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.
Hi, in an excess flow valve, normal flow is in the direction of closing of valve under excess flow. It will allow flow in both directions but when the flow is excessive in one direction, it closes. Assume an NRV whose flap is held back against closing by a spring. As flow increases, it closes the NRV overcoming the spring resistance. This NRV which is held by spring against closing is the excess flow valve. The spring offers no resistance in the opposite direction and hence excess flow valve always works in one direction only to prevent flow. Normally a small leakage ia allowed or a bypass valve is provided to open the u/s side valve initially in case the downstream is under very low pressure to start making the excess flow valve slam shut initially. Normally a loading/unloading hose for pressurized/liquified gas will have excess flow valves at both ends) but opposite direction of flow marking to protect against hose rupturing - the two valves will prevent flows from both ends to atmosphere but in opposite directions - but they will allow normal flow in both directions. Hope I could make it clear.
