Dual check valve

Abstract

A check valve comprising a section of a drill string, a flapper valve positioned within and concentric to the section of the drill string, and a piston valve positioned within and concentric to the section of the drill string. The flapper valve allows fluid flow in a first direction through an inner portion of a cross-section of the section of the drill string and not allowing fluid flow in a second direction, and the piston check valve allowing fluid flow in the first direction through an outer portion of the cross-section of the section of the drill string and not allowing fluid flow in the second direction, where the first portion and the second portion of the cross-section of the section of the drill string are mutually exclusive.

1 . A check valve comprising:
 a housing;   a first check valve positioned within the housing; and   a second check valve positioned within the housing, the second check valve being positioned in parallel to and at least partly within the first check valve.   
   
   
       2 . The check valve of  claim 1  in which the first check valve and the second check valve are each oriented to block flow in a first direction and allow flow in a second direction opposite to the first direction. 
   
   
       3 . The check valve of  claim 1  in which the first check valve has a first flow path and the second check valve has a second flow path, and the second check valve is oriented so that the second flow path is wholly contained within the first flow path. 
   
   
       4 . The check valve of  claim 2 , wherein each of the first check valve and the second check valve are selected from a group consisting of a flapper valve, a piston valve, a ball valve and a poppet valve. 
   
   
       5 . The check valve of  claim 2 , wherein the first check valve is a flapper valve and the second check valve is a piston valve. 
   
   
       6 . The check valve of  claim 2 , wherein the first check valve is configured to open when fluid pressure above a first threshold is applied, and the second check valve is configured to open when a fluid pressure above a second threshold that is higher than the first threshold is applied. 
   
   
       7 . The check valve of  claim 5 , wherein the piston valve is biased closed by either a spring or fluid pressure within the piston chamber. 
   
   
       8 . The check valve of  claim 7 , wherein the housing comprises a port to apply fluid pressure to the piston chamber. 
   
   
       9 . The check valve of  claim 2 , wherein the housing comprises a section of a drill string. 
   
   
       10 . The check valve of  claim 2 , wherein surfaces that redirect abrasive flows within the first and second check valves are tapered surfaces. 
   
   
       11 . The check valve of  claim 2 , wherein the second check valve is positioned within an inner wall of the housing, the second check valve having an annular shape, and the first check valve is positioned within the annular shape wall of the second check valve. 
   
   
       12 . The check valve of  claim 2  in which the first check valve and the second check valve are each positioned concentric to the housing. 
   
   
       13 . The check valve of  claim 1  in which the first check valve and the second check valve are oriented to block flow in opposite directions, and one of the first check valve and the second check valve is biased to resist opening in the direction in which the other of the first check valve and the second check valve blocks flow. 
   
   
       14 . The check valve of  claim 13  in which the first check valve has a first flow path and the second check valve has a second flow path, and the second check valve is oriented so that the second flow path is wholly contained within the first flow path. 
   
   
       15 . The check valve of  claim 11 , wherein each of the first check valve and the second check valve are selected from a group consisting of a flapper valve, a piston valve, a ball valve and a poppet valve. 
   
   
       16 . A check valve comprising:
 a section of a drill string;   a flapper valve positioned within the section of the drill string, the flapper valve allowing fluid flow in a first direction through an inner portion of a cross-section of the section of the drill string; and   a piston valve positioned within the section of the drill string, the piston valve allowing fluid flow in the first direction through an outer portion of the cross-section of the section of the drill string, where the first portion and the second portion of the cross-section of the section of the drill string are mutually exclusive.   
   
   
       17 . The check valve of  claim 16  in which the flapper valve and the piston valve are each positioned concentric to the housing. 
   
   
       18 . A check valve, comprising:
 a section of a drill string;   a ball valve positioned within and concentric to the section of the drill string, the ball valve allowing fluid flow in a first direction through an inner portion of a cross-section of the section of the drill string; and   a piston valve positioned within and concentric to the section of the drill string, the piston valve allowing fluid flow in a second direction opposed to the first direction through an outer portion of the cross-section of the section of the drill string and not allowing fluid flow in the second direction, where the first portion and the second portion of the cross-section of the section of the drill string are mutually exclusive.