HPU Design for Wellhead and Valve Actuation: Sizing the Accumulator

A hydraulic power unit (HPU) for wellhead and valve actuation spends most of its life doing very little: holding pressure on actuators that are already open. Its real test comes when a valve has to move and the pump cannot keep up on its own. That is the accumulator's job, and getting its size wrong either strands a valve part-stroked or adds cost and weight for nothing. This guide is for instrument and mechanical engineers sizing or reviewing an HPU, and it works through a full example.

What the accumulator must do

Define the duty before any calculation. Typical duties are to deliver the fluid needed to stroke one or more actuators without the pump running, to keep valves open through pump cycling and short supply interruptions, and to absorb pressure changes so the pump does not start and stop constantly. The volume the accumulator must deliver between its upper and lower working pressures is the usable volume, ΔV. Build it from actuator swept volumes, the number of strokes required, the fluid taken up by hoses and tubing as pressure changes, and a margin for valve and seal leakage.

On a wellhead control panel, a common duty is to hold all valves open through a pump stop and to open one valve without the pump. On a valve actuation HPU, it may be a defined number of full strokes on loss of power. Write the duty into the specification in those terms, so the vendor's calculation can be checked.

The three pressures that set the size

• P1, maximum working pressure: where the pump stops, usually a margin below the relief valve setting.

• P2, minimum working pressure: the lowest pressure at which every actuator still completes its stroke against its spring and process load.

• P0, gas precharge pressure: the nitrogen pressure in the empty accumulator. For bladder accumulators it is commonly set at around 0.9 × P2, so a little oil remains at P2 and the bladder does not close onto the oil port.

All gas-law calculations use absolute pressure, so add atmospheric pressure, about 1 bar, to gauge values.

Worked example

An HPU must deliver ΔV = 2.0 L to open a set of actuators without the pump. The pump stops at P1 = 210 barg, the actuators need at least P2 = 150 barg, and the precharge is P0 = 135 barg (≈ 0.9 × P2). In absolute terms, P1 = 211 bar, P2 = 151 bar and P0 = 136 bar.

Isothermal sizing, for slow discharge where the gas stays at ambient temperature:

V0 = ΔV / (P0/P2 − P0/P1) = 2.0 / (136/151 − 136/211) = 2.0 / (0.901 − 0.645) ≈ 7.8 L

Adiabatic sizing, for fast discharge where the gas cools as it expands, with n = 1.4 for nitrogen:

V0 = ΔV / ((P0/P2)^(1/n) − (P0/P1)^(1/n)) = 2.0 / (0.928 − 0.731) ≈ 10.1 L

Valve stroking in a shutdown or an opening sequence happens in seconds, so the adiabatic figure governs. A practical selection is 2 × 10 L bladder accumulators. That gives margin over 10.1 L and leaves room for the real-gas behaviour of nitrogen, which at these pressures typically makes the ideal-gas result optimistic. The accumulator maker's sizing software can confirm the final choice.

Temperature changes the precharge

Precharge is a gas pressure, so it moves with gas temperature. Between 15 °C and 55 °C, absolute temperature rises from about 288 K to 328 K, which shifts the precharge by about 14%. An accumulator precharged in a cool workshop and installed in a hot desert enclosure will sit at a higher precharge than intended, and its usable volume falls. Set and check precharge at a known temperature, correct it to the expected operating temperature, and record both on the nameplate or in the maintenance sheet. In practice, precharge is checked with the accumulator isolated and its oil side drained to zero pressure.

Bladder or piston accumulator

Bladder accumulators respond quickly, have no sliding seals and are the usual choice for wellhead and valve actuation duties. Piston accumulators suit larger volumes and higher ratios between maximum and minimum pressure, and can carry a position indicator, but seal friction makes them slower to respond and the seals need maintenance. Whichever type is used, it is a pressure vessel and must meet the pressure equipment rules that apply at site.

Pump, reservoir and fluid

• Pump capacity: size the pump to recharge the accumulators from P2 to P1 within the time the operator specifies, plus continuous leakage and a margin.

• Duty and standby pumps: electric, pneumatic or both, with a hand pump for start-up and emergencies.

• Unloading: a pressure switch or unloading valve stops the pump at P1 and restarts it at a set point above P2, so the pump does not run continuously.

• Reservoir: large enough to accept all fluid returning from actuators and accumulators, with level gauge, low-level switch, breather filter and drain.

• Filtration: pressure and return filters with differential indicators, sized to keep the fluid at the cleanliness the valve and actuator makers specify. Flush the system to that level before commissioning.

• Fluid: mineral hydraulic oil is usual; check seal compatibility, viscosity at the coldest and hottest site conditions, and any fire-resistance requirement.

• Relief and regulation: a relief valve on every pressure circuit, and regulators where actuators need different pressures.

Common sizing mistakes

• Using gauge instead of absolute pressures in the gas-law equations.

• Sizing isothermally for a discharge that happens in seconds.

• Ignoring the effect of site temperature on precharge.

• Leaving hose and tubing volume, and actuator leakage, out of ΔV.

• Setting P2 at the actuator's nominal pressure instead of the pressure it needs against the worst-case process load.

Standards and good practice

API Spec 16D governs control systems for drilling well control equipment, including accumulator sizing for BOP control systems. A wellhead or valve-actuation HPU is not a BOP control system, but some operators borrow its approach to usable volume and redundancy. Your company standards, the actuator makers' data and the pressure equipment rules for the accumulators themselves govern the final design.

Have your HPU sized and built

We design hydraulic power units in Vaughan, Canada and build them in Chennai, India, for wellhead control, actuator and valve control panels and test duties, and supply hydraulic oil flushing units to bring systems to cleanliness before start-up. Each HPU is factory acceptance tested, witnessed by the client or a third-party inspector, including accumulator precharge, pump capacity and relief settings. For the wellhead side, read SCSSV, master and wing valve sequencing and single-well or multi-well panels. Browse our hydraulic power and test units and the full oil and gas systems range. Send your actuator list, pressures and required stroke times to mail@techincresearch.com or call +91 739 749 8656 and we will return a P&ID, GA drawing and priced offer.