When Should the High Pressure Rules in Chapter IX of ASME B31.3 be Used?

When Should the High Pressure Rules in Chapter IX of ASME B31.3 be Used?

The simple answer to this question is they should be used when the owner selects the use of these rules for a particular piping system. Note that the definition of High-Pressure Fluid Service simply requires that the owner specify use of Chapter IX. However, it is probably useful to understand why such a selection may be made.

Some guidance is provided in K300 (a), which states that “High pressure is considered to be pressure in excess of that allowed by the ASME B16.5 PN 420 (Class 2500) rating for the specified design temperature and material group.” This is not a requirement, and the base Code may be satisfactorily used at pressures higher than ASME B16.5, PN 420 (Class 2500). However, the base Code rules become increasingly conservative and, in fact, impossible to use as the pressure approaches the allowable
stress (including quality factors). When this occurs, the wall thickness of straight pipe designed to the base Code becomes equal to the pipe outside radius, so the pipe would have no bore! It would be bar if designed to the base Code. By the same token, Chapter IX may be used at lower pressures; it has been used with high-strength steels with pressures as low as 28,000 kPa (5,000 psi).

A higher allowable stress, with a different pressure design equation can be used when the rules of Chapter IX are followed. As such, the wall thickness can be less. However, the tradeoff is that extensive additional and/or more stringent design/analysis, material testing (e.g. all material is required to be impact tested), examination and leak testing is required. Further, there are few engineering design firms that a properly qualified to design such systems.

The rules of Chapter IX should generally be used when there is sufficient economic justification based on reduced wall thickness to offset the other increased costs. Further, it is used when is becomes simply impractical to use the base Code because of the pressure design equation. Some consideration should also be given as to whether you will be engaging an engineering firm with the proper knowledge and experience to design the piping system using the Chapter IX rules.

Chapter IX bases the allowable stress on 2/3 of the yield strength of the material and gives no consideration to the tensile strength. As a result, considerable thinner piping systems can be designed using Chapter IX when the material has a high yield strength to tensile strength ratio. As a result, its use has been considered for piping systems to be constructed with high yield strength steel with pressures as low as 28,000 kPa (5,000 psi).

There is not a hard line at Class 2500, as some people assume because of the definition included in ASME B31.3, but rather it is a decision to be made by the owner, giving consideration to the economics. Both the base Code and Chapter IX result in safe piping systems, regardless of the pressure. It just becomes impossible to use the base Code when the pressure approaches the allowable stress (including quality factors).

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About The Author

Dr. Becht, Fellow ASME, former Chairman of the ASME B31.3, Process Piping Code, is a recognized authority in pressure vessels, piping, expansion joints, and elevated temperature design. He has more than 40 years of experience in design, design review, analysis, check-out, mechanical integrity, development, troubleshooting, and failure analysis. He has been a member of 14 Codes and Standards committees, five of which he has chaired. He has more than 60 publications including two books (on B31.3 and B31.1 piping) and seven patents and is a frequent speaker and chairman in technical forums. He received the ASME Dedicated Service Award in 2001, the 2009 ASME Pressure Vessel and Piping Medal, the 2014 J. Hall Taylor Medal, and the 2022 ASME B31 Forever Medal for Excellence in Piping. He is CEO of Becht, CEO of Helidex, LLC, and Chairman of Becht Industrial Group.

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When Should the High Pressure Rules in Chapter IX of ASME B31.3 be Used?

11 thoughts on “When Should the High Pressure Rules in Chapter IX of ASME B31.3 be Used?

  1. Author writes in simple manner to understand very easily. My hearty greetings to auther Chuck Becht.

  2. Is there any specific requirement for hydrotest or it is same as given in base code?

  3. The leak test requirements in Chapter IX are more stringent than the base code. While the test pressure is generally the same, it also applies to pneumatic tests. And a completed system tightness check is required. There are other differences as well.

  4. In general the test pressure shall be calculated as per equation 24 of para 345.4.2 ASME B31.3. The same paragraph also adds a paragraph “Alternatively, for CS piping with a MSYS 42ksi, the test pressure for the assembly of components, excluding pipe supporting elements and bolting (e.g., pipe, fittings, valves, flanges), may be based on Rr for any components in the assembly.” One can use this interpretation and use Rr for pipes and the test pressure will be 1.5 x design pressure. Under what circumstances this alternate method should be applied?

  5. I think the alternative is unlikely to apply to most high pressure piping systems due to the yield strength limit of 42 ksi. But, it can be used within the limitations stated in the paragraph, at least with how the code is currently written. There has been a great deal of debate on whether the leak test pressure should be based on the component that would require the highest or lowest test pressure due to ratio of allowable stresses or pressure rating. This change was made so that for typical carbon steel piping systems, the test could be based on the pipe, flanges, or any other component and there is no need to check the required test pressure for all components in the system and take the highest.

  6. I am in agreement with your interpretation. I have found that in the past and also at present designers use the hydrotest pressure as 1.5 x design pressure at design temperature without applying Rr, their interpretation is with alternative rule which allows to use Rr = 1 for CS pipe (Non Chapter IX piping). The argument is the componets are already in accordance to the relevant code tested and stamped, and now the spool need to be tested for leaks (weld joints). I am afraid there is confrontation with 3rd party due to this paragraph and I was looking for your interpretation. In my opinion this alternative idea of the code is debatable.

  7. Dr. Becht,

    I have a question about Chapter IX in B31.3.

    Around 2005 there was a single formula in ASME B31.3 Chapter IX for wall thickness, eqn (34a).

    In ASME B31.3-2006 edition, a change was made so that there was now two formulas for pipe wall thickness in Chapter IX:

    Eqn (34a) for carbon steel; and eqn (34c) for austenitic stainless steel and certain nickel alloys, note the carbon steel formula (34a) does not have the 1.155 factor in the ‘e’ exponent.

    I would like to understand the rationale behind these new two equations. I assume there was some research and testing around this. Where can I get this background?

  8. The equations in B31.3 are based on those developed for ASME Section VIII, Division 3 (Div. 3), the High Pressure Vessel Code. However, the allowable stress in B31.3 is 2/3 of specified minimum yield strength or 2/3 yield strength at temperature, whichever is less, except for austenitic stainless steels and those with similar stress-strain behavior, for which it is 2/3 specified minimum yield strength of 90% of the yield strength at temperature. When Section VIII Div 3 changed the equations, reducing margin, B31.3 followed suit. In both books, the design equations provide a margin on yielding through the entire thickness. In evaluating the reduction in design margins that Div. 3 had implemented for implementation in B31.3, the committee recognized that for certain austenitic and nickel alloys, at some temperatures, the allowable stress is based on 90% of the yield strength rather that 2/3 of the yield strength as for ferritic alloys. Therefore, if the design margin was reduced to follow Div. 3, the margin on through thickness yield for those austenitic and nickel alloys where the allowable stress was based on 90% of yield would be too low. Therefore, the Div. 3 margin was adopted, but an equation with the additional margin of 1.155 was adopted for those austenitic and nickel alloys. It should also be recognized that Div. 3 has recently made further changes to its equations and design margins.

  9. Because of the code wording which exempts listed components from code requirements (including testing) there is at this time a consensus in B31.3 that they do not need to be tested, even those that have never been tested. Personally, I don’t think this is appropriate, but that is how i believe a request for interpretation would presently be answered. It is an area of ongoing discussion.

  10. Is there any requirement by chapter IX of ASME B31.3 that DPI and/or MPI or any NDE is required for all components and welds supplied by vendors? (e.g. all components to be used for high pressure piping per chapter IX of ASME B31.3 required impact tested)
    Not able to interpret the requirement mentioned in para. K341.4, K341.5 and K344.
    Could you please clarify.

  11. See K302.2.1 for unlisted components and K326.3 for listed components. The NDE requirements are for the fabricated piping system and do not apply to the manufacture of components. Impact testing is not as clear, but the intent is that all components be subject to qualification by impact testing. The wording in K302.2.1 is being modified in the 2018 edition to make this clearly a requirement for unlisted components. It should already be clear for listed components.

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