Submersible pump

A submersible pump for attachment to the lower end of a well pipe discharge tube for insertion in liquid includes a stationary collecting tube for collecting the liquid and an upward and downward reciprocating pumping tube within the collecting tube for pushing and lifting the liquid into the discharge tube; source of pressurized gas, a casing assembly including a casing tube and a gas valve, and a piston assembly with the casing tube including a piston tube having a lower end connected to the pumping tube for reciprocating the pumping tube and an upper portion having pistons for moving in chambers with the inside of the casing tube. Preferably the same amount of liquid is pumped during the upward movement of the pumping tube as during the downward movement.