Once an excess flow valve shuts, the pressure upstream will keep the valve closed until the upstream side and downstream of the valve have equal pressure. When the pressure has equalized the valve will re-open. In the case of a hose rupture, line break situation the pressure will not equalize and the valve will stay closed. In the event of a surge closure with no rupture a bleed hole in the valve seat will allow gradual pressure equalization. This bleed also means that as excess flow valve does not provide 100% flow shutoff. A very small bleed flow will continue.

NOTE: certain piping configurations in which a large pressure drop occurs in the line prior to a line break can mask the break effect and the excess flow valve may not close. This is typically not the case in loading, unloading operations, but it should be considered. Also note that excess flow valves do not protect against small leaks – leaking hose joints, line cracks, etc. 

For more information:

Excess Flow Valve Sizing

Excess Flow Valve Technical Information

Excess Flow Valves