An excess flow valve allows liquid or vapor to flow in both directions; however, the flow is controlled in only one direction.

If the pressure in the pipeline downstream of the installed PLV drops suddenly for any reason (such as a pipe rupture), and the leakage through the gap between the plate and the valve center increases significantly, the force exerted on the valve plate exceeds the spring force, causing the valve to close. This prevents further uncontrolled leakage of the medium into the environment. In addition to preventing hazardous leaks, this mechanism ensures that the damaged pipeline can be repaired or replaced quickly and safely.

Once repairs are completed and all sections of the pipeline are sealed and connected, gas begins to flow slowly through a small orifice in the plate. When the pressure upstream and downstream of the PLV is nearly equalized, the spring engages at a specific moment, and the disc automatically opens. In this manner, the continuous flow of the medium is re-established.