There are two primary components to this unit – a slave cylinder assembly, which mounts on the wellhead above the tubing flange and a skid mounted power unit, which is placed about 20 feet from the well.
The power unit consists of the master cylinder, a hydraulic pump, motor and a control panel.
The master cylinder has four chambers, “A, B, C and D”, which are created by a free floating piston inside the master cylinder. At the top of the master cylinder (open end) is an accumulator which is used to store energy during the sucker rod downward movement and which releases energy during the sucker rod upstroke. The piston moves vertically within the cylinder in the opposite direction to the polished rod
Chamber A is connected to the slave cylinders by means of a pair of hoses. The operating principle of chamber A and the slave cylinders is comparable to the hydraulic brake system on an automobile or light truck. Oil is moved from the master cylinder to the wheel cylinders to apply the brakes when the brake pedal is depressed. As the brake pedal pressure is removed oil flows from the wheel cylinders to the master cylinder. The Series 9000-6 hydrostatic pump uses this same principle. Oil is displaced from chamber A to the slave cylinders causing them to rise when the piston in the master cylinder moves downward. The piston will rise when the polished rod moves down because oil is moved from the slave cylinders to chamber A of the master cylinder.
Chamber D of the master cylinder is the accumulator. It is a closed chamber that is filled with pressurized nitrogen. It acts to store energy during the down stroke of the polished rod for use in moving the polished rod upward.
Chambers B and C are where energy is added to the system. Chamber B is pressurized with oil from the hydraulic pump during the master piston down stroke. The hydraulic pump pressurizes chamber C during the polished rod down stroke. Chambers B and C are in a closed loop with the pump. The pump draws oil from Chamber C and discharges into Chamber B when the master piston is moving down. The direction of oil flow is reversed when the set limit of travel is reached.
A positive displacement swash plate-type pump provides the energy required for pumping. The volume of fluid passing and direction of flow through the pump is controlled by changing the angle of the swash plate. The angle of the swash plate is set by a signal received from the motion controller in the control panel. The pump is directly connected to the prime mover, an electrical motor or gas engine.
The Control Panel is the brains of the system. It tells the pump which direction to pass oil and at what flow rate. The control panel receives a continuous location signal from a probe that senses the position of the master piston within the master cylinder. When the probe senses that the master piston is at a preset travel position (such as the top of the stroke) the control panel delivers a signal to the pump to change flow direction. Stroke length, polished rod velocity, top turnaround and bottom turnaround acceleration are all considered by the control panel in delivering a signal to the hydraulic pump.
The slave cylinders mount on the wellhead and provide the power to stroke the polished rod. The assembly consists of twin cylinders mounted on either side of the polished rod, a traveling cross yoke (green) to pick up the polished rod. A fixed yoke (connector plate) ensures the rod is pulled straight and held in place. A rod clamp is applied above the traveling yoke.