Hydraulic accumulators

01

Set operating pressures, pre-charge and effective volume

HYDAC explains that accumulators exploit the compressibility of gas, with nitrogen as the compressible medium in its accumulators: the bladder accumulator draws in fluid as system pressure rises and the trapped gas is compressed, and when pressure drops the expanding gas forces fluid back into the circuit. The effective volume is the fluid available between the maximum operating pressure p2 and the minimum operating pressure p1. Gas pre-charge p0 must not exceed 0.9 × p1, with a permitted pressure ratio p2 : p0 of at most 4 : 1; these are maximum values, not a permanent load, and the tolerable ratio depends on geometry, temperature, fluid, flow rate and gas losses. Pre-charge must be stated at 20 °C. [1]

Editorial calculation (ideal isothermal gas, absolute pressures, ignoring temperature effects): a 10 L accumulator pre-charged to 90 bar, working between p1 = 100 bar and p2 = 200 bar, delivers about 10 × 90 × (1/100 − 1/200) = 4.5 L. It meets both HYDAC limits: 90 ≤ 0.9 × 100 and 200 : 90 ≈ 2.2 : 1. Real sizing should use the maker's method, as fast cycling is closer to adiabatic and gives less volume. [1]

Give the supplier the minimum and maximum operating pressures, fluid volume to deliver and the time to deliver it, the duty (energy storage, pulsation damping, constant pressure, volume compensation), hydraulic fluid and temperature range, installation position, port sizes and the design code or certification (for example PED).

02

Choose the type, bladder material and installation

HYDAC lists bladder elastomers by temperature range and fluid, for example NBR (standard grade −15 to +80 °C for mineral oil), ECO (−30 to +120 °C), IIR butyl (−50 to +100 °C) and FKM (−10 to +150 °C), noting these are elastomer ranges, not the accumulator's; its SB690 high-pressure range operates from −10 to +80 °C as standard. With a high p2/p0 ratio or rapid discharge the gas can cool below the permitted temperature and cause cold cracking of the bladder. Shells can be nickel-plated or coated inside and given an epoxy finish for offshore use. [1]

Bladder accumulators may be installed vertically, horizontally or slanted with the oil valve at the bottom; HYDAC prefers vertical for energy storage and volume compensation, and notes that horizontal or slanted mounting reduces effective fluid volume and permitted flow. The charging gas is nitrogen of at least class 2.8. HYDAC's US catalogue designates its standard SB330 range as 3,000 psi and lists applications from agricultural and mining machinery to oil field and offshore and machine tools. [1] [2]

Piston accumulators separate gas and fluid with a piston instead of a bladder; Drift, for example, lists a 60 L piston accumulator rated 315 bar working pressure. [3]

03

Check whether the accumulator falls under the Pressure Equipment Directive

For the European market, Directive 2014/68/EU (the Pressure Equipment Directive) applies its essential safety requirements to vessels for gases by thresholds on maximum allowable pressure PS and volume V. For Group 2 fluids the limits are a volume greater than 1 L and a PS × V product greater than 50 bar·L, or PS greater than 1,000 bar; for Group 1 fluids, volume greater than 1 L and PS × V greater than 25 bar·L, or PS greater than 200 bar. Equipment at or below these limits must be designed and made to the sound engineering practice of a Member State, come with adequate instructions for use, and must not bear the CE marking under the Directive. Group 1 covers substances classified as hazardous, such as flammable, oxidising or acutely toxic, and also contents held above their flashpoint; Group 2 is everything else. A vessel made of more than one chamber is classified in the highest category of its chambers, and a chamber holding more than one fluid by the fluid needing the highest category. [4]

Editorial calculation: a 10 L bladder accumulator rated PS = 330 bar has PS × V = 3,300 bar·L, far above the 50 bar·L Group 2 threshold; a 60 L, 315 bar piston accumulator gives 18,900 bar·L; a 1 L accumulator at any pressure up to 1,000 bar stays outside the gas-vessel limits because its volume is not greater than 1 L. [4] [3]

Editorial note: because the hydraulic fluid side may be held near its flashpoint and the gas side carries nitrogen, ask the supplier which fluid group and category it assigned, which conformity assessment module and notified body it used, and to supply the declaration of conformity with the pressure and temperature limits marked on the shell.

04

Compare what suppliers offer

The suppliers linked to this page list hydraulic accumulators; their cards link to the pages reviewed.

Ask any supplier for nominal and effective gas volume, maximum operating pressure and certification, separator type and elastomer or seal material, permitted temperature range, pre-charge as delivered, port and gas-valve connections, shell material and coating, and the safety block, mounting clamps and charging kit offered.

05

Trace the accumulator parts

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your circuit

An accumulator quotation needs values for your hydraulic circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the pressure, volume and fluid data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Pressures: [p1 minimum; p2 maximum]
Volume: [fluid to deliver; time]
Duty: [energy storage; damping; constant pressure; compensation]
Fluid: [type; temperature range]
Build: [bladder or piston; material; ports; position]
Certification: [PED; other]

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