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Engineering Encyclopedia Saudi Aramco DeskTop Standards

PIPE WALL THICKNESS CALCULATION

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-101.03

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

Engineering Encyclopedia

Piping, Pipelines & Valves Pipe Wall Thickness Calculation

Section

Page

INFORMATION ............................................................................................................... 5 INTRODUCTION............................................................................................................. 5 FACTORS AFFECTING WALL THICKNESS CALCULATION........................................ 6 BACKGROUND ON PIPEWALL THICKNESS................................................................ 7 STEPS FOR CALCULATING PIPE WALL THICKNESS................................................. 9 PIPE WALL THICKNESS FOR THE INTERNAL DESIGN PRESSURE ....................... 11 Transportation Piping: ASME B31.4 and B31.8 (Thickness for Internal Pressure) .. 11 Design Pressure [P]............................................................................................ 12 Pipe Diameter [D] ............................................................................................... 14 The allowable stress [SETF]............................................................................... 14 Longitudinal Joint Factor [E] ............................................................................... 16 Temperature Derating Factor [T] ........................................................................ 16 Design Factors [F] .............................................................................................. 17 RER & PDI.......................................................................................................... 18 Sample Problem 1: Transportation Piping .............................................................. 21 Solution .............................................................................................................. 22 PROCESS PIPING: ASME B31.3, MINIMUM THICKNESS FOR INTERNAL PRESSURE .................................................................................................................. 25 Design Pressure and Temperature .......................................................................... 26 Longitudinal Joint Factor [E] ............................................................................... 27 Basic Allowable Hoop Stress [S] ........................................................................ 28 The "Y" Factor for Plant Piping ................................................................................ 29 CORROSION, EROSION, AND THREAD ALLOWANCES........................................... 31

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Manufacturer Mill Tolerance .................................................................................... 31 SAES Limitations on Pipe Schedule ........................................................................ 32 Sample Problem 2 ................................................................................................... 33 Solution .............................................................................................................. 34 PIPE WALL THICKNESS FOR EXTERNAL PRESSURE............................................. 36 GUIDELINES FOR EXTERNAL PRESSURE CALCULATIONS ................................... 38 Sample Problem 3: External Pressure for Pipeline ................................................. 44 Solution .............................................................................................................. 44 Sample Problem 4: External Pressure for Plant Piping ........................................... 45 Solution .............................................................................................................. 45 TRAFFIC AND SOIL LOADS OVER BURIED PIPE ..................................................... 46 THE MAXIMUM ALLOWABLE OPERATING PRESSURE (MAOP).............................. 47 Guidelines for Calculating Maximum Design Pressure....................................... 47 MAOP of a Pipeline.................................................................................................. 49 Sample Problem 5. MAOP of a Pipeline ................................................................ 49 Solution .............................................................................................................. 50 Maximum Design Pressure for Process Plant Piping............................................... 51 Sample Problem 6: Process Piping......................................................................... 52 TYPICAL MISTAKES IN PIPE WALL THICKNESS CALCULATION ............................ 53 SUMMARY.................................................................................................................... 54 ADDENDUM ................................................................................................................. 55 ADDENDUM A .............................................................................................................. 56 ADDENDUM B .............................................................................................................. 62 ADDENDUM C.............................................................................................................. 69

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LIST OF FIGURES Figure 1. Stresses in the Pipe Shell due to Internal Pressure ....................................... 7 Figure 2. Schematic Diagram Explaining the Concept of RER and PDI ...................... 19 Figure 3. Variation of the Basic Allowable Stresses with Temperature for Grade B Material. ......................................................................................................... 28 Figure 4. Effects of External Pressure ........................................................................ 37 Figure 5. Geometrical Factor A for External Pressure Calculations ............................. 41 Figure 5. Geometrical Factor A for External Pressure Calculations (continued) ......... 42 Figure 6. Material Factor B for External Pressure Calculations ................................... 43

LIST OF TABLES Table 1. Level of Safety for Different Pipe Material Grade ......................................... 15 Table 2. Location Class and Design Factors for Transportation Piping ....................... 20 Table 3. The “Y” Factor as Extracted From ASME B1.3 TABLE 304.1.1 .................... 29 Table 4. Minimum Wall Thickness Schedule for Carbon Steel .................................... 32 Table A-1: ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) ............................................................................................... 56 Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E. ............................................................................................................... 59 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe ..................................................................................... 60 Table A-4. SAES B-064 Rupture Exposure Radius ..................................................... 61

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals ............................................................................................................. 62 Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E ............................................................................................ 67 Table C-1. Table of Properties of Pipe ......................................................................... 70

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INFORMATION INTRODUCTION This module discusses the process of determining pipe wall thickness, which is one of the first steps in specifying the design of piping system components. Pipe wall thickness is based on the internal pressure of a pipe and, if necessary, external pressure or any additional loads. The pipe wall thickness is calculated by using the equation for internal pressure thickness in the applicable ASME B31 Code, modifying the thickness for any external pressure or additional loads, selecting pipe schedule based on manufacturer's tolerance. Also, calculating the Maximum Allowable Operating Pressure (MAOP) for the pipe after determining the pipe wall thickness will be covered. The previous module discussed an early step in designing a piping system: selecting pipe material. Selecting pipe material sets parameters for the other facets of piping design and is required to determine allowable design stress, which is necessary to calculate the required wall thickness. Knowledge of the following is necessary to achieve the objectives of MEX 101.03: •

Scope and application of the ASME/ANSI B31 piping codes, as discussed in MEX 101.01.

•

Pipe material selection, as discussed in MEX 101.02.

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FACTORS AFFECTING WALL THICKNESS CALCULATION In the process of calculating the wall thickness of a pipe several parameters must be considered and accurately specified. These parameters are listed below. Pipe material: The pipe material will be specified as has been discussed in the previous chapter. The material type identifies basic design parameter, which is the allowable stress to be explained in the coming sections. Internal pressure: This the crucial parameter in piping design because the main purpose pressure piping Codes and Standards is to contain the pressurized fluid from escaping out or rupturing the pipe. Fluid temperature: The fluid temperature affects the allowable stresses of the material and the designated Code because each Code has temperature limitation. External loads: These loads could be caused by external pressure due to vacuum or water static head for submarine lines. Also, dead weight of soil, snow or sand over burden load on the pipe. Construction: Construction of pipelines whether on land or off shore could add additional construction requirement that may increase the wall thickness of the pipe. In fact for off shore pipeline, stresses on the pipe exerted on the pipe during lay-off from the barges are the detrimental factor of the pipe wall thickness. Another example is the elastic bend requirement to follow the pipeline terrain for on land pipelines. The industry Codes provide rules and guidelines for establishing these parameters, however Saudi Aramco have established its own rules that generally follow the industry Codes but with more stringent requirement.

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BACKGROUND ON PIPEWALL THICKNESS The main objective of piping codes is to insure that piping systems will not under pressure. Piping systems are designed and constructed to convey pressurized fluid, mostly flammable and could be toxic, therefore Codes tend to very conservative in respect to pressure containment of the pressurized fluids. This explains why there are few failures reported due to overpressure of the piping systems. The basic theory for designing of wall thickness of a piping system that contains the internal pressure is based on limiting the hope stress developed by this pressure to an acceptable value by the Code. The calculated hope stress in pipe shell, refer to Figure-1, is based on Lame’ equation as follows:

Figure 1. Stresses in the Pipe Shell due to Internal Pressure Equation 1.

Sh = P [0.5x(D/t) 2 –(D/t) + 1] / [(D/t)-1]

Boardman's empirical equation has further simplified Lame’ equation based on an intermediate diameter and introduced a Y factor to correct the difference. Boardman's equation has become the basis for calculating wall thickness B31 Codes for pressure containing piping. Boardman's equation is as follows:

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Equation 2.

Sh = P (D – Y t ) / 2 t

For thin pipe where D/t > 6, and outside diameter D, Y is considered 0.4. For thicker pipe Y can be calculated as will be discussed later. Each code provides an equation that is used to calculate internal pressure thickness. The equations may look different and / or the approach varies, but the basic concept is the same. The main concept is to limit the hoop stress, Sh, to an acceptable and safe level.

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STEPS FOR CALCULATING PIPE WALL THICKNESS Each engineer should be familiar with the steps followed to determine the wall thickness of the pipe. He should be aware that the thickness shall not be less than the minimum required by the Code otherwise this would be a basic violation to the relevant Code. At the same time it should not be over specified because that would be reflected as unnecessary capital cost. Over specifying wall thickness mostly will have significant impact on capital investment in terms of millions of dollars. The cost of line pipe is determined mainly by the tonnage, i.e. the pipe diameter, wall thickness, length of order, as well as the pipe grade and the type of alloying elements. The steps to be followed during the process of wall thickness calculation are as follow: 1. Determining the applicable ASME/ANSI B31 Code for the piping system of concern. This has been discussed in MEX 101.01. The latest SAES should be always referenced, and if there are potential savings by using different Code other than specified, the issue should be highlighted to the concerned specialists in CSD. 2. Identifying the applicable formula from the code for calculating the wall thickness to sustain the internal design pressure. 3. Setting the design parameters as specified in the design data and in compliance with SAES-L-002 and SAES-L-003, Design Stress Criteria for Pressure Piping. 4. Evaluating whether the minimum thickness is adequate for structural rigidity (excessive deflection or buckling under the pipe own weight.

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5. Increasing the calculated thickness, as needed, to account for corrosion allowance and mill tolerance. This is dependent on the Code and the SAES. 6. Checking if the calculated thickness for internal pressure is also acceptable for external pressure and other applied loads, as applicable such as traffic load, sand dunes dead weight, etc. 7. Selecting a thickness from an ANSI/API table of standard pipe thickness and checking the thickness against the Saudi Aramco minimum thickness requirements. Selecting a scheduled pipe is not always applicable for pipeline projects involved with long distances and large diameter pipe. The text of MEX 101.03 refers to ASME/ANSI B31.3 for plant piping and B31.8, for transportation piping. The process discussed in this module is consistent for all the B31 piping codes. However, the equations, variables, and definitions or values for allowable stress differ.

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PIPE WALL THICKNESS FOR THE INTERNAL DESIGN PRESSURE Calculating the required pipe wall thickness to contain internal pressure is the first step in determining pipe wall thickness. As explained earlier the applicable Code for a particular piping system will determine the method and rules governing the pipe wall thickness calculations. Identifying the Code has been already discussed in MEX-101.01. Therefore, in the following two sections the calculation procedure and requirement for the internal pressure design will be discussed. One section will focus on the transportation piping and the other one will explain internal pressure design for plant piping. Transportation Piping: ASME B31.4 and B31.8 (Thickness for Internal Pressure) This section outlines the method for calculating the pipe wall thickness for piping systems that are designated as transportation piping, ASME/ANSI B31.8, paragraph 841.11 states the rule for calculating the design pressure as follows: Equation 3.

P = [2 S t / D] x FET

Even though this equation is given in the AMSE B31.8 Code for gas transportation system, Saudi Aramco Standard calls for applying it for all other transportation piping systems regardless of the nature of the service. The differences will be in the design factors and de-rating factors as discussed bellow. This equation could be re-arranged to calculate the wall thickness required for internal pressure containment for gas transmission and distribution piping (as well as other transportation piping) as follows: Equation 4

t = P D / [ 2 SETF]

Where: t

= Internal pressure wall thickness, in.

P = Design pressure, psig. D = Outside diameter of pipe, in.

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S = Specified Minimum Yield Strength (SMYS), psi. E = Longitudinal-joint quality factor. T = Temperature derating factor. F = Design factor. Each of the above parameters will be discussed thoroughly next. Design Pressure [P] In order to understand the significance of the pressure term in the Equation - 4 the following topics will be discussed:

Pipeline optimization study

Pump & compressor shutoff

Static head and pressure drop

Pressure surge

Pipeline Optimization Study: Closer evaluation on Equation – 4, shows us that the wall thickness is directly proportional to the pressure, i.e. as the pressure becomes higher the thicker the pipe becomes. On the other hand, as the pipe diameter increases the pressure required to achieve certain flow drops down. Consequently, maybe lower wall thickness is needed. However, both the pipe wall and the diameter contribute to the pipe cost. On the other hand lowering the discharge pressure most probably will decrease the overall operating cost.

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Therefore, the designer should go through a very detailed exercise trying to optimize between the design pressure, pipe diameter and pipe wall thickness in one side and the operating cost in the other hand. These kind of studies are called “Pipeline Optimization Study”. Details of this studies are beyond the scope of this course. Pumps & compressors shutoff: The design pressure is dictated by the hydraulic requirement to achieve certain flow rate through the pipeline. Also, the source of the pressure upstream, such as the oil reservoir shut-in pressure, dictates the design pressure. For a pipeline system connected to a pump or a compressor, the design pressure will be affected by the pump or compressor shut-off pressure in case the flow downstream these rotating machinery was blocked and the pump or compressor continue to operate. The pipeline will be subjected to the highest pressure that the machine can produce. Static head and pressure drop: Also, the hydrostatic head in a liquid-filled piping system could also could be a detrimental factor in cases where there is a large difference in elevation along the pipeline route. In all cases the static head must be considered when it is positive and adding to the internal of the fluid. Our Standards do not allow taking advantage of or pressure reduction due to pressure drop or due to negative static head. Pressure surge: The pressure surge condition in liquid transportation pipeline must be considered, because sudden change in the flow velocity caused by closure of a down stream isolation or sudden stop of a downstream pump will create a pressure surge in the line. This pressure surge will be transmitted backward at the speed of sound in the transported fluid and will create radial pressure in the line. The development of pressure surges must be accounted in the design of a pipeline. There are several approaches to reduce the effect of pressure surges, for example increasing the closure time of the valve that is creating the surge, slowing down the rate at which the pump is stopped, or installing pressure surge relief systems.

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However, assuming that pressure surges are not avoidable, or it is not practical to install surge relief systems, then the wall thickness of the line must be designed to withstand these pressure surges. The ASME B31.4, paragraph 402.2.4 states that the MATP, maximum allowable transient pressure due to the pressure surges, shall not exceed 110% of the design pressure (MAOP) in the line. Saudi Aramco SAES requires that a formal surge analysis shall be made for liquid-packed services. In case that it is not economical to increase the pipe thickness and pressure surge could not be eliminated, then, surge protection systems shall be installed if surge pressures are calculated to exceed 110% of the MAOP. The Surge protection systems shall be of fail-safe design with an installed, spare, surge-relief valve for each surge protection system. Pipe Diameter [D] The design Equation-4 is based on the actual outside diameter of the pipe. The engineer must be aware that the designated outside diameter in the pipe data in API-5L or ASTM A53 or other source of information provides NPS, nominal pipe sizes. The nominal pipe size is less than the actual outside diameter for NPS 12 inches and lowers and it is equal to the actual outside diameter for 14 inches and above. Therefore, careful review should be exercised during the calculation otherwise fatal mistakes could happen. The table showing pipe dimensions and other data are provided in Addendum A of this module. The allowable stress [SETF] The combined term of SEFT is actually gives the limiting allowable hope stress for produced by internal pressure. Accordingly, this term will determine the minimum pipe wall thickness required for holding the internal pressure. A key parameter in this combined term is S. The parameter SA is the Specified Minimum Yield Strength (SMYS) of the material as explained previously in MEX-101-02. The ASME codes B31.4 & Saudi Aramco DeskTop Standards

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B31.8 provide the acceptable materials together with their SMYS. These tables are shown in Appendix A-1 of this chapter. The engineer should be aware that the allowable stress for transportation piping is based on material yield point. The main reason for that is economical, because pipelines usually extend for long distances therefore the intent of the Code is to utilize the material to its limits of yielding. This approach is simple which has some reasoning behind it because pipeline construction and configuration are also relatively simple and not complicated. However, grater caution should be exercised when dealing with higher-grade materials. To make this point clear, we must understand that the pipe bursting pressure is governed by the ultimate tensile strength of the material rather than the yield strength. Equation 5. Equation 1 for the design pressure can be re-arranged as follows: Pd

=

SMYS x [t ET/ D] x F

Pd

=

SMYS x [t / D] x F; Assuming E & T equal to 1

It is known fact and tested that the rupture pressure depend on the tensile strength of the material and defined as follows: Pb

=

U t x [t D]; Assume A= t D

P

=

S x A; (A is a factor combining all other parameters)

Table 1. Level of Safety for Different Pipe Material Grade Material Grade

SMYS psig

Ultimate Tensile U t psig

Design Pressure Pd

Bursting Pressure Pb

Pb / Pd

B

35000

60000

35000xAF

60000xA

1.71 / F

X52

52000

66000

52000xAF

66000xA

1.27 / F

X60

60000

75000

60000xAF

75000xA

1.25 / F

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X70

70000

82000

70000xAF

82000xA

1.17 / F

The above table provides parametric calculations for the design and burst pressure. The last column which gives the ratio between the burst pressure to the design pressure shows that as the pipe material become higher, the ration becomes less. This implies that the actual safety factor is jeopardized as materials with higher grade are used. Longitudinal Joint Factor [E] The longitudinal joint factor is safety factor that represents the quality of the pipe seam weld. As explained earlier in MEX101.2, this factor is dependent on both the manufacturing process and the intended Code. Seamless pipe has a longitudinal joint factor of on1 because in seamless pipes there is no seam weld. All other welded pipe must have an E factor ranging from 1 to 0.6. This factor will reduce the allowable stress; consequently the wall thickness will increase. Appendix A-2 of this chapter is an extract from AMSE B31.8, Table 841.115A. The engineer should always refer to the latest edition of the Code because this factor could be revised to be higher due to improvement in the manufacturing process and inspection techniques. At the same time it could be lowered due to lessons learned or a more conservative approach by the Code Committee. Temperature Derating Factor [T]

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In general, the design temperature of transportation piping is constant and relatively low. Also, we should remember that both ASME B31.4 & B31.8, as mentioned in MEX-101.01, are limited in the design temperature to 250o F and 450o F respectively. The temperature derating factor accounts for the fact that the yield strength of materials is reduced as the metal temperature increases. ASME B31.4 has no temperature derating factor because it is limited to 250o F while ASME B31.8 provide Table 841.116A for these values. This table is in Appendix A-3. The design temperature is mostly dictated by the upstream condition of the fluid transported in the pipeline. However, there are cases when the temperature will increase as the fluid progress due to turbulence in the flow or picking heat from sun radiation. These effects should be carefully studied because they could have adverse impact either on the safety of the piping system if ignored. On the other hand, they have significant cost impact if very conservative approach is utilized. For example, a conservative approach may lead to assigning a temperature derating factor which will be translated into heavier pipe wall and more cost to be spent. Also, it may lead to installing coolers downstream the pipeline that will never be operated but have added cost to the initial investment and additional cost for operation and maintenance. Design Factors [F] The design factor [F], sometimes called DF, is a safety factor that accounts for the relative hazard created by the presence of the pipeline to the surrounding population, environment, and facilities. This design factor is affected by many factor some of which are: 1. The nature of the fluid transported, service, whether it is farmable, toxic, explosive or pollutant. 2. The density of the population in the area where this line is passing through. The higher the population density the more safety factor should be imposed.

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3. Government laws and regulations. 4. Level of conservatism of the Committee members developing the Standards ASME B31.4 has identified one single design factor equals to 0.72, while ASME B 31.8 goes further step and assign a different design factor for a different area classification. Saudi Aramco is further step ahead in this issue, where the concept of location classification has been applied to liquid hydrocarbon pipelines as well as gas pipelines. To determine the design factor, F, both Standards SAES-B-064 and SAES-L-003 shall be used identify the piping system location class. RER & PDI In the case of pipelines, to determine the design another factor another two concepts need to be explained. Those are the Rupture Exposure Radius (RER) and the Population Density Index (PDI). As shown in Figure-2, RER is a measure of the extent of the risk that a pipeline could make to the surroundings when it ruptures. The second, PDI, is a measure of the vulnerability of people to that risk. The rules and guidelines for calculating RER and PDI are given is AMSE B31.8 paragraph 840.2 and further superseded by SAES-B-064. The, SAES-B064 specifies the values for RER based on pipeline service, True Vapor Pressure, H2S concentration and the line size. The values of RER based on SAES-B-064, issue 1997, are summarized in a table in Addendum A-4. PDI is measured by the number of existing buildings intended for human occupancy and the land area planned for future development, all falling within the RER as shown in Figure-2. It is worth mentioning that temporary facilities which will be in place for less than six consecutive months are not to be included in these calculations The method of calculation is detailed in the next Sample Problem 1.

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Figure 2. Schematic Diagram Explaining the Concept of RER and PDI The values of the design factor, F, corresponding to the location class and the PDI are provided in Table-2. Most probably, a single transportation pipeline may have multiple location classifications associated with it, based on the PDA results along its length. It is very essential that engineers must understand the intent of these factors rather than using them as they are. These factors if not well understood and regularly reevaluated, this may lead to situation where some of the pipelines may become safety hazard to people and environment. On the other hand, these factors could substantially increase the pipeline construction cost with no logical reason.

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Table 2. Location Class and Design Factors for Transportation Piping Location Class

Design Factor F

Population Density Index PDI

1

0.72

10

Commentary and Examples

Desert area non developed areas Water service lines.

2

0.60

11-29

3

0.50

30 and above

4

0.40

Special Cases

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Hydrocarbon service, in populated areas or parallel to highways Plant piping designed to B31.4 / B31.8 Highly populated complexes such as hospitals and malls.

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Sample Problem 1: Transportation Piping A 48-inch cross-country pipeline conveying sour gas from Uthmaniyah to Shedgum. Refer to Fihgure-2 for clarification. Most of the pipeline passes through a desert except for 2-Km section that traverses along the border of an industrial park. The industrial park is 400 meters away from the pipeline. Also, within this industrial park, there is an area planned for future development. Identify the design factor for this pipeline and calculate the minimum wall thickness required for internal pressure. The following data is given. Pipe Diameter: 48 inches Design Pressure: 740 psig Design temperature: 175oF Pipe specification: API 5L carbon steel, Material grade: X65 Pipe manufacturing: Double Submerge Arc Welded (DSAW) Two-story building is: 11 buildings Five-story building is: 4 buildings Future development: 1200 m2

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Solution Since this is a transportation piping system, Equation 2 applies as follow: t

= P D / [ 2 SETF]

P = 740 psig D = 48 inches S = 65000 psi (Addendum A-1 for API 5L X-65) E = 1 (Addendum A-2 for DSAW pipe) T = 1 (Addendum A-3 for T < 250 oF) What is left to identify is the design factor F. At this point we should be aware that there is a possibility that two design factors may be assigned to this pipeline. F for the undeveloped and desert area: Based on Table-1 the pipeline is considered in location class1, therefore F = 0.72 t

= 730 x 48 / ( 2 x 1.0 x 1.0 x 0.72 x 65000 ) t

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0.374 inches

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F for the section area close to the industrial Park: To determine the design factor, F, a population density analysis must be performed. RER = 5000 m for the 48” sour gas pipeline, (based on SAESB-064 shown in Addendum A-4) This means that the industrial park is located within the RER. Therefore, a formal population density analysis (PDA) must be conducted to determine the location class, thus the design factor F. PDI

= Existing DI + Virtual DI

PDI

= EDI + VDI

EDI

= N1 + N3

N1 = Number of 3-story N3 = Number of more than 3-sorty x Number of the stories / 3 (rounded to the next number) N1 = 11 N3 = 4 x 5 / 3 = 6.666 N3 = 7 EDI

= N1 + N3 = 11 + 7 = 18

VDI

= 0.00075 x 1200 = 0.9 = 1

PDI

= EDI + VDI =18 + 1

PDI

= 20

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Based on Table-1, for PDI = 20, this section of the pipeline is considered in location class-2, therefore F = 0.60 t = 730 x 48 / ( 2 x 1.0 x 1.0 x 0.60 x 65000 )

t

=

0.449 inches

Since no corrosion or other allowance was specified, the calculation is complete.

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PROCESS PIPING:

ASME B31.3, MINIMUM THICKNESS FOR INTERNAL PRESSURE This section outlines the method for calculating the minimum pipe wall thickness for plant piping systems designated as process plant piping ASME B31.3., as discussed in earlier in MEX-101.01. The minimum pipe wall thickness for Process Piping requires meeting three criteria. Those are: 1. Internal pressure requirement 2. Corrosion, erosion and mechanical requirement 3. Mill under-tolerance must be considered The first and the second factors are composed in the equation of paragraph 340 of ASME B31.3 as follows: Equation 6. t

tm

= t+c

= minimum wall thickness for pressure or mechanical strength

c = corrosion, erosion and mechanical allowance. Calculation of t for internal pressure is detailed as follows. ASME B31.3, paragraph 304 gives the rule for calculating the wall thickness requirement. Also, it provides several equations to conduct the calculation. The following equation is the mostly used one.

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Equation 7. t = PD / [ 2( SE + PY ) ] where: t

= Internal pressure design thickness, in.

P = Internal design pressure, psig. D = Outside diameter of pipe, in. E = Longitudinal-joint quality factor. S = Basic Allowable (hot) hoop stress, psi. Y = Wall thickness correction factor. For thickness t < D/6, the internal pressure thickness for straight pipe shall not be less than that calculated in the above equation. For t > D/6 or for P/SE > 0.385, calculation of pressure design thickness for straight pipe requires special consideration of factors such as theory of failure, effects of fatigue, and thermal stress. This module will not discuss this situation. The parameters for calculating wall thickness of the process plant piping are similar to the transportation piping, but the parameters are different. These parameters will be explained afterward. Design Pressure and Temperature The design pressure and temperature are used to calculate the internal pressure thickness of pipe. The design pressure is used directly in the thickness calculation equation, as previously shown. The design temperature is used to determine the allowable stresses from the ASME B31, 3 tables for Basic Allowable Stress. The worst combination of design pressure and temperature should be used for piping thickness calculations. The process engineer based on process requirements typically determines these values for design pressure and temperature

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Piping system design conditions generally are determined based on the design conditions of the equipment to which the piping is attached. Determining the piping design conditions consists of: 1. Identifying the equipment to which the piping system is attached. 2. Determining the design pressure and design temperature for the equipment. 3. Considering contingent design conditions, such as upsets not protected by pressure-relieving devices. 4. Verifying values with the process engineer. For example, a plant piping system that is attached to two process vessels, each with different design conditions, will have specified design pressure and design temperature based on the more severe design conditions of the two vessels. In another example regarding variation of T and P is a regeneration line which may be subjected to different combination of pressure and temperature. During steaming out the temperature is very high but the pressure is low, while during processing the pressure is high and the temperature is low. Longitudinal Joint Factor [E] As stated earlier the longitudinal joint factor is dependent on piping Code as well as manufacturing process itself. Therefore, there is possibility that for the same pipe manufacturing process, two different factors E will be assigned by the two different Codes. Generally, the values for E are lower for ASME B31.3 “Process Piping” Code than those for Transportation piping Codes. These values are listed in Table A-1B of the Code and attached in Addendum B-2 of this module. Comparing the value of E in this table we could tell that they are generally lower than the

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corresponding values for transportation piping. For ASME B31.3 piping, only seamless pipe has a longitudinal weld factor equivalent to one. Basic Allowable Hoop Stress [S] The basic allowable hoop stress (stress in the circumferential direction) is the allowable stress in tension for the pipe material It is defined by ASME B31.3 and appears in Table A-1 in an appendix-A of B31.3. An excerpt for these tables are shown in Addendum B-1. For plant piping, the allowable hoop stresses is a function of temperature and material, and considers the yield, tensile, and creep strengths of the material at the design temperature. Figure 3 gives graphical presentation of how these allowable stresses are defined and developed.

Figure 3. Variation of the Basic Allowable Stresses with Temperature for Grade B Material.

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Table A-1 is used in the following manner to determine allowable stress for plant piping. •

Pipe material and design temperature must be known.

•

Identify material Spec. No. and Grade in the table.

•

Obtain the allowable stress by looking under the appropriate temperature column at the specified material, and use linear interpolation between temperatures if required.

•

Using a pipe material at temperatures beyond the single solid line is not recommended. Going beyond the double solid line is prohibited.

The "Y" Factor for Plant Piping The Y factor is a correction factor for simplifying the original Lame’ equation. It accounts for geometrical relation between D & t. The "Y" factor is a function of the type of steel and the temperature, and is determined from Table 304.1.1 of ASME/ANSI B31.3, which is shown in Table 3 next. Table 3. The “Y” Factor as Extracted From ASME B1.3 TABLE 304.1.1 o Temperature, F

950

1,000

1,050

1,100

1150 and above

482 and below

510

538

566

593

621 and above

Ferritic Steels

0.4

0.5

0.7

0.7

0.7

0.7

Austenitic Steels

0.4

0.4

0.4

0.4

0.5

0.7

Other Ductile Metals

0.4

0.4

0.4

0.4

0.4

0.4

o Temperature, C

900 and below

General Note: The value of Y may be interpolated between 0

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The following notes should be clarified regarding Y factor in relation to Equation 4: For D/t > 6 , Y can be calculated as: Y= ( d + c ) / ( D + d + 2c ) where: D = outside diameter d = inside diameter c

= mechanical, erosion and corrosion allowance

For a conservative approach and simplicity, Y can be assumed equals to 0.4 or ignored. The Y factor becomes significant for the combination of very high pressure and small diameter. The factor should be used in the case of high temperature and pressure because significant saving in material could be achieved specially for alloy materials.

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CORROSION, EROSION, AND THREAD ALLOWANCES Allowances for corrosion, erosion, or threads must be accounted for in determining the required pipe wall thickness. This is more of a problem in plant piping because high fluid velocities or changes in the pressure of the fluid can corrode a pipe. Thread allowances apply only to smaller diameter pipes, which may be threaded. Corrosion, erosion, and thread allowances are determined in conjunction with the corrosion or process engineer and are often specified in a pipe specification. The appropriate allowance is added to the thickness that was calculated for internal pressure to arrive at a total required pipe wall thickness. The following Sample Problems-2 will be used to illustrate the method of calculating the pipe wall thickness for the process piping. Manufacturer Mill Tolerance The pipe specification for manufacturing pipe such as API 5L and ASTM A 53 or ASTM-A106, allow the manufacturer certain tolerance in the wall thickness positive and negative. There are some legitimate reasons for these tolerances because any manufacturing process always works within some specified range. The maximum manufacturer's under-tolerance for pipe wall thickness is 12.5% for carbon and low-alloy steels. For high-alloy steels it is 10%. Most seamless piping systems will be in the 12.5% category. When pipe is supplied, the actual thickness can be minus 12.5% of the nominal thickness. Therefore, for ASME B31.3 piping, after the required minimum wall thickness is determined, the next greater available standard pipe thickness must be selected, taking into account the manufacturer's under tolerance. In other word, the pipe wall thickness after deducting the negative mill tolerance must be m, equal to or more than t, specified in Equation 7.

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However, this process of correcting the wall thickness for mill tolerance was not addressed for transportation pipelines. One reason is that it is not economical to add for the mill tolerance, because pipelines usually extend for long distances, could be thousands of kilometers. Another reason is that the Code consider the mill tolerance indirectly in the design safety factor. SAES Limitations on Pipe Schedule SAES-L-005 and SAES-L-006 impose additional requirement over the calculated wall thickness. The minimum wall thickness (Schedule) of carbon steel pipe shall be as follows: Table 4. Minimum Wall Thickness Schedule for Carbon Steel Nominal Size

mm

Hydrocarbon Service

Low-Pressure Utility Service

in.

≤2

≤ 50

SCH 80

SCH 40 (see 3.9)

3-6

75 - 150

SCH 40

SCH 40

8 - 32

200 - 800

6.5 mm (0.250 in.)

6.5 mm (0.250 in.)

≥ 34

≥ 850

Diameter /135

Diameter/135

Note: Schedule 160 nipples shall be used for 50 mm (2 in.) and smaller pipe sizes in vibration service where bracing cannot be effectively provided.

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Sample Problem 2 A 16” re-generation line will be installed to connect to the reactor at Ras Tanura Refinery. The following data is provided: Pipe Diameter: 16 inches Pipe specification: ASTM 335, (1/1-4 Cr - 1/2 Mo) Pipe material grade: P11 Pipe manufacturing: Seamless

CASE 1: Design Pressure: 300 psig Temperature: 1000 oF

CASE 2: Design Pressure: 900 psig Temperature: 400 oF Corrosion allowance: 0.0625 inches Perform the following calculations: 1. Find the minimum wall thickness for the internal design pressure 2. The minimum wall thickness by ASME B31.3 code 3. Select the pipe schedule that meets the Code requirements.

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Solution The minimum wall thickness for the internal design pressure: Since this is a process plant piping system, Equation 6 applies as follows: t

= PD / [ 2( SE + PY ) ]

E = 1 (Addendum B-2, for seamless pipe, per B31.3 Appendix A-1B) In order to determine the thickness the calculation must be performed for both cases independently. CASE 1: P = 300 psig T = 1000 oF S = 6300 psi (Addendum B-1, B31.3 Appendix A-1) Y = 0.7 (based on Table-3) t = 300 x 16 / [2 x ( 6300x1 + 300x0.7 ) ] t = 0.369 inches

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CASE 2: P = 900 psig T = 400 oF S = 2000 psi (Addendum B-1, B31.3 Appendix A-1) Y = 0.4 (based on Table-3) t = 900 x 16 / [2 x ( 20000 x 1 + 900 x 0.4) ] t = 0.354 inches Therefore the required t, thickness for internal pressure case is t = 0.369 inches The minimum wall thickness by ASME B31.3 code: In this case, a 15 mm (0.0625 in.) corrosion allowance has been specified. Therefore: tm = t + c = 0.369 + 0.0625 tm = 0.4315 inches The pipe schedule which meets the Code requirements: Checking the pipe data available in Addendum C-3: The next pipe schedule above t 0.500

m

= 0.4315 “, is XS with T =

Tu = T x (1- 12.5/100) = 0.5 x 0.875 = 0.4375 inches Therefore 16” XS pipe meets the Code requirement.

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PIPE WALL THICKNESS FOR EXTERNAL PRESSURE A piping system may be exposed to an external pressure, and external pressure rather than internal pressure may govern the required wall thickness. This might be the case for largediameter/thin-walled process plant piping that is subject to vacuum conditions, or underwater pipelines, which must withstand the hydrostatic head of the water above them. Therefore, calculations must be conducted to ensure that the pipe wall thickness is adequate for a given external pressure. If it is not adequate, the thickness must be increased. Pipe is subject to compressive forces such as those caused by dead weight, wind, earthquake, and vacuum. The process engineer often identifies these forces. For example, a submarine pipeline may be exposed to an external pressure due to the liquid head of surrounding water being greater than the internal pressure. Piping components behave differently under these forces than when they are exposed to internal pressure. This difference in behavior is due to buckling or elastic instability that makes the pipe weaker in compression than in tension. In failure by elastic instability, the pipe may collapse or buckle. This applies particularly to pipe that has a fairly low internal pressure, large diameter, and thin wall. Figure 4 shows a section of pipe that bucked under external pressure.

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Figure 4. Effects of External Pressure The left photo shows the pipe that buckled under external pressure developed between the pipe and the sleeve. The right shows the section after removal from the sleeve.

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GUIDELINES FOR EXTERNAL PRESSURE CALCULATIONS The pressure piping Codes do not outline procedure to perform the calculation for external pressure but refer to the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Paragraph UG-28. That paragraph provides a procedure for evaluating cylindrical shells under external pressure. Pipe geometry factors, (unsupported length, outside diameter, and thickness), material strength, and design temperature are used to determine the thickness required to resist external pressure. 1. Determine the maximum unstiffened length of the pipe, L. 2. Use the value of “t” as determined for internal pressure thickness as a starting point. Calculate L/D and D/t (D is equivalent to DO in the ASME Code procedure). 3. Enter Figure G of part D, subpart 2 of the ASME Code, Section II, with L/D. For L/D greater than 50, use L/D = 50. This figure is excerpted in Figure 5. 4. Move horizontally to the line for D/t. Use linear interpolation for intermediate values of D/t. Move vertically downward to find Factor A. 5. Using the value of A, enter the applicable figure from Section II, Part D of the ASME code based on the pipe material, such as Figure CS-2, excerpted in Figure 8. Move vertically to an intersection with the material/temperature line. Use linear interpolation for intermediate temperatures. If A falls to the right of the material/temperature line, use the horizontal projection of the line. If A falls to the left of the material/temperature line, go to Step 8. Note that the ASME Code contains figures similar to Figure 6 for different material categories, and the correct figure must be used for this evaluation.

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6. From the intersection in Step 5, move horizontally to the right and read the value of Factor B. 7. Calculate the maximum allowable external pressures, Pa, as follows: Equation- 8 Pa = 4B/3(D/t) 8. If factor A falls to the left of the material/temperature line, then: Equation- 9 Pa = 2AE/3(D/t) 9. Note that in this case, “E” is equal to Young’s Modulus of Elasticity at the design temperature, not the longitudinal-joint quality factor. “E” is found from the applicable material figure from Section II of the ASME Code as shown in Figure 6. 10. If Pa is smaller than the external design pressure, select a larger value of t and repeat the design procedure until Pa is equal to or exceeds the external design pressure. As an alternative, external stiffening rings may be added or the distance between them reduced, to reduce the value of L that is used in the calculations. 11. Calculate tm = t + c to obtain the pipe thickness required for external pressure and mechanical, corrosion, and erosion allowances.

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Nomenclature t

= Pressure design thickness for external pressure, in.

L = Unstiffened length of pipe section, taken as largest of (1) distance between flanges or welded stiffening rings or ring girders, (2) distance between the point of tangency on an elbow or cap and a flange or stiffening ring, or (3) the distance between the points of tangency of two elbows or caps if there are no intermediate flanges or stiffening rings, in. D = Actual outside diameter of pipe, in. A = Factor from ASME Code, Section II. B = Factor from ASME Code, Section II, psi. E = Modulus of elasticity of material at design temperature, psi. Pa = Allowable external pressure, psi. tm = Required external pressure thickness, including allowances, in. c

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Figure 5. Geometrical Factor A for External Pressure Calculations

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Figure 5. Geometrical Factor A for External Pressure Calculations (continued)

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Figure 6. Material Factor B for External Pressure Calculations

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Sample Problem 3:

External Pressure for Pipeline Assume the pipeline discussed in Sample Problem 1 also must be capable of withstanding full vacuum [100 kPa (15 psi) external pressure].

Solution T

= 0.374 inches ( use the thinner section)

L / D > 50 because no stiffening is present. D / t = 48 / 0.374 = 128 Based on Figure-5 the A factor is found as: A = 0.00007 Refer to Figure-6 for B factor, use the upper curve: B = 9500 The A factor is on the left side of the curves, therefore Use Equation 9 Pa = 2AE/3(D/t) 6

Pa = 2 x 0.00007 x 29.7x10 / (3 x 128 ) = 10.8 psia The pipe is not adequate for vacuum, therefore alternative shall be evaluated a follows: 1. Increase the pipe wall thickness, which is very costly 2. For underground this may need more analytical study. 3. Impose operational guideline to avoid vacuum.

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Sample Problem 4:

External Pressure for Plant Piping For Sample Problem-1, assume the longest straight pipe section is 80 feet, otherwise there might be flanges, tee or valves acting as stiffener. Verify if the line is capable of withstanding full vacuum [100 kPa (15 psi) external pressure].

Solution t

= 0.449 inches

L = 80 x 12 = 960 inches L/D

= 960 / 16 = 60

D/t

= 16 / 0.4315 = 37

Based on Figure-5 the A factor is found as: A = 0.001 Refer to Figure-6 for B factor, use the upper chart. T = 900 F: B = 9500 Therefore, use Equation- 8: Pa = 4B/3(D/t) Pa = 4 x 9500 / (3 x 37 ) = 342 psia Therefore, the line is adequate for vacuum condition.

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TRAFFIC AND SOIL LOADS OVER BURIED PIPE Transportation pipelines often have buried sections of pipe. The required thickness of these buried sections will be affected by soil and traffic loads, in addition to the design pressure. These loads cause a circumferential bending stress in the pipe. The Saudi Aramco engineer needs to determine if the pipe is thick enough for these soil and traffic loads. Specific requirements for how traffic loads are determined are found in SAES-L-046, Pipeline Crossings Under Roads and Railroads. The pipe must be designed for the traffic load, soil weight, and passive soil reaction. At railroad and highway crossings where the loads may apply, the pipe must be designed according to API Recommended Practice 1102, Liquid Petroleum Pipelines Crossing Railroads and Highways. It provides the formula for determining circumferential stress in a carrier pipe with internal pressure due to external loads at highway and railroad crossings. The equation gives a stress that is based upon the thickness, internal pressure soil and traffic loads as follows: The stress calculated in accordance with this equation is limited to the Specified Minimum Yield Stress times the design factor, F, without considering the longitudinal joint factor. It should also be noted that SAES-L-046 contains criteria for when a protective casing is required, and how the casing should be designed. Saudi Aramco has a computer program that makes the calculation. This can be done through the Consulting Services Department (CSD.) All the load factors required by SAES-L-046 are in the computer program, as well as the required parameters. It is beyond the scope of this course to determine the stress.

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THE MAXIMUM ALLOWABLE OPERATING PRESSURE (MAOP) There are many cases where the purchased pipe wall is thicker than what is required for the design pressure. This would allow the designer to increase the actual design pressure of the pipe as long as it is not limited by other factors, such as flange rating as would be discussed in MEX-101.04. For transportation piping system, this maximum permissible pressure is called maximum allowable operating pressure (MAOP). For process plant piping this is called design pressure. The engineer must determine MAOP for pipe as well as other piping components. This module discusses MAOP for pipe. The MAOP of a pipe or other piping component will be at least equal to the design pressure. However, the MAOP can be higher than the design pressure since use of a standard wall thickness will typically provide an additional margin. Guidelines for Calculating Maximum Design Pressure 1. Subtract mill tolerance, (expressed as a decimal fraction), m, from the nominal pipe wall thickness, Tn, for ASME B31.3 piping to determine the minimum possible as - supplied thickness, T , as follows: T = ( 1 – m ) x Tn Where: Tn is the nominal pipe wall and m is the mill tolerance; “m” shall be taken as zero for ASME B31.4 and B31.8 piping systems.

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2. Subtract any other allowances, such as corrosion allowance, c, to calculate the minimum possible pipe thickness, t, as follows: t =T–c Usually c is zero for transportation piping. 3. Reverse the applicable internal pressure equation to calculate a value for maximum design pressure For ASME/ANSI B31.3, Process Piping, use the following equation to calculate Maximum Design Pressure:

MDP =

2tSE D −2tY

For ASME/ANSI B31.4 or B31.8, Transportation Piping, use the following equation to calculate MAOP:

2St MAOP= FET D 4. Calculate maximum design pressure with the factors identified earlier. 5. For ASME/ANSI B31.3, Process Piping, the following equation should be used to calculate maximum design pressure: Maximum Design Pressure = 2x t x SE / ( D – 2xtxY) 6. For ASME/ANSI B31.4 or B31.8, Transportation Piping, use the following equation to calculate MAOP: 2St MAOP = FET D

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MAOP of a Pipeline For transportation piping, the MAOP is calculated by Equation 3. The pipe wall thickness to be used in the equation is the nominal wall thickness. There should be no deduction for mill tolerance. This is illustrated in the next sample problem. Sample Problem 5.

MAOP of a Pipeline A 36 inch pipeline carrying crude oil passing through an area classified as Class 2. The required design pressure is 500 psig, and design temperature is 200 F. A double submerge arc welded (DSAW) API 5L grade X52 pipe was specified. A 36 inch pipe with 0.375 inch wall is available. Verify if this pipe is adequate for the design pressure by calculating the MAOP of this pipe.

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Solution Since this a pipeline, it would be designed to ASME B31.4/ B31.8, Equation-3 will be used to find The MAOP: P = [2 S t / D] x FET P = MAOP psig. D = 36 in. S = 52000 psi. E = 1 T = 1 F = 0.72 MAOP = 2 x 52000 x 0.375 (0.6x1x1) / 36 MAOP = 650 psig The MAOP is higher than the design pressure of 500 psig, therefore the line pipe is acceptable for the design condition.

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Maximum Design Pressure for Process Plant Piping From a terminology point of view, the word MAOP is not used by the Process Piping Code ASME B31.3. Paragraph 302.2.4 gives the rules on how to exceed the design pressure. The design pressure can be exceeded as follows:

by 33% for continuous 10 hours but not more than 100 hours per year

or by 20% for continuous 50 hours but not more than 500 hours in a year.

However, similar to what has been made for the transportation piping, a pipe could be purchased thicker to what is deeded for the actual design condition. The back calculation of the maximum design pressure (not the MAOP) will be illustrated in the following steps. This maximum design pressure can be used for the pressure exceeding criteria as set by the ASME B31.3.

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Sample Problem 6:

Process Piping Using the results of Sample Problems 2, determine the maximum design pressure that can be sustained by the pipe and maximum permissible pressure. T = (1-m) Tu = (1 - 0.125) (0.50) = 0.6563 in. t n = Tu - c t n = 0.0.4375 - 0.0625 = 0.375 in. MDP = 2 t n S E / ( D – 2 t n Y ) Apply Case-1 ( T = 1000 o F) MDP = 2 x 0.375 x 6300 x 1/ ( 16 – 2 x 0.375 x0.7 ) MDP = 305 psig 33% over MDp = 405 psig ( for 10 hours) 20% over MDp = 366 psig ( for 50 hours) Apply Case-2 ( T = 400) MDP = 2 x 0.375 x 20000 x 1/ ( 16 – 2 x 0.375 x0.4 ) MDP = 955 psig 33% over MDp = 1270 psig ( for 10 hours) 20% over MDp = 1146 psig ( for 50 hours)

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TYPICAL MISTAKES IN PIPE WALL THICKNESS CALCULATION Even though the calculation for wall thickness is relatively simple and straight forward, people do make mistakes that most of the time cost the company more capital investment than necessary. These mistakes are summarized as follows: 1. Specifying a design pressure that is higher than what is required to fulfill the operational requirement. 2. Selecting the wall thickness to match the flange rating and thickness. This might be acceptable for plant piping but for pipelines it could add additional cost. 3. Setting the design pressure based on a pre-selected or existing flange rating more what is actually required for operation. 4. Applying the wrong Code. 5. Specifying the wrong allowable stresses in the formula 6. Inaccurate interpretation of the SAES-L-006 regarding the minimum wall thickness requirement. Some of these requirements are intended for inventory purposes, therefore for line pipe purchased to be installed for a specific project the criteria should not be applied if there is great economic incentives. Following the standards without understanding the intent could lead to purchasing of line pipe thicker than what is required which means additional material and construction cost. 7. Surprisingly, pure mathematical mistakes are frequently made in the calculations reflected in possibly unsafe designs or costly material.

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SUMMARY MEX 101.03 discussed the process of determining pipe wall thickness, one of the first steps in specifying the design of piping system components. Pipe wall thickness is based on the internal pressure of a pipe, and if necessary, on any additional external pressure or loads. The pipe wall thickness is determined by using the equation for internal pressure thickness in the applicable code, adjusting the thickness as necessary for any applicable external pressure or additional loads, adding the corrosion or other allowances, accounting for the manufacturer's mill tolerance, and selecting pipe schedule. After selecting the pipe schedule, the Maximum Allowable Operating Pressure for the pipe may be calculated.

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ADDENDUM Index of Addendum

Page(s)

ADDENDUM A .............................................................................................................. 56 ADDENDUM B .............................................................................................................. 62 ADDENDUM C.............................................................................................................. 69

LIST OF TABLES Table A-1. ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) ................................................................................................. 56 Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E ..................................................................................................... 59 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe.................................................................................. 60 Table A-4. SAES B-064 Rupture Exposure Radius..................................................... 61 Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals................................................................................................... 62 Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E ............................................................................................ 67

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ADDENDUM A Table A-1: ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) SPEC. NO.

GRADE

TYPE (NOTE 1)

SMYS, PSI

API 5L (Note 2)

A25

BW, ERW, S

25,000

API 5L (Note 2)

A

ERW, S, DSA

30,000

API 5L (Note 2)

B

ERW, S, DSA

35,000

API 5L (Note 2)

X42

ERW, S, DSA

42,000

API 5L (Note 2)

X46

ERW, S, DSA

46,000

API 5L (Note 2)

X52

ERW, S, DSA

52,000

API 5L (Note 2)

X56

ERW, S, DSA

56,000

API 5L (Note 2)

X60

ERW, S, DSA

60,000

API 5L (Note 2)

X65

ERW, S, DSA

65,000

API 5L (Note 2)

X70

ERW, S, DSA

70,000

API 5L (Note 2)

X80

ERW, S, DSA

80,000

ASTM A 53

TYPE F

BW

25,000

ASTM A 53

A

ERW, S

30,000

ASTM A 53

B

ERW, S

35,000

ASTM A 106

A

S

30,000

ASTM A 106

B

S

35,000

ASTM A 106

C

S

40,000

EFW

(NOTE 3)

ERW

30,000

ASTM A 134 ASTM A 135

A

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Table A-1. ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe), Continued ASTM A 135

B

ERW

35,000

ASTM A 139

A

EFW

30,000

ASTM A 139

B

EFW

35,000

ASTM A 139

C

EFW

42,000

ASTM A 139

D

EFW

46,000

ASTM A 139

E

EFW

52,000

ASTM A 333

1

S, ERW

30,000

ASTM A 333

3

S, ERW

35,000

ASTM A 333

4

S

35,000

ASTM A 333

6

S, ERW

35,000

ASTM A 333

7

S, ERW

35,000

ASTM A 333

8

S, ERW

75,000

ASTM A 333

9

S, ERW

46,000

ASTM A 381

CLASS Y-35

DSA

35,000

ASTM A 381

CLASS Y-42

DSA

42,000

ASTM A 381

CLASS Y-46

DSA

46,000

ASTM A 381

CLASS Y-48

DSA

48,000

ASTM A 381

CLASS Y-50

DSA

50,000

ASTM A 381

CLASS Y-52

DSA

52,000

ASTM A 381

CLASS Y-56

DSA

56,000

ASTM A 381

CLASS Y-60

DSA

60,000

ASTM A 381

CLASS Y-65

DSA

65,000

GENERAL NOTE: This Table is not complete. For the minimum specified yield strength of other grades and grades in other approved specifications, refer to the particular specification. NOTES:

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(1) Abbreviations: BW - furnace butt-welded; ERW - electric-resistance welded; S seamless, FW - flash-welded; EFW - electric-fusion welded; DSA - doublesubmerged arc welded. (2) Intermediate grades are available in API 5L. (3) See applicable plate specification for SMYS.

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Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E. Spec. Number

Pipe Class

E Factor

ASTM A53

Seamless

1.00

Electric-Resistance Welded

1.00 0.60

Furnace Welded ASTM A106

Seamless

1.00

ASTM A134

Electric-Fusion Arc Welded

0.80

ASTM A135

Electric-Resistance Welded

1.00

ASTM A139

Electric-Fusion Welded

0.80

ASTM A211

Spiral-Welded Steel Pipe

0.80

ASTM A381

Double-Submerged Arc Welded

1.00

ASTM A671

Electric-Fusion Welded

1.00*

ASTM A672

Electric-Fusion Welded

1.00*

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Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E (Continued) Spec. Number

Pipe Class

E Factor

API 5L

Seamless

1.00

Electric-Resistance Welded

1.00

Electric-Flash Welded Submerged Arc Welded

1.00 1.00 0.60

Furnace Butt-Welded *1.00 for classes 12,22,32,42,52 0.80 for classes 13,23,43,53 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe TEMPERATURE o C

TEMPERATURE DERATING FACTOR, T o F

120 OR LESS

250 OR LESS

1.000

150

300

0.967

177

350

0.933

204

400

0.900

232

450

0.867

Note: For intermediate temperatures, interpolate for derating factor. Source: ASME/ANSI B31.8 - 1992. With permission from the American Society of Mechanical

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Table A-4. SAES B-064 Rupture Exposure Radius Service

Diameter

TVP

H2S

RER

inches

psig

% Mole

meters

Comments

Crude

< 24

< 14.5

< 1..5

200

RER study by LPD not required

Crude

> 24

< 14.5

< 1.5

400

RER study by LPD not required

Gas/ liquid hydrocarbon

< 24

> 14.5

< 1.5

1000

RER study can be done by LPD

Gas/ liquid hydrocarbon

< 24

> 14.5

> 1.5

3000

RER study can be done by LPD

Gas/ liquid hydrocarbon

> 24

> 14.5

< 1.5

2000

RER study can be done by LPD

Gas/ liquid hydrocarbon

> 24

> 14.5

> 1.5

5000

RER study can be done by LPD

General Notes: 1. LPD: Loss Prevention Department, which is responsible for SAES-B-064. 2. RER study can be done for services other than the first two. The RER shall not be less than the value specified in the first two items for the respective diameter. 3. Regardless of calculations, RER shall not be greater than the values specified in the last for items for the respective pipeline diameter. 4. TRV: is the true vapor pressure for flammable liquid lines

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ADDENDUM B Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 Table A-1 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued) 51. Special P-1, Sp-2, SP-3, SP-4, and SP-5 of carbon steels are not included in P-No 1 because of possible high-carbon, high-manganese combinations, or micro-alloying, which would require special consideration in qualification. Qualification of any high-carbon, high-manganese grade may be extended to other grades in its group. 52. Copper-silicon alloys are not always suitable when exposed to certain media and high temperature, particularly above 212°F. The user should satisfy himself that the alloy selected is satisfactory. 53. Stress relief treatment is required for service above 450°F. 54. The maximum operating temperature is arbitrarily set at 500°F because hard temper adversely affects design stress in the creep rupture ranges. 55. Pipe produced to this specification is not intended for hightemperature service. The stress values apply to either nonexpanded or cold-expanded material in the as-rolled, normalized, or normalized temperature conditions. 56. Because of thermal instability, this material is not recommended for service above 800°F. 57. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 800°F. 58. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 875°F. 59. For temperature above 900°F, consider the advantages of killed steel. Source: ASME/ANSI B31.3 - 1990. With permission from the American Society of Mechanical Engineers.

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Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E

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Table B-2. (TABLE A-1B) Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E (continued)

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ADDENDUM C Engineering and Design Data Table of Properties of Pipe References for Pipe Data Standard pipe wall thickness are specified in the following standards: •

ASME/ANSI B36.10, Welded and Seamless Wrought Steel Pipe (for carbon and low-alloy steel pipe).

•

ASME/ANSI B36.19, Stainless Steel Pipe.

•

API/5L, Specification for Line Pipe (only for carbon steel pipe that meets this specification).

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Table C-1. Table of Properties of Pipe

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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PIPE WALL THICKNESS CALCULATION

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-101.03

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

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Section

Page

INFORMATION ............................................................................................................... 5 INTRODUCTION............................................................................................................. 5 FACTORS AFFECTING WALL THICKNESS CALCULATION........................................ 6 BACKGROUND ON PIPEWALL THICKNESS................................................................ 7 STEPS FOR CALCULATING PIPE WALL THICKNESS................................................. 9 PIPE WALL THICKNESS FOR THE INTERNAL DESIGN PRESSURE ....................... 11 Transportation Piping: ASME B31.4 and B31.8 (Thickness for Internal Pressure) .. 11 Design Pressure [P]............................................................................................ 12 Pipe Diameter [D] ............................................................................................... 14 The allowable stress [SETF]............................................................................... 14 Longitudinal Joint Factor [E] ............................................................................... 16 Temperature Derating Factor [T] ........................................................................ 16 Design Factors [F] .............................................................................................. 17 RER & PDI.......................................................................................................... 18 Sample Problem 1: Transportation Piping .............................................................. 21 Solution .............................................................................................................. 22 PROCESS PIPING: ASME B31.3, MINIMUM THICKNESS FOR INTERNAL PRESSURE .................................................................................................................. 25 Design Pressure and Temperature .......................................................................... 26 Longitudinal Joint Factor [E] ............................................................................... 27 Basic Allowable Hoop Stress [S] ........................................................................ 28 The "Y" Factor for Plant Piping ................................................................................ 29 CORROSION, EROSION, AND THREAD ALLOWANCES........................................... 31

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Manufacturer Mill Tolerance .................................................................................... 31 SAES Limitations on Pipe Schedule ........................................................................ 32 Sample Problem 2 ................................................................................................... 33 Solution .............................................................................................................. 34 PIPE WALL THICKNESS FOR EXTERNAL PRESSURE............................................. 36 GUIDELINES FOR EXTERNAL PRESSURE CALCULATIONS ................................... 38 Sample Problem 3: External Pressure for Pipeline ................................................. 44 Solution .............................................................................................................. 44 Sample Problem 4: External Pressure for Plant Piping ........................................... 45 Solution .............................................................................................................. 45 TRAFFIC AND SOIL LOADS OVER BURIED PIPE ..................................................... 46 THE MAXIMUM ALLOWABLE OPERATING PRESSURE (MAOP).............................. 47 Guidelines for Calculating Maximum Design Pressure....................................... 47 MAOP of a Pipeline.................................................................................................. 49 Sample Problem 5. MAOP of a Pipeline ................................................................ 49 Solution .............................................................................................................. 50 Maximum Design Pressure for Process Plant Piping............................................... 51 Sample Problem 6: Process Piping......................................................................... 52 TYPICAL MISTAKES IN PIPE WALL THICKNESS CALCULATION ............................ 53 SUMMARY.................................................................................................................... 54 ADDENDUM ................................................................................................................. 55 ADDENDUM A .............................................................................................................. 56 ADDENDUM B .............................................................................................................. 62 ADDENDUM C.............................................................................................................. 69

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LIST OF FIGURES Figure 1. Stresses in the Pipe Shell due to Internal Pressure ....................................... 7 Figure 2. Schematic Diagram Explaining the Concept of RER and PDI ...................... 19 Figure 3. Variation of the Basic Allowable Stresses with Temperature for Grade B Material. ......................................................................................................... 28 Figure 4. Effects of External Pressure ........................................................................ 37 Figure 5. Geometrical Factor A for External Pressure Calculations ............................. 41 Figure 5. Geometrical Factor A for External Pressure Calculations (continued) ......... 42 Figure 6. Material Factor B for External Pressure Calculations ................................... 43

LIST OF TABLES Table 1. Level of Safety for Different Pipe Material Grade ......................................... 15 Table 2. Location Class and Design Factors for Transportation Piping ....................... 20 Table 3. The “Y” Factor as Extracted From ASME B1.3 TABLE 304.1.1 .................... 29 Table 4. Minimum Wall Thickness Schedule for Carbon Steel .................................... 32 Table A-1: ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) ............................................................................................... 56 Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E. ............................................................................................................... 59 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe ..................................................................................... 60 Table A-4. SAES B-064 Rupture Exposure Radius ..................................................... 61

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals ............................................................................................................. 62 Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E ............................................................................................ 67 Table C-1. Table of Properties of Pipe ......................................................................... 70

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INFORMATION INTRODUCTION This module discusses the process of determining pipe wall thickness, which is one of the first steps in specifying the design of piping system components. Pipe wall thickness is based on the internal pressure of a pipe and, if necessary, external pressure or any additional loads. The pipe wall thickness is calculated by using the equation for internal pressure thickness in the applicable ASME B31 Code, modifying the thickness for any external pressure or additional loads, selecting pipe schedule based on manufacturer's tolerance. Also, calculating the Maximum Allowable Operating Pressure (MAOP) for the pipe after determining the pipe wall thickness will be covered. The previous module discussed an early step in designing a piping system: selecting pipe material. Selecting pipe material sets parameters for the other facets of piping design and is required to determine allowable design stress, which is necessary to calculate the required wall thickness. Knowledge of the following is necessary to achieve the objectives of MEX 101.03: •

Scope and application of the ASME/ANSI B31 piping codes, as discussed in MEX 101.01.

•

Pipe material selection, as discussed in MEX 101.02.

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FACTORS AFFECTING WALL THICKNESS CALCULATION In the process of calculating the wall thickness of a pipe several parameters must be considered and accurately specified. These parameters are listed below. Pipe material: The pipe material will be specified as has been discussed in the previous chapter. The material type identifies basic design parameter, which is the allowable stress to be explained in the coming sections. Internal pressure: This the crucial parameter in piping design because the main purpose pressure piping Codes and Standards is to contain the pressurized fluid from escaping out or rupturing the pipe. Fluid temperature: The fluid temperature affects the allowable stresses of the material and the designated Code because each Code has temperature limitation. External loads: These loads could be caused by external pressure due to vacuum or water static head for submarine lines. Also, dead weight of soil, snow or sand over burden load on the pipe. Construction: Construction of pipelines whether on land or off shore could add additional construction requirement that may increase the wall thickness of the pipe. In fact for off shore pipeline, stresses on the pipe exerted on the pipe during lay-off from the barges are the detrimental factor of the pipe wall thickness. Another example is the elastic bend requirement to follow the pipeline terrain for on land pipelines. The industry Codes provide rules and guidelines for establishing these parameters, however Saudi Aramco have established its own rules that generally follow the industry Codes but with more stringent requirement.

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BACKGROUND ON PIPEWALL THICKNESS The main objective of piping codes is to insure that piping systems will not under pressure. Piping systems are designed and constructed to convey pressurized fluid, mostly flammable and could be toxic, therefore Codes tend to very conservative in respect to pressure containment of the pressurized fluids. This explains why there are few failures reported due to overpressure of the piping systems. The basic theory for designing of wall thickness of a piping system that contains the internal pressure is based on limiting the hope stress developed by this pressure to an acceptable value by the Code. The calculated hope stress in pipe shell, refer to Figure-1, is based on Lame’ equation as follows:

Figure 1. Stresses in the Pipe Shell due to Internal Pressure Equation 1.

Sh = P [0.5x(D/t) 2 –(D/t) + 1] / [(D/t)-1]

Boardman's empirical equation has further simplified Lame’ equation based on an intermediate diameter and introduced a Y factor to correct the difference. Boardman's equation has become the basis for calculating wall thickness B31 Codes for pressure containing piping. Boardman's equation is as follows:

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Equation 2.

Sh = P (D – Y t ) / 2 t

For thin pipe where D/t > 6, and outside diameter D, Y is considered 0.4. For thicker pipe Y can be calculated as will be discussed later. Each code provides an equation that is used to calculate internal pressure thickness. The equations may look different and / or the approach varies, but the basic concept is the same. The main concept is to limit the hoop stress, Sh, to an acceptable and safe level.

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STEPS FOR CALCULATING PIPE WALL THICKNESS Each engineer should be familiar with the steps followed to determine the wall thickness of the pipe. He should be aware that the thickness shall not be less than the minimum required by the Code otherwise this would be a basic violation to the relevant Code. At the same time it should not be over specified because that would be reflected as unnecessary capital cost. Over specifying wall thickness mostly will have significant impact on capital investment in terms of millions of dollars. The cost of line pipe is determined mainly by the tonnage, i.e. the pipe diameter, wall thickness, length of order, as well as the pipe grade and the type of alloying elements. The steps to be followed during the process of wall thickness calculation are as follow: 1. Determining the applicable ASME/ANSI B31 Code for the piping system of concern. This has been discussed in MEX 101.01. The latest SAES should be always referenced, and if there are potential savings by using different Code other than specified, the issue should be highlighted to the concerned specialists in CSD. 2. Identifying the applicable formula from the code for calculating the wall thickness to sustain the internal design pressure. 3. Setting the design parameters as specified in the design data and in compliance with SAES-L-002 and SAES-L-003, Design Stress Criteria for Pressure Piping. 4. Evaluating whether the minimum thickness is adequate for structural rigidity (excessive deflection or buckling under the pipe own weight.

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5. Increasing the calculated thickness, as needed, to account for corrosion allowance and mill tolerance. This is dependent on the Code and the SAES. 6. Checking if the calculated thickness for internal pressure is also acceptable for external pressure and other applied loads, as applicable such as traffic load, sand dunes dead weight, etc. 7. Selecting a thickness from an ANSI/API table of standard pipe thickness and checking the thickness against the Saudi Aramco minimum thickness requirements. Selecting a scheduled pipe is not always applicable for pipeline projects involved with long distances and large diameter pipe. The text of MEX 101.03 refers to ASME/ANSI B31.3 for plant piping and B31.8, for transportation piping. The process discussed in this module is consistent for all the B31 piping codes. However, the equations, variables, and definitions or values for allowable stress differ.

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PIPE WALL THICKNESS FOR THE INTERNAL DESIGN PRESSURE Calculating the required pipe wall thickness to contain internal pressure is the first step in determining pipe wall thickness. As explained earlier the applicable Code for a particular piping system will determine the method and rules governing the pipe wall thickness calculations. Identifying the Code has been already discussed in MEX-101.01. Therefore, in the following two sections the calculation procedure and requirement for the internal pressure design will be discussed. One section will focus on the transportation piping and the other one will explain internal pressure design for plant piping. Transportation Piping: ASME B31.4 and B31.8 (Thickness for Internal Pressure) This section outlines the method for calculating the pipe wall thickness for piping systems that are designated as transportation piping, ASME/ANSI B31.8, paragraph 841.11 states the rule for calculating the design pressure as follows: Equation 3.

P = [2 S t / D] x FET

Even though this equation is given in the AMSE B31.8 Code for gas transportation system, Saudi Aramco Standard calls for applying it for all other transportation piping systems regardless of the nature of the service. The differences will be in the design factors and de-rating factors as discussed bellow. This equation could be re-arranged to calculate the wall thickness required for internal pressure containment for gas transmission and distribution piping (as well as other transportation piping) as follows: Equation 4

t = P D / [ 2 SETF]

Where: t

= Internal pressure wall thickness, in.

P = Design pressure, psig. D = Outside diameter of pipe, in.

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S = Specified Minimum Yield Strength (SMYS), psi. E = Longitudinal-joint quality factor. T = Temperature derating factor. F = Design factor. Each of the above parameters will be discussed thoroughly next. Design Pressure [P] In order to understand the significance of the pressure term in the Equation - 4 the following topics will be discussed:

Pipeline optimization study

Pump & compressor shutoff

Static head and pressure drop

Pressure surge

Pipeline Optimization Study: Closer evaluation on Equation – 4, shows us that the wall thickness is directly proportional to the pressure, i.e. as the pressure becomes higher the thicker the pipe becomes. On the other hand, as the pipe diameter increases the pressure required to achieve certain flow drops down. Consequently, maybe lower wall thickness is needed. However, both the pipe wall and the diameter contribute to the pipe cost. On the other hand lowering the discharge pressure most probably will decrease the overall operating cost.

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Therefore, the designer should go through a very detailed exercise trying to optimize between the design pressure, pipe diameter and pipe wall thickness in one side and the operating cost in the other hand. These kind of studies are called “Pipeline Optimization Study”. Details of this studies are beyond the scope of this course. Pumps & compressors shutoff: The design pressure is dictated by the hydraulic requirement to achieve certain flow rate through the pipeline. Also, the source of the pressure upstream, such as the oil reservoir shut-in pressure, dictates the design pressure. For a pipeline system connected to a pump or a compressor, the design pressure will be affected by the pump or compressor shut-off pressure in case the flow downstream these rotating machinery was blocked and the pump or compressor continue to operate. The pipeline will be subjected to the highest pressure that the machine can produce. Static head and pressure drop: Also, the hydrostatic head in a liquid-filled piping system could also could be a detrimental factor in cases where there is a large difference in elevation along the pipeline route. In all cases the static head must be considered when it is positive and adding to the internal of the fluid. Our Standards do not allow taking advantage of or pressure reduction due to pressure drop or due to negative static head. Pressure surge: The pressure surge condition in liquid transportation pipeline must be considered, because sudden change in the flow velocity caused by closure of a down stream isolation or sudden stop of a downstream pump will create a pressure surge in the line. This pressure surge will be transmitted backward at the speed of sound in the transported fluid and will create radial pressure in the line. The development of pressure surges must be accounted in the design of a pipeline. There are several approaches to reduce the effect of pressure surges, for example increasing the closure time of the valve that is creating the surge, slowing down the rate at which the pump is stopped, or installing pressure surge relief systems.

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However, assuming that pressure surges are not avoidable, or it is not practical to install surge relief systems, then the wall thickness of the line must be designed to withstand these pressure surges. The ASME B31.4, paragraph 402.2.4 states that the MATP, maximum allowable transient pressure due to the pressure surges, shall not exceed 110% of the design pressure (MAOP) in the line. Saudi Aramco SAES requires that a formal surge analysis shall be made for liquid-packed services. In case that it is not economical to increase the pipe thickness and pressure surge could not be eliminated, then, surge protection systems shall be installed if surge pressures are calculated to exceed 110% of the MAOP. The Surge protection systems shall be of fail-safe design with an installed, spare, surge-relief valve for each surge protection system. Pipe Diameter [D] The design Equation-4 is based on the actual outside diameter of the pipe. The engineer must be aware that the designated outside diameter in the pipe data in API-5L or ASTM A53 or other source of information provides NPS, nominal pipe sizes. The nominal pipe size is less than the actual outside diameter for NPS 12 inches and lowers and it is equal to the actual outside diameter for 14 inches and above. Therefore, careful review should be exercised during the calculation otherwise fatal mistakes could happen. The table showing pipe dimensions and other data are provided in Addendum A of this module. The allowable stress [SETF] The combined term of SEFT is actually gives the limiting allowable hope stress for produced by internal pressure. Accordingly, this term will determine the minimum pipe wall thickness required for holding the internal pressure. A key parameter in this combined term is S. The parameter SA is the Specified Minimum Yield Strength (SMYS) of the material as explained previously in MEX-101-02. The ASME codes B31.4 & Saudi Aramco DeskTop Standards

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B31.8 provide the acceptable materials together with their SMYS. These tables are shown in Appendix A-1 of this chapter. The engineer should be aware that the allowable stress for transportation piping is based on material yield point. The main reason for that is economical, because pipelines usually extend for long distances therefore the intent of the Code is to utilize the material to its limits of yielding. This approach is simple which has some reasoning behind it because pipeline construction and configuration are also relatively simple and not complicated. However, grater caution should be exercised when dealing with higher-grade materials. To make this point clear, we must understand that the pipe bursting pressure is governed by the ultimate tensile strength of the material rather than the yield strength. Equation 5. Equation 1 for the design pressure can be re-arranged as follows: Pd

=

SMYS x [t ET/ D] x F

Pd

=

SMYS x [t / D] x F; Assuming E & T equal to 1

It is known fact and tested that the rupture pressure depend on the tensile strength of the material and defined as follows: Pb

=

U t x [t D]; Assume A= t D

P

=

S x A; (A is a factor combining all other parameters)

Table 1. Level of Safety for Different Pipe Material Grade Material Grade

SMYS psig

Ultimate Tensile U t psig

Design Pressure Pd

Bursting Pressure Pb

Pb / Pd

B

35000

60000

35000xAF

60000xA

1.71 / F

X52

52000

66000

52000xAF

66000xA

1.27 / F

X60

60000

75000

60000xAF

75000xA

1.25 / F

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X70

70000

82000

70000xAF

82000xA

1.17 / F

The above table provides parametric calculations for the design and burst pressure. The last column which gives the ratio between the burst pressure to the design pressure shows that as the pipe material become higher, the ration becomes less. This implies that the actual safety factor is jeopardized as materials with higher grade are used. Longitudinal Joint Factor [E] The longitudinal joint factor is safety factor that represents the quality of the pipe seam weld. As explained earlier in MEX101.2, this factor is dependent on both the manufacturing process and the intended Code. Seamless pipe has a longitudinal joint factor of on1 because in seamless pipes there is no seam weld. All other welded pipe must have an E factor ranging from 1 to 0.6. This factor will reduce the allowable stress; consequently the wall thickness will increase. Appendix A-2 of this chapter is an extract from AMSE B31.8, Table 841.115A. The engineer should always refer to the latest edition of the Code because this factor could be revised to be higher due to improvement in the manufacturing process and inspection techniques. At the same time it could be lowered due to lessons learned or a more conservative approach by the Code Committee. Temperature Derating Factor [T]

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In general, the design temperature of transportation piping is constant and relatively low. Also, we should remember that both ASME B31.4 & B31.8, as mentioned in MEX-101.01, are limited in the design temperature to 250o F and 450o F respectively. The temperature derating factor accounts for the fact that the yield strength of materials is reduced as the metal temperature increases. ASME B31.4 has no temperature derating factor because it is limited to 250o F while ASME B31.8 provide Table 841.116A for these values. This table is in Appendix A-3. The design temperature is mostly dictated by the upstream condition of the fluid transported in the pipeline. However, there are cases when the temperature will increase as the fluid progress due to turbulence in the flow or picking heat from sun radiation. These effects should be carefully studied because they could have adverse impact either on the safety of the piping system if ignored. On the other hand, they have significant cost impact if very conservative approach is utilized. For example, a conservative approach may lead to assigning a temperature derating factor which will be translated into heavier pipe wall and more cost to be spent. Also, it may lead to installing coolers downstream the pipeline that will never be operated but have added cost to the initial investment and additional cost for operation and maintenance. Design Factors [F] The design factor [F], sometimes called DF, is a safety factor that accounts for the relative hazard created by the presence of the pipeline to the surrounding population, environment, and facilities. This design factor is affected by many factor some of which are: 1. The nature of the fluid transported, service, whether it is farmable, toxic, explosive or pollutant. 2. The density of the population in the area where this line is passing through. The higher the population density the more safety factor should be imposed.

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3. Government laws and regulations. 4. Level of conservatism of the Committee members developing the Standards ASME B31.4 has identified one single design factor equals to 0.72, while ASME B 31.8 goes further step and assign a different design factor for a different area classification. Saudi Aramco is further step ahead in this issue, where the concept of location classification has been applied to liquid hydrocarbon pipelines as well as gas pipelines. To determine the design factor, F, both Standards SAES-B-064 and SAES-L-003 shall be used identify the piping system location class. RER & PDI In the case of pipelines, to determine the design another factor another two concepts need to be explained. Those are the Rupture Exposure Radius (RER) and the Population Density Index (PDI). As shown in Figure-2, RER is a measure of the extent of the risk that a pipeline could make to the surroundings when it ruptures. The second, PDI, is a measure of the vulnerability of people to that risk. The rules and guidelines for calculating RER and PDI are given is AMSE B31.8 paragraph 840.2 and further superseded by SAES-B-064. The, SAES-B064 specifies the values for RER based on pipeline service, True Vapor Pressure, H2S concentration and the line size. The values of RER based on SAES-B-064, issue 1997, are summarized in a table in Addendum A-4. PDI is measured by the number of existing buildings intended for human occupancy and the land area planned for future development, all falling within the RER as shown in Figure-2. It is worth mentioning that temporary facilities which will be in place for less than six consecutive months are not to be included in these calculations The method of calculation is detailed in the next Sample Problem 1.

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Figure 2. Schematic Diagram Explaining the Concept of RER and PDI The values of the design factor, F, corresponding to the location class and the PDI are provided in Table-2. Most probably, a single transportation pipeline may have multiple location classifications associated with it, based on the PDA results along its length. It is very essential that engineers must understand the intent of these factors rather than using them as they are. These factors if not well understood and regularly reevaluated, this may lead to situation where some of the pipelines may become safety hazard to people and environment. On the other hand, these factors could substantially increase the pipeline construction cost with no logical reason.

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Table 2. Location Class and Design Factors for Transportation Piping Location Class

Design Factor F

Population Density Index PDI

1

0.72

10

Commentary and Examples

Desert area non developed areas Water service lines.

2

0.60

11-29

3

0.50

30 and above

4

0.40

Special Cases

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Hydrocarbon service, in populated areas or parallel to highways Plant piping designed to B31.4 / B31.8 Highly populated complexes such as hospitals and malls.

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Sample Problem 1: Transportation Piping A 48-inch cross-country pipeline conveying sour gas from Uthmaniyah to Shedgum. Refer to Fihgure-2 for clarification. Most of the pipeline passes through a desert except for 2-Km section that traverses along the border of an industrial park. The industrial park is 400 meters away from the pipeline. Also, within this industrial park, there is an area planned for future development. Identify the design factor for this pipeline and calculate the minimum wall thickness required for internal pressure. The following data is given. Pipe Diameter: 48 inches Design Pressure: 740 psig Design temperature: 175oF Pipe specification: API 5L carbon steel, Material grade: X65 Pipe manufacturing: Double Submerge Arc Welded (DSAW) Two-story building is: 11 buildings Five-story building is: 4 buildings Future development: 1200 m2

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Solution Since this is a transportation piping system, Equation 2 applies as follow: t

= P D / [ 2 SETF]

P = 740 psig D = 48 inches S = 65000 psi (Addendum A-1 for API 5L X-65) E = 1 (Addendum A-2 for DSAW pipe) T = 1 (Addendum A-3 for T < 250 oF) What is left to identify is the design factor F. At this point we should be aware that there is a possibility that two design factors may be assigned to this pipeline. F for the undeveloped and desert area: Based on Table-1 the pipeline is considered in location class1, therefore F = 0.72 t

= 730 x 48 / ( 2 x 1.0 x 1.0 x 0.72 x 65000 ) t

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0.374 inches

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F for the section area close to the industrial Park: To determine the design factor, F, a population density analysis must be performed. RER = 5000 m for the 48” sour gas pipeline, (based on SAESB-064 shown in Addendum A-4) This means that the industrial park is located within the RER. Therefore, a formal population density analysis (PDA) must be conducted to determine the location class, thus the design factor F. PDI

= Existing DI + Virtual DI

PDI

= EDI + VDI

EDI

= N1 + N3

N1 = Number of 3-story N3 = Number of more than 3-sorty x Number of the stories / 3 (rounded to the next number) N1 = 11 N3 = 4 x 5 / 3 = 6.666 N3 = 7 EDI

= N1 + N3 = 11 + 7 = 18

VDI

= 0.00075 x 1200 = 0.9 = 1

PDI

= EDI + VDI =18 + 1

PDI

= 20

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Based on Table-1, for PDI = 20, this section of the pipeline is considered in location class-2, therefore F = 0.60 t = 730 x 48 / ( 2 x 1.0 x 1.0 x 0.60 x 65000 )

t

=

0.449 inches

Since no corrosion or other allowance was specified, the calculation is complete.

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PROCESS PIPING:

ASME B31.3, MINIMUM THICKNESS FOR INTERNAL PRESSURE This section outlines the method for calculating the minimum pipe wall thickness for plant piping systems designated as process plant piping ASME B31.3., as discussed in earlier in MEX-101.01. The minimum pipe wall thickness for Process Piping requires meeting three criteria. Those are: 1. Internal pressure requirement 2. Corrosion, erosion and mechanical requirement 3. Mill under-tolerance must be considered The first and the second factors are composed in the equation of paragraph 340 of ASME B31.3 as follows: Equation 6. t

tm

= t+c

= minimum wall thickness for pressure or mechanical strength

c = corrosion, erosion and mechanical allowance. Calculation of t for internal pressure is detailed as follows. ASME B31.3, paragraph 304 gives the rule for calculating the wall thickness requirement. Also, it provides several equations to conduct the calculation. The following equation is the mostly used one.

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Equation 7. t = PD / [ 2( SE + PY ) ] where: t

= Internal pressure design thickness, in.

P = Internal design pressure, psig. D = Outside diameter of pipe, in. E = Longitudinal-joint quality factor. S = Basic Allowable (hot) hoop stress, psi. Y = Wall thickness correction factor. For thickness t < D/6, the internal pressure thickness for straight pipe shall not be less than that calculated in the above equation. For t > D/6 or for P/SE > 0.385, calculation of pressure design thickness for straight pipe requires special consideration of factors such as theory of failure, effects of fatigue, and thermal stress. This module will not discuss this situation. The parameters for calculating wall thickness of the process plant piping are similar to the transportation piping, but the parameters are different. These parameters will be explained afterward. Design Pressure and Temperature The design pressure and temperature are used to calculate the internal pressure thickness of pipe. The design pressure is used directly in the thickness calculation equation, as previously shown. The design temperature is used to determine the allowable stresses from the ASME B31, 3 tables for Basic Allowable Stress. The worst combination of design pressure and temperature should be used for piping thickness calculations. The process engineer based on process requirements typically determines these values for design pressure and temperature

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Piping system design conditions generally are determined based on the design conditions of the equipment to which the piping is attached. Determining the piping design conditions consists of: 1. Identifying the equipment to which the piping system is attached. 2. Determining the design pressure and design temperature for the equipment. 3. Considering contingent design conditions, such as upsets not protected by pressure-relieving devices. 4. Verifying values with the process engineer. For example, a plant piping system that is attached to two process vessels, each with different design conditions, will have specified design pressure and design temperature based on the more severe design conditions of the two vessels. In another example regarding variation of T and P is a regeneration line which may be subjected to different combination of pressure and temperature. During steaming out the temperature is very high but the pressure is low, while during processing the pressure is high and the temperature is low. Longitudinal Joint Factor [E] As stated earlier the longitudinal joint factor is dependent on piping Code as well as manufacturing process itself. Therefore, there is possibility that for the same pipe manufacturing process, two different factors E will be assigned by the two different Codes. Generally, the values for E are lower for ASME B31.3 “Process Piping” Code than those for Transportation piping Codes. These values are listed in Table A-1B of the Code and attached in Addendum B-2 of this module. Comparing the value of E in this table we could tell that they are generally lower than the

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corresponding values for transportation piping. For ASME B31.3 piping, only seamless pipe has a longitudinal weld factor equivalent to one. Basic Allowable Hoop Stress [S] The basic allowable hoop stress (stress in the circumferential direction) is the allowable stress in tension for the pipe material It is defined by ASME B31.3 and appears in Table A-1 in an appendix-A of B31.3. An excerpt for these tables are shown in Addendum B-1. For plant piping, the allowable hoop stresses is a function of temperature and material, and considers the yield, tensile, and creep strengths of the material at the design temperature. Figure 3 gives graphical presentation of how these allowable stresses are defined and developed.

Figure 3. Variation of the Basic Allowable Stresses with Temperature for Grade B Material.

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Table A-1 is used in the following manner to determine allowable stress for plant piping. •

Pipe material and design temperature must be known.

•

Identify material Spec. No. and Grade in the table.

•

Obtain the allowable stress by looking under the appropriate temperature column at the specified material, and use linear interpolation between temperatures if required.

•

Using a pipe material at temperatures beyond the single solid line is not recommended. Going beyond the double solid line is prohibited.

The "Y" Factor for Plant Piping The Y factor is a correction factor for simplifying the original Lame’ equation. It accounts for geometrical relation between D & t. The "Y" factor is a function of the type of steel and the temperature, and is determined from Table 304.1.1 of ASME/ANSI B31.3, which is shown in Table 3 next. Table 3. The “Y” Factor as Extracted From ASME B1.3 TABLE 304.1.1 o Temperature, F

950

1,000

1,050

1,100

1150 and above

482 and below

510

538

566

593

621 and above

Ferritic Steels

0.4

0.5

0.7

0.7

0.7

0.7

Austenitic Steels

0.4

0.4

0.4

0.4

0.5

0.7

Other Ductile Metals

0.4

0.4

0.4

0.4

0.4

0.4

o Temperature, C

900 and below

General Note: The value of Y may be interpolated between 0

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The following notes should be clarified regarding Y factor in relation to Equation 4: For D/t > 6 , Y can be calculated as: Y= ( d + c ) / ( D + d + 2c ) where: D = outside diameter d = inside diameter c

= mechanical, erosion and corrosion allowance

For a conservative approach and simplicity, Y can be assumed equals to 0.4 or ignored. The Y factor becomes significant for the combination of very high pressure and small diameter. The factor should be used in the case of high temperature and pressure because significant saving in material could be achieved specially for alloy materials.

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CORROSION, EROSION, AND THREAD ALLOWANCES Allowances for corrosion, erosion, or threads must be accounted for in determining the required pipe wall thickness. This is more of a problem in plant piping because high fluid velocities or changes in the pressure of the fluid can corrode a pipe. Thread allowances apply only to smaller diameter pipes, which may be threaded. Corrosion, erosion, and thread allowances are determined in conjunction with the corrosion or process engineer and are often specified in a pipe specification. The appropriate allowance is added to the thickness that was calculated for internal pressure to arrive at a total required pipe wall thickness. The following Sample Problems-2 will be used to illustrate the method of calculating the pipe wall thickness for the process piping. Manufacturer Mill Tolerance The pipe specification for manufacturing pipe such as API 5L and ASTM A 53 or ASTM-A106, allow the manufacturer certain tolerance in the wall thickness positive and negative. There are some legitimate reasons for these tolerances because any manufacturing process always works within some specified range. The maximum manufacturer's under-tolerance for pipe wall thickness is 12.5% for carbon and low-alloy steels. For high-alloy steels it is 10%. Most seamless piping systems will be in the 12.5% category. When pipe is supplied, the actual thickness can be minus 12.5% of the nominal thickness. Therefore, for ASME B31.3 piping, after the required minimum wall thickness is determined, the next greater available standard pipe thickness must be selected, taking into account the manufacturer's under tolerance. In other word, the pipe wall thickness after deducting the negative mill tolerance must be m, equal to or more than t, specified in Equation 7.

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However, this process of correcting the wall thickness for mill tolerance was not addressed for transportation pipelines. One reason is that it is not economical to add for the mill tolerance, because pipelines usually extend for long distances, could be thousands of kilometers. Another reason is that the Code consider the mill tolerance indirectly in the design safety factor. SAES Limitations on Pipe Schedule SAES-L-005 and SAES-L-006 impose additional requirement over the calculated wall thickness. The minimum wall thickness (Schedule) of carbon steel pipe shall be as follows: Table 4. Minimum Wall Thickness Schedule for Carbon Steel Nominal Size

mm

Hydrocarbon Service

Low-Pressure Utility Service

in.

≤2

≤ 50

SCH 80

SCH 40 (see 3.9)

3-6

75 - 150

SCH 40

SCH 40

8 - 32

200 - 800

6.5 mm (0.250 in.)

6.5 mm (0.250 in.)

≥ 34

≥ 850

Diameter /135

Diameter/135

Note: Schedule 160 nipples shall be used for 50 mm (2 in.) and smaller pipe sizes in vibration service where bracing cannot be effectively provided.

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Sample Problem 2 A 16” re-generation line will be installed to connect to the reactor at Ras Tanura Refinery. The following data is provided: Pipe Diameter: 16 inches Pipe specification: ASTM 335, (1/1-4 Cr - 1/2 Mo) Pipe material grade: P11 Pipe manufacturing: Seamless

CASE 1: Design Pressure: 300 psig Temperature: 1000 oF

CASE 2: Design Pressure: 900 psig Temperature: 400 oF Corrosion allowance: 0.0625 inches Perform the following calculations: 1. Find the minimum wall thickness for the internal design pressure 2. The minimum wall thickness by ASME B31.3 code 3. Select the pipe schedule that meets the Code requirements.

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Solution The minimum wall thickness for the internal design pressure: Since this is a process plant piping system, Equation 6 applies as follows: t

= PD / [ 2( SE + PY ) ]

E = 1 (Addendum B-2, for seamless pipe, per B31.3 Appendix A-1B) In order to determine the thickness the calculation must be performed for both cases independently. CASE 1: P = 300 psig T = 1000 oF S = 6300 psi (Addendum B-1, B31.3 Appendix A-1) Y = 0.7 (based on Table-3) t = 300 x 16 / [2 x ( 6300x1 + 300x0.7 ) ] t = 0.369 inches

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CASE 2: P = 900 psig T = 400 oF S = 2000 psi (Addendum B-1, B31.3 Appendix A-1) Y = 0.4 (based on Table-3) t = 900 x 16 / [2 x ( 20000 x 1 + 900 x 0.4) ] t = 0.354 inches Therefore the required t, thickness for internal pressure case is t = 0.369 inches The minimum wall thickness by ASME B31.3 code: In this case, a 15 mm (0.0625 in.) corrosion allowance has been specified. Therefore: tm = t + c = 0.369 + 0.0625 tm = 0.4315 inches The pipe schedule which meets the Code requirements: Checking the pipe data available in Addendum C-3: The next pipe schedule above t 0.500

m

= 0.4315 “, is XS with T =

Tu = T x (1- 12.5/100) = 0.5 x 0.875 = 0.4375 inches Therefore 16” XS pipe meets the Code requirement.

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PIPE WALL THICKNESS FOR EXTERNAL PRESSURE A piping system may be exposed to an external pressure, and external pressure rather than internal pressure may govern the required wall thickness. This might be the case for largediameter/thin-walled process plant piping that is subject to vacuum conditions, or underwater pipelines, which must withstand the hydrostatic head of the water above them. Therefore, calculations must be conducted to ensure that the pipe wall thickness is adequate for a given external pressure. If it is not adequate, the thickness must be increased. Pipe is subject to compressive forces such as those caused by dead weight, wind, earthquake, and vacuum. The process engineer often identifies these forces. For example, a submarine pipeline may be exposed to an external pressure due to the liquid head of surrounding water being greater than the internal pressure. Piping components behave differently under these forces than when they are exposed to internal pressure. This difference in behavior is due to buckling or elastic instability that makes the pipe weaker in compression than in tension. In failure by elastic instability, the pipe may collapse or buckle. This applies particularly to pipe that has a fairly low internal pressure, large diameter, and thin wall. Figure 4 shows a section of pipe that bucked under external pressure.

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Figure 4. Effects of External Pressure The left photo shows the pipe that buckled under external pressure developed between the pipe and the sleeve. The right shows the section after removal from the sleeve.

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GUIDELINES FOR EXTERNAL PRESSURE CALCULATIONS The pressure piping Codes do not outline procedure to perform the calculation for external pressure but refer to the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Paragraph UG-28. That paragraph provides a procedure for evaluating cylindrical shells under external pressure. Pipe geometry factors, (unsupported length, outside diameter, and thickness), material strength, and design temperature are used to determine the thickness required to resist external pressure. 1. Determine the maximum unstiffened length of the pipe, L. 2. Use the value of “t” as determined for internal pressure thickness as a starting point. Calculate L/D and D/t (D is equivalent to DO in the ASME Code procedure). 3. Enter Figure G of part D, subpart 2 of the ASME Code, Section II, with L/D. For L/D greater than 50, use L/D = 50. This figure is excerpted in Figure 5. 4. Move horizontally to the line for D/t. Use linear interpolation for intermediate values of D/t. Move vertically downward to find Factor A. 5. Using the value of A, enter the applicable figure from Section II, Part D of the ASME code based on the pipe material, such as Figure CS-2, excerpted in Figure 8. Move vertically to an intersection with the material/temperature line. Use linear interpolation for intermediate temperatures. If A falls to the right of the material/temperature line, use the horizontal projection of the line. If A falls to the left of the material/temperature line, go to Step 8. Note that the ASME Code contains figures similar to Figure 6 for different material categories, and the correct figure must be used for this evaluation.

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6. From the intersection in Step 5, move horizontally to the right and read the value of Factor B. 7. Calculate the maximum allowable external pressures, Pa, as follows: Equation- 8 Pa = 4B/3(D/t) 8. If factor A falls to the left of the material/temperature line, then: Equation- 9 Pa = 2AE/3(D/t) 9. Note that in this case, “E” is equal to Young’s Modulus of Elasticity at the design temperature, not the longitudinal-joint quality factor. “E” is found from the applicable material figure from Section II of the ASME Code as shown in Figure 6. 10. If Pa is smaller than the external design pressure, select a larger value of t and repeat the design procedure until Pa is equal to or exceeds the external design pressure. As an alternative, external stiffening rings may be added or the distance between them reduced, to reduce the value of L that is used in the calculations. 11. Calculate tm = t + c to obtain the pipe thickness required for external pressure and mechanical, corrosion, and erosion allowances.

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Nomenclature t

= Pressure design thickness for external pressure, in.

L = Unstiffened length of pipe section, taken as largest of (1) distance between flanges or welded stiffening rings or ring girders, (2) distance between the point of tangency on an elbow or cap and a flange or stiffening ring, or (3) the distance between the points of tangency of two elbows or caps if there are no intermediate flanges or stiffening rings, in. D = Actual outside diameter of pipe, in. A = Factor from ASME Code, Section II. B = Factor from ASME Code, Section II, psi. E = Modulus of elasticity of material at design temperature, psi. Pa = Allowable external pressure, psi. tm = Required external pressure thickness, including allowances, in. c

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= Corrosion, erosion, other allowances, in.

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Figure 5. Geometrical Factor A for External Pressure Calculations

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Figure 5. Geometrical Factor A for External Pressure Calculations (continued)

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Figure 6. Material Factor B for External Pressure Calculations

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Sample Problem 3:

External Pressure for Pipeline Assume the pipeline discussed in Sample Problem 1 also must be capable of withstanding full vacuum [100 kPa (15 psi) external pressure].

Solution T

= 0.374 inches ( use the thinner section)

L / D > 50 because no stiffening is present. D / t = 48 / 0.374 = 128 Based on Figure-5 the A factor is found as: A = 0.00007 Refer to Figure-6 for B factor, use the upper curve: B = 9500 The A factor is on the left side of the curves, therefore Use Equation 9 Pa = 2AE/3(D/t) 6

Pa = 2 x 0.00007 x 29.7x10 / (3 x 128 ) = 10.8 psia The pipe is not adequate for vacuum, therefore alternative shall be evaluated a follows: 1. Increase the pipe wall thickness, which is very costly 2. For underground this may need more analytical study. 3. Impose operational guideline to avoid vacuum.

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Sample Problem 4:

External Pressure for Plant Piping For Sample Problem-1, assume the longest straight pipe section is 80 feet, otherwise there might be flanges, tee or valves acting as stiffener. Verify if the line is capable of withstanding full vacuum [100 kPa (15 psi) external pressure].

Solution t

= 0.449 inches

L = 80 x 12 = 960 inches L/D

= 960 / 16 = 60

D/t

= 16 / 0.4315 = 37

Based on Figure-5 the A factor is found as: A = 0.001 Refer to Figure-6 for B factor, use the upper chart. T = 900 F: B = 9500 Therefore, use Equation- 8: Pa = 4B/3(D/t) Pa = 4 x 9500 / (3 x 37 ) = 342 psia Therefore, the line is adequate for vacuum condition.

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TRAFFIC AND SOIL LOADS OVER BURIED PIPE Transportation pipelines often have buried sections of pipe. The required thickness of these buried sections will be affected by soil and traffic loads, in addition to the design pressure. These loads cause a circumferential bending stress in the pipe. The Saudi Aramco engineer needs to determine if the pipe is thick enough for these soil and traffic loads. Specific requirements for how traffic loads are determined are found in SAES-L-046, Pipeline Crossings Under Roads and Railroads. The pipe must be designed for the traffic load, soil weight, and passive soil reaction. At railroad and highway crossings where the loads may apply, the pipe must be designed according to API Recommended Practice 1102, Liquid Petroleum Pipelines Crossing Railroads and Highways. It provides the formula for determining circumferential stress in a carrier pipe with internal pressure due to external loads at highway and railroad crossings. The equation gives a stress that is based upon the thickness, internal pressure soil and traffic loads as follows: The stress calculated in accordance with this equation is limited to the Specified Minimum Yield Stress times the design factor, F, without considering the longitudinal joint factor. It should also be noted that SAES-L-046 contains criteria for when a protective casing is required, and how the casing should be designed. Saudi Aramco has a computer program that makes the calculation. This can be done through the Consulting Services Department (CSD.) All the load factors required by SAES-L-046 are in the computer program, as well as the required parameters. It is beyond the scope of this course to determine the stress.

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THE MAXIMUM ALLOWABLE OPERATING PRESSURE (MAOP) There are many cases where the purchased pipe wall is thicker than what is required for the design pressure. This would allow the designer to increase the actual design pressure of the pipe as long as it is not limited by other factors, such as flange rating as would be discussed in MEX-101.04. For transportation piping system, this maximum permissible pressure is called maximum allowable operating pressure (MAOP). For process plant piping this is called design pressure. The engineer must determine MAOP for pipe as well as other piping components. This module discusses MAOP for pipe. The MAOP of a pipe or other piping component will be at least equal to the design pressure. However, the MAOP can be higher than the design pressure since use of a standard wall thickness will typically provide an additional margin. Guidelines for Calculating Maximum Design Pressure 1. Subtract mill tolerance, (expressed as a decimal fraction), m, from the nominal pipe wall thickness, Tn, for ASME B31.3 piping to determine the minimum possible as - supplied thickness, T , as follows: T = ( 1 – m ) x Tn Where: Tn is the nominal pipe wall and m is the mill tolerance; “m” shall be taken as zero for ASME B31.4 and B31.8 piping systems.

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2. Subtract any other allowances, such as corrosion allowance, c, to calculate the minimum possible pipe thickness, t, as follows: t =T–c Usually c is zero for transportation piping. 3. Reverse the applicable internal pressure equation to calculate a value for maximum design pressure For ASME/ANSI B31.3, Process Piping, use the following equation to calculate Maximum Design Pressure:

MDP =

2tSE D −2tY

For ASME/ANSI B31.4 or B31.8, Transportation Piping, use the following equation to calculate MAOP:

2St MAOP= FET D 4. Calculate maximum design pressure with the factors identified earlier. 5. For ASME/ANSI B31.3, Process Piping, the following equation should be used to calculate maximum design pressure: Maximum Design Pressure = 2x t x SE / ( D – 2xtxY) 6. For ASME/ANSI B31.4 or B31.8, Transportation Piping, use the following equation to calculate MAOP: 2St MAOP = FET D

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MAOP of a Pipeline For transportation piping, the MAOP is calculated by Equation 3. The pipe wall thickness to be used in the equation is the nominal wall thickness. There should be no deduction for mill tolerance. This is illustrated in the next sample problem. Sample Problem 5.

MAOP of a Pipeline A 36 inch pipeline carrying crude oil passing through an area classified as Class 2. The required design pressure is 500 psig, and design temperature is 200 F. A double submerge arc welded (DSAW) API 5L grade X52 pipe was specified. A 36 inch pipe with 0.375 inch wall is available. Verify if this pipe is adequate for the design pressure by calculating the MAOP of this pipe.

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Solution Since this a pipeline, it would be designed to ASME B31.4/ B31.8, Equation-3 will be used to find The MAOP: P = [2 S t / D] x FET P = MAOP psig. D = 36 in. S = 52000 psi. E = 1 T = 1 F = 0.72 MAOP = 2 x 52000 x 0.375 (0.6x1x1) / 36 MAOP = 650 psig The MAOP is higher than the design pressure of 500 psig, therefore the line pipe is acceptable for the design condition.

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Maximum Design Pressure for Process Plant Piping From a terminology point of view, the word MAOP is not used by the Process Piping Code ASME B31.3. Paragraph 302.2.4 gives the rules on how to exceed the design pressure. The design pressure can be exceeded as follows:

by 33% for continuous 10 hours but not more than 100 hours per year

or by 20% for continuous 50 hours but not more than 500 hours in a year.

However, similar to what has been made for the transportation piping, a pipe could be purchased thicker to what is deeded for the actual design condition. The back calculation of the maximum design pressure (not the MAOP) will be illustrated in the following steps. This maximum design pressure can be used for the pressure exceeding criteria as set by the ASME B31.3.

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Sample Problem 6:

Process Piping Using the results of Sample Problems 2, determine the maximum design pressure that can be sustained by the pipe and maximum permissible pressure. T = (1-m) Tu = (1 - 0.125) (0.50) = 0.6563 in. t n = Tu - c t n = 0.0.4375 - 0.0625 = 0.375 in. MDP = 2 t n S E / ( D – 2 t n Y ) Apply Case-1 ( T = 1000 o F) MDP = 2 x 0.375 x 6300 x 1/ ( 16 – 2 x 0.375 x0.7 ) MDP = 305 psig 33% over MDp = 405 psig ( for 10 hours) 20% over MDp = 366 psig ( for 50 hours) Apply Case-2 ( T = 400) MDP = 2 x 0.375 x 20000 x 1/ ( 16 – 2 x 0.375 x0.4 ) MDP = 955 psig 33% over MDp = 1270 psig ( for 10 hours) 20% over MDp = 1146 psig ( for 50 hours)

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TYPICAL MISTAKES IN PIPE WALL THICKNESS CALCULATION Even though the calculation for wall thickness is relatively simple and straight forward, people do make mistakes that most of the time cost the company more capital investment than necessary. These mistakes are summarized as follows: 1. Specifying a design pressure that is higher than what is required to fulfill the operational requirement. 2. Selecting the wall thickness to match the flange rating and thickness. This might be acceptable for plant piping but for pipelines it could add additional cost. 3. Setting the design pressure based on a pre-selected or existing flange rating more what is actually required for operation. 4. Applying the wrong Code. 5. Specifying the wrong allowable stresses in the formula 6. Inaccurate interpretation of the SAES-L-006 regarding the minimum wall thickness requirement. Some of these requirements are intended for inventory purposes, therefore for line pipe purchased to be installed for a specific project the criteria should not be applied if there is great economic incentives. Following the standards without understanding the intent could lead to purchasing of line pipe thicker than what is required which means additional material and construction cost. 7. Surprisingly, pure mathematical mistakes are frequently made in the calculations reflected in possibly unsafe designs or costly material.

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SUMMARY MEX 101.03 discussed the process of determining pipe wall thickness, one of the first steps in specifying the design of piping system components. Pipe wall thickness is based on the internal pressure of a pipe, and if necessary, on any additional external pressure or loads. The pipe wall thickness is determined by using the equation for internal pressure thickness in the applicable code, adjusting the thickness as necessary for any applicable external pressure or additional loads, adding the corrosion or other allowances, accounting for the manufacturer's mill tolerance, and selecting pipe schedule. After selecting the pipe schedule, the Maximum Allowable Operating Pressure for the pipe may be calculated.

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ADDENDUM Index of Addendum

Page(s)

ADDENDUM A .............................................................................................................. 56 ADDENDUM B .............................................................................................................. 62 ADDENDUM C.............................................................................................................. 69

LIST OF TABLES Table A-1. ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) ................................................................................................. 56 Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E ..................................................................................................... 59 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe.................................................................................. 60 Table A-4. SAES B-064 Rupture Exposure Radius..................................................... 61 Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals................................................................................................... 62 Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E ............................................................................................ 67

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ADDENDUM A Table A-1: ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe) SPEC. NO.

GRADE

TYPE (NOTE 1)

SMYS, PSI

API 5L (Note 2)

A25

BW, ERW, S

25,000

API 5L (Note 2)

A

ERW, S, DSA

30,000

API 5L (Note 2)

B

ERW, S, DSA

35,000

API 5L (Note 2)

X42

ERW, S, DSA

42,000

API 5L (Note 2)

X46

ERW, S, DSA

46,000

API 5L (Note 2)

X52

ERW, S, DSA

52,000

API 5L (Note 2)

X56

ERW, S, DSA

56,000

API 5L (Note 2)

X60

ERW, S, DSA

60,000

API 5L (Note 2)

X65

ERW, S, DSA

65,000

API 5L (Note 2)

X70

ERW, S, DSA

70,000

API 5L (Note 2)

X80

ERW, S, DSA

80,000

ASTM A 53

TYPE F

BW

25,000

ASTM A 53

A

ERW, S

30,000

ASTM A 53

B

ERW, S

35,000

ASTM A 106

A

S

30,000

ASTM A 106

B

S

35,000

ASTM A 106

C

S

40,000

EFW

(NOTE 3)

ERW

30,000

ASTM A 134 ASTM A 135

A

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Table A-1. ASME/ANSI B31.8 (Appendix D: Specified Minimum Yield Strength for Steel Pipe), Continued ASTM A 135

B

ERW

35,000

ASTM A 139

A

EFW

30,000

ASTM A 139

B

EFW

35,000

ASTM A 139

C

EFW

42,000

ASTM A 139

D

EFW

46,000

ASTM A 139

E

EFW

52,000

ASTM A 333

1

S, ERW

30,000

ASTM A 333

3

S, ERW

35,000

ASTM A 333

4

S

35,000

ASTM A 333

6

S, ERW

35,000

ASTM A 333

7

S, ERW

35,000

ASTM A 333

8

S, ERW

75,000

ASTM A 333

9

S, ERW

46,000

ASTM A 381

CLASS Y-35

DSA

35,000

ASTM A 381

CLASS Y-42

DSA

42,000

ASTM A 381

CLASS Y-46

DSA

46,000

ASTM A 381

CLASS Y-48

DSA

48,000

ASTM A 381

CLASS Y-50

DSA

50,000

ASTM A 381

CLASS Y-52

DSA

52,000

ASTM A 381

CLASS Y-56

DSA

56,000

ASTM A 381

CLASS Y-60

DSA

60,000

ASTM A 381

CLASS Y-65

DSA

65,000

GENERAL NOTE: This Table is not complete. For the minimum specified yield strength of other grades and grades in other approved specifications, refer to the particular specification. NOTES:

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(1) Abbreviations: BW - furnace butt-welded; ERW - electric-resistance welded; S seamless, FW - flash-welded; EFW - electric-fusion welded; DSA - doublesubmerged arc welded. (2) Intermediate grades are available in API 5L. (3) See applicable plate specification for SMYS.

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Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E. Spec. Number

Pipe Class

E Factor

ASTM A53

Seamless

1.00

Electric-Resistance Welded

1.00 0.60

Furnace Welded ASTM A106

Seamless

1.00

ASTM A134

Electric-Fusion Arc Welded

0.80

ASTM A135

Electric-Resistance Welded

1.00

ASTM A139

Electric-Fusion Welded

0.80

ASTM A211

Spiral-Welded Steel Pipe

0.80

ASTM A381

Double-Submerged Arc Welded

1.00

ASTM A671

Electric-Fusion Welded

1.00*

ASTM A672

Electric-Fusion Welded

1.00*

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Table A-2. ASME/ANSI Code B31.8, Table 841.115A, (Excerpt) Longitudinal Joint Factor E (Continued) Spec. Number

Pipe Class

E Factor

API 5L

Seamless

1.00

Electric-Resistance Welded

1.00

Electric-Flash Welded Submerged Arc Welded

1.00 1.00 0.60

Furnace Butt-Welded *1.00 for classes 12,22,32,42,52 0.80 for classes 13,23,43,53 Table A-3. ASME/ANSI B31.8 Table 841.116A (Excerpt)Temperature Derating Factor for Steel Pipe TEMPERATURE o C

TEMPERATURE DERATING FACTOR, T o F

120 OR LESS

250 OR LESS

1.000

150

300

0.967

177

350

0.933

204

400

0.900

232

450

0.867

Note: For intermediate temperatures, interpolate for derating factor. Source: ASME/ANSI B31.8 - 1992. With permission from the American Society of Mechanical

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Table A-4. SAES B-064 Rupture Exposure Radius Service

Diameter

TVP

H2S

RER

inches

psig

% Mole

meters

Comments

Crude

< 24

< 14.5

< 1..5

200

RER study by LPD not required

Crude

> 24

< 14.5

< 1.5

400

RER study by LPD not required

Gas/ liquid hydrocarbon

< 24

> 14.5

< 1.5

1000

RER study can be done by LPD

Gas/ liquid hydrocarbon

< 24

> 14.5

> 1.5

3000

RER study can be done by LPD

Gas/ liquid hydrocarbon

> 24

> 14.5

< 1.5

2000

RER study can be done by LPD

Gas/ liquid hydrocarbon

> 24

> 14.5

> 1.5

5000

RER study can be done by LPD

General Notes: 1. LPD: Loss Prevention Department, which is responsible for SAES-B-064. 2. RER study can be done for services other than the first two. The RER shall not be less than the value specified in the first two items for the respective diameter. 3. Regardless of calculations, RER shall not be greater than the values specified in the last for items for the respective pipeline diameter. 4. TRV: is the true vapor pressure for flammable liquid lines

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ADDENDUM B Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued)

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Table B-1. ASME/ANSI B31.3 Table A-1 (Excerpt) Basic Allowable Stresses In Tension for Metals (Continued) 51. Special P-1, Sp-2, SP-3, SP-4, and SP-5 of carbon steels are not included in P-No 1 because of possible high-carbon, high-manganese combinations, or micro-alloying, which would require special consideration in qualification. Qualification of any high-carbon, high-manganese grade may be extended to other grades in its group. 52. Copper-silicon alloys are not always suitable when exposed to certain media and high temperature, particularly above 212°F. The user should satisfy himself that the alloy selected is satisfactory. 53. Stress relief treatment is required for service above 450°F. 54. The maximum operating temperature is arbitrarily set at 500°F because hard temper adversely affects design stress in the creep rupture ranges. 55. Pipe produced to this specification is not intended for hightemperature service. The stress values apply to either nonexpanded or cold-expanded material in the as-rolled, normalized, or normalized temperature conditions. 56. Because of thermal instability, this material is not recommended for service above 800°F. 57. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 800°F. 58. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 875°F. 59. For temperature above 900°F, consider the advantages of killed steel. Source: ASME/ANSI B31.3 - 1990. With permission from the American Society of Mechanical Engineers.

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Table B-2. Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E

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Table B-2. (TABLE A-1B) Basic Quality Factors for Longitudinal Weld Joints In Pipes, Tubes, and Fittings E (continued)

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ADDENDUM C Engineering and Design Data Table of Properties of Pipe References for Pipe Data Standard pipe wall thickness are specified in the following standards: •

ASME/ANSI B36.10, Welded and Seamless Wrought Steel Pipe (for carbon and low-alloy steel pipe).

•

ASME/ANSI B36.19, Stainless Steel Pipe.

•

API/5L, Specification for Line Pipe (only for carbon steel pipe that meets this specification).

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Table C-1. Table of Properties of Pipe

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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Table C-1. Table of Properties of Pipe (Cont’d)

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