Sizing a hydraulic accumulator - volume and precharge?
A hydraulic accumulator absorbs pressure peaks, buffers volume and keeps holding or emergency functions alive. This guide shows how to size bladder and diaphragm accumulators by useful volume, precharge pressure and pressure range, and which inspection duties apply under Pressure Equipment Directive 2014/68/EU.
View accumulatorsBladder or diaphragm accumulator - what is the difference?
A hydraulic accumulator separates a nitrogen gas cushion from the hydraulic fluid by an elastic barrier. As system pressure rises the gas is compressed and stores energy; when pressure drops the gas cushion pushes oil back into the circuit. Depending on the barrier you distinguish bladder, diaphragm and piston accumulators.
Diaphragm accumulators are compact, low-maintenance and suited to small volumes up to about 4 litres with pressure ratios up to 8:1. Bladder accumulators cover larger volumes and higher flow rates, but work economically only up to a p2/p0 pressure ratio of around 4:1 so the bladder is not over-stretched.
How do you calculate useful volume and precharge?
Sizing begins with the required useful volume ΔV between minimum working pressure p1 and maximum pressure p2. The state change of the nitrogen then gives the necessary nominal volume V0. The precharge pressure p0 is measured with the accumulator empty and is the single most important setting.
For slow processes (pressure holding, leakage make-up) use the isothermal case with exponent n = 1; for fast shocks use the adiabatic case with n = 1.4. The nominal volume follows from the gas law referenced to p0, but in practice the manufacturer sizing charts give V0 directly.
- Always set p0 below p1 so the barrier never seats fully on the oil port.
- Diaphragm: keep p0 ≥ 0.25 x p2 to protect the diaphragm from over-stretching.
- Bladder: keep p0 between 0.25 x p2 and 0.9 x p1.
- Precharge is temperature-dependent - always convert values to operating temperature.
What inspection duties apply under the Directive?
Accumulators are pressure equipment and fall under Pressure Equipment Directive 2014/68/EU. The decisive figure is the product of allowable pressure and volume (PS x V). Depending on category I to IV, the requirements for conformity assessment, marking and recurring inspection increase.
- CE marking and declaration of conformity to 2014/68/EU when placed on the market.
- Nameplate stating PS, PT, volume, test pressure, year and fluid group.
- Inspection before commissioning and recurring inspections by a competent person or notified body.
- Protection against exceeding the allowable working pressure (safety valve, isolation and relief block).
Frequently asked questions
How high should the precharge pressure p0 be?
The rule of thumb is p0 = 0.9 x p1, just below the minimum working pressure. This keeps a residual oil volume in the accumulator and stops the barrier from seating hard on the oil port.
Bladder or diaphragm accumulator - which is better?
Diaphragm accumulators are compact, low-maintenance and ideal for small volumes up to about 4 litres. Bladder accumulators deliver larger useful volumes and higher flow, but work economically only up to a pressure ratio of around 4:1.
Which gas goes into a hydraulic accumulator?
Nitrogen only. Oxygen or compressed air are strictly forbidden because of the explosion risk from compressed oil vapour.
Does a hydraulic accumulator need recurring inspection?
Yes. As pressure equipment under 2014/68/EU it requires, depending on category, an inspection before commissioning and recurring inspections by a competent person or a notified body.
Accumulator sized correctly?
We supply bladder and diaphragm accumulators including charging and testing unit and safety block - matched to your pressure range and useful volume.
Per 2014/68/EU
Accumulators with CE marking and declaration of conformity.
Correctly sized
Nominal volume and p0 matched to your pressure range.
Verifiably safe
With charging and testing unit for the precharge.
Expert advice
Our specialists support your selection.