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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 Aramcos
employees. 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, 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 EncyclopediaSaudi Aramco DeskTop Standards
Pipewall Thickness Calculation
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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 Saudi
Aramco and is intended for the exclusive use of Saudi Aramcosemployees. 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
CONTENTS PAGE
OVERVIEW: DETERMINING PIPEWALL THICKNESS ............................................1
Normal Operating Conditions............................................................................................2
Design Conditions .............................................................................................................2
Contingent Design Conditions...........................................................................................2
CALCULATING THE MINIMUM REQUIRED THICKNESS
FOR THE INTERNAL DESIGN PRESSURE..................................................................3
Equation for Internal Pressure Thickness for Transportation Piping:
ASME/ANSI B31.8 and B31.4..........................................................................................3
Equation for Internal Pressure Thickness for Plant Piping: Code ASME/ANSI B31.3.....4
Determining Design Pressure and Temperature ................................................................5
Determining Allowable Piping Hoop Stress and Joint Quality Factor ..............................6
Transportation Piping ........................................................................................................6
Plant Piping .......................................................................................................................9
Determining Design Factors and Temperature Derating Factor for
Transportation Pipelines ..................................................................................................14
Design Factor ..................................................................................................................14
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 THE
MANUFACTURERS TOLERANCES AND SAUDI ARAMCOS MINIMUM
REQUIREMENTS FOR PIPEWALL THICKNESS.......................................................23
CALCULATING THE MAXIMUM ALLOWABLE OPERATING
PRESSURE (MAOP) ......................................................................................................24
WORK AID 4: GUIDELINES FOR CALCULATING MAOP ......................................26
GLOSSARY ....................................................................................................................27
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OVERVIEW: DETERMINING PIPEWALL THICKNESS
Given the design temperature, design pressure, pipe diameter, and pipe material, the Saudi
Aramco engineer can determine the required thickness of the pipewall. Pipewall thickness is
a function of the allowable hoop stress, established by each code and of SAES-L-003, Design
Stress Cr i teria For Pressure Pipi ng. Each code provides the equation that is used to calculate
internal 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 and
mill tolerance.
Selecting a thickness from an ANSI/API table of standard pipe thicknesses, and
checking 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 B31
piping codes. However, the equations, variables, and definitions or values for allowable
stress differ.
It is important for the engineer to keep in mind the design conditions, normal operating
conditions, and contingent conditions of the piping system when determining the pipewall
thickness. These conditions establish the necessary parameters for pipewall thickness
calculations.
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Normal Operating Conditions
Based on SAES-L-002, Design Condi tions For Pressure Piping, normal operating conditions
are those expected to occur during normal operation per design, excluding failure of any
operating device, operator error, and the occasional, short-term variations stated in the
applicable code. Startup and controlled shutdown of plants, shut-in of wells at the GOSP, and
similar foreseeable events also are included with normal operation.
Design Conditions
Based on SAES-L-002, design conditions are all conditions which govern the design and
selection of pressure piping components, and are based on the most severe conditions
expected to occur in service, in accordance with the code. A margin is used between the
normal 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 of
less 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 pipewall
thickness. To calculate internal pressure thickness given certain design conditions, the Saudi
Aramco 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 pressure
thickness. 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 of
gas 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 be
used 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 thickness
for 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 in
subsequent sections.
For thicknesses 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.
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Determining 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, especially for
plant piping. The values for design pressure and temperature typically are determined by the
process engineer based on process requirements. The values used for piping thickness
calculations allow for the worst combination of design pressure and temperature.
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.
For a transportation piping system attached between two pump or compressor stations,
normal operating conditions and potential contingent design conditions (such as pump failure
at a downstream pumping station which causes a pressure surge) will be determined. The
hydrostatic head in a liquid-filled piping system could also prove to be a significant factor in
cases 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 Pipeli ne Safety,
Paragraph 8, identifies design pressure considerations for transportation piping in which surge
can occur. These are highlighted as follows:
The design pressure used to determine the minimum pipewall thickness per
ASME/ANSI B31.4/B31.8 in location class 3 and 4 zones shall be established by
determining the maximum expected surge pressure from a single contingency, such as
inadvertent closure of a valve, or failure of a sensing or regulating device. Self-actuated
surge protection systems, if provided, shall be assumed to operate as intended, to
mitigate the single worst contingency.
Surge analysis shall be made for liquid-packed services. Surge protection systems shall
be installed if surge pressures are calculated to exceed 110% of the MAOP.
Surge protection systems shall include duplicate, critical subassemblies sufficient to
overcome 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 in
tension for the pipe material, as modified by the joint quality factor. The joint quality factor
depends upon the pipe manufacturing process. The allowable hoop stress is defined by each
code. 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 Specified
Minimum Yield Strength (SMYS). The SMYS for commonly used piping materials may be
found by using Appendix D of ASME/ANSI B31.8. The joint quality factor, E, is determined
by 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 pipe
material 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 the
code.
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ASME/ANSI B31.8 APPENDIX D SPECIFIED MINIMUM YIELD STRENGTH FORSTEEL 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 25,000ASTM 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
ASTM A 134 EFW (NOTE 3)
ASTM A 135 A ERW 30,000
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 52,000ASTM 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
Source: ASME/ANSI B31.8 - 1989. With permission from the American Society of Mechanical Engineers.
FIGURE 1
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ASME/ANSI B31.8 APPENDIX D SPECIFIED MINIMUM YIELD STRENGTH FORSTEEL PIPE, CONT'D
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:
(1) Abbreviations: BW - furnace butt-welded; ERW - electric-resistance welded; S - seamless, FW - flash-
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
Spec. Number Pipe Class E Factor
ASTM A53 Seamless
Electric-Resistance Welded
Furnace Welded
1.00
1.00
0.60
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*
API 5L Seamless
Electric-Resistance Welded
Electric-Flash WeldedSubmerged Arc Welded
Furnace Butt-Welded
1.00
1.00
1.001.00
0.60
*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 hoop
stress. For transportation piping, SAES-L-003 states:
For cross-country and submarine pipelines in hydrocarbon service within the scope of
ASME/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 the
Specified 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, and
considers 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. An
excerpt 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 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.
Obtain the value of the joint quality factor, E, based on pipe material and manufacturing
process 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 the
allowable hoop stress. For plant piping, SAES-L-003 states the allowable stresses as shown
in Appendix A of the code shall be used for wall thickness calculations.
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ASME/ANSI B31.3 TABLE A-1 (EXCERPT) BASIC ALLOWABLE STRESSES INTENSION FOR METALS
Source: ASME/ANSI B31.3 - 1990. With permission from the American Society of Mechanical Engineers.
FIGURE 3
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ASME/ANSI B31.3 TABLE A-1 (EXCERPT) BASIC ALLOWABLE STRESSES INTENSION FOR METALS, CONT'D
FIGURE 3, CONT'D
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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 microalloying, 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 212F. The user should satisfy himself that the alloy selected is satisfactory.
53. Stress relief treatment is required for service above 450F.
54. The maximum operating temperature is arbitrarily set at 500F 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 temperature
conditions.
56. Because of thermal instability, this material is not recommended for service above 800F.
57. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 800F.
58. Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 875F.
59. For temperature above 900F, 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 (or
SMYS), and joint quality factor, E, can be determined for use in the equation for internal
pressure 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 using
SAES-B-064 to determine a pipeline location class, and then SAES-L-003 to determine the
design 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 a
population density analysis (PDA) of the population located within the Rupture
Exposure Radius (RER) along a pipeline route. RER is the distance on either side of a
pipeline that must be included in a PDA, and is a measure of the zone that could be
potentially affected by a pipeline rupture. The PDA, as defined in Paragraph 6 of
SAES-B-064, supersedes instructions in ASME/ANSI B31.4 and B31.8 pertaining to the
population density index. Paragraph 5 of SAES-B-064 defines RER, as follows:
- For pipelines carrying liquid hydrocarbons having a true vapor pressure less than
100 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 vapor
pressure of 100 kPa (15 psig) or greater, and an H2S concentration of less than 1.5
mol %, 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 H2S
concentration 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, per
SAES-B-062. For other producing lines, the RER shall be set equal to the largest
RER of any well that is connected by that line.
The boundaries of areas in which building/development is present or planned within the
RER 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 for
Saudi Aramco facilities shall be processed by the Facilities Planning Department. No
Saudi Aramco-controlled land shall be developed or released for development unless the
requirements of this standard are met.
The population density index for a pipeline is the sum of the existing density index and
the virtual density index for each segment of the line, and shall be used as the design
basis of the line.
Buildings having more than four occupied stories shall be included in the density index
as 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 up
to a whole number.
The existing density index for a location shall be determined from a count of the number
of buildings lying within the RER of the pipeline.
- An existing density index shall be calculated for each 1 km (0.6 mile) section of
the pipeline.
- To determine the existing density index for a pipeline, establish a zone that extends
1 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 and
equivalent buildings in each of the segments. The whole number count is the
existing density index for each segment.
In areas where development is planned, the estimated number and function of future
buildings 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) of
pipeline.
- To determine the virtual density index for a pipeline, establish a zone that extends
one RER wide to each side of the pipeline route. Divide the pipeline and
associated RER zone into 1 km (0.6 mile) long segments.
- Calculate the land area in square meters for any development planned for this
segment.
- Multiply the included area by 0.00075 (exactly) and round up. The resulting
whole number is the virtual density index for this segment.
Temporary facilities which will be in place for less than six consecutive months are not
to be included in these calculations.
The extent of RER zones, the boundaries between location class areas, and the location
class designations shall be marked on plan drawings. Additionally, the population
density 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, based
on the PDA:
Class 1: Locations are undeveloped areas for which the population density index
for any 1 km (0.6 mile) segment is 10 or less.
Class 2: Locations are areas for which the population density index is 11 through
30. The portion of subsea pipelines located between Lowest
Astronomical Tide (LAT) and points 0.4 km (0.25 mile) on the seaward
side of the LAT-line shall be designated for Construction Type 2.
Construction Type 2 shall be the minimum used for the portion of these
pipelines located between the LAT-line and the onshore anchor.
Class 3: Locations are areas for which the population density index is more than
30, or which include primary or secondary highways as defined by the
Saudi Arabian Government Ministry of Communications.
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Class 4: Locations are areas in which a school, hospital, hotel, prison, or shopping
mall or similar retail complex is located, as well as any Class 3 areas
which include buildings of more than four occupied floors.
A single transportation pipeline typically will have multiple location classifications associated
with 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. For
cross-country and submarine pipelines in hydrocarbon service, the design factor is an
follows:
Location class 1: F = 0.72.
Location class 2: F = 0.60.
Location class 3: F = 0.50.
Location class 4: F = 0.40.
Check additional requirements regarding design factor in SAES-L-003, Paragraphs 3.2.2
through 3.2.8.
For instance, Paragraph 3.2.6 of SAES-L-003 states that for any buried piping in
hydrocarbon 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 and
accounts for the reduction in pipe material yield strength as temperature increases. Table
841.116A of ASME/ANSI B31.8, which is shown in Figure 5, provides values for T. T may
be 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 is
120C (250oF).
ASME/ANSI B31.8 TABLE 841.116A TEMPERATURE DERATING FACTOR T FORSTEEL PIPE
TEMPERATURETEMPERATURE
DERATING FACTOR, ToC oF120 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 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 from
Table 304.1.1 of ASME/ANSI B31.3. This is shown in Figure 6.
ASME/ANSI B31.3 TABLE 304.1.1 VALUES OF Y
Temperature, oF 900 and
below
950 1,000 1,050 1,100 1150 and
above
Temperature, oC 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 DuctileMetals
0.4 0.4 0.4 0.4 0.4 0.4
General Note: The value of Y may be interpolated between the 28oC (50oF) values shown in the Table. For
cast 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 the
required pipewall 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 pipewall thickness.
Calculating the Thickness Value
Everything necessary to calculate the required pipe thickness for internal pressure has been
discussed. To determine the internal pressure thickness, substitute the values discussed into
the 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 may
be governed by external pressure rather than internal pressure. This might be the case for
large-diameter/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, the Saudi Aramco engineer must ensure that the pipewall 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. These forces are often identified by the process engineer. 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 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 thin
wall.
The ASME Boi ler and Pr essure Vessel Code, Section VIII, Division 1, Paragraph UG-28,
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.
Work Aid 2 outlines the procedure for calculating the required pipe thickness for external
pressure.
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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 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 Paragraph 2.7 of
SAES-L-046, Pipeli ne Crossings Under Roads and Rai lr oads. 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
Rail roads and Hi ghways. 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 and
traffic loads as follows:
S =6 K b WERT
ET3
+ 24 K zPR3
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 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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SELECTING PIPE SCHEDULE THAT TAKES INTO ACCOUNT THEMANUFACTURERS TOLERANCES AND SAUDI ARAMCOS MINIMUMREQUIREMENTS FOR PIPEWALL THICKNESS
After the required pressure thickness is determined, the next greater available standard pipe
thickness must be selected, taking into account the manufacturer's tolerance. For
transportation pipelines, it is sometimes advantageous to specify the exact thickness required
rather than using a standard pipe thickness. Because a transportation pipeline can be many
miles long, the cost increase associated with ordering a special thickness is far outweighed by
the savings associated with not paying for excess thickness. Standard pipewall thicknesses
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, Stai nless Steel Pipe.
API/5L, Specification for Line Pipe (only for carbon steel pipe that meets this
specification).
The maximum manufacturer's undertolerance for pipewall 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. This undertolerance must be accounted for in B31.3 piping, but does not
need to be considered for B31.4 and B31.8 piping systems. The design factors used in B31.4
and 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 is
manufactured 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 for
transportation piping. 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.
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 where
bracing 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 wall
thickness,T
, for ASME B31.3 piping to determine the minimum possible as - supplied
thickness, 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 minimum
possible 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 to
calculate 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 to
a single act or operating decision. Failure of a device to
function, fire, or ambient conditions.
design factor Factor used for transportation piping that depends onpopulation density or other factors.
location
classification
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 per
design, excluding failure of any operating device, operator
error, and the occasional, short-term variations stated in the
applicable code.
population densityindex
A value based on the number of buildings within the area
defined by the rupture exposure radius. Used when doing a
population 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 on
the pipe material and the pipe manufacturing process.
Recommended