58231654 Pipewall Thickness Calculation

7/28/2019 58231654 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 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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Engineering Encyclopedia Piping & Valves

Pipewell Thickness Calculation

Saudi Aramco DeskTop Standards

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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Saudi Aramco DeskTop Standards 1

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








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









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.


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


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.


FIGURE 3, CONT'D


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TABLE A-1B BASIC QUALITY FACTORS FOR LONGITUDINAL WELD JOINTSIN PIPES, TUBES, AND FITTINGS, E


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.)


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.)





- 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.




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.


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.


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.

58231654 Pipewall Thickness Calculation

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