EPA and OSHA put out an Alert in 2007 regarding the use of Excess Flow Valves (EFV) as a “safeguard”. Although that alert has been “archived” the failures we see today align with what that alert warned us about 16 years ago!
While excess flow valves (EFV) are in extensive service and have prevented numerous pipe or hose breaks from becoming much more serious incidents, experience has shown that in some cases the EFV did not perform as intended, usually because of misapplication. Also, undue reliance must not be placed on EFVs as the sole or primary protection to control accidental chemical releases from tanks or piping.
Undoubtedly, our #1 finding in using EFVs is a “design error”! As stated in the alert, the NPGA stated…
Piping system restrictions, such as pipe length, branches, reduction in pipe size, and partially closed shut-off valve, could limit the flow rate through the EFV.
So, the Compressed Gas Association (CGA) explicitly calls out this design flaw in its standard for anhydrous ammonia. (emphasis by me):
5.5.4 Excess flow valves shall be designed to close automatically at the rated flows of vapor or liquid as specified by the manufacturer. The piping, including valves , fittings, and hose, being protected by an excess flow valve SHALL HAVE A GREATER CAPACITY THAN THE RATED FLOW OF THE EXCESS FLOW VALVE, so the valve will likely close in case of failure at any point in the line or fittings.
It is common to find processes where there are ample pipe runs that, if severed, the free flow from the pipe would NOT trip the EFV. And that small pipe failure, that was not enough to trip the EFV, could ultimately drain the entire process of its contents.
Almost all of our findings result from “modifications” made to the original design of the process; hence, understanding the limitations of the EFVs by those evaluating the change will have on safety and health and process safety is CRITICAL.
I like the WARNING that Emerson/Fisher provides on their Spec Sheet:
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WARNING! A break or leak down stream of an excess flow valve that does not allow a flow equal to the valve flow rating will not actuate the excess flow valve and could result in a fire or explosion from leaking gas. After the excess flow valve closes, the leakage through the equalizing hole must be controlled or a hazard can be created. For this reason, the operator must be familiar with the system shutoff valves and close the system if an emergency occurs. Reference NPGA-113–“The Limitations of Excess Flow Check Valves for LP-Gas” |