Drill string safety valve

A safety valve for allowing controlled normal flow and preventing uncontrolled abnormal back flow of fluid pressure through an internal passage of a drill string casing comprising:

• a valve seat situated in and around a portion of the internal passage in a portion of the drill string casing;
• a valve body retained within an adjoining portion of the internal passage in the drill string casing and adapted to allow fluid to pass around and through the internal passage comprising
• a cylinder including an outer sidewall and an internal chamber within the outer side wall extending between opposite ends of the cylinder, and
• closure means attached to and closing off opposite ends of the cylinder and internal chamber;
• a movable valve stem including opposite end portions each slideably mounted in an aperature extending through each of the closure means of the valve body;
• a valve head attached to an opposite end portion of the valve stem, adapted for sealing engagement with the valve seat and normally maintained in a partially open position relative to the valve seat sufficient to allow passage of fluid and suspended cuttings during tripping of the drill string casing into and out of the borehole, but displacable to a wide open position in response to a predetermined normal amount of differential fluid pressure in the internal passage directed downstream against one side of the valve head and movable into sealing mating engagement with the valve seat in response to a predetermined abnormal amount of differential fluid back pressure in the internal passage directed back upstream against an opposite side of the valve head;
• and
• biasing means of predetermined opposing force comprising
• abutment means projecting from an intermediate portion of the movable valve stem within the internal chamber, and
• resilient means extending about each of the opposite end portions of the valve stem and compressible between each of the closure means and the abutment means on and movable with the valve stem for opposing movement of, returning and maintaining the valve stem in a predetermined normal position and the attached valve head in the partially open position in absence of and until the predetermined amount of differential fluid pressure directed against one side of the valve head is sufficiently greater than the opposing force of the resilient means and any fluid pressure directed against an opposite side of the valve head and thereby displace the valve head from the partially open position toward either the wide open position or a closed position.