Question: You ask whether strict compliance with 49 CFR is mandatory or not.

Yes Virginia, you must comply with the regulations.

While we now have over  40 years experience with federal pipeline safety regulations, compliance and enforcement, in the early years there were many questions about the application of these new rules.  Prior to 1970 gas operators in many states were required to meet the requirements set out in ASME B31.8, yet there were some differences as to the application of compliance.

With the advent of the regulations in 49 CFR there was now a consistent set of safety standards that had to be followed.

Questions about application of the regulations also were common, comparing B31.8 to the new Part 192.

The following interpretation reinforces the fact that compliance with at least the minimums in the regulations is required and further explains a difference in Part 192 from the B31.8 standard.

Interpretation 192.103  4
September 26, 1980

Mr. J. K. Campbell, P. E.
Venart Industries Limited
215-52nd Street
Delta, British Columbia, Canada V4M 2Y3

Dear Mr. Campbell:

This refers  to your letter dated August 11, 1980, in which you requested an interpretation of 49 CFR Part 192, Subpart C – Pipe Design, as it relates to compressor station main piping.
Question: You ask whether strict compliance with 49 CFR is mandatory or not.
Response: All pipelines subject to the Natural Gas Pipeline Safety Act (NGPSA) must comply with the prescribed requirements of 49 CFR Part 192, “Transportation of Natural and Other Gas By Pipeline,”  (copy enclosed).  These requirements are considered as minimums that must be met.  Under Section 11(a) of the NGPSA, operators of pipelines found not in compliance with 49 CFR Part 192 are subject to a civil penalty not to exceed $1,000 for each violation for each day the violation persists up to a maximum of $200,000 for any related series of violations.

Question: You ask if we agree with your interpretation of the maximum stress levels allowable under 49 CFR Part 192.
Response: No. It appears that you have not considered the performance requirements contained in §192.103, General, which states: “Pipe must be designed with sufficient wall thickness, or must be installed with adequate protection, to withstand anticipated external pressures and loads that will be imposed on the pipe after installation.”  The purpose of this performance requirement is to take care of those stresses which are not relieved after construction and proof testing that become additive to operating stresses.  Some of the stresses which are of concern specifically with compressor station design are those caused during blow downs and repressurization of discharge lines, by effects of soil settlement, by major temperatures changes, by mechanical and sonic vibration, and pulsation.

We wish to emphasize that the design factor (F) contained in §192.111 is required to be used only in determining the pressure containment abilities of the pipe.  However, a person responsible for designing a pipeline, including compressor station piping, must use appropriate design factors and /or provide protection against the accumulative effects of all relevant stresses. Part 192 does not specify the design factors to be used in considering stresses other than those produced by internal pressure although the design factor (F) contained in §192.111 would be considered appropriate.

Thank you for your very helpful analysis and discussion of MOHR circle diagrams explaining the complexity involved in analyzing combined stresses in compressor station piping.


Melvin A. Judah
Acting Associate Director for
Pipeline Safety Regulation
Materials Transportation Bureau

Venart Industries Limited
215-52nd Street
Delta, British Columbia, Canada V4M 2Y3

August 11th 1980

Office of Pipeline Safety
400 Seventh Street                                                          By Registered Mail
Washington, D.C. 20590

Attention: Mr. Paul Quarrie, Safety Engineering Group
cc: Mr. Cesar De Leon, Director of Pipeline Safety Regulation

Dear Mr. Quarrie,

Re: Safety Regulations Governing The Permissable Stress Levels In Natural Gas Compressor Station Main Piping.

You may recall our telephone conversation of May 13th. 1980, during which we discussed the intent of the Department of Transportation under the Code of Federal Regulations, Part 192, Title 49, specifically with reference to Subpart C, Pipe Design.

Since we are close to concluding our investigations and analyses of the applicable codes and regulations etc., on behalf of a client who wishes to participate in U.S. natural gas compressor station projects, we would appreciate your comments on the contents of this letter, which elaborates on the subject matter we discussed during our earlier telephone conversation.
Our interpretation of Subpart C, Pipe Design, for compressor station main piping, and with which you concurred, is that the Design Factor ‘F’ is to be used as the overall controlling factor in determining the safe maximum stress in the pipe, taking into account,(a) The hoop stress in

the pipe wall generated by the internal pressure, and (b) The compressive stresses from bending and anchor restraints etc., along with torsional and shear stresses, all resulting from temperature changes,(and pressure at changes in direction), etc., the so called “expansion” stresses.

The industry code, B31.8, appears to assume that the stresses described under (a) and (b), have mutually exclusive rights to the same territory, taking no account of the accumulative effect of these combined stresses, other than to require that the summation of all stresses should not exceed the SMYS of the pipe material. Such situations of course, totally negate the intent and effect of Design Factor ‘F’.

Our understanding, taking compressor station piping as the example, and with ‘E’ and ‘T’ factors both equal to one, is that CFR 192 requirements are that the effects of the ‘combined stresses’, hoop, compressive, torsion and shear, must not exceed 50% of the SMYS of the pipe material. In more accurate engineering terms, this could perhaps be rephrased two other ways: Either (1)”When combined stresses exist,  the summation of the compressive and tensile principal stresses,(at 90° to each other, and where the shear stress is zero), should not exceed 50% of the SMYS of the pipe material;” or (2) “When combined stresses exist, the resultant maximum shear stress should not exceed 25% of the SMYS of the pipe material,” since ductile materials are considered to fail in shear under all load conditions, and the shear yield strength is half of the SMYS in tension.

Among our reasons for requesting your confirmation of our interpretation of permissable stress levels allowable under CFR 192, are that we are somewhat confused in that according to our enquiries, the formula in Paragraph 192.105(a) appears to be widely used to establish the wall thickness of compressor station piping, with the 0.50 Design Factor from Paragraph 192.111. However, the remaining 50% of the SMYS is considered to be available and is used to absorb the other stresses which are always present.

In addition, the computer programs used by the industry to analyse the stress levels in the pipe wall, as well as to generate information on the forces and moments etc. imposed by the piping on the station machinery and allied equipment, do not consider the additive effects of the combined stresses. The computer outputs indicate only whether or not the various component stresses are in compliance with the industry codes.
We have enclosed a group of MOHR circle diagrams which illustrate the real effects of combined stresses when the 0.50 Design Factor is used to determine the pipe wall thickness, and when the same wall thickness is subjected to the other combined ‘expansion’ type stresses, etc.

Sketch # 1 shows the maximum stress levels which result when the hoop stress level is 50% of the SMYS, and when the effects of all expansion type stresses are neutralized. The resultant maximum shear stress of 25% of the SMYS is at the level intended by the Design Factor (F) of 0.50 in Paragraph 192.111.
Sketch # 2 shows the combined effects of a hoop stress level of 50% of the SMYS and a compressive stress level of 72% of the SMYS, with zero torsion/direct shear stress imposed. In this case, not only is the safety consideration of the Design Factor of 0.50 eclipsed, the resultant shear stress exceeds the shear yield strength by a considerable margin.

Sketch # 3 shows the combined effects of a hoop stress level of 50% of the SMYS and an imposed torsion/direct shear stress level of 36% of the SMYS, with zero compressive stress imposed. In this case, while the shear yield strength of the pipe material is not exceeded, the safety considerations of the Design Factor of 0.50 are not in effect.
Sketch #’s 4,5,6,7 & 8 show the combined effects of hoop stress levels of 50% of the SMYS and the maximum permissible shear stress levels which can be imposed in conjunction with imposed compressive stress levels of 12%, 24%, 36%, 48% and 60% of the SMYS respectively.

From the above, it would seem that in order to preserve the safety intended by the Design Factor of 0.50,(which, recognizing the consequences of a failure in a compressor station piping system, cannot be considered as an overly conservative Design Factor), then either additional wall thickness should be provided in the pipe to accomodate these additional stress loads, or, as illustrated in Sketch # 1, the effects of these additional stress loads should be neutralized.
Fortunately, equipment is now available for use in compressor stations which limits the stress in the pipe to the hoop stress tangentially and the corresponding longitudinal stress which is always half the level of the hoop stress. With the effects of all the expansion type stresses neutralized, then the wall thickness established by the formula in Paragraph 192.105 would in fact experience the stress levels intended by the Design Factor of Paragraph 192.111, which would then leave the intended safety considerations intact.

The Office of Pipeline Safety may feel that nothing should be done to correct any compressor station installations which are operating with stress levels higher than intended by the 0.50 Design Factor. However, with the increased worldwide sensitivity of the public, including labor organizations, to the relevant government safety regulations governing major industries, and recognizing the current plans for the construction of a large number of new natural gas compressor stations, the timing may be appropriate for clarifying the permissable stress levels discussed in this letter, and the order of precedence of CFR 192 with regard to the industry codes.

We trust you will find this letter of interest, and would appreciate a response at your earliest convenience, confirming, (or otherwise), our interpretation of the maximum stress levels allowable under CFR192, and whether strict compliance with CFR 192 is mandatory or not.

Yours very truly,
J.K Campbell, P. Eng.

All this and more in WinDOT, The Pipeline Safety Encyclopedia


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