Yes… here we go again! If we wish to claim an Excess Flow Valve as a “layer of protection” against a hose/pipe failure, we then MUST ensure the device is SIZED properly, INSTALLED properly, OPERATED properly, and MAINTAINED properly. This is what is called process safety management and for the life of me, I just can’t understand this out-of-sight = out-of-mind concept, along with how much reliance so many place on these safety valves. Heck, EVEN the manufacturers of these safety valves tell us excess flow valves should NOT be relied upon as the ONLY means of controlling the escape of product in the event of piping damage. When possible, shut-off protection by quick closing valves, with shut-off controls accessible in spite of likely line damage, should be provided IN ADDITION TO, or instead of excess flow valves.
Rightfully so, many of our codes/standards REQUIRE these excess flow valve; unfortunately, it is a rare occasion for us to find them being applied properly. In the past 5-years, I can not even count the number of audit and PHA findings we have suggested regarding Excess Flow Valves and every time we use the manufacturer’s own requirements, so this is not some out of this world “Haywood Process Safety Requirement”. This is the 20th article in which I have discussed Excess Flow Valves and it is common for me to get many questions regarding my articles as they always seem to trigger someone to realize they are like the rest of us… they too have a flaw in their system. These items are taken directly from my favorite EFV manufacturer… Rego (not an endorsement, just my favorite)
An excess flow valve is not designed to close and thus may not provide protection if any of the following conditions are present:
- The piping system restrictions (due to pipe length, branches, reduction in pipe size, or number of other valves) decrease the flow rate to less than the valve’s closing flow. (Valve should be selected by closing flow rating — not just by pipe size).
- The break or damage to the downstream line is not large enough to allow enough flow to close the valve.
- A shut-off valve in the line is only partially open and will not allow enough flow to close the excess flow valve.
- LP-Gas pressure upstream of the excess flow valve, particularly due to low temperature, is not high enough to produce a closing flow rate.
- Foreign matter (such as welding slag) is lodged in the valve and prevents its closing.
- A buildup of process material (sludge), which may be found in LP-Gas, may occur over a period of time and cause the valve to stick open.
- The piping break or damage occurs upstream of an in-line excess flow valve, so the escaping product is not passing through the valve.
- The flow through the valve is in the wrong direction. (Excess flow valves only respond to flow in one direction.)
- The excess flow valve has been damaged, or is otherwise not in operating condition.
- Excess flow valves should be tested and proven at the time of installation and at periodic intervals NOT TO EXCEED ONE YEAR. CAUTION: Testing an excess flow valve in the summer when tank pressures are high will NOT prove that the same valve will also function under low pressure conditions in the winter. ONCE A YEAR TESTING SHOULD BE CONDUCTED DURING THE WINTER.
- The test should include a simulated break in the line by the quick opening of a shut-off valve at the FARTHEST POINT in the piping that the excess flow valve is intended to protect. If the excess flow valve closes under these conditions, it is reasonable to assume that it will close in the event of accidental breakage (clean break) of the piping at any point closer to the excess flow valve.
- In order to test an excess flow valve in a piping system, the flow through the valve must be made to exceed the valve’s closing rating. This testing should ONLY be attempted by trained personnel familiar with the process. If no one at the facility has experience in proper testing, outside expert help should be obtained. The exact procedure used may vary with the installation, advisability of gas discharge and availability of equipment. [SAFTENG NOTE: this testing MUST be done per a Written Procedure, whether it be an Operating Procedure or Maintenance Procedure.]
Documented Limitations of these Emergency Valves
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Not a clean break – Hoses with a split or tear, and pipe lines not completely severed may be emitting LP Gas [NH3] gas in an amount insufficient to cause an “excess” flow. The amount of Gas which can escape through such breaks may be even less than the flow during normal transfer service and under these conditions, the excess flow valve could not be expected to close. |
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Line Restriction Too Great – An excess flow valve installed in a tank outlet will not close if the line beyond it is reduced or if the flow is otherwise restricted by too many fittings or too long a run because the line is incapable of passing the amount of LP-Gas necessary to create an “excess” flow. This condition should be corrected when testing a system by simulating a break at the farthest possible point and replacing any restrictive hose, pipe or fittings. |
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Improper Operating Practice – A restriction can also be imposed upon the excess flow valve by an improperly opened valve at the tank outlet. The shutoff valve should be either fully opened or fully closed. If “throttled,” the valve could reduce the amount of LP-Gas passing through the excess flow valve in a sufficient amount to keep it from closing. Throttling operations should not be performed in the lines being protected by excess flow valves. |
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Impurities in the Line – Dirt, weld slag, broken drill taps, and various other foreign objects have been found jammed between the valve disc and valveseat to prevent excess flow valves from closing. A pre-test of the system would also discover this. |