18 Major Differences between ASME B31.3 and B31.1 | B31.3 vs B31.1

What is ASME B31.3 or Process piping Code?

ASME B31.3 or Process Piping Code provides rules for piping design for petroleum refineries; onshore and offshore petroleum and natural gas production facilities; chemical, pharmaceutical, textile, paper, ore processing, semiconductor, and cryogenic plants; food and beverage processing facilities; and related processing plants and terminals. This code is known as the Bible for Process Piping Professionals. So this code dictates the design considerations of process plants.

What is ASME B31.1 or Power Piping Code?

ASME B31.1 or Power Piping Code provides rules for piping typically found in electric power generating stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems. This code is very important for Power piping Professionals as this code governs the design rules for power generation plants.

During piping stress analysis of a piping system, it sometimes seems that there is no major difference between ASME B31.3 and ASME B31.1. As simply changing the codes in the input spreadsheet is sufficient for analysis. But if we take a closer look at both B31.3 vs B31.1, we can understand that there are some major differences in the rules, applications, and guideline considerations between ASME B31.1 and ASME B31.3. In this article, we have listed 18 major differences between the process piping (B31.3) and the power piping (B31.1) code.

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Difference Between ASME B31.3 and ASME B31.1 (B31.1 vs B31.3)

From the above discussions, it is clear that both ASME B31.3 and ASME B31.1 codes i.e Process Piping Code and Power Piping code are different. Both are related to piping design aspects but vary widely in design considerations. The following table lists the major differences between ASME B31.3 and ASME B31.1.

Sr. NoParameterASME B31.3-Process PipingASME B31.1-Power Piping
1Scope (B31.3 vs B31.1)ASME B31.3 provides rules for Process or Chemical Plant piping.ASME B 31.1 provides rules for Power Plant piping.
2Basic Allowable Material StressAs per ASME B31.3, the basic allowable material stress value is higher (For example the allowable stress value for A 106 B material at 250 Deg C is 132117.328 Kpa as per ASME B 31.3) than the same as per B31.1.The basic allowable material stress value as per ASME B31.1 is lower  (For example the allowable stress value for A 106 B material at 250 Deg C is 117900.344 Kpa as per ASME B 31.1) than that of ASME B31.3.
3Allowable Sagging (Sustained)ASME B31.3 code does not specifically say about any limit of allowable sagging. An allowable sagging of up to 15 mm is acceptable in general. B31.3 does not provide a suggested support span.ASME B31.1 clearly specifies the allowable sagging value as 2.5 mm. Table 121.5-1 of ASME B 31.1 provides a suggested support span.
4SIF on ReducersProcess Piping Code ASME B31.3 does not use SIF (SIF=1.0) for reducer stress calculationPower Piping code ASME B31.1 uses a maximum SIF of 2.0 for reducers while pipe stress calculation.
5Factor of SafetyASME B31.3 uses a factor of safety of 3; relatively lower than ASME B31.1.ASME B31.1 uses a safety factor of 4 to have higher reliability as compared to Process plants
6SIF for Butt Welded JointsB31.3 uses a SIF of 1.0 for butt-welded jointsB31.1 uses a SIF of up to 1.9 max in stress calculation.
7Approach towards SIFASME B31.3 uses a complex in-plane, out-of-plane SIF approach.ASME B31.1 uses a simplified single SIF Approach.
8Maximum values of Sc and ShAs per the Process Piping code ASME B31.3, the maximum value of Sc and Sh are limited to 138 Mpa or 20 ksi.For the Power piping code (ASME B31.1), the maximum value of Sc and Sh are 138 Mpa only if the minimum tensile strength of the material is 70 ksi (480 Mpa) otherwise it is dependent on the values provided in the mandatory Appendix A as per temperature.
9Allowable Stress for Occasional StressesThe allowable value of occasional stress as per ASME B31.3 is 1.33 times ShAs per ASME B31.1, the allowable value of occasional stress is 1.15 to 1.20  times Sh
10Equation for Pipe Wall Thickness CalculationThe equation for pipe wall thickness calculation in B31.3 is valid for t<D/6.There is no such limitation in the Power Piping (ASME B31.1) wall thickness calculation. However, they add a limitation on maximum design pressure.
11Section Modulus, Z for Sustained and Occasional StressesWhile Sustained and Occasional stress calculation, the Process Piping code ASME B31.3 reduces the thickness by corrosion and other allowances.ASME B31.1 calculates the section modulus using nominal thickness. Thickness is not reduced by corrosion and other allowances.
12Rules for material usage below -29 Deg. CB31.3 provides extensive rules for use of materials below -29 Deg CThe power piping code, B31.1 does not provide any such rules for pipe materials below -29 deg C.
13Maximum Value of Cyclic Stress Range FactorThe maximum value of cyclic stress range factor, f as per B31.3 is 1.2.As per ASME B31.1, the maximum value of f is 1.0
14Allowance for Pressure Temperature VariationAs per clause 302.2.4 of ASME B31.3, occasional pressure-temperature variation can exceed the allowable by (a) 33% for no more than 10 hours at any one time and no more than 100 hours/year or (b) 20% for no more than 50 hours at any one time and no more than 500 hours/year.As per clause 102.2.4 of ASME B31.1, occasional pressure-temperature variation can exceed the allowable by (a) 15% if the event duration occurs for no more than 8 hours at any one time and not more than 800 hours/year, or (b) 20% if the event duration occurs for not more than 1 hour at any one time and not more than 80 hours/year
15Design LifeProcess Piping following ASME B31.3 is normally designed for 20 to 30 years of service life.Power Piping using ASME B31.1 is generally designed for 40 years or more of service life.
16PSV reaction forceB31.3 code does not provide specific equations for PSV reaction force calculation.ASME B31.1 provides specific equations for PSV reaction force calculation.
17Hydrostatic Test PressureAs per ASME B31.3, the hydrostatic test for the piping system need to be performed at 1.5 times the design pressure corrected for temperature means the design pressure must be multiplied by ST/S in the case of process piping. Here, ST=pipe material allowable stress at test temperature, and S=pipe material allowable stress at component design temperature. (Clause 345.4.2)The hydrostatic test pressure following ASME B31.1 is 1.5 times the piping design pressure. (Clause 137.4.5)
18Pneumatic Test PressureThe pneumatic test pressure as per ASME B31.3 is (1.1 to 1.33) times the design pressure of the piping system. (Clause 345.5.4)B31.1 instructs to use a pneumatic test pressure between (1.2 to 1.5) times the design pressure for the piping system. (Clause 137.5.5)
Differences between process piping and power piping codes (ASME B31.1 vs ASME B31.3)

Other differences between ASME B31.1 and ASME B31.3:

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The other differences between ASME B31.3 and ASME B31.1 are

  • There is a difference in bending and forming requirements in both codes.
  • The welder and brazer qualifications are not identical.
  • The limitations for use of cast irons are different in both codes.
  • The criteria for using soldered, brazed and threaded joints are different in both codes.

The following image (Fig. 1 ) shows a rough estimate of different stress values of the same system with code change in Caesar II-2018 software:

Stress values for the same system with code change
Fig. 1: Stress values for the same system with code change

Few other useful differences for you.

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Difference between Stub-in and Stub-on Piping Connection
Difference between Centrifugal and Reciprocating Compressor
Difference between PDMS and PDS
Difference between Piping and Pipeline
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Difference between Caesar II and Start-Prof
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

17 thoughts on “18 Major Differences between ASME B31.3 and B31.1 | B31.3 vs B31.1

  1. your contents are always useful and valuable. please continue, such that it will be helpful for lot of freshers and entry level engineers.

    1. In case of power piping operating pressure and temperature is high as compared to process piping , so to add extra safety factor we generally go for conservative design in case of power piping

  2. Anup Sir please Explain why sustain and expansion stresses they have different allowable limits in codes. Is there any derivation for that
    ? Please Explain in brief.

    1. I currently work for a district heating utility, and was written up for what I believed was the proper action. Was told cut out a 4’ section of 6” pipe with two threaded 45 fitting, that had small leak at the threads on a low pressure line in a customer basement on a line that feeds three others. After the asbestos was removed it exposed a threaded 6” tee that was blanked. My concern was welding in piping 2” to 3”inches away from a cast threaded fitting would cause the threads to leak or possibly damage the fitting from thermal restricted expansion. That the 6” threaded fitting was not to code and it is a code violation to blank a tee. I removed the tee and had my welder installed a 90. I was told that it was not part of the plan to remove the tee and that because it was low pressure code doesn’t apply. Note: the LP steam is 15psi however I have see it in the 20s and it’s reduced from 150psi without safety valves.

    1. Kindly go to the end of the respective article. Buttons for creating pdf and downloading is provided at the end of each article. However, as each article is dynamic and updated frequently it is preferable to read from the website. You can search for any article and read it directly from the website.

  3. I have a question. In the comparison of B31.1 and B31.3, where can the standard of design life be seen in the code?

  4. dear Anup,
    Thank you for summarizing some very important aspects/rules about ASME B31.3 and B31.1.
    Your article motivates to learn and improve knowledge.

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