Patent ID: 6854705

Claim:
A valve for controlling flow of a fluid comprising: (a) a housing defining a piston chamber, a plunger chamber, and a valve seat in the plunger chamber; (b) a piston slideably disposed in the piston chamber; (c) a plunger slideably disposed in the plunger chamber for blocking flow of the fluid when the plunger is engaged with the valve seat in the housing; (d) a shaft connected to the piston and to the plunger for communicating axial motion from the piston to the plunger; (e) a fluid outlet port in the housing that is in fluid communication with the plunger chamber; (f) a fluid inlet port in the housing that is in fluid communication with the plunger chamber when the piston is upwardly disposed and the plunger is disengaged from the valve seat and wherein the plunger blocks such fluid communication when the piston is downwardly disposed and the plunger is engaged with the valve seat; (g) piston seals defining an upper piston volume in the piston chamber above the piston, a lower piston volume in the piston chamber below the piston, and a vent port volume in the piston chamber below the piston; (h) an upper pressure port in the housing that is in fluid communication with the upper piston volume; (i) a lower pressure port in the housing that is in fluid communication with the lower piston volume; and, (j) a vent port in the housing that is in fluid communication with the vent port volume.