A method of controlling drilling mud flow in a drilling mudchoke which choke includes:
- a hollow valve body having a passage therethrough;
- a valve seat having an encircling, tapered face surrounding an axial passage for directing mud flow past said tapered face and along said passage within said valve body;
- an encircling circular valve seat shoulder in a plane perpendicular to the flow through said axial passage which shoulder defines an upstream located closure face cooperatively positioned relative to said valve seat tapered face;
- a valve element having an external tapered face conforming to said valve seat tapered face wherein said valve element, on movement, moves into a concentric, centrally positioned throttling location within said valve seat opposite said valve seat tapered face and wherein 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;
- said valve element and said valve seat tapered faces are defined by straight line segments in axial section;
- a surrounding circular 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, wherein said valve element and valve seat define a minimum gap at closure of said valve element relative to said valve seat;
- means for moving said valve element into a fully closed position relative to said valve seat;
- inlet and outlet means communicating with said passage through said valve body;
- and
- wherein the method includes the steps of:
- (a) positioning the valve element in the hollow valve body and spaced from the valve seat such that the encircling circular valve seat shoulder and the surrounding shoulder on the valve element are facing one another and moved toward or away from one another on movement of the valve element in the valve body and such movement varies the volume of mud flowing therepast;
- (b) further positioning the valve element in the valve seat to enable the tapered face of the valve element to move into a throttling position relative to the encircling tapered face of the valve seat between first and second throttling locations relative to the valve seat to define a gap therebetween wherein mud flow between the tapered faces in the gap is reduced in the second throttling position as the gap is narrowed on movement;
- (c) defining a flow path from the inlet means of the hollow valve body and then between the encircling and facing circular shoulders, and which flow path then flows through the gap defined by the valve element and valve seat facing tapered faces, and thereafter extending along the passage to the outlet means;
- (d) selectively closing the upstream encircling and facing circular shoulders fully to interrupt mud flow therepast;
- and
- (e) wherein the step of closing the upstream encircling and facing circular shoulders creates a cleaning and flushing flow effect which prevents particles of a certain size in the drilling mud from flowing through the gap.