Abstract: A pump includes a two-stepped cylinder with cavities and a lower intake valve, and is connected to a tubing-string via a sub, which includes an upper intake valve and discharge valve. A hollow plunger is arranged in the cylinder below and above the cavities. The plunger is connected to a hollow rod and has a lower discharge valve and a through-hole provided above the lower discharge valve to provide communication between the plunger cavity and a chamber formed as said plunger is moved downwards in a lower step of the cylinder. An upper step of the cylinder includes an inner diameter which is smaller than the inner diameter of the tubing-string. The upper discharge valve is a bush arranged on the rod with longitudinal movement upwards along said rod with excess pressure in the cylinder cavity above the plunger, and downward movement in the liquid under its own weight.

Abstract: A pump includes a two-stepped cylinder with cavities and a lower intake valve, and is connected to a tubing-string via a sub, which includes an upper intake valve and discharge valve. A hollow plunger is arranged in the cylinder below and above the cavities. The plunger is connected to a hollow rod and has a lower discharge valve and a through-hole provided above the lower discharge valve to provide communication between the plunger cavity and a chamber formed as said plunger is moved downwards in a lower step of the cylinder. An upper step of the cylinder includes an inner diameter which is smaller than the inner diameter of the tubing-string. The upper discharge valve is a bush arranged on the rod with longitudinal movement upwards along said rod with excess pressure in the cylinder cavity above the plunger, and downward movement in the liquid under its own weight.