How does an excess flow valve work?

Excess flow valves are spring loaded check valves. The valve seat is held open by a spring. When the flow of the fluid exceeds a certain preset limit the valve seat, or disc in this case, closes and shuts off flow.

Each valve model has been designed, given its spring size, for a maximum flow in gallons per hour. When the flow exceeds that specified rate, the fluid pressure will overcome the spring resistance and the valve will close, cutting off the flow through the valve body. 

In normal operations of loading and unloading, the in-line flow is restricted by the pressure drop through the hose and or pipeline. In the event of a rupture, disconnection, or line break, the pressure will drop, the flow will surge exceeding the spring load limit, and the valve will close.

For more information:

Excess Flow Valve Sizing 

Excess Flow Valve Technical Information

Excess Flow Valve Limitations