Hydraulic Power Unit, together with Well Control Module, is part of Wellhead Control Panel (WHCP) which provides hydraulic power to operate Production Wing Valve, Production Master Valve and Downhole Safety Valve (DHSV) for each wellhead.
Hydraulic Power Unit supplies hydraulic to these valves which are designed to shut-in production of wells and isolate the processing facilities in response to shutdown requirement and emergency condition.
In some plant, production choke valve is also hydraulic operated hence powered from Hydraulic Power Unit.
Hydraulic Power Unit consists of the following mechanical equipment and devices, but not limited to:
Hydraulic Reservoir
Reservoir for hydraulic liquid storage with capacity sufficient to the requirements of valve actuators total volume, accumulator volume, hydraulic control lines volume, plus safety factor consideration
Hydraulic Pump
Hydraulic pump pressurizes each hydraulic supply header to the valves. Pump could of air driven pump or motor driven pump type which depends on application.
It is common to have high pressure pump and medium pressure pump as the pressure to stroke the Downhole Safety Valve is higher than the other two valves. It is preferred to have more than one pump for redundancy for each header.
Hydraulic accumulator
Hydraulic accumulator keeps the hydraulic supply header above the minimum hydraulic operating pressure to allow the wellhead valves to move without running the hydraulic pumps. The size of hydraulic accumulator depends on actuator volume and the number of valve strokes.
Ancillaries equipment such as, hydraulic tubing, filter, pressure gauge, pressure transmitter, check valve, ball valve (isolation valve), pressure safety valve (PSV).
Below picture is typical Hydraulic Power Unit schematic yet may differs for each project.
Downhole Safety Valve may also named as SCSSV (Surface Controlled Sub-Surface Safety Valve)