How to Charge HYDAC Hydro-pneumatic Bladder Accumulator

How to Charge HYDAC Hydro-pneumatic Bladder Accumulator
12 Apr

This video has been prepared in order to explain the process of charging the Nitrogen pre-charge pressure of a bladder type accumulator.

HYDAC Australia host regular training courses on many topics, including hydraulic accumulator technology in Melbourne, Australia. For more information, please visit our website,

What is an accumulator?

Hydraulic accumulators are specifically designed to store and then discharge pressurized fluid as needed. They are classified as pressure vessels. Hydropneumatic accumulators are charged with nitrogen, which is separated from the fluid by a piston, bladder or diaphragm. There are risks involved when working with high-pressure gasses and fluids. On no account must any welding, soldering or any mechanical work be carried out on the accumulator shell. Work on systems employing hydraulic accumulators must only be carried out once the pressure in the fluid and/or the gas have been released. This tutorial video has been prepared in order to explain the process of charging the nitrogen pre-charge pressure of a bladder type accumulator.


Hydropneumatic accumulators need to be pre-charged with an inert gas in order to work at all. Selection of the correct gas pressure is vital, as it defines the stored hydraulic fluid volume and pressure, which are important for the safe and correct operation of any machine. It is important that the correct gas pressure is maintained, and periodic inspection is therefore necessary.

This gas used in hydro-pneumatic accumulators is almost always industrial grade nitrogen. The use of other gasses in these applications is dangerous, and should not be considered.

The nitrogen pre-charge pressure of an accumulator can only be assessed when it has been fully depressurised of hydraulic fluid. This means that the accumulator must be positively isolated from a live system and released of all hydraulic energy, or that the entire system has been completely shut-down and fully depressurised, or that the accumulator is not currently connected to a system, for example, it is on a work bench.

The HYDAC FPU-1 Universal charging and testing unit can be fitted directly to HYDAC diaphragm and piston accumulators, but it uses an adaptor to connect to a bladder type accumulator. This is known as the A3 adaptor. It is clearly stamped A3.

The accumulator gas valve will be opened and closed by pushing the valve plunger down. This will be operated by the centre screw of the A3 adaptor. Due to the high pressures involved, this should never be done by hand. Due to the high pressures involved, this valve should never be opened by hand. Use only the correct equipment, that is the universal charging and testing unit, and follow this process:

  1. Remove the plastic protective cap.
  2. Remove the metal seal cap.
  3. Ensure that the socket head cap screw of the A3 adaptor has been wound back, so as to not immediately open the gas valve upon fitting it to the stem. After ensuring that the surfaces are clean, screw the adaptor on hand-tight.
  4. Before fitting the universal charging and testing unit to the A3 adaptor, first ensure that the surfaces are clean and that the sealing O-ring has been correctly fitted.
  5. Align the spring-loaded hex drive to the screw, and screw the universal charge head to the A3 adaptor, hand-tight.
  6. Ensure that the gas release valve on the side of the charge head has been closed firmly. It closes clock-wise, like a tap.
  7. Before connecting a nitrogen regulator to a nitrogen bottle, ensure that the regulator will be closed, by backing the pressure setting down, winding the adjustment counter-clockwise.
  8. Make sure that the threads and seats are clean, and connect the nitrogen regulator to a bottle of industrial grade nitrogen.
  9. Connect the hose in-between the regulator and the universal test and charge head.
  10. Open the isolation valve of the gas bottle.
  11. Slowly turn the main spindle of the charge head clockwise, until you feel a resistance to movement. From this point, back the spindle off about ¼ of a turn.
  12. Begin the gas flow from the bottle to the accumulator by slowly raising the pressure at the regulator.
  13. When shipped, the majority of HYDAC bladder accumulators are charged to 2 bar. At this pressure, the bladder is inflated, and is in contact with the inside of the shell, and there is little chance of damaging the bladder. However, if the bladder is new, or has been deflated, you must ensure that the initial inflation is undertaken as slowly as possible, otherwise the bladder may stretch too quickly and rupture. You can assess whether the bladder has been inflated through checking the position of the fluid port poppet stem. If it has extended, the inflated bladder is pushing the poppet valve closed.
  14. The rate of nitrogen flow may be increased, only once the bladder has been properly inflated.
  15. Raise the gas pressure in the vessel to the desired pre-charge pressure.
  16. Stop the flow of gas by backing off the nitrogen regulator fully.
  17. Small pressure changes may occur in the accumulator assembly with temperature changes. It is best practice to charge the vessel to a pressure slightly higher than what is needed. Allow the vessel to cool, and then de-pressurise to the final setting slowly.
  18. Once the pressure is at the desired level, you can begin disconnection by winding the main spindle counter-clockwise to re-close the gas valve. The spindle has been fully wound-back once a faint clicking noise is heard.
  19. Release the gas from the charge head and hose, by opening the gas release valve in a counter-clockwise direction.
  20. Once de-pressurised, you can remove the charge head from the A3 adaptor.
  21. Then remove the A3 adaptor from the gas stem.
  22. Use a leak detection fluid to ensure that no gas is escaping from the accumulator.
  23. Replace the metal seal cap, tightening it to 30Nm.
  24. Replace the plastic protective cap.
  25. Close the main isolation valve on the nitrogen bottle.
  26. Vent the remaining gas out of the regulator by adjusting the pressure regulator up.
  27. You can now remove the hose from the regulator, and the regulator from the nitrogen bottle.

We recommend that the gas pre-charge pressure is re-checked the following week. If there is no gas loss observed, check again after four months. Again, if there are no losses observed, a six monthly inspection cycle should be sufficient.

HYDAC host regular training courses on many topics, including hydraulic accumulator technology in Melbourne, Australia. For more information, please visit our website,