Differential pressure overflow valve

Claim 1

A differential pressure overflow valve having a housing, an inlet and an outlet to said housing, an inlet and outlet pressure chamber in said housing connected respectively to said inlet and outlet, a valve seat in said housing between said inlet and outlet chambers, a flexible diaphragm in said outlet pressure chamber attached to said housing, a valve plate supported by said diaphragm, a first spring bearing on said diaphragm and biasing said valve plate against said valve seat to effect closure of the latter, a differential pressure indicator in said housing, said indicator having a cylinder, a first passage connecting said inlet pressure chamber to said cylinder, a second passage connecting said outlet chamber to said cylinder, a piston in said cylinder and spaced therefrom to provide an annular gap, stop means in said cylinder for said piston located between said first passage and said piston, a second spring acting on said piston to bias the latter against said stop means, said cylinder being in the form of a transparent tube carrying a scale, a lift on said piston due to pressure exerted thereon through the first passage indicating the differential pressure between said inlet and outlet, the cross sectional area of said annular gap being substantially smaller than the total cross sectional area of said first and second passages to provide a choke effect.