLE STRESSES

One of the major factors in the design of pressure vessels is the relationship between thestrength of the components and the external loads (pressure, weight, etc.) that are imposedupon the components. These external loads cause internal stresses in the components. Stressis the force-per-unit area in a solid material that resists the separation, compaction, or slidingthat is induced by external forces. The design of a pressure vessel must ensure that theseinternal stresses never exceed the strength of the components that make up the pressurevessel. Pressure vessel components are designed such that the component stresses that arecaused by the loads are limited to maximum allowable values that will ensure safe operationof the pressure vessel. Maximum allowable stress is the maximum force-per-unit area thatmay be safely applied to a pressure vessel component. The maximum allowable stressincludes an adequate safety margin between the maximum stress level in a component due tothe applied loads and the stress level that could actually cause a failure. The use of maximumallowable stress in the design of pressure vessel components will be discussed in MEX202.03.

CSE 110 briefly introduced the concept of maximum allowable stress and the ASME Codemaximum allowable stress tables based on Section VIII, Division 1. The paragraphs thatfollow provide additional detail by describing the ASME Code rationale for determiningmaximum allowable stress. The description highlights the differences between Division 1 andDivision 2 of Section VIII in determining maximum allowable stress and discusses themaximum allowable stress tables and maximum allowable compressive stress in more detail.

ASME Criteria for Determining Maximum Allowable Stress

Appendices 1 and 2 of the ASME Code, Section II, Part D - Properties, contain the criteriathat are used to establish the maximum allowable stresses for most ferrous and nonferrousmaterials. Appendix 1 provides the criteria for Division 1 pressure vessels for all materialsother than bolting. Appendix 2 provides the criteria that are used to establish the maximumallowable stresses for materials that are used in Division 2 pressure vessels (including bolting)and the maximum allowable stresses for bolting materials in Division 1 pressure vessels.Appendix P of the ASME Code, Section VIII, Division 1, Pressure Vessels, presents thecriteria that are used to establish maximum allowable stress values for low-temperature steelsthat are used in cryogenic applications for cast and nodular iron materials.




Division 1 Criteria

The following data is from the ASME Code, Section II, Part D, Appendix 1, Non-mandatoryBasis for Establishing Stress Values in Tables 1A and 1B. A similar discussion is containedin Section II, Part D, Appendix 2 for bolting, and Section VIII, Division 1, Appendix P forlow-temperature, cast or ductile iron materials.

When evidence of satisfactory performance is available, successful experience in serviceguides the determination of maximum allowable stress values for pressure vessel parts. Suchevidence is considered equivalent to test data where operating conditions are known withreasonable certainty. In the evaluation of new materials, engineers must compare testinformation with available data on successful applications of similar materials. Figure 5, isbased on the ASME Code, Section II, Part D, Appendix 1, Table 1-100, and shows thecriteria/equations that are used to compute the maximum allowable stresses for wrought orcast ferrous and nonferrous materials, other than bolting, for Division 1 pressure vessels.Below room temperature, the yield and tensile strengths of the material must be used todetermine its maximum allowable stress. Above room temperature, the material's creep andrupture strength must be considered as well in determining maximum allowable stress. Referto the ASME Code for the criteria that are used for welded pipe or tube and for structuralquality steel.




Temperature Criteria (1)

Below Room Temperature Tensile Strength Yield Strength

Room Temperature and Above Tensile Strength

Yield Strength

Stress Rupture

Creep Rate

Criteria for Determining Allowable Stress for Division 1 Pressure Vessels(Wrought or Cast, Ferrous and Nonferrous Materials)

Figure 5




The following nomenclature is used in the allowable stress computations that are shown inFigure 5:

St = Specified minimum tensile strength at roomtemperature (ksi).

Rt = Ratio of the average temperature-dependent trendcurve value of tensile strength to the room temperaturetensile strength.

Sy = Specified minimum yield strength at roomtemperature.

Ry = Ratio of the average temperature-dependent trendcurve value of yield strength to the room temperatureyield strength.

SRavg = Average stress to cause rupture at the end of 100 000hr.

SRmin = Minimum stress to cause rupture at the end of 100 000hr.

Sc = Average stress to produce a creep rate of 0.01%/1 000hr.

Two sets of allowable stress values are provided in Table 1A of the ASME Code, Section II,Part D, Appendix 1, for austenitic materials and in Table 1B for specific non-ferrous alloys.The higher alternative allowable stresses are identified by a footnote. These stresses exceedtwo-thirds but do not exceed 90% of the minimum yield strength of the material attemperature. The higher allowable stress values should be used only where slightly higherdeformation of the component is not in itself objectionable. These higher allowable stressesare not recommended for the design of flanges or other strain-sensitive applications. In thecase of flanges, for example, the larger deformation that would be expected if the higherallowable stresses were used could cause flange leakage problems even though a major flangefailure would not occur.

The maximum allowable stress for materials other than bolting for Division 1 pressure vesselsis the lowest value that is obtained from the criteria that are stated in Figure 5. Note that thesecriteria are based on specified fractions of the stated material strength properties. Thesefractions can be considered as safety factors between the maximum allowable stress and thestress that would cause material failure.




Division 2 Criteria

The maximum allowable stress criteria for materials other than bolting that are contained inAppendix 2 of the ASME Code, Section II, Part D for Division 2 pressure vessels willtypically yield somewhat higher values than Appendix 1 will yield for Division 1 vessels.However, the Appendix 2 criteria consider only the material yield and tensile strengths, sinceDivision 2 does not permit the use of materials at temperatures that are in the creep range.Participants should refer to Appendix 2 for additional details.

ASME Maximum Allowable Stress Tables

As discussed in CSE 110, tables that are included in the ASME Code, Section II, Part D,contain the maximum allowable tensile stresses of materials that are acceptable for use inASME Code, Section VIII, pressure vessels. The maximum allowable stress varies withtemperature because material strength is a function of temperature. The maximum allowablestress values that are contained in these tables are based on the criteria that were previouslydiscussed.

Figure 6 (adapted from Table 1A of the ASME Code, Section II, Part D) shows examples ofmaximum allowable Division 1 tensile stress data for three different material specifications:

Carbon steel plates and sheets (Spec. No. SA-515 and SA-516).

Low-alloy steel plates (Spec. No. SA-387).

The first part of Figure 6 identifies the Spec. No. (material specification number), the grade (amaterial specification may have multiple grades), the nominal chemical composition, the P-No. and Group No., and the minimum yield and tensile strengths in thousands of pounds persquare inch (ksi). This first part of Figure 6 also helps identify any similarities that may existamong the material specifications, such as in nominal alloy composition or yield and tensilestrengths. In some cases, these similarities may be used to help select the material to use forpressure vessel fabrication, given specific process conditions. The maximum allowable stressvalues as a function of temperature are presented in the second part of Figure 6.

The information that is contained in the ASME Code Table 1A has been condensed andreorganized in Figure 6 in two parts to help the Participants to compare the material types andto note variances in maximum allowable stress that are determined by temperature and alloycomposition. The actual tables that are contained in the ASME Code should be used for allpractical work applications.




TABLE 1A (excerpt)

ALLOWABLE STRESS IN TENSION FOR CARBON ANDLOW-ALLOY STEEL

Spec No. Grade NominalComposition

P-No. GroupNo.

Min.Yield(ksi)

Min.Tensile

(ksi)

Carbon Steel Plates and Sheets

SA-515 55 C-Si 1 1 30 55

60 C-Si 1 1 32 60

65 C-Si 1 1 35 65

70 C-Si 1 2 38 70

SA-516 55 C-Si 1 1 30 55

60 C-Mn-Si 1 1 32 60

65 C-Mn-Si 1 1 35 65

70 C-Mn-Si 1 2 38 70




Plate - Low Alloy Steels

SA-387 2 Cl.1 1/2Cr-1/2Mo 3 1 33 55

2 Cl.2 1/2Cr-1/2Mo 3 2 45 70

12 Cl.1 1Cr-1/2Mo 4 1 33 55

12 Cl.2 1Cr-1/2Mo 4 1 40 65

11 Cl.1 1 1/4Cr-1/2Mo-Si 4 1 35 60

11 Cl.2 1 1/4Cr-1/2Mo-Si 4 1 45 75

22 Cl.1 2 1/4Cr-1Mo 5 1 30 60

22 Cl.2 2 1/4Cr-1Mo 5 1 45 75

ASME Maximum Allowable Stress Tables (Excerpt)

Figure 6




TABLE 1A (excerpt)ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL

Max Allowable Stress, ksl (Multiply by 1,000 to Obtain psi)for Metal Temperature, F, Not Exceeding

650 700 750 800 850 900 950 1000 1050 1100 1150 1200SpecNo.

Carbon Steel Plates and Sheets13.8 13.3 12.1 10.2 8.4 6.5 4.5 2.5 -- -- -- -- SA-51515.0 14.4 13.0 10.8 8.7 6.5 4.5 2.5 -- -- -- -- SA-51516.3 15.5 13.9 11.4 9.0 6.5 4.5 2.5 -- -- -- -- SA-51517.5 16.6 14.8 12.0 9.3 6.5 4.5 2.5 -- -- -- -- SA-515

13.8 13.3 12.1 10.2 8.4 6.5 4.5 2.5 -- -- -- -- SA-51615.0 14.4 13.0 10.8 8.7 6.5 4.5 2.5 -- -- -- -- SA-51616.3 15.5 13.9 11.4 9.0 6.5 4.5 2.5 -- -- -- -- SA-51617.5 16.6 14.8 12.0 9.3 6.5 4.5 2.5 -- -- -- -- SA-516

Plate-Low Alloy Steels (Cont'd)13.8 13.8 13.8 13.8 13.8 13.3 9.2 5.9 -- -- -- -- SA-38717.5 17.5 17.5 17.5 17.5 16.9 9.2 5.9 -- -- -- -- SA-38713.8 13.8 13.8 13.8 13.4 12.9 11.3 7.2 4.5 2.8 1.8 1.1 SA-38716.3 16.3 16.3 16.3 15.8 15.2 11.3 7.2 4.5 2.8 1.8 1.1 SA-38715.0 15.0 15.0 15.0 14.6 13.7 9.3 6.3 4.2 2.8 1.9 1.2 SA-38718.8 18.8 18.8 18.8 18.3 13.7 9.3 6.3 4.2 2.8 1.9 1.2 SA-38715.0 15.0 15.0 15.0 14.4 13.6 10.8 8.0 5.7 3.8 2.4 1.4 SA-38717.7 17.2 17.2 16.9 16.4 15.8 11.4 7.8 5.1 3.2 2.0 1.2 SA-387

Figure 6, cont'd




Note that the allowable stresses at temperatures between -29C and 343C (-20F and 650F)are the same as the allowable stress at 343C (650F) for each material presented in Figure 6except for SA-387, Grade 22 Cl. 2. The allowable stress increases to 129.6 MPa (18.8 ksi)for SA-387, Grade 22 Cl. 2 material at 38C (100F) and below. See the ASME Code,Section II, Part D, Table 1A for details.

Note that each material specification has different Types, Grades, and/or Classes within it. Insome cases, these differences are due to different chemical compositions, while in other casesthey may be due to the particular steelmaking process that was employed. Higher strengthgrades of a particular material specification have higher maximum allowable stresses.Therefore, if higher strength material is used for a pressure vessel, the vessel can be fabricatedof thinner material. For example, SA-516, Grade 60 has a higher maximum allowable stressthan Grade 55 at 371C (700F). As a result, a vessel made from SA-516, Grade 60 materialcan be fabricated from thinner plate and can still have an acceptable reliability. When morethan one material specification is acceptable based on strength considerations alone, materialcost and availability will then determine which material specification will be used. Thedashed columns in Figure 6 indicate that SA-516 cannot be used to construct pressure vesselswith design temperatures above 537C (1 000F).

The maximum allowable stress for most ferritic materials does not change for designtemperatures through 343C (650F). As the design temperature increases above 343C(650F), the thickness that is required for pressure vessel components increases because thematerial strength and maximum allowable stress decrease. For example, the maximumallowable stress for SA-516, Grade 55 decreases from 13.8 ksi to 8.4 ksi in going from 650Fto 850F. The addition of alloying elements to carbon steel typically increases the high-temperature strength of the material. Therefore, a thinner alloy component can typically beused at higher temperatures when its high-temperature strength is compared to that of plaincarbon steel. For example, in Figure 6, compare the maximum allowable stress of SA-516,Grade 70 material with that of SA-387, Grade 11 Cl. 1 at 427C (800F). Note that the SA-387 material may be used through 648C (1 200F) but that the SA-516 material cannot beused over 537C (1 000F). Therefore, based on strength considerations, alloy construction isoften justified on economic grounds for high-temperature service because alloy componentswill be thinner than if carbon steel were used. This reduced quantity of required material willoften offset the higher cost of alloy versus carbon steel material on a weight basis.

By using the various tables that are contained in the ASME Code, comparisons can be madeamong the various material types, grades, compositions, and maximum allowable stressvalues to select the most cost-effective pressure vessel materials for the specific vesselapplication.

Work Aid 1 provides a general procedure that may be used to determine maximum allowablestress and whether contractor-specified values for maximum allowable stress are correct.




Maximum Allowable Compressive Stress

The ASME Code maximum allowable stress criteria and tables that were previously discussedare valid for pressure vessel components that are in tension under applied loads, such asinternal pressure. Pressure vessel components may also be placed into compression by loadssuch as weight, wind, or earthquake. The maximum allowable compressive stress for apressure vessel component is the smaller of the following:

The maximum allowable tensile stress as determined from the appropriatemaximum allowable stress table discussed above.

The value of the factor B determined using the appropriate external pressurechart presented in the ASME Code. This chart will be discussed in MEX202.03 as part of the discussion of external pressure design of vesselcomponents.




DETERMINING WHETHER PRESSURE VESSEL MATERIALS MEET SAUDIARAMCO MATERIAL SELECTION REQUIREMENTS

The ASME Code contains general design rules. When additional design requirements for aparticular application are needed, the user and pressure vessel designer must specify them.These additional requirements are based on design and operating experience that are relevantto the particular applications. SAES-D-001 and 32-SAMSS-004 are the primary documentsthat specify Saudi Aramco requirements for pressure vessels.

The Saudi Aramco engineer will typically not specify the materials that are to be used forpressure vessel components. Instead, the Saudi Aramco engineer will typically reviewContractor Design Packages to determine whether contractor-specified material specificationsare acceptable based on Saudi Aramco requirements.

The sections that follow summarize the overall scope and use of SAES-D-001 and 32-SAMSS-004, the material selection requirements that these Saudi Aramco documents contain,and the typical contents of a Contractor Design Package.

SAES-D-001

SAES-D-001 is Saudi Aramco's basic engineering standard for pressure vessel design.SAES-D-001 contains additional design requirements that are beyond ASME Code rules.This standard, plus other SAESs that are referenced in it, represent the main body ofrequirements that are used by Saudi Aramco's engineering contractor in the preparation of apressure-vessel purchase order. Relevant requirements are extracted from SAES-D-001 bythe contractor and added to the pressure-vessel purchase specification, as needed.

SAES-D-001's additional requirements remain within the scope of the ASME Boiler andPressure Vessel Code, Section VIII, Divisions 1 and 2. If a pressure vessel has a maximumdesign pressure of less than 1380 kPa (ga) (200 psig) and a volume less than 1.98 m3 (70 ft.3),SAES-D-001 requirements do not apply unless specified in the purchase order. Therefore, thebasic ASME Code requirements are acceptable without change for a small relatively low-pressure vessel. Pressure vessels that are part of packaged units; liquid nitrogen vessels andair surge drums, when code stamped, also need not conform to SAES-D-001 unless stated inthe purchase order.

If a pressure vessel is fabricated within Saudi Arabia, it does not need to be stamped with theASME Code stamp. However, the pressure vessel must meet all other requirements of theASME Code and SAES-D-001. In this case, the Saudi Aramco Inspection Departmentverifies conformance to ASME requirements, and the issue of a formal stamp is waived.




SAES-D-001 references several other Saudi Aramco standards, material systemspecifications, and drawings. The requirements that are contained in these additionaldocuments must also be met. One of these references is to Saudi Aramco Materials SystemSpecification 32-SAMSS-004, Pressure Vessels.

SAES-D-001 contains material selection requirements that must be observed. Work Aid 2contains a material selection procedure which includes these requirements. The paragraphsthat follow discuss these requirements.

Rimmed Steels

Rimmed steels must not be used for any pressure vessels. Rimmed steel is characterized by amarked difference in chemical composition across the section and from the top to the bottomof the ingot that was produced during the steelmaking process. This pattern of variedcomposition persists from the rolling process to the final product form. This variation inchemical composition also makes rimmed steels unsuitable for pressure vessel construction,which requires more uniformity in material properties.

Nozzle Reinforcing Plates and Shell Stiffener Rings

Nozzle reinforcing plates and shell-stiffener rings must be either of the same or equivalentmaterial specification as the shell or head material to which they are attached. Because of thisrequirement, welding considerations are simplified, and all the materials are of equal strengthand fracture toughness.

Corrosion Allowance

As discussed earlier, materials selection must also consider the corrosion that takes placeduring operation of the pressure vessel. A corrosion allowance must be added to all carbonsteel pressure-containing parts, including the shell, heads, nozzle necks, and covers. Therequired corrosion allowance will be specified in the Contractor Design Package. WhileSAES-D-001 specifically addresses only carbon steel material, the need for a corrosionallowance must be considered for all material types, especially ferritic materials. COE 105discussed corrosion allowance requirements for various steels in different processenvironments.

Pressure Vessel Internals - Removable pressure vessel internals that are subject to corrosionshould have a corrosion allowance equal to that of the shell. The design of removableinternals considers only half of the expected total corrosion.




The rationale for this approach is that removable internals that are designed for only theexpected total corrosion will cost less initially and can easily be replaced later, based on theactual corrosion that occurs.

Nonremovable internals must have a corrosion allowance that is equal to twice that of theshell. Most pressure vessel internals, such as downcomers, weirs, and tray supports, cancorrode on both sides. From a strength-design viewpoint, corrosion from both sides should beconsidered with regard to nonremovable internals.

Determination of Corrosion Allowance - Unless an alternative approach is specified in thepurchase order, the amount of corrosion allowance for carbon steel pressure-containing partsis determined on the basis of the information that is contained in the paragraphs that follow:

When corrosion rates are known from the histories of pressure vessels insimilar service, the corrosion allowance is based on a 20-year service life. Apressure vessel may remain in service longer than 20 years if periodicinspections confirm that the component thicknesses are still adequate for thedesign conditions. The minimum corrosion allowance is 1.6 mm (1/16 in.).

If considerable material erosion is expected, the next higher pipe schedule fromthat required for the applied loads should be used for nozzles. This higher pipeschedule effectively increases the corrosion allowance for the nozzles. Flowvelocities through the nozzles are higher than in the overall vessel. Thus, iferosion is a concern in the particular service, such as when entrained solids arepresent, the erosion has a greater effect on the nozzles due to the higher flowvelocity through the nozzles.

No more than 6.4 mm (1/4 in.) corrosion allowance may be specified. If morethan 6.4 mm (1/4 in.) corrosion allowance is required for carbon steel parts toachieve a 20-year service life, the use of a more corrosion-resistant material orthe use of cladding or lining (metal or synthetic material) must be considered.

32-SAMSS-020, Column Trays, specifies corrosion allowances for parts covered by thisspecification. For these parts, the corrosion allowances in 32-SAMSS-020 should be used,rather than those in SAES-D-001.




32-SAMSS-004

32-SAMSS-004 must be a part of all pressure vessel purchase documents. 32-SAMSS-004covers the requirements for Saudi Aramco pressure vessels that are within the scope of theASME Boiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2. The requirementsof 32-SAMSS-004 plus the ASME Code requirements must be adhered to by vendors whosupply pressure vessels to Saudi Aramco. All other specifications, drawings, and forms thatare referenced in 32-SAMSS-004 must also be followed. Among the items that thesedocuments cover are the following:

Material requirements for carbon and low-alloy steel vessels.

Design details for standard vessel components, such as flanges.

Additional fabrication, inspection, and testing requirements.

Forms to be completed by the vendor.

The ASME Code contains a variety of materials that are acceptable for pressure vesselapplications. Saudi Aramco has simplified the material selection process by the identificationof carbon and low-alloy steel materials that are suitable for services and design conditionsnormally encountered in Saudi Aramco operations. Table 1 from 32-SAMSS-004 containsthese material selections. Vendors may propose alternatives to these materials. However,first they must furnish the material mechanical properties and chemical analysis, and thenSaudi Aramco must approve the substitution before it is used. Occasionally, the materials thatare identified in Table 1 are not suitable for a particular service. In these cases, materialselections are handled on an exception basis and are not covered by SAES or SAMSSrequirements. Table 1 from 32-SAMSS-004 is reproduced in Work Aid 2 for reference asFigure 11.




Figure 11 is used to select, on the basis of cost and availability, the appropriate vesselcomponent materials from the alternatives that are listed. The intent is for the primarypressure-containing components to have the same material chemistry and comparable strengthin a given pressure vessel. For example, if a low-alloy plate material is selected for the shelland head of a pressure vessel in a high-temperature service, comparable low-alloy materialmust be used for the nozzles, flanges, forgings, and fittings. Work Aid 2 may also be used tohelp determine whether contractor-specified pressure-vessel materials meet Saudi Aramcorequirements. The paragraphs that follow discuss the content of Figure 11.

Figure 11 identifies six major categories of pressure-vessel components. The materialspecifications that are indicated are all ASME Code approved materials for the specificcomponent form. Component form refers to plate, pipe, flanges and forgings, fittings,bolting, or supports and attachments.

Plates are used for shells, heads, rolled nozzles (for example, larger-diameter nozzles that arefabricated from plate rather than from pipe material), reinforcing pads, stiffeners, supports,and attachments.

Pipes are used for small-diameter nozzles that are not rolled from plate. The choice betweenusing pipe material or rolled plate for nozzles is based on economics.

Forged material is used for nozzle flanges and forged fittings, such as couplings. Wroughtmaterials are used for other fittings, such as elbows.

Bolting materials (for example, bolts and nuts) are used at nozzle flanges for pressure vessels.

Support and attachment materials are used for skirts and any nonpressure-containingcomponents that connect to the pressure vessel itself.

Vessel service classification is based primarily on design temperature.

General Service category covers the design-temperature range from 0C through 350C (32Fthrough 650F). This category includes most pressure vessels that are in typical process plantand production applications. The material specifications in this category for plate, pipe,forgings, and fittings are all carbon steel.




High-Temperature Service category covers design temperatures from 351C through 485C(651F through 850F). In this temperature range, the higher corrosion rate and lowermaterial strength become more significant factors in the mechanical design of pressure vesselcomponents. Therefore, low-alloy materials are shown as options for plate, pipe, forgings,and fittings. If the corrosion rate of carbon steel is too high, or if the alloy could be made thinenough to compensate for alloy steel's greater cost per pound, it is more economical to uselow-alloy material. An example of a low-alloy material is SA-335 Gr. P11 pipe (1 1/4 Cr-1/2Mo). Previous sections of this module discussed the increased corrosion resistance andmaterial strength of alloy material at elevated temperatures.

If carbon steel is not suitable for the combination of temperature and hydrogen partialpressure that is required for the particular application, the use of an alloy material may also benecessary due to hydrogen attack considerations. Previous sections of this module discussedmaterial selection based on hydrogen attack considerations.

Some Saudi Aramco applications exceed a 454C (850F) design temperature. Designtemperatures that are above this level are in the creep range for ferritic materials. Materialselection for services that are in the creep range is beyond the scope of Figure 11 and 32-SAMSS-004 and must be made on an individual basis. Alloy materials will typically be usedat temperatures that are above 454C (850F) in order to have adequate creep strength andmaximum allowable stress. The selection of the particular alloy to use will be based on itsmaximum allowable stress at design temperature, relative cost, and corrosion resistance atelevated temperature. 1 1/4 Cr-1/2 Mo and 2 1/4 Cr-1 Mo steels are the most commonly usedpressure-vessel materials for high-temperature applications. Additional requirements mayalso be specified on the steel manufacturing process, vessel fabrication, and inspection thatwill improve overall vessel material and fabrication quality, as well as long-term reliability atelevated temperature.

Low-Temperature Service category is divided into two ranges: 0C to -46C (32F to-50F) and -47C to -101C (-51F to -150F). Brittle fracture is a major concern for thisservice. The materials are selected to ensure that they have adequate fracture toughness atthese low temperatures.

Materials that are suitable at temperatures to -46C (-50F) are unlikely to have adequatefracture toughness at temperatures below -46C (-50F). Therefore, materials with greaterfracture toughness are specified for the lower temperature range. Material selections fortemperatures that are below -101C (-150F) are beyond the scope of Figure 11 and 32-SAMSS-004 and must be made on an individual basis.




Wet, Sour Service is the last category. The specified materials apply to a maximum designtemperature of 203C (400F). If the service has a higher design temperature, materialselections must be made on an individual basis.

Saudi Aramco imposes special requirements on the materials that are used in wet, sour servicebeyond the material specifications that are shown in Figure 11. Cracking at welds, calledsulfide stress corrosion cracking, is possible in this process environment. Sulfide stresscorrosion cracking is a form of brittle fracture and occurs under the combined action of tensilestress and corrosion in the presence of water (wet) and hydrogen sulfide (sour). SAES-D-001and 32-SAMSS-004 contain additional Saudi Aramco requirements for wet, sour service.Saudi Aramco requirements are based on the following factors:

Hydrogen Sulfide (H2S) partial pressure

Fluid state (for example, liquid or gas)

Operating pressure

SAES-L-033, Corrosion Protection for Pipelines/Piping, defines wet, sour service. Termsrelevant to wet, sour service are contained in the Glossary. Participants are referred to SAES-D-001 and 32-SAMSS-004 for specific additional requirements for materials in wet, sourservice and to COE 105 for additional information.

Contractor Design Package

In most situations, the Saudi Aramco engineer will not take the lead role in the initial materialspecification and mechanical design of pressure vessel components. Lead roles are taken bythe prime contractor that Saudi Aramco has employed for the particular project and thespecific pressure vessel manufacturer. The job of a Saudi Aramco engineer will normally beto review the work that is performed by the prime contractor and pressure vessel manufacturerfor acceptability with respect to Saudi Aramco requirements. The term Contractor DesignPackage, as used in this course, describes the total of all the detailed design information forthe pressure vessel that is prepared by both the prime contractor and the pressure vesselmanufacturer. The Saudi Aramco engineer will use the information that is contained in aContractor Design Package in order to perform his review function.




A complete Contractor Design Package will include the following items:

A completed Pressure Vessel Design Sheet, Form 2682 for Division 1 pressure vesselsor Form 2683 for Division 2 pressure vessels. This data sheet will normally beprepared by the prime contractor. The content and use of these forms are discussed inMEX 202.03, and blank copies are contained in Course Handout 3 for reference.

Detailed fabrication drawings and welding requirements for all the pressure vesselcomponents, such as the shell, heads, nozzles, support, and internals. These drawingsand welding requirements will be prepared by the pressure vessel manufacturer.

Pressure vessel inspection plan. This plan will be prepared by the pressure vesselmanufacturer.

Pressure vessel hydrotest procedure, in the form of a drawing or a written stepwiseprocedure. This procedure will be prepared by the pressure vessel manufacturer.

Pressure vessel design calculations. The initial design calculations for the pressurevessel shell and heads will be prepared by the prime contractor on the Pressure VesselDesign Data Sheet. The final and complete calculations will be prepared by thepressure vessel manufacturer.

Safety Instruction Sheet, Form 2694. Note that this form may actually be completedby either a Saudi Aramco engineer or the prime contractor, depending on the particularsituation. Completion of the Safety Instruction Sheet will be discussed in MEX202.03.

The information that Participants will use to solve the Exercises and Evaluations in this andthe next two modules is contained in Contractor Design Packages that are contained in CourseHandout 4.

Refer to the Pressure Vessel Design Sheet, Form 2682, that is contained in Course Handout 3.Figure 7 shows the area on this form where information that is related to material selection isspecified. Note that this area includes items such as service, design temperature, materialspecifications for the major components, maximum allowable stresses, and corrosionallowance. This section of the form must be reviewed to help determine if the materials thatare specified by the contractor meet Saudi Aramco requirements.




OPERATING CONDITIONS

INTERNAL Normal F PSIG

PRESSURE Maximum F PSIG

Minimum (when




WORK AID 1: PROCEDURE FOR DETERMINING MAXIMUM ALLOWABLESTRESSES

This Work Aid may be used to determine the maximum allowable stress in tension forpressure vessel materials in accordance with the ASME Code, Section VIII, Division 1, and toverify the maximum allowable stresses that are specified in Contractor Design Packages.

1. On the basis of information that is contained in the Contractor Design Package,determine the generic material type (ferrous, nonferrous, bolting, etc.) for the pressure-vessel component under consideration. The vast majority of Saudi Aramcoapplications will use ferrous material for the primary components. Bolting will bespecified at connections such as flanges that must be disassembled periodically.

2. Determine the appropriate ASME Code allowable-stress table that coincides with thegeneric type of material that will be used for the pressure-vessel component. UseFigure 9:

GENERIC MATERIAL TYPE MAXIMUM ALLOWABLE-STRESS TABLE

Ferrous Section II, Part D, Table 1A

Nonferrous Section II, Part D, Table 1B

Bolting Section II, Part D, Table 3

Nickel, Type 304, or Aluminum Alloyused at cryogenic temperatures Section VIII, Table ULT-23

Cast Iron Section VIII, Table UCI-23

Cast Ductile Iron Section VIII, Table UCD-23

Allowable-Stress Table Based on Material TypeFigure 9

3. Locate the material specification number and Type/Grade that will be used for thepressure-vessel component in the maximum allowable-stress table that was determinedin Step 2.




4. Determine the design temperature specified for the pressure vessel from the pressure-vessel design data sheet that is contained in the Contractor Design Package.

5. Locate the pressure-vessel design temperature in the maximum allowable-stress table.Use linear interpolation for design temperatures that are between those that are shownin the table.

6. Determine the ASME Code, Section VIII, Division 1 maximum allowable stress intension at the intersection of the material specification found in Step 3, in combinationwith the pressure-vessel design temperature found in Step 5. Use linear interpolationto determine the maximum allowable stress for temperatures that are between thestated values.

7. Verify that the maximum allowable stress that was specified in the Contractor DesignPackage coincides with the value that was found in Step 6.




WORK AID 2: PROCEDURE FOR DETERMINING WHETHER PRESSUREVESSEL MATERIALS MEET SAUDI ARAMCO REQUIREMENTS

This Work Aid may be used in conjunction with SAES-D-001 and 32-SAMSS-004, todetermine whether materials that are specified in a Contractor Design Package for pressure-vessel components meet Saudi Aramco requirements. For convenience, the Nelson Curvesand Table 1 of 32-SAMSS-004 are reproduced in this Work Aid as Figures 10 and 11respectively. Pressure vessel design information that is required to verify material selectionis obtained from the Contractor Design Package.

Nelson CurvesFigure 10




Acceptable Materials for Carbon and Low-Alloy Steel Vessels

Figure 11




Legend and Notes for Figure 11

Legend

* Sour service above 205C (400F) is not within scope of the Specification.

** Grade of material must be the same classification as pipe and plate for theindicated service.

*** Avoid prolonged exposures to temperatures above 425C (800F), because thecarbide phase of carbon steel may be converted to graphite.

Notes:

1. These temperatures are limiting design temperatures and are not operatingtemperatures.

2. That section of attachments extending 305 mm (12 in.) or less from the shell head orpressure-containing part of any Division 2 pressure vessel or low-temperature servicevessel shall be of the same material as the item to which it is attached. Beyond the 305mm (12 in.) or any attachments to Division 1 pressure vessels, the material may be asshown in Table 1.

3. Shall not be welded directly to shell.

4. Non-resulfurized, special quality only. Merchant quality ('M' grades) are notpermitted.




1. Pressure-vessel design temperature _______C (_______F)

If the design temperature is below -101C (-150F) or above 485C (850F), materialselection requirements are beyond the scope of SAES-D-001 and 32-SAMSS-004requirements. Consult the Consulting Services Department.

2. Is the pressure vessel in wet, sour service? Yes No

If yes, the design temperature must be no more than 205C (400F). Otherwise,consult the Consulting Services Department.

3. Is the pressure vessel in hydrogen service? Yes No

If yes, what is the hydrogen partial pressure?

Hydrogen partial pressure _____________ MPa(a), (psia)

4. Identify the "Vessel-Service Classification" based on the above information.

General Service (0C to 350C [32F to 650F]) _______________

High-Temperature Service

(351C to 485C [651F to 850F]) _______________

Low-Temperature Service _______________

Below 0C to -46C (32F to -50F) _______________

-47C to -101C (-51F to -150F) _______________

Wet, Sour Service _______________




5. Identify the material specifications that are specified by the contractor or vendor in theContractor Design Package for the following vessel components.

Plate for:

- Shell _______________

- Heads _______________

- Rolled nozzles _______________

- Reinforcing pads _______________

- Stiffeners _______________

Pipe for nozzles _______________

Flanges and forgings _______________

Fittings _______________

Bolts _______________

Nuts _______________

Supports and attachments _______________

6. Refer to Figure 11, Acceptable Material for Carbon and Low-Alloy Steel Vessels.Confirm that the material specifications found in Step 5 are acceptable for the "Vessel-Service Classification" found in Step 4. If the vessel is in hydrogen service, confirmthat the combination of hydrogen partial pressure and design temperature is acceptablein accordance with Figure 10, Nelson Curves.

7. If the proposed materials are not contained in Figure 11, further review is required.Consult the Consulting Services Department as needed.




8. Confirm that the following additional requirements are met:

No rimmed steels are used. As long as the specified material is shown inFigure 11, this requirement is met.

Materials used for reinforcing plates and stiffener rings are either the same orequivalent to the shell or head material to which they are attached.

A suitable corrosion allowance is specified. A 1.6 mm (1/16 in.) minimumcorrosion allowance is required for all carbon steel pressure containing parts.

9. For pressure vessels that are in wet, sour service, confirm that the following additionalrequirements are met:

01-SAMSS-016 must be specified if controlled rolled steel is used.

A minimum 3.2 mm (1/8 in.) corrosion allowance is required unless the vesselis internally coated. A 1.6 mm (1/16 in.) corrosion allowance is required forinternally-coated pressure vessels.

NACE MR-01-75 must be specified.




GLOSSARY

AISI American Iron and Steel Institute.

alloy An intentional combination that consists of two or moresubstances, of which at least one is a metal, and that exhibitsmetallic properties. An alloy can be either a mixture of twotypes of crystalline structures or a solid solution.

ASME American Society of Mechanical Engineers.

austenitic Either a chromium-nickel or a chromium-nickel-manganesealloy that has a minimum alloy content of 18% chromium plus8% nickel and that is nonmagnetic.

austenitizing Forming austenite by heating a ferrous alloy into thetransformation range (partial austenitizing) or above thetransformation range (complete austenitizing).

brittle fracture A sudden break that is not preceded by deformation oryielding.

Charpy V-notch test The most popular method of qualitatively determining thefracture toughness of steel.

cold forming Shaping material into the required geometry without the use ofheat.

corrosion Deterioration of a material, usually a metal, due to its reactionwith the environment. Corrosion may be caused either bydirect chemical attack or by an electrochemical action.

creep A condition that occurs at elevated temperature wheredeformation continues to increase without any increase inapplied load.

creep strength The limit of the ability to resist creep. Expressed in terms ofthe stress that is required to cause continuous elongation of amaterial that is subjected to elevated temperature.




downcomer A method of conveying liquid from one tray to the tray belowit in a vertical column.

ductility The ability of a material to deform plastically withoutfracturing, as measured by elongation or reduction of area in atensile test.

ferritic An iron alloy steel, used for pressure vessel construction, thathas a carbon content less than 0.4% and that is magnetic.

fracture toughness The ability of a material to withstand the conditions that couldcause a brittle fracture.

hardness The resistance of a metal to plastic deformation, usually byindentation.

heat-affected zone(HAZ)

That portion of the base metal that is not melted duringbrazing, cutting, or welding, but whose microstructure andproperties are altered by the heat of brazing, cutting, orwelding.

heat treatment Heating and cooling o

Documents

Materials of Construction for Pressure Vessels