BIZOMAT MAKEOS · Hydraulics
Hydraulic Accumulator Size
What gas-charged accumulator size delivers this volume between the working pressures?
Nominal gas volume of a bladder or piston accumulator to deliver a required oil volume between minimum and maximum working pressures, for isothermal or adiabatic gas behaviour.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Oil volume to deliver | L | Oil volume needed between maximum and minimum pressure. |
| Gas precharge pressure (gauge) | bar | Nitrogen precharge (often about 90% of the minimum working pressure). |
| Minimum working pressure (gauge) | bar | Lowest pressure at which the system still works. |
| Maximum working pressure (gauge) | bar | Highest working pressure (relief or pump cut-out). |
| Polytropic exponent | 1.0 for slow (isothermal) cycles over several minutes; 1.4 for fast (adiabatic) discharges in seconds. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Oil volume to deliver
- What it is
- The volume of oil the accumulator must supply to the circuit as its pressure falls from the maximum to the minimum working pressure.
- Units and format
- Enter a number in L: litres. The form accepts 0.000001 L to 100000 L. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 L as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Accumulator size rises in direct proportion.
- Get it right
- This is the oil delivered, not the accumulator size; the tool works out the size from it.
What this means: Gas precharge pressure (gauge)
- What it is
- The nitrogen pressure charged into the accumulator with no oil in it, measured at the gas valve.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter gauge pressure; it must be below the minimum working pressure. The tool adds atmospheric pressure itself.
What this means: Minimum working pressure (gauge)
- What it is
- The lowest pressure at which the circuit still does its job, reached when the accumulator has delivered its oil.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher minimum pressure, closer to maximum, means a larger accumulator.
- Get it right
- Enter gauge pressure, as read on the system gauge.
What this means: Maximum working pressure (gauge)
- What it is
- The highest working pressure in the circuit when the accumulator is full, usually the pump cut-out or relief setting.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher maximum widens the pressure band and reduces the size needed.
- Get it right
- Enter gauge pressure; it must be above the minimum working pressure.
What this means: Polytropic exponent
- What it is
- The exponent describing how the gas behaves as it expands: slow cycles let it stay at constant temperature, fast discharges do not allow time for heat exchange.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1.67. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher exponent, for faster discharges, needs a larger accumulator.
- Get it right
- Choose it from how fast the oil is drawn off; the guide explains the two limiting cases.
Method and formulas
- Absolute pressures: P = gauge + 1.013 bar.
- V₀ = ΔV ÷ ((P₀ ÷ P₁)^(1/n) − (P₀ ÷ P₂)^(1/n)).
- Gas volume at a pressure = V₀ (P₀ ÷ P)^(1/n).
- Example: 2 L between 100 and 200 bar with 90 bar precharge needs about 5.5488 L of accumulator.
Limitations
- Ideal gas; real nitrogen deviates at high pressure, so allow a margin.
- Temperature changes between charging and operating alter the precharge.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size an accumulator for emergency or peak flow
- Check the precharge pressure
- Compare slow and fast discharge
Who it is for
- Hydraulic and process engineers
- Maintenance engineers
- Design engineers
What you get
- Required nominal gas volume
- Gas volumes at each pressure
- Precharge ratio
Interpreting the result
- Choose the next larger standard size; pressure vessels must comply with your pressure equipment regulations.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 352–368. Topic reference; no source tables or recommended settings are reproduced.
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