Fluidlexikon

Hydraulic accumulator

Accumulator which stores a fluid under pressure and is therefore able to release hydraulic energy. Pressurisation is mainly based on gas pressure (air, nitrogen, "hydropneumatic accumulator") and, more rarely, springs or weights (spring accumulator, weighted accumulator). The latter is the only accumulator which keeps the pressure constant during withdrawal of the volume.

With hydropneumatic accumulators and spring accumulators by contrast, the fluid pressure decreases - due to the element applying the load expanding - as the spring characteristic becomes stronger (hydropneumatic accumulator).

With a hydraulic accumulator, the following characteristics are significant:

Precharge pressure:

Pressure required, during filling with gas, in order to achieve the final pressure for the system when admitting the usable volume.

Minimum operating pressure:

The lower value to which the accumulator pressure may decrease following withdrawal of the usable volume.

Maximum operating pressure:

The upper value to which the accumulator pressure may increase following admission of the usable volume (equal to the nominal pressure for the system).

Usable volume ΔV:

Volume which the accumulator, during aworking cycle, must feed the circuit in addition for it to be able to perform its allotted task.

Pressure difference Δp:

Difference between the accumulator pressure at the start and end of volume withdrawal. It depends on the relationship between the gas capacity applying the load and the usable volume, bearing in mind that an adiabatic withdrawal is involved. It also means a reduction in energy for the control element acted on.

The flow rate from an accumulator is dependent on the line cross section and the difference between the (decreasing) accumulator pressure p 1 and the respective load pressure p L:

hydrospeicher_01.gif

This pressure difference also means a loss of energy (Figure H 25).

Tasks of an accumulator:

  • Storing energy to cover peak needs or to support the pump delivery flow
  • Damping of pulsations in the hydraulic system and weight compensation
  • Compensation for leakage oil and volume
  • Compensation of pressure fluctuations due to temperature changes
  • Emergency actuation (safety reserve)
  • Improving the system efficiency

Designs

Given how dangerous they can be, the accident prevention provisions set out in UVV 17 "Hydraulic accumulators" must be complied with when using hydraulic accumulators!

0001_hydrospeicher

Figure H 25: Pressure losses due to throttles in the accumulator based on the system load