Hydraulic-pneumatic drive for down hole oil well pump connecting rods - has pneumatic constant pressure cylinders to balance weight of assembly and hydraulic accelerating and retarding cylinders

摘要

Reciprocating downhole oil well pump is driven via a connecting rod string by means of a hydro-pneumatic drive comprising =1, single acting pneumatic cylinder coupled to a constant pressure gas reservoir and =1 double acting hydraulic cylinder. The gas cylinder piston exerts an upward force on the pump con-rods equal to the average of the maximum tension force on the up stroke and the maximum tension force on the downstroke, whilst the hydraulic cylinder drives both up and down in such a manner that the velocity/time characteristic of the pump is approximately sinusoidal. The pref. drive assembly comprises a series of alternate gas and hydraulic pistons distributed in a circle around the pump rods and attached to the upper end of the rods via a cross-head. The gas cylinders are connected to a nitrogen buffer vessel with a capacity several times greater than the total capacity of the gas cylinder and the buffer vessel is coupled via a pressure regulating device to a nitrogen reservoir. The double acting hydraulic cylinders are coupled to two horizontally opposed single acting pump cylinders driven by a crank shaft via a step-down gearbox, flywheel and electric motor. The flywheel is on the motor side to reduce its weigth and is of sufficient mass to ensure its constant rotational speed and, consequently, a sinusoidal output from the hydraulic pumping cylinders. The hydraulic system is also connected to an oil pump and reservoir which compensate for fluid leakage. The drive is much lighter than the traditional mechanical crank arm and balancing wheel drive.