I believe in Excess Flow Valves and Check Valves, even though I have seen them fail more often than work as hoped. However, these failures are “human failures” in the incorrect design (e.g., sizing and placement) of the process where these valves are utilized. The valves are highly reliable when they are DESIGNED, INSTALLED, and TESTED per the manufacturer’s requirements/recommendations. But in nearly every LOPC event I have been involved with where the EFV or Check Valve did not function as intended, it came down to poor engineering and literally NO testing of these safety-critical valves.
I learned in 1996 that as part of, before, the PSSR, these EXFVs and Check Valves MUST be functionally tested. We also must ensure these valves are in our Preventive Maintenance testing program at some specified testing and replacement frequency based on the OEM. Since 1996, in every PSSR I have asked to participate in or facilitate, I had required this functional testing when the PHA claimed the EXFV and/or Check Valve as a “safeguard.” In most tests, these valves have failed to close due to incorrect design downstream of the valves.
For example, one of my favorite EXFV OEM states the following in their spec sheet…
(Emphasis by me)
3. Manually operate the excess flow valve’s poppet BEFORE installation to assure parts were not damaged in shipment or blocked with dirt or foreign material.
…
5. AFTER the excess flow valve is installed, the system should be tested for excess flow valve operation by simulating a break downstream in the system at the farthest point being protected. To test the unit, pressure the system and then open a shutoff valve quickly at the farthest point in the piping that the excess flow valve is intended to protect. There should be a sudden decrease in flow, indicating that the valve has closed and is working properly.
I also learned the hard way that these safety-critical valves must be included in a PM program for functional testing at the OEM’s prescribed frequency. Another significant miss by many facilities. The OEM I use most often in my designs states the following:
“Excess flow closure should be checked annually or on a regularly scheduled basis to ensure that the valve is still functional. Excess flow valves are non-repairable. Replace non-functioning valves.”
Sizing Matters, much like our PRV sizing exercise
A rule of thumb for sizing excess flow valves is to choose a valve with a closing flow of 1.5 times the maximum operating flow. Two (2) times maximum flow may be required for surge conditions. However, the excess flow valve’s closing flow rate MUST BE LESS THAN the capacity of the system in which the valve is being used.
The flow rating of the piping, fittings, pump, valves, and hose on both the inlet and outlet of the excess flow valve must be GREATER than the flow rating of the excess flow valve. If branching, piping length, additional valves, reduction in pipe size, elbows, or other necessary components in the piping system create restrictions which reduce the flow rating to less than that of the excess flow valve rating, the valve will not give excess flow protection, and additional excess flow valves must be installed at these points.