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Hydraulic accumulator sizing for wellhead control panel hydraulic power unit (hpu)

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03-15-2013, 12:33 AM

Hydraulic accumulator sizing for wellhead control panel hydraulic power unit (hpu)

In the wellhead control panel there is a Hydraulic Power

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Unit (HPU) that utilize hydraulic reservoir, hydraulicpumps, and accumulator as it components. To make thehydraulic demand from SCSSV (Surface Controlled Subsea Safety Valves) or SSV (Surface Safety Valves) isachievable, the downstream of the hydraulic pump(hydraulic header) is equipped with an accumulator. Ithelps the hydraulic pump to supply the hydraulic demandquickly (See[Only registered and activated users can

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accumulator plays a vital role as a hydraulic pressurestorage (or energy storage), careful sizing andcalculation must be done so that the well operationrequirement is achieved. So actually what is the hydraulicaccumulator and how to calculate the quantity required?Hydraulic accumulator is an energy storage device thathas gas section and hydraulic oil section. At initialconditions the gas section shall be pre-charged withsome gases (usually dry nitrogen) at some pressure.When the hydraulic oil pumped into the accumulator, thehydraulic oil pressure will press the gas section (gas iscompressible while liquid isn’t compressible) into certainvolume and certain pressure. If there is a hydraulicdemand, then the accumulator will release the hydraulicoil and makes the gas section expanded. See belowdrawing for accumulator working principle illustration.

Pre-Charge Conditions:P1 = 0.9 P3 for Bladder accumulators (90% of minimumsystem pressure to prevent bladder damage)P1 = P3 for Piston accumulatorsCharged Conditions:P3 = Minimum system operating pressureP2 = Maximum system operating pressureAs per above description, the goal of the sizing is to findthe effective volume of the accumulator V1. To avoidany deficiency due to something that we don’t know (un-accurate valve actuator swept volume etc) we need toput some margin (lets say 10% of V1) so the accumulatorvolume will be 1.1 x V1.The filling or compression process by the hydraulic pump(from P1 to P2) can be considered as isothermal processsince there is no temperature change after compression.While the expansion process (from P2 to P3) can beconsidered as adiabatic process since the expansionprocess is fast due to the usage in the valve actuator.

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This fast expansion will make the change in heat doesn’thave sufficient time to transferred outside theaccumulator. From this assumption, we can develop thefollowing equation to calculate the required effectivevolume V1 of the accumulator.

Where:P1 = 0.9 P3 for Bladder accumulators (90% of minimumsystem pressure to prevent bladder damage)P1 = P3 for Piston accumulatorsP2 = Maximum system operating pressureP3 = Minimum system operating pressureV1 = Effective gas volume of the accumulatorV2 = Maximum gas volume compressed by the hydraulicoilV3 = Expanded volume of gas at the minimum pressurerequired (P2)VHYD = Amount of oil to be supplied to valve actuator

(swept volume of hydraulic valve actuator)n = ratio of specific heat or CP/CV = 1.4 for nitrogenExample:The following data was provided by the x-mast treevendor:- 10 well were drilled and supplied complete with SCSSV,Master, and Wing Valve.- Operating supply pressure to control line of SCSSVactuator is 4500 – 5500 psi- Operating supply pressure to control line of Master &Wing Valve actuators is 2500 – 3500 psi- The control line fluid to SCSSV, Master, and Wing Valveactuator is Shell Tellus 32 Hydraulic Oil- Swept volume of SCSSV is 0.05 ltr, Master & Wingvalve is 2.5 ltr.As per data above, we will have two hydraulic header,one for high pressure (4500 – 5500 psi operatingpressure) and one for medium pressure (2500 – 3500 psioperating pressure). Each header will have its ownaccumulator to storage the hydraulic pressure so that allvalves can be opened twice (this twice opening cycleusually comes from project specification, may be differ oneach project) without the present of the hydraulic pump.The selected accumulator type is bladder type.

Accumulator for High Pressure HeaderP1 = 0.9 * P3 = 4050 psiP2 = 5500 psiP3 = 4500 psiVHYD = 0.05 ltr x 10 SCSSV x 2 opening cycle = 1 Ltr

Then we can get V1 (accumulator gas effective volume)as follow:V1 = 5500 * (1*(4500/5500)^(1/1.4))/(4050*(1-(4500/5500)^(1/1.4)))V1 = 8.81 LtrBy putting 10% margin the V1 will becomeV1 = 1.1 x 8.81 Ltr = 9.69 LtrBy looking at the bladder accumulator vendor catalogue,we can conclude that we need two bladder typeaccumulators with the following data:Nominal Capacity = 10 LtrEffective Gas Volume = 9.4 LtrWith this design, we will have almost twice of theeffective gas volume surplus. This is good design.Accumulator for Medium Pressure HeaderP1 = 0.9 * P3 = 2250 psiP2 = 3500 psiP3 = 2500 psiVHYD = 2.5 ltr x 10 Master Valve x 10 Wing Valve x 2

opening cycle = 100 LtrThen we can get V1 (accumulator gas effective volume)as follow:V1 = 3500 * (100*(2500/3500)^(1/1.4))/(2250*(1-(2500/3500)^(1/1.4)))V1 = 572.6 LtrBy putting 10% margin the V1 will becomeV1 = 1.1 x 572.6 Ltr = 629.8 LtrBy looking at the bladder accumulator vendor catalogue,we can conclude that we need 13 bladder typeaccumulators with the following data:Nominal Capacity = 54 LtrEffective Gas Volume = 49.2 LtrWith this design, we will have the following surplus of thegas effective volume:49.2 Ltr x 13 = 9.77 Ltr

Last edited by info; 03-15-2013 at 12:39 AM.

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