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 : Piping & Valves For additional information on this subject, contact File Reference: MEX10103 K.S. Chu on 873-2648 or R. Hingoraney on 873-2649 Engineering Encyclopedia Saudi Aramco DeskTop Standards Pipewall Thickness Calculation
Transcript
Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready 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, EngineeringServices, Saudi Aramco.
Chapter : Piping & Valves For additional information on this subject, contactFile Reference: MEX10103 K.S. Chu on 873-2648 or R. Hingoraney on 873-2649
Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is not alreadyin the public domain may not be copied, reproduced, sold, given, ordisclosed to third parties, or otherwise used in whole, or in part, withoutthe written permission of the Vice President, Engineering Services, SaudiAramco.
Chapter : Piping & Valves For additional information on this subject, contactFile Reference: MEX10103 K.S. Chu on 873-2648 or R. Hingoraney on 873-2649
Temperature Derating Factor...........................................................................................18
Determining the Proper "Y" Factor for Plant Piping .......................................................19
ADJUSTING PIPEWALL THICKNESS FOR EXTERNAL PRESSURE.....................21
IDENTIFYING THE PROCEDURE FOR EVALUATING OTHER
LOADS THAT ARE APPLIED TO BURIED PIPE .......................................................22
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SELECTING PIPE SCHEDULE THAT TAKES INTO ACCOUNT THEMANUFACTURER’S TOLERANCES AND SAUDI ARAMCO’S MINIMUMREQUIREMENTS FOR PIPEWALL THICKNESS.......................................................23
Given the design temperature, design pressure, pipe diameter, and pipe material, the SaudiAramco engineer can determine the required thickness of the pipewall. Pipewall thickness isa function of the allowable hoop stress, established by each code and of SAES-L-003, DesignStress Criteria For Pressure Piping. Each code provides the equation that is used to calculateinternal pressure thickness.
Pipewall thickness is calculated by:
• Determining the applicable ASME/ANSI B31 Code, as discussed in MEX 101.01.
• Calculating the required thickness for internal pressure.
• Checking the calculated thickness to determine its acceptability for external pressure andother applied loads, as applicable.
• Increasing the calculated thickness, as needed, to account for corrosion allowance andmill tolerance.
• Selecting a thickness from an ANSI/API table of standard pipe thicknesses, andchecking the thickness against the Saudi Aramco minimum thickness requirements.
The MAOP for the pipe can be calculated after the final pipewall thickness is determined.
The text of MEX 101.03 refers to ASME/ANSI B31.3 for plant piping and B31.8, fortransportation piping. The process discussed in this module is consistent for all the B31piping codes. However, the equations, variables, and definitions or values for allowablestress differ.
It is important for the engineer to keep in mind the design conditions, normal operatingconditions, and contingent conditions of the piping system when determining the pipewallthickness. These conditions establish the necessary parameters for pipewall thicknesscalculations.
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Normal Operating Conditions
Based on SAES-L-002, Design Conditions For Pressure Piping, normal operating conditionsare those expected to occur during normal operation per design, excluding failure of anyoperating device, operator error, and the occasional, short-term variations stated in theapplicable code. Startup and controlled shutdown of plants, shut-in of wells at the GOSP, andsimilar foreseeable events also are included with normal operation.
Design Conditions
Based on SAES-L-002, design conditions are all conditions which govern the design andselection of pressure piping components, and are based on the most severe conditionsexpected to occur in service, in accordance with the code. A margin is used between thenormal operating and design conditions to account for normal operating variations.
Contingent Design Conditions
Based on SAES-L-002, contingent design conditions are:
• Uncontrolled shutdown of plants.
• Improper operation due to a single act or operating decision.
• Failure of a device or function.
• Fire.
• Ambient conditions, such as storms, which have an expected average return interval ofless than 100 years.
• Certain multiple, coincident, unrelated contingencies or failures.
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CALCULATING THE MINIMUM REQUIRED THICKNESS FOR THE INTERNALDESIGN PRESSURE
Calculating the required internal pressure thickness is the first step in determining pipewallthickness. To calculate internal pressure thickness given certain design conditions, the SaudiAramco engineer must use the applicable code (as discussed in MEX 101.01), SAES-L-002,and SAES-L-003. Work Aid 1 outlines the procedure for calculating internal pressurethickness. The sections that follow highlight several aspects of this procedure.
Equation for Internal Pressure Thickness for Transportation Piping: ASME/ANSIB31.8 and B31.4
ASME/ANSI B31.8, gives the equation for calculating the internal pressure wall thickness ofgas transmission and distribution piping (transportation piping) as follows:
t = PD
2 SEFT
where: t = Internal pressure wall thickness, in.
P = Design pressure, psig.
S = Specified Minimum Yield Strength (SMYS), psi.
D = Outside diameter of pipe, in.
F = Design factor.
E = Longitudinal-joint quality factor.
T = Temperature derating factor.
Saudi Aramco Engineering Standard SAES-L-003 provides values for the design factor, F.The standard also should be referred to for other considerations for each value in the equation.The method for determining each of these values will be discussed in subsequent sections.
ASME/ANSI B31.4 uses this same basic equation in a more simplified form. However,design requirements that are contained in SAES- L-003 require that this same equation beused for B31.4 systems.
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Equation for Internal Pressure Thickness for Plant Piping: Code ASME/ANSI B31.3
ASME/ANSI B31.3 gives the equation for calculating the internal pressure design thicknessfor Chemical Plant and Petroleum Refinery Piping (plant piping) as:
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 = Allowable hoop stress, psi.
Y = Wall thickness correction factor.
The method for determining each of these values in the equation will be discussed insubsequent sections.
For thicknesses t < D/6, the internal pressure thickness for straight pipe shall not be less thanthat calculated in the above equation. For t _ D/6 or for P/SE > 0.385, calculation of pressuredesign thickness for straight pipe requires special consideration of factors such as theory offailure, effects of fatigue, and thermal stress. This module will not discuss this situation.
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Determining Design Pressure and Temperature
The design pressure and temperature are used to calculate the internal pressure thickness ofpipe. The design pressure is used directly in the thickness calculation equation, as previouslyshown. The design temperature is used to determine the allowable stresses, especially forplant piping. The values for design pressure and temperature typically are determined by theprocess engineer based on process requirements. The values used for piping thicknesscalculations allow for the worst combination of design pressure and temperature.
Piping system design conditions generally are determined based on the design conditions ofthe equipment to which the piping is attached. Determining the piping design conditionsconsists 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 differentdesign conditions, will have specified design pressure and design temperature based on themore severe design conditions of the two vessels.
For a transportation piping system attached between two pump or compressor stations,normal operating conditions and potential contingent design conditions (such as pump failureat a downstream pumping station which causes a pressure surge) will be determined. Thehydrostatic head in a liquid-filled piping system could also prove to be a significant factor incases where there is a large difference in elevation between sections of the system.
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Saudi Aramco Engineering Standard SAES-B-064, Onshore and Nearshore Pipeline Safety,Paragraph 8, identifies design pressure considerations for transportation piping in which surgecan occur. These are highlighted as follows:
• The design pressure used to determine the minimum pipewall thickness perASME/ANSI B31.4/B31.8 in location class 3 and 4 zones shall be established bydetermining the maximum expected surge pressure from a single contingency, such asinadvertent closure of a valve, or failure of a sensing or regulating device. Self-actuatedsurge protection systems, if provided, shall be assumed to operate as intended, tomitigate the single worst contingency.
• Surge analysis shall be made for liquid-packed services. Surge protection systems shallbe installed if surge pressures are calculated to exceed 110% of the MAOP.
• Surge protection systems shall include duplicate, critical subassemblies sufficient toovercome single-mode system failures.
• An installed, spare, surge-relief valve is required for each surge protection system.
• Surge protection systems shall be of fail-safe design.
Determining Allowable Piping Hoop Stress and Joint Quality Factor
Allowable hoops stress (stress in the circumferential direction) is the allowable stress intension for the pipe material, as modified by the joint quality factor. The joint quality factordepends upon the pipe manufacturing process. The allowable hoop stress is defined by eachcode. For plant piping, the allowable stress appears in tables in an appendix of B31.3.
Transportation Piping
For transportation piping, the allowable hoop stress is a function of the material's SpecifiedMinimum Yield Strength (SMYS). The SMYS for commonly used piping materials may befound by using Appendix D of ASME/ANSI B31.8. The joint quality factor, E, is determinedby using Table 841.115A of ASME/ANSI B31.8 (or Table 402.4.3 of ASME/ANSI B31.4).The pipe material specification and grade, are required to determine its SMYS. The pipematerial specification and grade are required to determine E. These tables are shown, in part,as Figures 1 and 2.
ASME/ANSI B31.4 determines allowable hoop stress in a similar manner, as specified by thecode.
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ASME/ANSI B31.8 APPENDIX D SPECIFIED MINIMUM YIELD STRENGTH FORSTEEL PIPE
ASTM A 53 TYPE F BW 25,000ASTM A 53 A ERW, S 30,000ASTM A 53 B ERW, S 35,000ASTM A 106 A S 30,000ASTM A 106 B S 35,000ASTM A 106 C S 40,000ASTM A 134 EFW (NOTE 3)ASTM A 135 A ERW 30,000ASTM A 135 B ERW 35,000ASTM A 139 A EFW 30,000ASTM A 139 B EFW 35,000ASTM A 139 C EFW 42,000ASTM A 139 D EFW 46,000ASTM A 139 E EFW 52,000ASTM A 333 1 S, ERW 30,000ASTM A 333 3 S, ERW 35,000ASTM A 333 4 S 35,000ASTM A 333 6 S, ERW 35,000ASTM A 333 7 S, ERW 35,000ASTM A 333 8 S, ERW 75,000ASTM A 333 9 S, ERW 46,000ASTM A 381 CLASS Y-35 DSA 35,000ASTM A 381 CLASS Y-42 DSA 42,000ASTM A 381 CLASS Y-46 DSA 46,000ASTM A 381 CLASS Y-48 DSA 48,000ASTM A 381 CLASS Y-50 DSA 50,000ASTM A 381 CLASS Y-52 DSA 52,000ASTM A 381 CLASS Y-56 DSA 56,000ASTM A 381 CLASS Y-60 DSA 60,000ASTM A 381 CLASS Y-65 DSA 65,000
Source: ASME/ANSI B31.8 - 1989. With permission from the American Society of Mechanical Engineers.
This Table is not complete. For the minimum specified yield strength of other grades and grades in otherapproved specifications, refer to the particular specification.
welded; EFW - electric-fusion welded; DSA - double-submerged arc welded.(2) Intermediate grades are available in API 5L.(3) See applicable plate specification for SMYS.
Source: ASME/ANSI B31.8 - 1989. With permission from the American Society of Mechanical Engineers.
ASME/ANSI CODE B31.8, TABLE 841.115A, (EXCERPT) LONGITUDINAL JOINTFACTOR, E
*1.00 for classes 12,22,32,42,52 0.80 for classes 13,23,43,53
FIGURE 2
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After determining the allowable hoop stress and the joint quality factor from the code, SAES-L-003 must be checked to determine if there are any other restrictions on the allowable hoopstress. For transportation piping, SAES-L-003 states:
For cross-country and submarine pipelines in hydrocarbon service within the scope ofASME/ANSI B31.4 or B31.8, near populated areas as defined and classified in SAES-B-064, the maximum allowable hoop stress due to internal pressure shall not exceed theSpecified Minimum Yield Strength (SMYS) times the design factor, F.
SAES-L-003 specifies values for F based on location class, and for other specific situations.
Plant Piping
For plant piping, the allowable hoop stress is a function of temperature and material, andconsiders the yield, tensile, and creep strengths of the material at the design temperature.Allowable hoop stress is determined directly from Table A-1 of ASME/ANSI B31.3. Anexcerpt from this table is shown in Figure 3.
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 thespecified 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.
Obtain the value of the joint quality factor, E, based on pipe material and manufacturingprocess from Table A-1B in B31.3, as shown in Figure 4.
Once these values are determined, SAES-L-003 must be referred to for restrictions on theallowable hoop stress. For plant piping, SAES-L-003 states the allowable stresses as shownin Appendix A of the code shall be used for wall thickness calculations.
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 possiblehigh-carbon, high-manganese combinations, or microalloying, which would require special considerationin qualification. Qualification of any high-carbon, high-manganese grade may be extended to other gradesin 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 high-temperature service. The stress values apply to
either nonexpanded or cold-expanded material in the as-rolled, normalized, or normalized temperatureconditions.
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.
FIGURE 3, CONT'D
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TABLE A-1B BASIC QUALITY FACTORS FOR LONGITUDINAL WELD JOINTSIN PIPES, TUBES, AND FITTINGS, E
Source: ASME/ANSI B31.3 - 1990. With permission from the American Society of Mechanical Engineers.
FIGURE 4
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Determining Design Factors and Temperature Derating Factor for TransportationPipelines
Up to this point, the pipe diameter, D, design pressure, P, allowable hoop stress, S (orSMYS), and joint quality factor, E, can be determined for use in the equation for internalpressure thickness. The last two values that need to be determined are the design factor, F,and temperature derating factor, T, which must be used for transportation piping systems.
Design Factor
For transportation piping, the procedure for determining the design factor, F, requires usingSAES-B-064 to determine a pipeline location class, and then SAES-L-003 to determine thedesign factor.
The procedure for determining the design factor for transportation piping is as follows:
• Refer to SAES-B-064 to determine a location class. Location class is based upon apopulation density analysis (PDA) of the population located within the RuptureExposure Radius (RER) along a pipeline route. RER is the distance on either side of apipeline that must be included in a PDA, and is a measure of the zone that could bepotentially affected by a pipeline rupture. The PDA, as defined in Paragraph 6 ofSAES-B-064, supersedes instructions in ASME/ANSI B31.4 and B31.8 pertaining to thepopulation density index. Paragraph 5 of SAES-B-064 defines RER, as follows:
- For pipelines carrying liquid hydrocarbons having a true vapor pressure less than100 kPa gauge (15 psig) and an H2S concentration of less than 1.5 mol %, theRER is 400 m (1,300 ft) for all sizes of lines.
- For pipelines carrying combustible gas or liquid hydrocarbons with a true vaporpressure of 100 kPa (15 psig) or greater, and an H2S concentration of less than 1.5mol %, the following RER values shall be used:
Pipe size less than 12 in. diameter: 500 m (1,640 ft.)12 in. to less than 18 in. diameter: 800 m (2,625 ft.)18 in. to less than 26 in. diameter: 1,000 m (3,280 ft.)26 in. to less than 36 in. diameter: 1,200 m (3,940 ft.)36 in. to less than 48 in. diameter: 2,100 m (6,890 ft.)48 in. to less than 60 in. diameter: 2,200 m (7,260 ft.)
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- For pipelines carrying liquid hydrocarbons or combustible gas, with an H2Sconcentration of 1.5 mol % or greater, the following RER values shall be used:
Pipe size less than 18 in. diameter: 4,500 m (14,800 ft.)18 in. to less than 26 in. diameter: 4,700 m (15,400 ft.)26 in. to less than 36 in. diameter: 5,400 m (17,700 ft.)36 in. to less than 48 in. diameter: 7,100 m (23,300 ft.)48 in. to less than 60 in. diameter: 11,000 m (36,100 ft.)
- The RER for a flowline shall be equal to the RER of the well that is served, perSAES-B-062. For other producing lines, the RER shall be set equal to the largestRER of any well that is connected by that line.
• The boundaries of areas in which building/development is present or planned within theRER of the pipeline shall be indicated or approved by the Land and Lease Departmentof the Saudi Government Affairs Organization. Approval for the use of such land forSaudi Aramco facilities shall be processed by the Facilities Planning Department. NoSaudi Aramco-controlled land shall be developed or released for development unless therequirements of this standard are met.
• The population density index for a pipeline is the sum of the existing density index andthe virtual density index for each segment of the line, and shall be used as the designbasis of the line.
• Buildings having more than four occupied stories shall be included in the density indexas a number of equivalent buildings. The number of equivalent buildings shall becalculated by dividing the number of stories in those buildings by three and rounding upto a whole number.
• The existing density index for a location shall be determined from a count of the numberof buildings lying within the RER of the pipeline.
- An existing density index shall be calculated for each 1 km (0.6 mile) section ofthe pipeline.
- To determine the existing density index for a pipeline, establish a zone that extends1 RER wide to each side of the pipeline. Divide the pipeline and associated RERzone into 1 km (0.6 mile) long segments. Count the number of buildings andequivalent buildings in each of the segments. The whole number count is theexisting density index for each segment.
• In areas where development is planned, the estimated number and function of futurebuildings are used to determine the virtual density index.
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- A virtual density index shall be calculated for each specific 1 km (0.6 mile) ofpipeline.
- To determine the virtual density index for a pipeline, establish a zone that extendsone RER wide to each side of the pipeline route. Divide the pipeline andassociated RER zone into 1 km (0.6 mile) long segments.
- Calculate the land area in square meters for any development planned for thissegment.
- Multiply the included area by 0.00075 (exactly) and round up. The resultingwhole number is the virtual density index for this segment.
• Temporary facilities which will be in place for less than six consecutive months are notto be included in these calculations.
• The extent of RER zones, the boundaries between location class areas, and the locationclass designations shall be marked on plan drawings. Additionally, the populationdensity index for each km of pipeline shall be provided in all pipeline project proposals.
• Pipeline location classification is determined using Paragraph 7 of SAES-B-064, basedon the PDA:
Class 1: Locations are undeveloped areas for which the population density indexfor any 1 km (0.6 mile) segment is 10 or less.
Class 2: Locations are areas for which the population density index is 11 through30. The portion of subsea pipelines located between LowestAstronomical Tide (LAT) and points 0.4 km (0.25 mile) on the seawardside of the LAT-line shall be designated for Construction Type 2.Construction Type 2 shall be the minimum used for the portion of thesepipelines located between the LAT-line and the onshore anchor.
Class 3: Locations are areas for which the population density index is more than30, or which include primary or secondary highways as defined by theSaudi Arabian Government Ministry of Communications.
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Class 4: Locations are areas in which a school, hospital, hotel, prison, or shoppingmall or similar retail complex is located, as well as any Class 3 areaswhich include buildings of more than four occupied floors.
A single transportation pipeline typically will have multiple location classifications associatedwith it, based on the PDA results along its length.
• Refer to SAES-L-003 to determine the design factor, F, based on the location class. Forcross-country and submarine pipelines in hydrocarbon service, the design factor is anfollows:
Location class 1: F = 0.72.Location class 2: F = 0.60.Location class 3: F = 0.50.Location class 4: F = 0.40.
For instance, Paragraph 3.2.6 of SAES-L-003 states that for any buried piping inhydrocarbon service within 150 m (500 ft.) of critical plant equipment, the design factor,F, shall not exceed 0.50.
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Temperature Derating Factor
The temperature derating factor for transportation piping, T, is a function of temperature andaccounts for the reduction in pipe material yield strength as temperature increases. Table841.116A of ASME/ANSI B31.8, which is shown in Figure 5, provides values for T. T maybe taken as 1.0 for ASME/ANSI B31.4 systems. This is consistent with B31.8 at 120oC(250oF) or less, since the maximum permitted design temperature for a B31.4 system is120°C (250oF).
ASME/ANSI B31.8 TABLE 841.116A TEMPERATURE DERATING FACTOR T FORSTEEL PIPE
TEMPERATURETEMPERATURE
DERATING FACTOR, ToC oF
120 OR LESS 250 OR LESS 1.000150 300 0.967177 350 0.933204 400 0.900232 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 Engineers.
FIGURE 5
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Determining the Proper "Y" Factor for Plant Piping
The "Y" factor is a function of the type of steel and the temperature, and is determined fromTable 304.1.1 of ASME/ANSI B31.3. This is shown in Figure 6.
General Note: The value of Y may be interpolated between the 28oC (50oF) values shown in the Table. Forcast iron, Y equals 0.0.
Source: ASME/ANSI B31.3 - 1990. With permission from the American Society of Mechanical Engineers.
FIGURE 6
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Identifying Corrosion, Erosion, and Thread Allowances
Allowances for corrosion, erosion, or threads must be accounted for in determining therequired pipewall thickness. This is more of a problem in plant piping because high fluidvelocities or changes in the pressure of the fluid can corrode a pipe. Thread allowances applyonly to smaller diameter pipes which may be threaded. Corrosion, erosion, and threadallowances are determined in conjunction with the corrosion or process engineer and are oftenspecified in a pipe specification. The appropriate allowance is added to the thickness that wascalculated for internal pressure to arrive at a total required pipewall thickness.
Calculating the Thickness Value
Everything necessary to calculate the required pipe thickness for internal pressure has beendiscussed. To determine the internal pressure thickness, substitute the values discussed intothe appropriate equation. Work Aid 1 summarizes the overall calculation procedure.
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ADJUSTING PIPEWALL THICKNESS FOR EXTERNAL PRESSURE
A piping system may be exposed to an external pressure, and the required wall thickness maybe governed by external pressure rather than internal pressure. This might be the case forlarge-diameter/thin-walled process plant piping that is subject to vacuum conditions, orunderwater pipelines, which must withstand the hydrostatic head of the water above them.Therefore, the Saudi Aramco engineer must ensure that the pipewall thickness is adequate fora 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. These forces are often identified by the process engineer. For example, asubmarine pipeline may be exposed to an external pressure due to the liquid head ofsurrounding water being greater than the internal pressure. Piping components behavedifferently under these forces than when they are exposed to internal pressure. Thisdifference in behavior is due to buckling or elastic instability which makes the pipe weaker incompression 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 thinwall.
The ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Paragraph UG-28,provides a procedure for evaluating cylindrical shells under external pressure. Pipe geometryfactors, (unsupported length, outside diameter, and thickness), material strength, and designtemperature are used to determine the thickness required to resist external pressure.
Work Aid 2 outlines the procedure for calculating the required pipe thickness for externalpressure.
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IDENTIFYING THE PROCEDURE FOR EVALUATING OTHER LOADS THATARE APPLIED TO BURIED PIPE
Transportation pipelines often have buried sections of pipe. The required thickness of theseburied 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 engineerneeds 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 Paragraph 2.7 ofSAES-L-046, Pipeline Crossings Under Roads and Railroads. The pipe must be designed forthe traffic load, soil weight, and passive soil reaction.
At railroad and highway crossings where the loads may apply, the pipe must be designedaccording to API Recommended Practice 1102, Liquid Petroleum Pipelines CrossingRailroads and Highways. It provides the formula for determining circumferential stress in acarrier 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 and soil andtraffic loads as follows:
S = 6 K b WERT
ET 3 + 24 K z PR 3
where: S = Circumferential stress due to external loads, psi.P = Internal pressure, psi.R = Outside radius, in.T = Wall thickness, in.Kb = Bending parameter.Kz = Deflection parameter.E = Modulus of elasticity of metal.W = Traffic load (SAES-L-046), lb.
The stress calculated in accordance with this equation is limited to the Specified MinimumYield 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 isrequired, and how the casing should be designed.
Saudi Aramco has a computer program that makes the calculation. This can be done throughthe Consulting Services Department (CSD.) All the load factors required by SAES-L-046 arein the computer program, as well as the required parameters. It is beyond the scope of thiscourse to determine the stress.
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SELECTING PIPE SCHEDULE THAT TAKES INTO ACCOUNT THEMANUFACTURER’S TOLERANCES AND SAUDI ARAMCO’S MINIMUMREQUIREMENTS FOR PIPEWALL THICKNESS
After the required pressure thickness is determined, the next greater available standard pipethickness must be selected, taking into account the manufacturer's tolerance. Fortransportation pipelines, it is sometimes advantageous to specify the exact thickness requiredrather than using a standard pipe thickness. Because a transportation pipeline can be manymiles long, the cost increase associated with ordering a special thickness is far outweighed bythe savings associated with not paying for excess thickness. Standard pipewall thicknessesare 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 thisspecification).
The maximum manufacturer's undertolerance for pipewall thickness is 12.5% for carbon andlow-alloy steels. For high-alloy steels it is 10%. Most seamless piping systems will be in the12.5% category. When pipe is supplied, the actual thickness can be minus 12.5% of thenominal thickness. This undertolerance must be accounted for in B31.3 piping, but does notneed to be considered for B31.4 and B31.8 piping systems. The design factors used in B31.4and B31.8 systems inherently account for the mill tolerance. In addition, piping that is usedfor transportation pipelines is often rolled from plate material specifications. Plate ismanufactured to more stringent thickness tolerances than pipe manufacturing standards.
The procedure for selecting pipe schedule is outlined in Work Aid 3.
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CALCULATING THE MAXIMUM ALLOWABLE OPERATING PRESSURE(MAOP)
MAOP establishes permissible operating limits to withstand internal pressure, especially fortransportation piping. The engineer must determine MAOP for pipe as well as other pipingcomponents. This module discusses MAOP for pipe. The MAOP of a pipe or other pipingcomponent will be at least equal to the design pressure. However, the MAOP can be higherthan the design pressure since use of a standard wall thickness will typically provide anadditional margin.
The procedure for calculating MAOP is outlined in Work Aid 4.
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SAES-L-006, PARAGRAPH 2.3
The minimum wall thickness (Schedule) of carbon steel pipe shall be as follows:
Nominal Size Hydrocarbon ServiceLow-PressureUtility Service
mm in._ 50 _ 2 SCH 80 SCH 40 (see 3.9)
75 - 150 3 - 6 SCH 40 SCH 40
200 - 800 8 - 32 6.5 mm (0.250 in.) 6.5 mm (0.250 in.)
_ 850 _ 34 Diameter /135 Diameter/135
Note: Schedule 160 nipples shall be used for 50 mm (2 in.) and smaller pipe sizes in vibration service wherebracing cannot be effectively provided.
FIGURE 10
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WORK AID 4: GUIDELINES FOR CALCULATING MAOP
1. Subtract mill tolerance, (expressed as a decimal fraction), x, from the nominal pipe wallthickness, T , for ASME B31.3 piping to determine the minimum possible as - suppliedthickness, T , as follows:
T = ( 1 − x ) T
“x” may be taken as zero for ASME B31.4 and B31.8 piping systems.
2. Subtract any other allowances, such as corrosion allowance, c, to calculate the minimumpossible pipe thickness, t, as follows:
t = T - c
3. Reverse the applicable internal pressure equation to calculate a value for MAOP.
4. Calculate MAOP with the factors identified earlier.
For ASME/ANSI B31.3, Plant Piping, use the following equation to calculate MAOP:
MAOP = 2 tSE
D − 2 tY
For ASME/ANSI B31.4 or B31.8, Transportation Piping, use the following equation tocalculate MAOP:
MAOP = 2 St
D
FET
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GLOSSARY
allowable hoopstress
The limit on stresses due to internal pressure.
ANSI American National Standards Institute
API American Petroleum Institute
carrier pipe The pipe used to transport any liquid or gas.
casing A pipe through which the carrier pipe is installed.
circumferentialbending stress
A stress caused by the bending of pipe caused by acircumferential moment applied locally to the pipe.
contingentoperatingconditions
Uncontrolled shutdown of plants. Improper operation due toa single act or operating decision. Failure of a device tofunction, fire, or ambient conditions.
design factor Factor used for transportation piping that depends onpopulation density or other factors.
locationclassification
A classification for pipelines going through populated areas,based on the actual or expected population density index.
normal operatingconditions
Conditions expected to occur during normal operation perdesign, excluding failure of any operating device, operatorerror, and the occasional, short-term variations stated in theapplicable code.
population densityindex
A value based on the number of buildings within the areadefined by the rupture exposure radius. Used when doing apopulation density analysis.
rupture exposureradius
The distance on either side of a pipeline, used for conductinga population density analysis.
weld-jointefficiency factor
Factor used for internal pressure calculations, that depends onthe pipe material and the pipe manufacturing process.
