Membrane accumulators for hydraulics

Accumulators in general

One of the main tasks of a hydraulic accumulator is storing a certain amount of (hydraulic) fluid under pressure and wholly or partially discharging this when necessary. Because of the way a hydraulic accumulator works, they are seen as a pressure vessel that should be able to operate at maximum working pressure and should comply with regulations that are required in different countries.

Accumulators with a partition between the fluid side and the gas side which is usually pre-filled with nitrogen, are often applied in hydraulic systems. There are different types of partition available that determine the design and shape of the accumulator.

WA membrane accumulators

For many hydraulic accumulators a membrane is used as partition between the fluid side and the gas side of the accumulator. These maintenance free accumulators are completely maintenance free, which also means that they cannot be repaired. Membrane accumulators are produced in large quantities and are well-suited for relatively simple/cheap hydraulic systems. A membrane accumulator is made of 2 strong steel halves that are welded together. The U-formed membrane separates the gas from the hydraulic fluid. Membranes are available in nitrile rubber NBR, epichlorhydrin ECO and nitrile rubber suitable for low temperatures (-40° C). When pre-filled a robust steel knob seals off the hole at the fluid side and this prevents the membrane from being pushed out of the accumulator.

The charging valve is available in standard size M28x1,5, which uses a securing pin in the charging valve. For pre-filling an accumulator a DP100 device for pre-charging should be used. In case of a 5/8” charging valve a DP200 device for pre-charging should be used. Accumulators that have been pre-filled with nitrogen to a specific pressure by the manufacturer, are sometimes equipped with a welded plug which means it is not possible to change the pre-filled pressure in a later stage. There are various ways of connecting membrane accumulators. Findynamica can supply membrane accumulators from stock.

Properties

  • Compact design
  • Quick and easy installation
  • Can be mounted in any position
  • Long lifetime
  • High energy efficiency
  • Quick response time (less than 25 millisecond)
  • High compression ratio max 8:1
  • Can be used together with lubricants
  • High dirt tolerance (parts present in the fluid)
  • Limited weight
  • Low costs

Applications

  • Energy storage in systems with interrupted operation for power reduction of the pump
  • Energy storage for emergency back-up systems when the pump does not work or during electricity failure.
  • Compensation for loss through leakage
  • Pressure compensation
  • Damping of vibration
  • Volume compensation in case of pressure and temperature fluctuations
  • Hydraulic suspension for vehicles
  • Shock absorbers for mechanical influences

Collars for accumulators

An accumulator should be properly attached. Most accumulators are quite heavy. Because of this Findynamica has a large range of collars in stock so every accumulator can be mounted safely and securely.

Pre-filling your accumulator at Findynamica

When you need an accumulator it is quite easy to arrange for the accumulator to be pre-filled with nitrogen. At Findynamica we have the possibility to fill your accumulator which makes it directly available for mounting within your hydraulic system. Findynamica also sells complete devices for pre-charging.

 

Calculations

The status of the accumulator

To make the right calculations different factors concerning accumulator, working pressure, pre-fill pressure, required volume and working temperatures should be taken into account.

  • P­0 = (nitrogen pre-fill pressuren) and V0 = (actual gas volume)  indicate the pre-fill status. The hydraulic pressure is lower than the pre-fill pressure, the pre-fill pressure is so high that there is no fluid present in the accumulator.
  • P1 = (minimal working pressure) and V1 = (gas volume at pressure P1) indicate the status at minimal working pressure. Always make sure there is a slight amount of oil present in the accumulator to prevent the membrane from hitting the outside shell.
  • P2 = (maximum working pressure) en V2 = (gas volume at pressure P2)   indicate the status at the membrane at maximum working pressure. At that point the membrane has been fully pushed in and the accumulator is filled with oil.

 

Calculating pre-fill pressure

The pre-fill pressure depends on the application it is used for.

  • In case of energy storage, emergency function, hydraulic suspension, pressure compensation, leakage compensation and volume compensation the pre-fill pressure (P0)  is usually 0,9 to 0,95 times the minimal working pressure (P1) (at maximum working temperature). The compression ratio must be the same or lower as the maximum compression ratio indicated in the product catalogue.
  • In case of pulsation damping, the pre-fill pressure is 0,7 to 0,9 times the working pressure.

Temperature fluctuations

The working temperature can strongly influence the pre-fill pressure of the accumulator. When the temperature is increased, the pre-fill pressure will also increase. When the temperature is lowered, the pre-fill pressure will also be lower. To optimally use an accumulator when the temperature fluctuates it is important that the pre-fill pressure is calculated in the right way.      

P0 (T20) = P0 (Tx) x (20 + 273 / Tx + 273)

P0 (Tx) = gas pressure P0 at measured temperature Tx
P0 (T20) = nitrogen filling pressure P0 at 20°C

 

Extensive product documentation


Membrane accumulators