Choke for controlling the flow of drilling mud

Claim 1

A choke for controlling the rate of flow of mud comprising: a hollow housing; a movable valve element in said housing; an encircling valve seat in said housing; contacting surfaces on said valve element and valve seat which include tapered throttling surfaces which, on separation, define a controllable throttling passage and which contact one another on closure wherein said throttling surfaces include: (a) a tapered surface on said valve seat cooperative with a surface on said valve element, which, on closing movement of said valve element relative to said valve seat, defines a flow path between said valve element and said valve seat which is narrowed sufficiently to permit flow of mud while preventing entry of particles above a certain size in the mud; (b) means for directing the flow from said tapered surface in a flushing manner across the surface of said valve element; downstream transverse shoulders on both said valve element and valve seat which are parallel to one another and which facially close and contact against one another to close the flow through said housing, said shoulders being sheltered from fluid flow through said tapered surfaces by their respective locations on said valve element and valve seat, said shoulders being relatively spaced on said valve element and valve seat to close and contact after said tapered surfaces move to their narrowest spacing therebetween and wherein said tapered surfaces are concentric of one another and wherein one of said tapered surfaces connects immediately to said transverse shoulder.