The primary purpose of an excess flow valve is to protect against excessive flow when breakage of pipe lines or hose rupture takes place. When we refer to breakage or rupture, a clean and complete separation is assumed. It is obvious that, if the damage is only a crack or if the piping is crushed at the point of failure, the escaping flow will be restricted and may or may not pass sufficient vapor or liquid to cause the excess flow valve to close.

An excess flow valve, while in its normal open position permits the flow of liquid or gas in either direction. Flow is controlled in one direction only. Each excess flow valve is stamped with an arrow showing the direction in which the flow is controlled. If the flow in that direction exceeds a predetermined rate, the valve automatically closes. Manufacturers' catalogues show the closing flow rating both in terms of liquid and vapour. Since excess flow valves depend on flow for closure, the line leading away from the excess flow valve should be large enough so that it will not excessively restrict the flow if the pipe run is unusually long or restricted by numerous elbows, tees or other fittings. Consideration should be given to the use of larger size pipe and fittings. Never use a pipe size smaller than that of the excess flow valve.

It is considered good practice to select an excess flow valve with a rated closing flow approximately 50% greater than the anticipated normal flow. This is important because valves which have a closing flow very close to the normal flow may chatter or slug closed when surges in the line occur either during normal operation or due to the rapid opening of a control valve.

Excess flow valves should be tested and proven at the time of installation and at periodic intervals not to exceed one year. The tests should include a simulated break in the line by the quick opening of a shut off valve at the farthest possible point in the piping which the excess flow valve is intended to protect. If the valve closes under these conditions, it is reasonable to assume that it will close in the event of accidental breakage of the piping at any point closer to the excess flow valve.

See Rego's Products Safety Warning WB-3 for important warning information.