Choke for controlling the flow of drilling mud

A choke for controlling the flow of mud which comprises:

• a hollow valve body having a passage therethrough;
• a valve seat having an encircling, tapered face surrounding an axial passage past said tapered face for directing mud flow past said tapered face within said valve body;
• an encircling valve seat shoulder axially spaced from said tapered face lying in a plane perpendicular to the flow through said axial passage;
• a valve element having an external tapered face conforming to said valve seat tapered face which said valve element, on movement, moves from a spaced position into a concentric, centrally positioned first throttling location within said valve opposite said valve seat tapered face and which said valve element and said valve seat tapered faces define a gap therebetween for throttling mud flow passing through the gap, and wherein said valve element controllably blocks the axial passage flow of mud past said valve seat on further movement of said valve element toward said valve seat shoulder defining a second throttling location having reduced flow compared to said first throttling location;
• a surrounding shoulder on said valve element which shoulder is adapted to conform to and seat against said valve seat shoulder to plug and close the flow of mud through the gap between said valve element and said valve seat, said shoulder further moving toward said valve seat shoulder for closure;
• means for moving said valve element into a fully closed position relative to said valve seat;
• and
• inlet and outlet means communicating with said passage through said valve body.