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,SAFETY VALVES 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, Plug Valvesbut 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,Hydraulic control valves 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. A three position fluid pressure responsive down hole safety valve adapted for mounting in and controlling the flow of fluid through the internal passage of a drill string pipe or casing situated in the bore hole. The safety valve comprises a valve body including end caps axially retained and centralized within an enlarged portion of the internal passage and surrounding wall of a lower section of the drill string. Angularly spaced fingers extend radially between the end caps and the surrounding wall to provide a fluid passage around the valve body clamped between axially spaced annular shoulders of the lower and an attached adjoining upper section of the drill string casing. The upper section has an internal fluid passage and valve seat therein engageable by the mating surface of a valve head attached to and moveable with a valve stem slideably mounted in the end caps and valve body. Within the valve body is a sealed chamber divided into a large upper bore and a smaller lower bore by an intermediate surface or shoulder normally engaged by a collar slideably mounted in the large bore and about the valve stem for engaging and compressing an upper return spring surrounding an upper portion of the valve stem. The valve stem has an enlarged intermediate portion including an upper shoulder for engaging and displacing the collar to compress the spring and a lower shoulder for engaging and compressing a return spring surrounding a lower portion of the valve stem in the lower smaller bore of the valve chamber. The opposing upper and lower return springs act against the opposite end caps of the valve body and the intermediate portion to maintain the valve stem and hence the attached valve head in a normal substantially counter balanced intermediate partially opened position allowing fluid to pass through the drill string when tripping into and out of the bore hole. Once in the bore hole the additional downward force of drilling fluid circulated under pump pressure downstream through the drill string and upstream in the surrounding bore hole annulus acts against the valve head to overcome the resistance of and compress the lower return spring and displace the valve head to a full open position. Any abnormal or greater amount of opposing differential back pressure of fluid acts in conjunction with the lower return spring, upwardly against areas of the valve stem and under-side of the valve head to overcome the opposing lesser differential downward fluid pressure against the valve head and resistance of the upper return spring to compress the upper return spring and close the valve to the back flow of fluid therethrough.

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