# Memo: Internal — Pipeline Safety Interpretation

- **operation:** document
- **citation:** PI-76-077
- **title:** Memo: Internal — Pipeline Safety Interpretation
- **source type:** guidance
- **agency:** Pipeline and Hazardous Materials Safety Administration
- **status:** guidance
- **official:** true
- **published on:** 1976-12-15
- **effective on:** Not available
- **summary:** PI-76-077 response to Memo: Internal concerning 192.161.
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- **source url:** https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/legacy/interpretations/Interpretation%20Files/Pipeline/1976/PI76077.pdf
**body:**

<<<PAGE 1>>>

December 15, 1976
Chief, Southern Region, MTP-50-SO
Acting Director, Office of Pipeline
Safety Operations
Interpretation of 192.161(d)(1)
Your memorandum of August 20, 1976, asks, "Are pipe stacks installed for the purpose of
controlling relieved gas, under overpressure conditions, subject to rule 192.161(d)(1)?"
Section 192.161(d)(1) requires:
(d) Each support on an exposed pipeline operated at a stress level of 50 percent or
more of SMYS must comply with the following:
(1) A structural support must not be welded directly to the pipe.
Section 192.3 defines the word "pipeline" as "all parts of those physical facilities through which
gas moves in transportation including pipe, valves, and other appurtenances attached to pipe,
compressor units, metering stations, regulator stations, delivery stations, holders and fabricated
assemblies."
These pipe stacks do not contain gas in transportation but are only used to safely vent gas to
atmosphere that has been removed from the transportation system by the operation of an
emergency relief valve. Thus, the vent pipe for a relief valve is not subject to these restrictions.
When designing these pipe stacks, consideration must be given to the extreme forces and
vibrations that often result from the discharge of a high pressure jet of gas. Stresses developed as
a result of this jet can cause failure of the stacks and may also be transferred to the relief valves
and other pipeline facilities with a resulting possibility of failure. Considering the usual industry
practice of designing discharge stacks for relief valves using pipe of equal or larger diameter than
the relief valve size and the fact that relief valves in a full open condition are operating at critical
flow rates, it is doubtful that stress level due to internal gas pressure would reach 50 percent of
SMYS. On a high pressure relief, the combined forces of valve vibration, gas stream thrust, and
bending moments are very severe and must be considered in the design of the relief stack to
prevent the failure of such piping.
Cesar DeLeon, MTP-1
dal/192.161
76-12-15
1

<<<PAGE 2>>>

MEMORANDUM
DATE: 8/20/76
TO: Acting Director, MTP-1
Office of Pipeline Safety Operations
FROM: Chief, Southern Region
SUBJ: Interpretation of 192.161(d)(1)
Are pipe stacks installed for the purpose of controlling relieved gas, under
overpressure conditions, subject to rule 192.161(d)(1)?
During normal operating conditions, these stacks would be under atmospheric
pressure.
If the answer is affirmative, I would also like a technical opinion as to whether,
under relieving conditions, it is reasonable to expect stack pressures similar to line
pressures when the stacks are essentially open ended.
James C. Thomas
dal/192.161
76-12-15
2
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