Pipeline Pressure Primer

Samuel Tirer, MD; Mike Mahan, PE

Numerous questions to the Committee on Technology are individually and quickly answered each quarter by knowledgeable committee members. Many of those responses would be of value to the general readership, but are not suitable for the Dear SIRS column. Therefore, we have created this simple column to address the needs of our readership.

Pipeline Pressure Primer

Q Dear Q&A,

I have always thought that, in a hospital central gas supply system, the oxygen pipeline should operate at a slightly higher pressure than the air and nitrous oxide lines in order to mitigate the effects of a possible cross connection. I am working at a new hospital and the pipeline person is asking for documentation. Is this just an informal safety measure or is it mandated by code? Thanks for your help.

Samuel Tirer, MD

ADear Dr. Tirer,

The National Fire Protection Association (NFPA) 99 Standard for Health Care Facilities, 2005 Edition, states, “Piping systems, with the exception of nitrogen systems, shall be capable of maintaining 50-55 psig (345-380 kPa gauge) to all outlets at the maximum flow rate.” Reading the NFPA is like reading IRS 1040 instructions, so perhaps there is a qualifier (regarding different oxygen pressure) somewhere else in the document that I missed in my scan. But I have never heard of different wall pressure for O2.

I believe there is not a standard driving this, but a local preference. I have heard of hospitals setting oxygen at the top of the allowable range, such that if a check valve failed somewhere in the system, the oxygen would prevent potentially hypoxic scenarios.

Using the highest pressure for oxygen in a pipeline system is a very old practice for 2 reasons:

  1. It allows one to certify that day-to-day running of a mixed pipeline system is “safe” without using an oxygen analyzer on each outlet. In many parts of the world this is the routine safety check. Where gas-mixing devices are used, as with nitrous/oxygen for analgesia, it could be part of the basic design.

  2. If there is any sort of link between 2 lines, better that oxygen dilutes the other. I think you’ll find this rule originates in old British standard safe practice that preceded people writing specs.

The APSF Committee on Technology

ADear Dr. Tirer,

The pressure ranges listed in Table 5.1.11 from NFPA 99-2005 for medical air, oxygen, nitrous oxide, helium, and carbon dioxide are the same (50-55 psi). I can find nothing that says one should be greater than the other to avoid cross connections. NFPA 99 also requires stringent initial testing, and specifies testing that is necessary after working on the system.

There are 2 tests acceptable to NFPA to initially verify that there are no cross connections in a system. One is called the Individual Pressurization Test, whereby the system being checked shall be pressurized to 50 psi, while all other disconnected, atmospheric lines are simultaneously checked to determine that test gas is being dispensed only from the outlets/inlets of the piping system being tested.

What some of your contacts may be referring to is the other acceptable method, or Pressure Differential Test. This test does require that the oxygen system and the medical air system (and others) be pressurized and maintained to different, specified psi (50 psi, 60 psi, respectively for oxygen and medical air), after which a pressure test is made at every outlet to check for cross-connections.

NFPA does NOT say that the system needs to be operated during normal use with those pressures—rather only for the verification test.

Mike Mahan, PE
North Carolina Baptist Hospital
Engineering
Winston-Salem, NC


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