What prevents excess flow valves from closing when the line breaks? There are plenty of things that can prevent EFVs from operating as designed. These include design failures, mechanical failures due to poor or no maintenance, and “human failures.”
1. Not a Clean Break
For one or a combination of the following reasons, excess flow valves have been prevented from closing in emergencies:
Hoses with a split or tear and pipelines not completely severed may emit hazardous material insufficiently to cause an “excess” flow. The amount of HAZMAT that 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.
2. 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 cannot pass the amount of HAZMAT 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.
3. Improper Operating Practice
An improperly opened valve at the tank outlet can also restrict the excess flow valve. The shutoff valve should be either fully opened or fully closed. If “throttled,” the valve could reduce the amount of HAZMAT passing through it sufficiently to keep it from closing. Throttling operations should not be performed in the lines being protected by excess flow valves.
4. Improper Selection
The many types of excess flow valves available are designed for specific jobs. The excess flow valve selected should remain open during normal flow but close at “excess” flow. An inspection that simulates a line break prior to start-up operations will determine if the proper valve has been selected.
5. Tampering with Excess Flow Valves
Sometimes, operators, annoyed with frequent closures of an excess flow valve with too low a rating, mutilate the valve and forget to replace it with a properly rated one. A pre-test of the system would reveal this and allow the excess flow valve to be replaced.
6. Impurities in the Line
Dirt, weld slag, broken drill taps, and other foreign objects have been found jammed between the valve disc and seat to prevent excess flow valves from closing. A pre-test of the system would also discover this.
Because of these limitations, it is good industry practice not to rely entirely on excess flow valves for protection. In addition to excess flow valves, emergency shut-off valves with remote controls are recommended.
Testing
The National Propane Gas Association Safety Bulletin #113-78 states:
“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.
In general, most testing makes use of the fact that excess flow valves are “surge sensitive” and will close quicker under a sudden flow surge than under steady flow. A sufficient surge can often be created by using a quick open/close valve to control sudden, momentary flow into a tank or piping section containing very low pressure. An audible click from the excess flow valve (and corresponding stoppage of flow) indicates its closure.
A test involving venting gas to the atmosphere is hazardous and may be impractical, or illegal. Any test of any excess flow valve will not prove that the valve will close in an emergency situation, due to reasons cited before. This test will only check the valve’s condition, and the flow rate sizing for those test conditions.”