US Patent 7874367 Low clearance downhole pump

A pump has a housing having a top plate and a bottom plate and a shaft slidingly extending through an aperture in the top plate. A piston is connected to the shaft. The piston separates a volume within the housing into a first stage chamber defined in the housing below the piston and a second stage chamber defined in the housing above the piston. A fluid transfer port in the piston is in fluid communication with the second stage chamber and, through a check valve, with the first stage chamber. Another stage check valve allows one-way flow of fluid from a suction area below the bottom plate into the first stage chamber. A pressure control valve may be provided for selectively allowing fluid to flow out of the first stage chamber.