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Hydraulic Power Units in the Chemical Industry

Hydraulic Power Units (HPUs) provide a critical function to offshore oil drilling as well as onshore oilfield operations by supplying the hydraulic power (pressure, flow, and duty-cycle) for the entire system. For offshore operations, HPUs provide both the high pressure (HP) fluid power to operate the subsea or downhole safety valve system and the low pressure (LP) fluid power to operate the subsea tree, manifold and ancillary systems. For oilfields, hydraulic power units or hydraulic power packs can be mobile systems that power slickline, injectors and snubbing units, among others. It is worth noting that HPUs have broad application in many applications and industries, including petro-chemical, agricultural, manufacturing.

System Description

Hydraulic power units are composed of a system of components typically including, electric motor (or diesel engine) as a prime mover, a pump, accumulator, reservoir tank, suction strainer, return filter, relief valves, pressure gauge, hydraulic fluid, fittings and piping. Common HPU features include programmable logic control, emergency automatic shutdown, operational warning annunciators and remote operator controls. While tank capacities vary with the application, tanks can hold about 2,000 gallons of hydraulic fluid. The ratings of the LP and HP supplies vary as well, with the LP supply typically rated up to 5K psi and the HP supply rated up to 10k psi.

Offshore Applications

For offshore oil drilling applications, hydraulic power units, typically located on the surface drilling platform via an umbilical, provide a supply of both HP and LP hydraulic fluid power to the subsea safety valves & control module or system and the subsea hydraulic distribution tree, respectively, to operate the subsea hydraulic actuators.

The HPU is a totally enclosed system, constructed with redundant normal and backup hydraulic pumps, accumulators, filters, and instrumentation for both the LP and HP supplies. It is controlled by a programmable logic controller (PLC) and interlocked with the pump motor controls as well as the subsea control system. The accumulators provide several important functions, including pump pressure damping as well as sufficient hydraulic power to handle variable cycle rates and pump recharging operations. More importantly, the accumulators can provide power for a period of time in the event of an electric power outage.

The HPU uses two, separate tank reservoirs for each of the LP and HP supply systems. The reservoirs consist of circulating pumps, filters, level transmitters & gauges, drain ports, air vents and inspection holes for maintenance. The hydraulic control fluids can be oil- or-water-based; the use of synthetic control fluids is not typical and, when they are used, they are limited to electro-hydraulic control systems. The HPU pumps are electrically driven and powered from the platform’s electrical power grid. The pumps are selected based on their ability to rapidly return to normal operating pressure after a depressurization.

The LP and HP hydraulic supplies are fed from the HPU through internal subsea headers to a directional control valve that connects the header to either supply based upon control commands. The HPU can be integrated into the subsea control system in a variety of ways, such as, directly with the subsea control, to a separate HPU, or to manual valves connected to the subsea control system.