Materials of Construction for Pressure Vessels

Engineering Encyclopedia Saudi Aramco DeskTop Standards

MATERIALS OF CONSTRUCTION FOR PRESSURE VESSELS

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Mechanical File Reference: MEX-202.02

For additional information on this subject, contact PEDD Coordinator on 874-6556

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

Content

Page

INTRODUCTION............................................................................................................ 3 HOW MATERIAL SELECTION FACTORS INFLUENCE MATERIAL SELECTION ....... 4 Strength, Including Creep .................................................................................... 5 Resistance to Corrosion ...................................................................................... 6 Increasing Resistance to Corrosion .......................................................... 8 Resistance to Hydrogen Attack............................................................... 10 Fracture Toughness .......................................................................................... 12 Material Fractures................................................................................... 12 Fracture Toughness Determination ........................................................ 12 Factors That Influence Fracture Toughness ........................................... 14 Control of Fracture Toughness ............................................................... 15 ASME Code and Brittle-Fracture Evaluation........................................... 16 Fabricability ....................................................................................................... 20 Requirements for Fabricability ................................................................ 20 Postweld Heat Treatment ....................................................................... 21 DETERMINING MAXIMUM ALLOWABLE STRESSES ............................................... 25 ASME Criteria for Determining Maximum Allowable Stress............................... 25 Division 1 Criteria.................................................................................... 26 Division 2 Criteria.................................................................................... 29 ASME Maximum Allowable Stress Tables......................................................... 29 Maximum Allowable Compressive Stress.......................................................... 34 DETERMINING WHETHER PRESSURE VESSEL MATERIALS MEET SAUDI ARAMCO MATERIAL SELECTION REQUIREMENTS.................................... 35 SAES-D-001 ...................................................................................................... 35 Rimmed Steels ....................................................................................... 36 Saudi Aramco DeskTop Standards

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Nozzle Reinforcing Plates and Shell Stiffener Rings............................... 36 Corrosion Allowance ............................................................................... 37 32-SAMSS-004.................................................................................................. 38 Contractor Design Package............................................................................... 42 SUMMARY................................................................................................................... 45 GLOSSARY ................................................................................................................. 55

List of Figures Figure 1: Corrosion Allowance ..................................................................................... 7 Figure 2: Typical Impact Energy Transition Curves.................................................... 13 Figure 3: Impact Test Exemption Curves for Carbon Steels ...................................... 17 Figure 4: PWHT Requirements for Carbon and Low-Alloy Steels .............................. 21 Figure 5: Criteria for Determining Allowable Stress for Division 1 Pressure Vessels (Wrought or Cast, Ferrous and Nonferrous Materials) .... 27 Figure 6: ASME Maximum Allowable Stress Tables (Excerpt) ................................... 31 Figure 7: Form 9527 Excerpt...................................................................................... 44 Figure 9: Allowable-Stress Table Based on Material Type......................................... 46 Figure 10: Nelson Curves........................................................................................... 48 Figure 11: Acceptable Materials for Carbon and Low-Alloy Steel Vessels ................. 49

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INTRODUCTION This module describes the primary factors that influence the selection of materials for pressure vessel components. The factors that influence the selection of materials for pressure vessel components include strength (including creep), resistance to corrosion (including hydrogen attack resistance), fracture toughness, and fabricability. The module then describes the ASME criteria for determining allowable stress, the ASME allowable stress tables, and allowable compressive stress. Allowable stresses are required for the detailed design of all pressure vessel components. The mechanical design of pressure vessel components will be discussed in MEX 202.03. Finally, the module describes the methods that are used to determine whether pressure vessel materials meet Saudi Aramco material selection requirements. These methods include the use of Saudi Aramco Engineering Standards (SAESs) and Saudi Aramco Material System Specifications (SAMSSs) to select the materials for major pressure vessel components. COE 105, Material Selection, discussed general material selection considerations and requirements for process equipment. This module briefly reviews the information on the mechanical properties of steel and general material selection considerations and then focuses on Saudi Aramco pressure vessel requirements. CSE 110, Introduction to Storage Tanks, Pressure Vessels, and Piping Systems for Civil and Mechanical Engineers, briefly discussed the concept of allowable stress, its use in the mechanical design of pressure vessels, and the ASME Code allowable stress tables. This module goes further in that it discusses the material strength bases that are used to determine allowable stresses, the variations in allowable stress based on material specification and temperature, and how allowable stress could influence material selection.

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HOW MATERIAL SELECTION FACTORS INFLUENCE MATERIAL SELECTION Materials that are used to construct ASME Code pressure vessels must be selected from material specifications that are approved under the Code. A materials engineer normally makes material selections for specific applications after the process environment and the required design conditions have been defined. However, a mechanical engineer must also be familiar 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 and expanded upon here. Other factors that influence material selection are cost, availability of materials, and ease of maintenance. Alloys of carbon steel may be used to construct pressure vessels because of the suitability of these alloys in terms of the first three material selection factors. Fabricability considerations must also be evaluated, based on the particular alloy used. Alloys have the following characteristics: •

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

•

Increase the steel's fracture toughness.

•

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

•

May allow the steel to withstand extremes in operating pressure or temperature that may be encountered during normal use of a pressure vessel, without component failure.

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The primary alloying elements that are used in carbon and lowalloy steels are chromium, magnesium, silicon, molybdenum, vanadium, nickel, copper, and columbium (also called niobium). The specific alloying elements that are used and their quantities directly influence material properties. Strength, Including Creep Strength is a material's ability to withstand an imposed force or stress. Strength is a significant factor in the selection of a material for a particular application inasmuch as strength determines how thick a pressure vessel component must be in order to withstand the imposed loads. Inasmuch as the yield and ultimate tensile strengths of materials are relatively low at elevated temperatures, creep and rupture strengths of materials may determine allowable stress values. At elevated temperatures in the creep range (above about 427°C [800°F]), a material will continue to deform without an increase in the applied load and resultant stress. Creep resistance is increased by the addition of alloying elements such as chromium, molybdenum, and/or nickel to carbon steel. Therefore, in elevated temperature applications, alloy materials are often employed for the sole purpose of increasing creep resistance. 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 of the material. Material strength determines the allowable stresses that are used in the ASME Code for detailed component design. Allowable stress values are discussed later in this module.

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Resistance to Corrosion Corrosion is the deterioration of metals by chemical action. A material's resistance to corrosion is probably the most important factor that influences its selection for a specific application. The corrosion rates of various metals in established processes are determined on the basis of experience, while laboratory tests are used to determine the corrosion rates for new processes. The corrosion resistance of a particular metal can be significantly changed by a 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 a corrosion allowance. A corrosion allowance is supplemental metal thickness which is added to the minimum thickness that is required for the component to resist applied loads. This added thickness compensates for thinning (corrosion) that will take place during service. Saudi Aramco requirements for corrosion allowance are discussed later in this module. The concept of corrosion allowance is illustrated in Figure 1.

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Pressure Vessel Component

Tmin c T

20202.F01

20202.01

Where: Tmin

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

c

= Corrosion allowance

T

= Total required component thickness Figure 1: Corrosion Allowance

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Increasing Resistance to Corrosion

The corrosion resistance of carbon steel is increased through the addition of alloying elements such as chromium, molybdenum, or nickel. To determine whether the use of an alloy is appropriate, and to determine the particular alloy material to use, both cost and an acceptable corrosion rate must be considered. Before a final material selection is made, the cost increase in going from plain carbon steel to alloy steel must be compared with the corrosion rate and higher required corrosion allowance for carbon steel. Stainless steels are the most common, readily available, and among the more expensive corrosion-resistant materials. Stainless steel can be used either as a solid plate or as a lining that is bonded to a carbon or low-alloy steel baseplate. The use of solid stainless-steel plate is the 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 than another depends on current cost and material availability, and these factors vary based on market conditions and location. If a stainless-steel lining is used, the following three choices are 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 backing plate together with a corrosion-resistant sheet. The two layers that form this lined plate are then welded at the edges.

Strip or Sheet Lining is fabricated by welding alloy strip or sheet to

the vessel shell. This lining method is normally used in retrofit applications rather than in new vessel construction.

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Weld Overlay is a lining method in which corrosion-resistant weld

metal is added directly over a carbon or low-alloy steel backing material. Weld overlay is frequently more economical than cladding, based on the choice of vendor and the availability of material. Weld overlay also often supplements other lining methods. For example, when the cladding on clad plate is locally removed to make an attachment directly to the base material (such as for a nozzle), the corrosion-resistant layer is restored by weld overlay. Standard Drawing No. AB-036367, Joint Preparation and Welding Details, Alloy and Clad Pressure Vessels and Heat Exchangers, provides standard weld overlay details for nozzle attachments to clad pressure vessels. Corrosion resistance in pressure vessels may also be increased through the use of a nonmetallic, internal coating. In this application, the coating is bonded to the metallic surface and protects the metal from the corrosive process environment. Maximum temperature limitations of internal coatings prohibit their wide use in pressure vessels. However, Saudi Aramco does use internal coatings in production applications where pressure vessel temperatures are within the limitations of the coating material. SAES-H-001, Selection Requirements for Industrial Coatings, specifies the requirements for the following: •

Acceptable coating systems based on the type of structure or equipment to be coated 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.

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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 the thickness of the component, rather than just at its surface, and this damage occurs without any metal loss. Thus, it is not practical to provide a corrosion allowance to allow for hydrogen attack. In addition, once hydrogen attack has occurred, the metal cannot be repaired and must be replaced. Monatomic hydrogen easily diffuses through steel. However, molecular hydrogen does not diffuse through steel. The diffusion of monatomic hydrogen through steel depends on the following factors: •

Hydrogen partial pressure in the process environment

•

Material composition

•

Temperature

•

Time

At intermediate temperatures, from approximately 150°C to 200°C (300°F to 400°F), monatomic hydrogen diffuses into voids that are normally present in steel. In these voids the monatomic hydrogen forms molecular hydrogen, which cannot diffuse out of the steel. If this hydrogen diffusion continues, pressure can build to high levels within the steel, and the steel can crack. At elevated temperatures, over approximately 315°C (600°F), monatomic hydrogen not only causes cracks to form but also attacks the steel. Plain carbon steel is especially susceptible to hydrogen attack. At these elevated temperatures, monatomic hydrogen forms and combines with 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. The steel then becomes spongy and embrittled, and permanent damage results. Over time, the steel loses tensile strength, hardness, notch toughness, and ductility. Hydrogen attack, plus the stresses in the steel that are caused by operating conditions and residual fabrication stresses, have caused catastrophic failure of pressure vessels.

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Protection Against Hydrogen Attack - Plain carbon steels are

satisfactory materials for hydrogen service at low operating temperatures and high hydrogen partial pressures or at high operating temperatures and low hydrogen partial pressures. The addition of carbide stabilizing elements, such as chromium and molybdenum, decreases the reaction of hydrogen with the carbides in steels. Therefore, engineers must often use lowalloy steel that contains chromium, molybdenum, or both elements to provide adequate protection against hydrogen attack in refinery and petrochemical services. When hydrogen attack is a factor, API Publication 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants, is used 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 temperature increases as the alloy content (chromium, molybdenum, or both elements) increases, given a specific hydrogen partial pressure. Figure 10 shows why it is common to use low-alloy steels for pressure vessels that are used in hydrogen service and that operate at elevated temperatures and pressure. Note that Figure 10 shows 1.0 Cr-0.5 Mo steel as equivalent to 1.25 Cr-0.5 Mo steel at hydrogen partial pressures above about 8.28 MPa(a) (1200 psia). The 1.0 Cr-0.5 Mo steel is also adequate for somewhat lower temperatures at lower hydrogen partial pressures. From a practical standpoint, 1.0 Cr-0.5 Mo material is rarely used in pressure vessel construction. 1.25 Cr0.5 Mo is normally the lowest alloy that would be considered for use in situations where carbon steel is not adequate due to hydrogen attack considerations. SAES-D-001 does not directly address hydrogen attack considerations. However, 32-SAMSS-004 in Table 1, Note A requires adherence to API 941 for pressure components in hydrogen service. Specific material selection requirements are discussed later in this module.

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Fracture Toughness Fracture toughness refers to the ability of a material to withstand conditions that could cause brittle fracture. Pressure vessel components that are constructed of ferrous material have occasionally fractured at a pressure that was well below the design value. Such fractures generally occurred at low temperatures and were brittle rather than ductile in nature. Brittle fractures are characterized by the lack of deformation or yielding before the component fails completely. 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 particular location in a pressure vessel: •

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

•

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 act as 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 the necessary 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 qualitatively determine the fracture toughness of steel. In this test, an impact test is performed on a notched specimen that is taken from a specific location in the material; and the impact energies 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 impact energy that is required to fracture the specimen. Figure 2 shows the typical shape of impact energy transition curves for low- and high-strength steels.

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A - Low- Strength Steel Upper shelf energy (ductile)

Impact Energy

Lower shelf energy (brittle)

B - High- Strength Steel

Low

NDT for lowstrength steels

Temperature

High

NDT for high- strength steels

20202.F02

Figure 2: Typical Impact Energy Transition Curves

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Factors That Influence Fracture Toughness

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

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

•

Cold forming of thick plates may cause brittle fractures in areas with stress raisers 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 fracture toughness with temperature. The "lower shelf" is the lower section of the impact energy curve, and the "upper shelf" is the upper section. A material is very brittle at lower shelf energy temperatures and can behave like a piece of glass. Fracture at lower shelf energy temperatures is very abrupt, as when a piece of glass is dropped. A material is ductile at upper shelf energy temperatures. Fracture at upper shelf energy occurs after a small amount of yielding has taken place. Low-strength steels have a significant increase in fracture toughness as the temperature increases (see curve A of Figure 2). High-strength steels show only a slight increase in fracture toughness as temperature increases (see curve B of Figure 2). The dotted lines in Figure 2 show the nil ductility transition (NDT) temperatures for both high- and low-strength steels. The NDT temperatures are the starting points of the transitions between brittle and ductile fractures. Below the NDT temperatures, material fracture is brittle in nature. Above the NDT temperatures, material fracture is ductile in nature. The rate of change 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 fracture toughness when going from low to high temperature.

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Material selection must confirm that the material has adequate fracture toughness at the lowest expected metal temperature. The lowest One-Day Mean Temperature for the site and the lowest temperature to which the vessel may be exposed during any phase of its operation determine the lowest expected temperature that the vessel must be designed for. This lowest temperature 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 to ensure that pressure vessels have adequate fracture toughness. SAES-D-001 defines

the basis for Minimum Design Metal Temperature (MDMT) and the Lowest Possible Fluid Temperature (LPFT) of a pressure vessel. The first step in the specification of material with adequate fracture toughness is to set the MDMT. The MDMT is defined as the minimum metal temperature occurring under all possible scenarios to which the vessel may be exposed at normal operating pressure. For example, autorefrigeration shock chilling and ambient conditions must be considered, where this is possible. Identification of various possible operating scenarios and the minimum design temperatures associated with them is the responsibility of the process design engineer. SAES-D-001 also requires the data sheet to contain the Lowest Possible Fluid Temperature (LPFT) which can occur at any pressure including atmospheric. This requirement is intended to indicate low temperature conditions which may occur in depressurization conditions. If the pressure vessel is immediately repressurized before the pressure components can regain their proper operating temperautres, a brittle fracture situation is possible. Either suitable materials may be specified or process controls must prevent such an occurrence.

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Saudi Aramco requires that all material for pressure-containing components be impact tested, if required by the ASME Code or 32-SAMSS-004. When required, impact testing must meet the procedures and acceptance criteria of the relevant Division of ASME Section VIII, either Division 1 or Division 2, and the additional restrictions and listed in 32-SAMSS-004. 32-SAMSS-004 specifies the following requirements in addition to the ASME, based on material fracture toughness considerations: •

When the MDMT is less than –18°C (0°F), all pressure boundary materials and materials welded to pressure boundary components shall be impact tested.

•

When the MDMT is less than –29°C (-20°F) all pressure boundary materials, materials attach to pressure components, pressure retaining welds and attachment welds shall be impact tested.

•

When the LPFT is less than –45°C (–50°F) all pressure boundary materials, pressure retaining welds, attachment welds and attachment materials shall be impact tested.

•

ASME impact testing exemptions per UG-20(f), VCS-66(b), UCS-68(c), UG-84(b)(2) and Table UG-84.4 are not allowed.

ASME Code and Brittle-Fracture Evaluation

The ASME Code, Section VIII, contains a simplified approach to evaluate brittle fracture in carbon and low-alloy steel. Material specifications are classified within Groups A through D, for the purpose of brittle fracture evaluation (Figure UCS-66 of the ASME Code, Section VIII, Division 1). The Code contains exemption curves for those Material Groups that identify the acceptable minimum design metal temperature versus thickness, 0 mm through 100 mm for welded construction (0 in. through 4 in.), where impact testing (Charpy V-notch) is not required. The curves that are shown in Figure 3 are taken from Figure UCS-66, ASME Code, Section VIII, Division 1. The curves are based on both experience and test data. If the design conditions do not permit exemption in accordance with this basis, then material impact testing at the specified minimum design temperature is required to permit its use. Saudi Aramco DeskTop Standards

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MinimumDesign Metal Temperature. oF

140 120 A

100

B

80 60

C

40

D

20 0 - 20 - 40 - 50 - 60 - 80

Impact Testing Required 0.394

1

2

3

4

5

Nominal Thickness, in. (Limited to 4 in, for Welded Construction) Source: ASME Chart, Section VIII, Division 1, UCSA-66.

Figure 3: Impact Test Exemption Curves for Carbon Steels

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A letter that designates the corresponding Material Group appears above each curve in Figure 3. If the minimum design metal temperature of a pressure vessel is equal to or above that shown by the intersection of the Material Group curve and of the component thickness, then impact testing is not required. For example, a Group B material that is 38 mm (1.5 in.) thick and operates at 16°C (60°F) does not require impact testing. It should be noted that the exemption of a material from impact testing through the use of this basis does not mean that the ASME Code ignores brittle fracture. Impact-test exemption means that there is sufficient data to conclude that the combinations of material, temperature, and thickness defined by the exemption curves result in material that has sufficient fracture toughness, without the need for additional impact testing. The minimum design temperature at which impact testing is not required increases with the material thickness. Thick material is more prone to brittle fracture than thin material, and a higher temperature is required to prevent brittle fracture in thicker material. For all welded construction over 100 mm (4 in.) thick and with a minimum design temperature below 49°C (120°F), impact testing is required. The ASME Code also contains impact-testing procedures and impact-energy requirements for cases that are subject to impact testing. Participants should refer to the ASME Code for details. Figure UCS-66(b) of the ASME Code, Section VIII, Division 1 provides an additional exemption from impact testing if the actual design stress is less than that which is allowed by the Code. Saudi Aramco does not allow this exemption since stresses in the pressure components could still achieve fracture levels from external loads such as piping loads or relief valve reactions.

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The Material Groups in Figure 3 (based on Figure UCS-66) covers mainly carbon steel materials. The addition of alloying elements will typically improve the toughness of steels. In general, the addition of manganese, silicon, and/or nickel to carbon steel will improve its fracture toughness. The following list highlights some of the ASME Code, Section VIII, Division 1 impact-test requirements for alloy steels: •

Base metal impact tests are not required for low-allow steels and product forms when the minimum design temperature is not below the Group A curve of Figure 3. 2 1/4 Cr-1 Mo material that conforms to SA-387, Grade 21 and 22, and SA-182, Grades F21 and F22 (forgings) are included in Material Group C.

•

For high-alloy steels: -

Austenitic chromium stainless steels Types 304, 304L, 316, 316L, 321 and 347 require impact testing at minimum design temperatures below -254°C (425°F). All other grades must be impact tested at minimum design temperatures below -198°C (-325°F) if their carbon content is less than 0.10%.

-

Austenitic chromium stainless steels with carbon content above 0.10% must be impacted tested with MDMT’s below –45°C (–50°F).

-

Other stainless steels such as duplex, ferritic chromium and nontensitic chromium have various impact testing requirements below –29°C (–20°F). See ASME, Section VIII, Division 1, UHA-51.

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

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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 are required in the use of the material. Fabricability includes the following considerations: •

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

•

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

Specific fabrication requirements vary among material types. The sections that follow discuss these considerations. Requirements for Fabricability

Pressure vessels that are of interest to the Participants use welded construction. Welding procedures are used to ensure that welded joints are of acceptable strength and quality. The material chemistry of the weld area must be equivalent to the material chemistry of the base material so that the material properties and corrosion resistance of the weld area will be the same as those of the base material. Special concerns arise where a ferritic material is welded to an austenitic material, resulting in a bimetallic weld. In the case of a bimetallic weld, the difference in the thermal expansion coefficient between the two materials causes high local stresses at elevated temperatures. These local stresses must be considered in the detailed mechanical design. Sometimes in the case of bimetallic welds, a welding electrode material is selected that has a thermal expansion coefficient that is between the coefficient of the two base materials that are to be welded. This welding electrode selection reduces the localized thermal stresses. In all cases, the ASME Code requires that written and tested welding procedures be followed.

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All welders must be tested to verify their capabilities. In order to achieve the required finished quality, only qualified welding procedures and welders are used to fabricate ASME Code equipment. Welding, welding procedures, and welder qualification are discussed further in MEX 202.04. Postweld Heat Treatment

Postweld Heat Treatment (PWHT), because it adds to total cost, is another consideration in the fabricability of pressure vessels. In PWHT, the pressure vessel is heated to a high temperature after the completion of all welding, and the high temperature is maintained for a specified 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 then contract as they cool. This contraction causes stresses due to the uneven cooling and constraint of the overall structure. PWHT is used to relieve these stresses so that a vessel failure does not occur. The ASME Code contains rules which determine when PWHT is necessary. These rules are based on the material type and wall thickness. Figure 4 is an excerpt from the ASME Code, Section VIII, Division 1, Table UCS-56, and gives PWHT requirements for a particular material class. While Figure 4 and the discussion that follows focus on a specific material class, similar considerations apply to the other material classes as well.

MATERIAL

NORMAL HOLDING TEMPERATURE, °F, MINIMUM

MINIMUM HOLDING TIME AT NORMAL TEMPERATURES FOR NOMINAL THICKNESS Up to 2 in.

P-No. Gr. 1, 2, 3

1 Nos.

Gr. No. 4

1100 Not applicable

Over 2 in. to 5 in.

Over 5 in.

2 hr. plus 15 min. 2 hr. plus 15 min. for for each 1 hr./in., 15 min. each additional inch additional inch minimum over 2 in. over 2 in. None

None

None

Figure 4: PWHT Requirements for Carbon and Low-Alloy Steels

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The material in Figure 4 is identified by "P-No." ("P" Number) and "Gr. No." (Group Number). The allowable stress tables in the ASME Code provide these numbers for every material. The P-No. and Gr. No. are ASME designations for materials that have common welding and heat-treating characteristics. The PNo. 1 material that is shown in Figure 4 corresponds to the carbon steel material specifications. The ASME Code contains similar tables for all materials that may be used. Figure 4 specifies the minimum PWHT temperature and the minimum holding time at temperature, based on wall thickness. When the vessel is heated to this elevated temperature, the residual welding stresses relax and the vessel reaches an initial stressfree state. The minimum holding time at the PWHT temperature increases with wall thickness. More time is needed to relax the welding stresses in large volumes of weld metal since more weld shrinkage occurs. Another reason for using PWHT is to reduce the weld hardness for particular materials. The welding process produces locally hard regions in the weld and in adjacent areas of certain materials (for example, low chrome-alloy materials). The locally hard areas are less ductile and more prone to the formation of cracks. The PWHT softens the hard areas and restores ductility. The ASME Code does not have specific requirements for weld hardness, and it does not require PWHT for the purpose of hardness reduction. Therefore, the pressure vessel user must specify weld hardness limitations separately. 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 stressed welds in carbon steel material. Residual stresses that remain after welding may cause crack formation in this environment. As noted above, the ASME Code does not require PWHT for this purpose, and the user must specify PWHT.

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Notes that accompany Table UCS-56 provide several exemptions to the PWHT requirements that are specified. These exemptions are only valid if the PWHT is required for relief of residual stress. The exemptions do not apply if the PWHT is required for reduction of weld hardness or because of process considerations. Exemptions from the PWHT requirements of Table UCS-56 are based on material, weld type, and weld size. For example, PWHT is not mandatory for groove or fillet welds in P-No. 1 material not over 13 mm (0.5 in.) size that attach nonpressure parts to pressure parts provided a minimum preheat temperature of 93°C (200°F) is used and the pressure part is no more than 32 mm (1.25 in.) thick. Refer to Table UCS-56 for the details of the PWHT exemptions. The requirements in the paragraphs that follow expand on the discussion of requirements shown 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 not necessary, provided that the new welds are within the PWHT exemptions of Table UCS-56, and provided that the original PWHT was not necessary for hardness 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 the final hydrotest without the need for another PWHT. The ASME Code limits the size of the repair that is permissible without subsequent PWHT. The total repair 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 3 materials. Additional weld procedure and inspection requirements must also be met.

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•

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

•

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

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

DETERMINING MAXIMUM ALLOWABLE STRESSES One of the major factors in the design of pressure vessels is the relationship between the strength of the components and the loads (pressure, weight, etc.) that are imposed upon the components. These loads cause internal stresses in the components. Stress is the force-per-unit area in a solid material that resists the separation, compaction, or sliding that is induced by external forces. The design of a pressure vessel must ensure that these internal stresses never exceed the strength of the components that make up the pressure vessel. Pressure vessel components are designed such that the component stresses that are caused by the loads are limited to maximum allowable values that will ensure safe operation of the pressure vessel. Maximum allowable stress is the maximum force-perunit area that may be safely applied to a pressure vessel component. The maximum allowable stress includes an adequate safety margin between the maximum stress level in a component due to the applied loads and the stress level that could actually cause a failure. The use of maximum allowable stress in the design of pressure vessel components will be discussed in MEX 202.03. CSE 110 briefly introduced the concept of maximum allowable stress and the ASME Code maximum allowable stress tables based on Section VIII, Division 1. The paragraphs that follow provide additional detail by describing the ASME Code rationale for determining maximum allowable stress. The description highlights the differences between Division 1 and Division 2 of Section VIII in determining maximum allowable stress and discusses the maximum 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 criteria that are used to establish the maximum allowable stresses for most ferrous and nonferrous materials. Appendix 1 provides the criteria for Division 1 pressure vessels for all materials other than bolting. Appendix 2 provides the criteria that are used to establish the maximum allowable 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 the criteria that are used to establish maximum allowable stress values for low-temperature steels that are used in cryogenic applications for cast and nodular iron materials.

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Division 1 Criteria

The following data is from the ASME Code, Section II, Part D, Appendix 1, Non-mandatory Basis for Establishing Stress Values in Tables 1A and 1B. A similar discussion is contained in Section II, Part D, Appendix 2 for bolting, and Section VIII, Division 1, Appendix P for low-temperature, cast or ductile iron materials. When evidence of satisfactory performance is available, successful experience in service guides the determination of maximum allowable stress values for pressure vessel parts. Such evidence is considered equivalent to test data where operating conditions are known with reasonable certainty. In the evaluation of new materials, engineers must compare test information with available data on successful applications of similar materials. Figure 5, is based on the ASME Code, Section II, Part D, Appendix 1, Table 1-100, and shows the criteria/equations that are used to compute the maximum allowable stresses for wrought or cast 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 to determine its maximum allowable stress. Above room temperature, the material's creep and rupture strength must be considered as well in determining maximum allowable stress. Refer to the ASME Code for the criteria that are used for welded pipe or tube and for structural quality steel.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

Temperature Below Room Temperature

Criteria (1) Yield Strength

Tensile Strength

2

St 4 Room Temperature and Above

3

Sy

Tensile Strength

1.1 S tR t 4

St 4 Yield Strength 2

3

2

Sy

3

S yRy or 0. 9SyRy (2)

Stress Rupture

0.8SR min

0.67SRavg Creep Rate

1.0Sc

Figure 5: Criteria for Determining Allowable Stress for Division 1 Pressure Vessels (Wrought or Cast, Ferrous and Nonferrous Materials)

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

•

St

=

Specified minimum tensile strength at room temperature (ksi).

•

Rt

=

Ratio of the average temperature-dependent trend curve value of tensile strength to the room temperature tensile strength.

•

Sy

=

Specified minimum yield strength at room temperature.

•

Ry

=

Ratio of the average temperature-dependent trend curve value of yield strength to the room temperature yield strength.

•

SRavg

=

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

•

SRmin

=

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

•

Sc

=

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

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 exceed two-thirds but do not exceed 90% of the minimum yield strength of the material at temperature. The higher allowable stress values should be used only where slightly higher deformation of the component is not in itself objectionable. These higher allowable stresses are not recommended for the design of flanges or other strain-sensitive applications. In the case of flanges, for example, the larger deformation that would be expected if the higher allowable stresses were used could cause flange leakage problems even though a major flange failure would not occur.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

The maximum allowable stress for materials other than bolting for Division 1 pressure vessels is the lowest value that is obtained from the criteria that are stated in Figure 5. Note that these criteria are based on specified fractions of the stated material strength properties. These fractions can be considered as safety factors between the maximum allowable stress and the stress that would cause material failure. Division 2 Criteria

The maximum allowable stress criteria for materials other than bolting that are contained in Appendix 2 of the ASME Code, Section II, Part D for Division 2 pressure vessels will typically 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, since Division 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 in ASME Code, Section VIII, pressure vessels. The maximum allowable stress varies with temperature because material strength is a function of temperature. The maximum allowable stress values that are contained in these tables are based on the criteria that were previously discussed. Figure 6 (adapted from Table 1A of the ASME Code, Section II, Part D) shows examples of maximum 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).

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The first part of Figure 6 identifies the Spec. No. (material specification number), the grade (a material specification may have multiple grades), the nominal chemical composition, the PNo. and Group No., and the minimum yield and tensile strengths in thousands of pounds per square inch (ksi). This first part of Figure 6 also helps identify any similarities that may exist among the material specifications, such as in nominal alloy composition or yield and tensile strengths. In some cases, these similarities may be used to help select the material to use for pressure vessel fabrication, given specific process conditions. The maximum allowable stress values 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 and reorganized in Figure 6 in two parts to help the Participants to compare the material types and to note variances in maximum allowable stress that are determined by temperature and alloy composition. The actual tables that are contained in the ASME Code should be used for all practical work applications.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

TABLE 1A (excerpt) ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW-ALLOY STEEL Spec No. Grade Nominal P-No. Group Min. Composition No. Yield (ksi) Carbon Steel Plates and Sheets SA-515 55 C-Si 1 1 30 60 C-Si 1 1 32 65 C-Si 1 1 35 70 C-Si 1 2 38

SA-516

55 60 65 70

C-Si C-Mn-Si C-Mn-Si C-Mn-Si

Plate - Low Alloy Steels SA-387 2 Cl.1 1/2Cr-1/2Mo 2 Cl.2 1/2Cr-1/2Mo 12 Cl.1 1Cr-1/2Mo 12 Cl.2 1Cr-1/2Mo 11 Cl.1 1 1/4Cr-1/2mo-Si 11 Cl.2 1 1/4Cr-1/2mo-Si 22 Cl.1 2 1/4Cr-1Mo 22 Cl.2 2 1/4Cr-1Mo

Min. Tensile (ksi)

55 60 65 70

1 1 1 1

1 1 1 2

30 32 35 38

55 60 65 70

3 3 4 4 4 4 5 5

1 2 1 1 1 1 1 1

33 45 33 40 35 45 30 45

55 70 55 65 60 75 60 75

Figure 6: ASME Maximum Allowable Stress Tables (Excerpt)

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ASME Maximum Allowable Stress Tables, cont'd TABLE 1A (excerpt) ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL Max Allowable Stress, ksi (Multiply by 1,000 to Obtain psi) for Metal Temperature, °F, Not Exceeding 650

700

750

800

13.8 15.0 16.3 17.5

13.3 14.4 15.5 16.6

12.1 13.0 13.9 14.8

10.2 10.8 11.4 12.0

8.4 8.7 9.0 9.3

6.5 6.5 6.5 6.5

4.5 4.5 4.5 4.5

2.5 2.5 2.5 2.5

Spec 1050 1100 1150 1200 No. Carbon Steel Plates and Sheets ----SA-515 ----SA-515 ----SA-515 ----SA-515

13.8 15.0 16.3 17.5

13.3 14.4 15.5 16.6

12.1 13.0 13.9 14.8

10.2 10.8 11.4 12.0

8.4 8.7 9.0 9.3

6.5 6.5 6.5 6.5

4.5 4.5 4.5 4.5

2.5 2.5 2.5 2.5

-----

13.8 17.5 13.8 16.3 15.0 18.8 15.0 17.7

13.8 17.5 13.8 16.3 15.0 18.8 15.0 17.2

13.8 17.5 13.8 16.3 15.0 18.8 15.0 17.2

13.8 17.5 13.8 16.3 15.0 18.8 15.0 16.9

13.8 17.5 13.4 15.8 14.6 18.3 14.4 16.4

13.3 16.9 12.9 15.2 13.7 13.7 13.6 15.8

9.2 9.2 11.3 11.3 9.3 9.3 10.8 11.4

5.9 5.9 7.2 7.2 6.3 6.3 8.0 7.8

Plate-Low Alloy Steels (Cont'd) ----SA-387 ----SA-387 4.5 2.8 1.8 1.1 SA-387 4.5 2.8 1.8 1.1 SA-387 4.2 2.8 1.9 1.2 SA-387 4.2 2.8 1.9 1.2 SA-387 5.7 3.8 2.4 1.4 SA-387 5.1 3.2 2.0 1.2 SA-387

850

900

950

1000

-----

-----

-----

SA-516 SA-516 SA-516 SA-516

Figure 6: ASME Maximum Allowable Stress Tables (Excerpt), cont'd

Note that the allowable stresses at temperatures between -29°C and 343°C (-20°F and 650°F) are the same as the allowable stress at 343°C (650°F) for each material presented in Figure 6 except 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 38°C (100°F) and below. See the ASME Code, Section II, Part D, Table 1A for details.

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Note that each material specification has different Types, Grades, and/or Classes within it. In some cases, these differences are due to different chemical compositions, while in other cases they may be due to the particular steelmaking process that was employed. Higher strength grades 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 fabricated of thinner material. For example, SA-516, Grade 60 has a higher maximum allowable stress than Grade 55 at 371°C (700°F). As a result, a vessel made from SA-516, Grade 60 material can be fabricated from thinner plate and can still have an acceptable reliability. When more than one material specification is acceptable based on strength considerations alone, material cost and availability will then determine which material specification will be used. The dashed columns in Figure 6 indicate that SA-516 cannot be used to construct pressure vessels with design temperatures above 537°C (1 000°F). The maximum allowable stress for most ferritic materials does not change for design temperatures through 343°C (650°F). As the design temperature increases above 343°C (650°F), the thickness that is required for pressure vessel components increases because the material strength and maximum allowable stress decrease. For example, the maximum allowable stress for SA-516, Grade 55 decreases from 13.8 ksi to 8.4 ksi in going from 650°F to 850°F. The addition of alloying elements to carbon steel typically increases the hightemperature strength of the material. Therefore, a thinner alloy component can typically be used at higher temperatures when its high-temperature strength is compared to that of plain carbon steel. For example, in Figure 6, compare the maximum allowable stress of SA-516, Grade 70 material with that of SA387, Grade 11 Cl. 1 at 427°C (800°F). Note that the SA-387 material may be used through 648°C (1 200°F) but that the SA516 material cannot be used over 537°C (1 000°F). Therefore, based on strength considerations, alloy construction is often justified on economic grounds for high-temperature service because alloy components will be thinner than if carbon steel were used. This reduced quantity of required material will often offset the higher cost of alloy versus carbon steel material on a weight basis.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

By using the various tables that are contained in the ASME Code, comparisons can be made among the various material types, grades, compositions, and maximum allowable stress values to select the most cost-effective pressure vessel materials for the specific vessel application. Work Aid 1 provides a general procedure that may be used to determine maximum allowable stress and whether contractorspecified values for maximum allowable stress are correct. Maximum Allowable Compressive Stress

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

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

•

The value of the factor B determined using the appropriate external pressure chart presented in the ASME Code. This chart will be discussed in MEX 202.03 as part of the discussion of external pressure design of vessel components.

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DETERMINING WHETHER PRESSURE VESSEL MATERIALS MEET SAUDI ARAMCO MATERIAL SELECTION REQUIREMENTS

The ASME Code contains general design rules. When additional design requirements for a particular application are needed, the user and pressure vessel designer must specify them. These additional requirements are based on design and operating experience that are relevant to the particular applications. SAES-D-001 and 32-SAMSS-004 are the primary documents that specify Saudi Aramco requirements for pressure vessels. The Saudi Aramco engineer will typically not specify the materials that are to be used for pressure vessel components. Instead, the Saudi Aramco engineer will typically review Contractor Design Packages to determine whether contractorspecified material specifications are 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 of requirements that are used by Saudi Aramco's engineering contractor in the preparation of a pressure-vessel purchase order. Relevant requirements are extracted from SAES-D-001 by the contractor and added to the pressurevessel purchase specification, as needed. SAES-D-001's additional requirements remain within the scope of the ASME Boiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2. Pressure vessels that are mass produced and stamped with the “UM” symbol in accordance with ASME Section VIII, Division 1 are not within the scope of SAES-D-001 and 32-SAMSS-004 and therefore do not have to meet their additional requirements.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

All pressure vessels within the scope of ASME Section VIII, Divisions 1 and 2 must be stamped with the ASME Code stamp. SAES-D-001 references several other Saudi Aramco standards, material system specifications, and drawings. The requirements that are contained in these additional documents must also be met. One of these references is to Saudi Aramco Materials System Specification 32-SAMSS-004, Pressure Vessels. SAES-D-001 contains material selection requirements that must be observed. Work Aid 2 contains a material selection procedure which includes these requirements. The paragraphs that follow discuss these requirements. Rimmed Steels

Rimmed steels must not be used for any pressure vessels. Rimmed steel is characterized by a marked difference in chemical composition across the section and from the top to the bottom of the ingot that was produced during the steelmaking process. This pattern of varied composition persists from the rolling process to the final product form. This variation in chemical 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 equivalent material specification as the shell or head material to which they are attached. Because of this requirement, welding considerations are simplified, and all the materials are of equal strength and fracture toughness.

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Corrosion Allowance

As discussed earlier, materials selection must also consider the corrosion that takes place during operation of the pressure vessel. A corrosion allowance must be added to all carbon steel pressure-containing parts, including the shell, heads, nozzle necks, and covers. The required corrosion allowance will be specified in 32-SAMSS-004 and the Contractor Design Package. While SAES-D-001 specifically addresses only carbon steel material, the need for a corrosion allowance must be considered for all material types, especially ferritic materials. COE 105 discussed corrosion allowance requirements for various steels in different process environments. Pressure Vessel Internals - Removable pressure vessel internals

that are subject to corrosion should have a corrosion allowance equal to that of the shell. The design of removable internals considers only half of the expected total corrosion. The rationale for this approach is that removable internals that are designed for only the expected total corrosion will cost less initially and can easily be replaced later, based on the actual corrosion that occurs.

Nonremovable internals must have a corrosion allowance that is equal to twice that of the shell. Most pressure vessel internals, such as downcomers, weirs, and tray supports, can corrode on both sides. From a strength-design viewpoint, corrosion from both sides should be considered with regard to nonremovable internals. Determination of Corrosion Allowance - The amount of corrosion

allowance for carbon steel pressure-containing parts is specified in SAES-D-001 and has been 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 in similar service, the corrosion allowance is based on a 20-year service life. A pressure vessel may remain in service longer than 20 years if periodic inspections confirm that the component thicknesses are still adequate for the design conditions. The minimum corrosion allowance is 1.6 mm (1/16 in.).

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•

If considerable material erosion is expected, the next higher pipe schedule from that required for the applied loads should be used for nozzles. This higher pipe schedule effectively increases the corrosion allowance for the nozzles. Flow velocities through the nozzles are higher than in the overall vessel. Thus, if erosion is a concern in the particular service, such as when entrained solids are present, the erosion has a greater effect on the nozzles due to the higher flow velocity through the nozzles.

•

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

32-SAMSS-020, Column Trays, specifies corrosion allowances for parts covered by this specification. 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-004 covers the requirements for Saudi Aramco pressure vessels that are within the scope of the ASME Boiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2. The requirements of 32-SAMSS-004 plus the ASME Code requirements must be adhered to by vendors who supply pressure vessels to Saudi Aramco. All other specifications, drawings, and forms that are referenced in 32-SAMSS-004 must also be followed. Among the items that these documents cover are the following: •

Material requirements for carbon and low-alloy steel vessels.

•

Design details for standard vessel components.

•

Additional fabrication, inspection, and testing requirements.

•

Forms to be completed by the vendor.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

The ASME Code contains a variety of materials that are acceptable for pressure vessel applications. Saudi Aramco has simplified the material selection process by the identification of carbon and low-alloy steel materials that are suitable for services and design conditions normally encountered in Saudi Aramco operations. Table 1 from 32-SAMSS-004 contains these material selections. Vendors may propose alternatives to these materials. However, first they must furnish the material mechanical properties and chemical analysis, and then Saudi Aramco must approve the substitution before it is used. Occasionally, the materials that are identified in Table 1 are not suitable for a particular service. In these cases, material selections are handled on an exception basis and are not covered by SAES or SAMSS requirements. Table 1 from 32SAMSS-004 is reproduced in Work Aid 2 for reference as Figure 11. Figure 11 is used to select, on the basis of cost and availability, the appropriate vessel component materials from the alternatives that are listed. The intent is for the primary pressure-containing components to have the same material chemistry and comparable strength in a given pressure vessel. For example, if a low-alloy plate material is selected for the shell and head of a pressure vessel in a high-temperature service, comparable low-alloy material must be used for the nozzles, flanges, forgings, and fittings. Work Aid 2 may also be used to help determine whether contractor-specified pressure-vessel materials meet Saudi Aramco requirements. The paragraphs that follow discuss the content of Figure 11. Figure 11 identifies nine major categories of pressure-vessel components. The material specifications that are indicated are all ASME Code approved materials for the specific component form. Component form refers to plate, pipe, flanges, forgings, fittings, pressure bolting, supports, attachments and anchor bolts. Plates are used for shells, heads, rolled nozzles (for example,

larger-diameter nozzles that are fabricated 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 between using pipe material or rolled plate for nozzles is based on economics.

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Forged material is used for nozzle flanges and forged fittings,

such as couplings. Wrought materials 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-containing pressure vessel itself.

components

that

connect

to

the

Design Metal Temperature is separated into four temperature

ranges.

General Service category covers the design-temperature range from 0°C through 425°C (32°F through 800°F). This category includes most pressure vessels that are in typical process plant and 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 425°C through 645°C (800°F through 1,200°F). In this temperature range, the higher corrosion rate and lower material strength become more significant factors in the mechanical design of pressure vessel components. Therefore, low-alloy materials are shown as options for plate, pipe, forgings, and fittings. Carbon steels are not recommended for continuous use above 425°C (800°F) due to graphitization or loss of carbon atoms. An example of a low-alloy material is SA-335 Gr. P11 pipe (1 1/4 Cr-1/2 Mo). Previous sections of this module discussed the increased corrosion resistance and material strength of alloy material at elevated temperatures. If carbon steel is not suitable for the combination of temperature and hydrogen partial pressure that is required for the particular application, the use of an alloy material may also be necessary due to hydrogen attack considerations. Previous sections of this module discussed material selection based on hydrogen attack considerations.

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Design of Pressure Vessels Materials of Construction for Pressure Vessels

Alloy materials are specified at temperatures above 425°C (800°F) in order to have adequate creep strength and maximum allowable stress. The selection of the particular alloy to use will be based on its maximum allowable stress at design temperature, relative cost, and corrosion resistance at elevated temperature. 1 1/4 Cr-1/2 Mo and 2 1/4 Cr-1 Mo steels are the most commonly used pressure-vessel materials for hightemperature applications. Additional requirements may also be specified on the steel manufacturing process, vessel fabrication, and inspection that will improve overall vessel material and fabrication quality, as well as long-term reliability at elevated temperature. Low-Temperature Service category is divided into two ranges: 0°C

to -46°C (32°F to -50°F) and -47°C to -101°C (-51°F to -150°F). Brittle fracture is a major concern for this service. The materials are selected to ensure that they have adequate fracture toughness at these low temperatures. Materials that are suitable at temperatures to -46°C (-50°F) are unlikely to have adequate fracture toughness at temperatures below -46°C (-50°F). Therefore, materials with greater fracture toughness are specified for the lower temperature range. Material selections for temperatures that are below -101°C (150°F) are beyond the scope of Figure 11 and 32-SAMSS-004 and must be made on an individual basis.

Wet, Sour Service receives special considerations. The specified materials in Table 1 apply to a maximum design temperature of 203°C (400°F) with certain limitations as outlined in the notes to Table 1 of 32-SAMSS-004. If the service has a higher design temperature, material selections must be made on an individual basis.

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

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Hydrogen Sulfide (H2S) partial pressure

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Fluid state (for example, liquid or gas)

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Operating pressure

SAES-D-001 defines wet, sour service. Terms relevant 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, sour service 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 material specification and mechanical design of pressure vessel components. Lead roles are taken by the prime contractor that Saudi Aramco has employed for the particular project and the specific pressure vessel manufacturer. The job of a Saudi Aramco engineer will normally be to review the work that is performed by the prime contractor and pressure vessel manufacturer for acceptability with respect to Saudi Aramco requirements. The term Contractor Design Package, as used in this course, describes the total of all the detailed design information for the pressure vessel that is prepared by both the prime contractor and the pressure vessel manufacturer. The Saudi Aramco engineer will use the information that is contained in a Contractor 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 9527 for pressure vessels. This data sheet will normally be prepared by the prime contractor. The content and use of this form is discussed in MEX 202.03, and blank copies are contained in Course Handout 3 for reference.

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

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

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

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

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Safety Instruction Sheet, Form 2694. Note that this form may actually be completed by either a Saudi Aramco engineer or the prime contractor, depending on the particular situation. Completion of the Safety Instruction Sheet will be discussed in MEX 202.03.

The information that Participants will use to solve the Exercises and Evaluations in this and the next two modules is contained in Contractor Design Packages that are contained in Course Handout 4. Refer to the Pressure Vessel Design Sheet, Form 9527, that is contained in Course Handout 3. Figure 7 shows the area on this form where information that is related to material selection is specified. Note that this area includes items such as service, design temperature, material specifications for the major components, maximum allowable stresses, and corrosion allowance. This section of the form must be reviewed to help determine if the materials that are specified by the contractor meet Saudi Aramco requirements.

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OPERATING CONDITIONS INTERNAL PRESSURE

Normal Maximum Minimum (when