Determining Acceptability of Materials for Storage Tanks

Engineering Encyclopedia Saudi Aramco DeskTop Standards

DETERMINING ACCEPTABILITY OF MATERIALS FOR STORAGE TANKS

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 o f Saudi Aramco’s employees. employees . Any material contained in this document which is not already in the public p ublic 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-203.02

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

Engineering Encyclopedia

Evaluating Storage Tank Design and Installation Determining Acceptability of Materials for Storage Tanks

Section

Page

INTRODUCTION........................ INTRODUCTION................................................... ....................................................... ....................................................... .............................. ... 2 EFFECT OF MATERIAL STRENGTH AND FRACTURE TOUGHNESS PROPERTIES PROPERTIES ON MATERIAL MATERIAL SELECTION.............................. SELECTION......................................................... ..................................... .......... 3 Strength ................................................... ............................................................................. .................................................... ..................................... ........... 3 Fracture Toughness Toughness ................................................... .............................................................................. ............................................. .................. 4 Determining Determining Fracture Toughness.................................................................... Toughness.................................................................... 6 Factors that Influence Fracture Toughness.................................... Toughness..................................................... ................. 7 Material Selection................................ Selection......................................................... .................................................. ........................................... .................. 8 SPECIFYING MATERIAL STANDARDS THAT APPLY TO THE PRIMARY STRUCTURAL STRUCTURAL COMPONENTS COMPONENTS OF STORAGE STORAGE TANKS .............................................. .............................................. 10 SAES-D-100.............. SAES-D-100........................................ .................................................... .................................................... ........................................ .............. 11 32-SAMSS-005 32-SAMSS-005 .................................................. ............................................................................ ................................................... ......................... 13 SAES-W-017.................................. SAES-W-017............................................................ .................................................... ............................................. ................... 14 API-650......................... API-650 ................................................... .................................................... .................................................... .................................... .......... 14 Strength Considerations........................... Considerations.................................................... .................................................. ............................ ... 15 Fracture Toughness Toughness Considera Considerations tions ............................................. ............................................................. ................ 16 SUMMARY........................................................... SUMMARY................................. .................................................... .................................................... ............................... ..... 21 WORK AID 1: MATERIAL STANDARDS THAT APPLY TO PRIMARY STRUCTURAL STRUCTURAL COMPONENTS COMPONENTS OF STORAGE STORAGE TANKS .............................................. .............................................. 22 GLOSSARY GLOSSARY .................................................... .............................................................................. .................................................... .................................... .......... 25

LIST OF TABLES Table 1. Location of Materials Selection Selection Information..................................................... Information..................................................... 10 Table 2. SAES-D-100 Corrosion Corrosion Allowance Requirements Requirements........................ ........................................... ................... 12 Table 3. Permissible Permissible Plate Materials and Allowable Allowable Stresses Stresses ....................................... ....................................... 16 Table 4. API-650 Material Groups................................................................... Groups................................................................................. .............. 17

Saudi Aramco DeskTop Standards

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INTRODUCTION MEX 203.02, Determining Acceptability of Materials for Storage Tanks, describes the material properties that are significant in selecting materials. This module also provides an overview of the overall process that a tank design contractor uses to select storage tank materials. This module also explains the standards and requirements that must be met in the selection of materials for the primary components of storage tanks. As the module points out, the tank design contractor must observe these standards and requirements in the material selection process. The Saudi Aramco engineer must use these standards and requirements to determine whether the selected materials are acceptable. Materials selection begins the process of overall tank design, which will be discussed further in MEX 203.03.


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EFFECT OF MATERIAL STRENGTH AND FRACTURE TOUGHNESS PROPERTIES ON MATERIAL SELECTION The materials that are to be used for the primary structural components of a storage tank must first be selected before these components can be designed. This section discusses the following items: Material strength and fracture toughness properties

•

•

Note:

General steps that are used to select appropriate materials

COE 105 discussed the strength and fracture toughness properties of materials and how these properties are considered in selecting construction materials for process equipment. This section briefly reviews that information, and it then applies it to atmospheric storage tanks.

Strength Strength is the ability of a material to withstand an imposed force or stress. The strength of a material is defined by its yield strength, ultimate tensile strength, and creep strength. Material creep strength is only a design factor at temperatures that are above approximately 427°C (800°F). Because the design temperatures for storage tank applications are never more than 260°C (500°F), the tensile and yield strengths are the only strength properties that are of concern for storage tanks. The yield strength and the tensile strength of a material decrease as the material temperature increases. The tank material strength is a minimum at the maximum temperature to which the tank will be exposed. Therefore, material strength considerations are pertinent to ensure that tank components will not fracture at the maximum operating temperature of the tank. Component fracture that is caused by exceeding the material strength is ductile in nature and is preceded by permanent deformation. Therefore, there is normally time to take some form of remedial action to reduce the imposed loads before a component fracture occurs.


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The maximum operating temperature of the tank is specified on either the Storage Tank Design Specification Sheet (Drawing 2696) or the Storage Tank Data Sheet (Appendix L of API-650). Specification of the maximum operating temperature ensures that material strength is properly considered in tank design. Copies of Drawing 2696 and API-650 are in Course Handouts 3 and 1 respectively. Both of these items were discussed in MEX 203.01. The strength of the material, as defined by its tensile and yield properties, has a direct impact on the structural design of tank components. Storage tank components are designed to have a margin between the maximum stress that is permitted under the imposed load conditions and both the yield strength and tensile strength of the material. The allowable stress is the maximum stress that is permitted in a component for design purposes. Because the yield strength and tensile strength define failure limits of a material, a safety factor is used between the allowable stress and the stress at which a tank component is considered to fail. API-650 specifies safety factors for the determination of allowable stresses. For example, when a tank shell is designed for the loads that are imposed by the stored liquid, the allowable stress is limited to the lower of the following two values: twothirds of the yield strength, or two-fifths of the tensile strength of the material. API-650 specifies allowable stresses for each material specification that may be used to construct an atmospheric storage tank. Allowable stresses are discussed again later in this module. MEX 203.03 discusses the application of allowable stress to storage tank design.

Fracture Toughness Fracture toughness is the ability of a material to withstand conditions that could cause a brittle fracture. Brittle fracture is characterized by the lack of deformation or yielding in the material prior to failure. When a brittle fracture occurs, there is no leak or warning prior to complete failure of the component. These failure characteristics of brittle fracture are in contrast to the ductile type of failure that occurs when the material strength is exceeded.


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Brittle fracture occurs only when the following three conditions occur simultaneously: •

•

•

The material has insufficient fracture toughness at the temperature. There is sufficient stress in the component to cause a crack to initiate and grow. There is a critical size defect in the component that can act as a local stress concentration point and site for crack initiation such as at a weld.

The brittle fracture occurs without warning the first time that the component is exposed to the necessary combination of low temperature, high stress, and critical size defect; therefore, it is extremely important that material selection eliminate the possibility of brittle fracture. The fracture toughness of a material decreases as the material temperature decreases. Tank material fracture toughness is a minimum at the minimum temperature to which the tank will be exposed. The design metal temperature is the minimum temperature to which the tank will be exposed; therefore, material fracture toughness considerations are pertinent to ensure that tank components will not experience a brittle fracture at temperatures that are as low as the design metal temperature. The design metal temperature of the tank is specified on the Storage Tank Data Sheet (Appendix L of API-650 for atmospheric tanks). Specification of the design metal temperature ensures that brittle fracture and material fracture toughness are properly considered in tank material selection. Refer to API-650 in the Course Handout.


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Determining Fracture Toughness

The Charpy V-notch test (Cv) is commonly used to qualitatively determine the fracture toughness of steel. The test consists of performing an impact test on a notched specimen that is taken from a specific location in the material, and then recording the impact energy that is required to fracture the specimen at various temperatures. The magnitude of the measured impact energy, the shape of the impact energy curve, and the appearance of the specimen cross-section are significant factors in evaluating the material fracture toughness. Figure 1 illustrates the typical shape of impact energy transition curves for low- and high-strength steel s.

A - Low Strength Steel

Upper shelf energy (ductile)

Lower shelf energy (brittle)

B - High-Strength Steel

Temperature

Low

High

NDT for low-strength steels NDT for high-strength steels

Figure 1. Typical Impact Energy Transition Curves


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Factors that Influence Fracture Toughness

The impact energy level at a given temperature varies with different steels and with different manufacturing and fabrication processes. Additional factors such as torch cutting, arc strikes, and cold forming also affect brittle fracture behavior. Torch cutting or beveling of plate edges may lead to hard and brittle areas that make the edges more prone to cracking. Arc strikes can cause a brittle fracture, especially if the strike is made over a repaired area. Cold forming of thick plates may cause fractures in areas that have local stress concentration points or plate scratches. The slope of the impact energy curve in Figure 1 indicates the rate of change of the 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 it 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 only after excessive yielding takes place. Low-strength steels have a significant increase in fracture toughness as the temperature increases, as shown in Curve A of Figure 1. High-strength steels show only a slight increase in fracture toughness as the temperature increases, as shown in Curve B of Figure 1.

The dotted lines in Figure 1 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. Material fracture is brittle in nature at temperatures that are below the NDT temperature. Material fracture is ductile in nature at temperatures that are above the NDT temperature. The rate of change of fracture toughness is significantly different between high-and lowstrength 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 tank may be exposed during any phase of its operation determine the lowest expected temperature for which the tank must be designed. This lowest temperature identification must also consider temperatures that will occur during precommissioning, startup, shutdown, or upsets. The mechanical design of a storage tank must avoid either a brittle fracture or a ductile fracture. However, because 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.

Material Selection Tank materials are selected to provide the most economical design that is suitable for the specified design conditions. This materials selection considers both the material strength fracture and toughness properties that were previously discussed. Although several materials can be technically acceptable, the ultimate choice of what materials to use is based on current economic conditions, material availability, and Saudi Aramco's desire to standardize on a relatively small number of materials. Materials standardization simplifies storage requirements for spare material, reduces spare material costs, and minimizes the probability of using incorrect material during repair and maintenance activities. As an example of the basis for material selection, the use of a higher strength steel may reduce the thickness that is required for shell sections, but the cost per pound of the higher-strength steel may be so much higher than the cost of a lower-strength material that it could result in a higher overall cost for the tank. Generally speaking, an initial material selection can be made based on strength considerations. Then, shell wall thicknesses can be calculated, and a check can then be made to confirm that the fracture toughness of the selected material is acceptable for the specified design metal temperature. Shell wall thicknesses are normally kept to a maximum of 40 mm (1.5 in.) in order to avoid the need for any special fabrication or heat treatment considerations.


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Normally, Saudi Aramco engineers do not select materials. Saudi Aramco engineers usually review for acceptability the materials selections that are made by a design contractor or a tank supplier. The material selection process that a design contractor or tank supplier uses may be divided into several steps. These steps are based on the considerations that were previously discussed. A design contractor or a storage tank supplier can generally combine these steps because of his experience. The steps in the material selection process are explained below. •

•

•

•

•

Make an initial material selection for the tank primary components (roof, shell, bottom) based on strength and material standardization considerations. As a later section of this module explains, API-650 contains a list of acceptable material specifications. Typically, first select the highest strength steel that is acceptable. Determine preliminary thicknesses for the primary tank components. Procedures for this determination will be discussed in MEX 203.03. Determine whether the selected materials have adequate fracture toughness in the calculated thicknesses for the required design metal temperature of the tank. As previously discussed, fracture toughness becomes a more significant consideration as the design metal temperature of the tank decreases. Change the selected materials as needed, based on their fracture toughness. Recalculate the component thicknesses based on the new materials used. Also determine if the thicknesses should be reduced by using stronger material to minimize fabrication difficulties. Review the cost and availability of the selected materials, and adjust the material selections as appropriate.

The final materials selections are specified on the Storage Tank Data Sheet (Appendix L of API-650).


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SPECIFYING MATERIAL STANDARDS THAT APPLY TO THE PRIMARY STRUCTURAL COMPONENTS OF STORAGE TANKS This section discusses the material standards that apply to the primary structural components of storage tanks. These material standards, along with additional material requirements, are specified in the following: SAES-D-100

•

32-SAMSS-005

•

SAES-W-017

•

API-650

•

Table 1 summarizes the locations within these documents that contain material selection information.

Engineering Document

Location of Material Selection Information

SAES-D-100

Section 7

32-SAMSS-005

Section 2.0

SAES-W-017 API-650

Section 2, Appendix N

Table 1. Location of Materials Selection Information

The sections that follow briefly discuss several material selection requirements that are contained within each of these documents. Participants are referred for additional information to the copies of these documents that are in Course Handouts 1 and 2. Work Aid 1 contains a procedure that may be used to review a Contractor Design Package for acceptability, based on material selection considerations.


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SAES-D-100 SAES-D-100, Atmospheric and Low-Pressure Tanks, specifies several factors that affect material selection for storage tanks. These factors include reference to specific materials specifications and design parameters that are required in order to permit correct material specification. Several of these factors are discussed in the paragraphs that follow. •

The minimum design metal temperature is 0°C (32°F) for tanks that are located in the Eastern Province. Tanks that are located in the Western Province have a minimum design metal temperature of 10°C (50°F). The minimum design metal temperature for other locations must be determined. The minimum design metal temperature must always be specified on the Tank Data Sheet. As previously discussed, the minimum design metal temperature is a parameter that affects the fracture toughness of a material. Therefore, the proper selection of shell plate material based on fracture toughness considerations requires that this temperature be specified. A later section in this module discusses API-650 fracture toughness requirements that relate to the minimum design metal temperature.

•

•

The annular bottom plate must have the same material specification and grade as that of the lowest shell course. The annular bottom plate (or annular ring) is a specially designed portion of the tank bottom that is located directly under the tank shell. This plate is a very critical area of the tank because it experiences a complex combination of loads and stresses. A failure in this area can result in a significant spill of the stored liquid. Use of the same material specification for both the annular bottom plate and the lowest shell course ensures that both the annular bottom plate and the lowest shell course have the same strength, fracture toughness properties, and load-carrying capacity. SAES-D-100 specifies corrosion allowance requirements for new storage tanks, as summarized in Table 2.


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Stored Liquid

Corrosion Allowance Requirements

Petroleum Products or Crude Oil

None unless specified by the proponent Department based on experience with other tanks that store the same liquid.

Water

1.6 mm (1/16 in.) for shell plates if no internal cathodic protection system is installed None for roof and bottom plates 1.6 mm (1/16 in.) added to thickness of roof support structures (columns, rafters, girders) Table 2. SAES-D-100 Corrosion Allowance Requirements

Note that atmospheric storage tanks that store petroleum products do not require a corrosion allowance. Saudi Aramco's philosophy for these applications is that it is preferable to reduce the initial cost of the tank by not using a corrosion allowance because these services are not very corrosive. Any corrosion that does occur will normally be localized, and it can be evaluated on an individual basis as part of the tank maintenance program. MEX 203.08 discusses the evaluation of corrosion in existing storage tanks.


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Water service is known to cause corrosion of tank components; therefore, corrosion allowances are specified. A corrosion allowance is not specified for either the bottom or the roof because the thicknesses of these tank components are not based on stress considerations. Any corrosion that occurs in the bottom or the roof will be evaluated as a part of the tank maintenance program.

32-SAMSS-005 32-SAMSS-005, Atmospheric Storage Tanks, states that materials must comply with API-650 requirements, but it does allow the vendor to propose the use of alternative materials for consideration by Saudi Aramco. For example, it may be advantageous to use a German or Japanese plate material for particular cases due to its technical properties, availability, or cost. When materials that are not listed in API-650 are proposed, the material must at least comply with a recognized national standard material specification. If the proposed material specification is not ASTM , the chemical and mechanical properties of the material must be furnished by the vendor for evaluation by Saudi Aramco. Saudi Aramco must approve the use of any material that is not specified in API-650. The use of materials that are listed in API-650 results in an established level of material quality, but Saudi Aramco does not want to eliminate the potential for the use of other materials. The review and approval process that is specified ensures that material quality that is equivalent to that of API-650 materials will be achieved. 32-SAMSS-005 does not permit the use of rimmed steel s or capped steel s for the roof, bottom, and shell plates. Rimmed steels or capped steels may only be used for tank components that are not highly stressed. These types of steel do not have uniform material chemistries and properties throughout the component, and they should not be used for components that are critical to the structural integrity of the tank.


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SAES-W-017 SAES-W-017, Welding Requirements for Tanks, specifies general welding requirements that are applicable to storage tanks. Welding requirements are affected by the materials that are used in the storage tank. For example, the material that is used for the welding electrode, the specific procedure that is used to make the welds, and some of the weld inspection requirements are all affected by the specified construction material.

API-650 API Standard 650, Welded Steel Tanks for Oil Storage , covers material, design, fabrication, erection, and testing requirements for vertical, cylindrical, aboveground, closed- and open-top, welded steel storage tanks for internal pressures that approximate atmospheric pressure. The specific pressure and temperature limits of API-650 were discussed in MEX 203.01. Section 2 of API-650 specifies acceptable material specifications that may be used for tank construction, and it covers the following material categories: Plates

•

Sheets

•

•

Structural shapes

•

Piping and forgings

Flanges

•

Bolting

•

•

Welding electrodes

Section 2 also specifies special manufacturing and testing requirements and any limitations that apply to the materials.


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Strength Considerations

Table 3-2 of API-650 lists the plate materials that may be used for the shell, roof, and bottom of the tank. Table 3 contains part of Table 3-2. Note in Table 3 that the minimum yield strength and the minimum tensile strength are listed for each of the acceptable material specifications. Table 3-2 (Table 3) also specifies the Product Design Stress Sd and Hydrostatic Test Stress St for each of the acceptable material specifications. These two stress values are the allowable stresses that are used for the calculation of the required thickness of the tank shell. These allowable stresses account for the required safety factor between allowable stress and material strength. The concepts of material strength, safety factor, and allowable stress were previously discussed. MEX 203.03 discusses the use of these two allowable stress values in the calculation of tank shell thickness; however, recall from CSE 110 that the required shell plate thickness decreases as the material allowable stress increases. As an example of the application of this table, assume that a Contractor Design Package specifies that the tank shell is fabricated from A 516 Grade 60 plate. Table 3 contains the following information for A 516 Grade 60 plate: •

Minimum Yield Strength

32,000 psi

•

Minimum Tensile Strength

60,000 psi

•

Product Design Stress S d

21,300 psi

•

Hydrostatic Test Stress

24,000 psi


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Plate Specification

Grade

Minimum Yield Strength

Minimum Tensile Strength

Product Design Stress Sd

Hydrostatic Test Stress St

20,000 20,000 22,700 23,200 28,400 20,000 21,300 21,300 23,300 28,000 20,000 21,300 23,300 25,300 26,000 28,000 28,000 32,000 28,000 28,000 32,000 28,000

22,500 22,500 24,900 24,900 30,400 22,500 24,000 24,000 26,300 30,000 22,500 24,000 26,300 28,500 27,900 30,000 30,000 34,300 30,000 30,000 34,300 30,000

ASTM Specifications A 283 A 285 A 131 A 36 A 131 A 442 A 442 A 573 A 573 A 573 A 516 A 516 A 516 A 516 A 662 A 662 A 537 A 537 A 633 A 678 A 678 A 737

C C A, B, CS EH 36 55 60 58 65 70 55 60 65 70 B C 1 2 C, D A B B

30,000 30,000 34,000 36,000 51,000 30,000 32,000 32,000 35,000 42,000 30,000 32,000 35,000 38,000 40,000 43,000 50,000 60,000 50,000 50,000 60,000 50,000

55,000 55,000 58,000 58,000 2 71,000 55,000 60,000 58,000 65,000 2 70,000 55,000 60,000 65,000 70,000 65,000 2 70,000 2 70,000 2 80,000 2 70,000 2 70,000 2 80,000 2 70,000

Source: API Standard 650 , Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993, p.3-7.

Table 3. Permissible Plate Materials and Allowable Stresses

Fracture Toughness Considerations

API-650 places each of the acceptable material specifications into one of eight different Material Groups, based on their fracture toughness properties. The Material Groups are specified in Table 2-3 of API-650. Table 3 contains part of Table 2-3.


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Group IAs Rolled, Semikilled

Group II As Rolled Killed or Semikilled

Material

Notes

Material

Notes

Material

A 283 C A 285 C A 131 A A 36 Fe 42 B Grade 37 Grade 41

2 2 2 2, 3 4 3, 5 6

A 131 B A 36 A 442-55 A 442-60 G40.21M-260W Fe 42 C Grade 41

7 2, 6

A 573-58 A 516-55 A 516-60 G40.21M-260W Fe 42 D Grade 41

Group IV As Rolled, Killed Fine-Grain Practice

Material A 573-65 A 573-70 A 516-65 A 516-70 A 662 B G40.21M-300W G40.21M-350W Fe 44 B, C, D Fe 52 C, D Grade 44

Notes

4 5, 8

Group IVA As Rolled, Killed Fine-Grain Practice

Group III As Rolled, Killed Fine-Grain Practice Notes

9 4, 9 5, 9

Group V Normalized, Killed Fine-Grain Practice

Group IIIA Normalized, Killed Fine-Grain Practice Material

Notes

A 131 CS A 573-58 A 516-55 A 516-60 G40.21M-260W Fe 42 D Grade 41

10 10 10 9, 10 4, 9, 10 5, 9, 10

Group VI Normalized or Quenched and Tempered, Killed FineGrain Practice Reduced Carbon

Material

Notes

Material

Notes

Material

A 662 C A 573-70 G40.21M-300W G40.21M-350W

11 9, 11 9, 11

A 573-70 A 516-65 A 516-70 G40.21M-300W G40.21M-350W

10 10 10 9, 10 9, 10

A 131 EH 36 A 633 C A 633 D A 537 I A 537 II A 678 A A 678 B A 737 B

9 9 4, 9 9 5, 9

Notes


Table 4. API-650 Material Groups

Note in Table 4 that the A516 Grade 60 material that was used in the previous example appears in both Group III and Group IIIA. The difference between these two Material Groups is that the materials that are in Group IIIA must be normalized during the steel-making process, but the materials that are in Group III are supplied in the as-rolled condition. Normalizing is a heattreating process, and it can be done as an option after the plate has been rolled to its final thickness. Normalizing enhances the fracture toughness properties of the steel. If the A516 Grade 60 material must have the better fracture toughness properties of Group IIIA material, the material must be ordered in the normalized condition. The Contractor Design Package must clearly specify any special steel-making requirements that are needed for the specified materials. Saudi Aramco DeskTop Standards

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Figure 2-1 of API-650 specifies the minimum acceptable design metal temperature at which a plate material may be used in the following tank components without being impact tested prior to fabrication of those components: Shell

•

•

Shell reinforcing plates or insert plates

•

Bottom plates that are welded to the shell

•

Plates that are used for nozzle or manhole necks

•

Flanges that are made from plate

This minimum temperature is based on the Material Group of the plate material and the thickness of the plate. Figure 6 contains part of Figure 2-1. API-650 does not require that the material be impact-tested for situations that satisfy the criteria that are contained in Figure 2-1 (Figure 2). There is sufficient experience that shows that the material has acceptable fracture toughness if it satisfies the Figure 2-1 criteria.


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Figure 2. Minimum Permissible Design Metal Temperature

If the A516 Grade 60 material of the earlier examples is supplied in the normalized condition, it is a Group IIIA material. A Group IIIA material may be used at a design metal temperature that is as low as -22°C (-40°F) without being impact tested, regardless of its thickness. If the A516 Grade 60 material is not normalized , it is a Group III material. The minimum design metal temperature of a Group III material that is not impact tested varies with its thickness. For example, if the plate thickness is 25 mm (1 in.), it may only be used down to a temperature of -6°C (-10°F).


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If the specified material is not exempt from impact testing, two choices are available. Either the material must be impact-tested at the specified design metal temperature or a different material specification that does not require impact testing must be used. The choice of which approach to take is based on experience. This experience must consider whether the specified material will pass the impact testing and how much changing to a better material will cost. Materials engineers in the Consulting Services Department can help in this determination. It should also be noted that all controlled rolled plate s must be impact-tested regardless of thickness or design metal temperature. The impact testing of controlled rolled plates ensures that they have acceptable fracture toughness even in the nonpreferred rolling direction. The previous discussion clearly illustrates the following points: •

•

•

There is an interaction among material specification, fracture toughness, and material strength. The material specification of the shell plate establishes both its fracture toughness properties and its strength properties. Any change that is made to a material specification in the interest of changing plate thickness must also lead to the consideration of the potential impact of this change on minimum acceptable design metal temperature.

The design metal temperature and the shell plate thicknesses must be specified in the Contractor Design Package on either Drawing 2696 or in the Storage Tank Data Sheet. API-650 requires that the design metal temperature for a tank be no higher than 8°C (15°F) above the lowest one-day mean temperature for the site, unless there is sufficient experience that justifies the use of another assumption. As previously noted, Saudi Aramco specifies this temperature based on tank location. API-650 also contains additional fracture toughness requirements. These requirements include impact testing procedures, minimum impact energy requirements, and fracture toughness requirements for the tank structural components other than the shell. Participants are referred for additional information to the copy of API-650 that is in Course Handout 1.


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SUMMARY On the basis of the information that MEX 203.02 has provided, the Participant should be able to describe the material properties that must be considered in the selection of appropriate materials for the structural components of storage tanks. The Participant should also be able to specify the specifications and requirements that determine whether materials to be used for the primary structural components of a storage tank are acceptable and to review the contents of a Contractor Design Package to determine if the specified materials are acceptable. Now that materials for storage tanks have been selected, and it has been determined whether these materials are acceptable, the tank components may be designed. Mechanical design of tank components is discussed in MEX 203.03.


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WORK AID 1: MATERIAL STANDARDS THAT APPLY TO PRIMARY STRUCTURAL COMPONENTS OF STORAGE TANKS The procedure that is contained in this Work Aid may be used to specify the materials standards that apply to the primary structural components of storage tanks. This Work Aid may also be used to review the acceptability of materials that are specified by a storage tank supplier for specific tank components based on information that is contained in a Contractor Design Package. Note that a copy of API-650 is contained in Course Handout 1, and copies of SAES-D-100, 32-SAMSS-005, and SAES-W-001 are contained in Course Handout 2. Material selection requirements are specified in the following sections of the referenced engineering documents: •

SAES-D-001

--

Section

•

32-SAMSS-005

--

Section 2.0

•

SAES-W-017

--

•

API-650

--

Section 2, Appendix N

Refer to these sections as needed in the application of the following procedure. 1.

Identify the design metal temperature and maximum operating temperature for the tank. These values are stated in the Contractor Design Package. If the maximum operating temperature exceeds 93°C (200°F) but does not exceed 260°C (500°F), additional requirements that are contained in API-650, Appendix M must be applied. Confirm that the specified design metal temperature meets SAES-D-100, Para. 6.1.3.

2.

Identify the plate material specifications for the tank shell, roof, bottom and annular plate that are specified in the Contractor Design Package. Note that different materials may be used for each component, and that more than one material may be used for the shell. Component

Material

Shell Roof Bottom Annular Plate Any Saudi Aramco materials standardization requirements that are currently in effect should also be considered at this point.


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3.

Confirm that the material specifications that were identified in Step 2 are each listed in Table 3-2 of API-650. Confirm that the annular plate material meets SAES-D-100, Para. 5.2. If any material specifications are not listed in API-650, confirm that they meet the requirements of 32-SAMSS-005, Para. 2.2.1 and 2.2.5, and API-650, Para. 2.2.1 and 2.2.5.

4.

For the shell material specifications that were identified in Step 2, determine the applicable API-650 Material Group from Table 2-3 of API-650.

5.

Identify the shell plate thicknesses that are specified in the Contractor Design Package.

6.

Refer to Figure 2-1 in API-650. Use the Material Groups that were identified in Step 4, and the plate thicknesses that were identified in Step 5, to determine the minimum permissible design metal temperature. This is at the intersection of the wall thickness and Material Group lines. The following table may be used to summarize the shell material information that has been identified.

Shell Course

Material Specification

Material Group

Thickness, mm (in.)

Permitted Design Metal Temperature, °C (°F)

1 2 3 4 5 6 7 8


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7.

If the tank design metal temperature identified in Step 1 is less than the values that are summarized in Step 6, the following options are available: •

•

Change the material specification of the unacceptable shell plate to one that is in a Material Group with better fracture toughness (a lower group line in Figure 2-1). The required wall thickness must be recalculated for the new material, and Steps 2 through 6 must be repeated. Impact-test the material at the specified minimum design metal temperature. Confirm that its fracture toughness meets API-650 requirements at this temperature.

The choice of which option is used depends on economics and the likelihood that the material will pass the impact test requirements. This choice is based on current market conditions and past experience with other impact test results. 8.

Identify the tank service and the corrosion allowance that has been specified in the Contractor Design Package. Confirm that the specified corrosion allowance meets SAES-D-100, Para. 6.3.


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GLOSSARY allowable stress

The limiting stress (maximum stress) that is specified in a component’s design that includes an appropriate safety factor.

API

American Petroleum Institute

ASTM

American Society for Testing and Materials

brittle fracture

A sudden break that is not preceded by deformation or yielding.

capped steel

Capped steels have characteristics that are similar to rimmed steel, but they have them to a degree that is between the characteristics of rimmed and semi-killed steel. A deoxidizing agent may be added to control rimming when the ingot is cast.

Charpy V-notch test

A qualitative test for determining the toughness of a steel.

cold forming

A forming operation performed on unheated metal.

controlled rolled plate

Plate with enhanced properties in a preferred rolling direction.

creep

The gradual continuous distortion of a material under continued load, usually at high temperatures.

creep strength

The stress that is required to cause continuous elongation of a material that is subjected to elevated temperature.

CSA

Canadian Standards Association

ductility

The property of being permanently deformed by tension without rupture; that is, the ability to be drawn from a large to a small size.

fracture toughness

The ability of a material to withstand conditions that could cause a brittle fracture.

high-strength steel

Steel having a specified minimum yield strength greater than 296 MPa (43 000 psi) and a specified maximum tensile strength equal to or less than 689 MPa (100 000 psi).

impact energy

The energy required to fracture a material.

impact strength

The ability of a material to absorb energy and deform plastically prior to fracture.

impact test

A procedure that is used to measure the impact energy of a material at a specified temperature.


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ISO

International Organization for Standardization

low-strength steel

Steel having a specified minimum yield strength equal to or less than 296 MPa (43 000 psi) and a specified maximum tensile strength equal to or less than 586 MPa (85 000 psi).

nil ductility transition

The temperature below which a metal exhibits

temperature

brittle rather than ductile failure in toughness testing.

rimmed steel

No deoxidizing agents are added in the furnace during the production of rimmed steels. These steels are characterized by marked differences in chemical composition across the section and from the top to the bottom of the ingot.

rupture

To break.

strain

The change in dimensions of a material due to the application of stress. Linear strain is the ratio of the change in length to the original length.

strength

The ability of a material to withstand an imposed force or stress (referred to as load).

tensile strength

The greatest stress that a metal can tolerate without breaking apart. Calculated by dividing the maximum load by the original cross-sectional area. Also known as ultimate strength or ultimate tensile strength.

yield strength

The stress that causes permanent deformation in a material. It is usually defined as the stress required to cause 0.2% offset strain.


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Determining Acceptability of Materials for Storage Tanks

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