Hydraulic accumulators are energy storage devices in a hydraulic circuit. They are the hydraulic equivalent of a capacitor in an electrical circuit. Accumulators can be used in a variety of ways in a hydraulic system. The most common use is to deliver a high volume of oil very rapidly to extend and retract cylinders at fast speed. Accumulators can also be used to absorb shock, reduce noise levels, maintain pressure, and to provide fluid in the event of a power failure condition.

Accumulators that are used for volume are pre-charged with dry nitrogen normally to 1/2 – 2/3rds the maximum system pressure. For example, if the maximum system pressure, as determined by the pump compensator or system relief valve, is set at 1500 PSI, the accumulator should be pre-charged between 750 – 1000 PSI. The pre-charge is the pressure of the nitrogen in the gas-side of the accumulator when the accumulator is devoid of fluid. Gas filled accumulators have the dry nitrogen separated from the fluid by a bladder, piston, or diaphragm, depending on the accumulator’s construction.

Without fluid pressure in the system, the dry nitrogen will fill the entire void of the accumulator. Either the bladder or diaphragm will be fully extended, or the piston will be fully pressed against the fluid inlet cap. As the pump is turned on, the fluid pressure builds to a point above the pre-charge pressure, and the nitrogen gas starts to be compressed. This function continues until the maximum system pressure is reached, possibly at 1500 PSI. If the pre-charge is 750 psi in a 10-gallon accumulator, there will now be 5 gallons of fluid in the accumulator and the gas pressure will be at 1500 psi. When directional valves are shifted in the system, the system and accumulator pressure will begin to drop if more fluid is used than what the pump can produce. In a bladder accumulator, the nitrogen forces the bladder to expand and force the oil out of the shell. In a piston type, the piston is forced against the fluid side, again delivering a high flow rate of oil to the system. In most systems, the accumulator volume combines with the pump volume to power the actuators.

Accumulators that are constantly functioning in a system will generally have a higher temperature at the fluid end than at the gas end. This is one way to verify that an accumulator is operating properly. If the shell is the same temperature all over then it is possible that the bladder may have ruptured, the pre-charge may be above the compensator or relief valve setting or the nitrogen may have leaked out of the valve stem. This heat differential will vary with accumulator size, the number of cycles per minute and the amount of volume delivered. The accumulator has the capability of delivering portion of its rated gas capacity, depending on the pre-charge/system pressure ratio.

Besides checking for temperature differences, accumulators can be verified by observing the fluid pressure gauge as the system is turned off. Often the accumulators have manual or solenoid operated valves to release their fluid back to tank when the system is turned off. As the fluid leaves the accumulators back through the system and to tank, the system pressure will slowly drop until all fluid is out of the accumulators. At this point, the system pressure gauge will rapidly drop to 0 PSI. The last pressure reading, before the gauge needle went to 0 PSI, is very close to the pre-charge pressure. You can see a video of this demonstration. Click here.

The most common way to check an accumulator’s pre-charge is to use a check/charge head. The head has a valve that is threaded onto the gas valve when the accumulator is devoid of fluid. It head also has a pressure gauge which allows the gas pressure to be read directly out of the accumulator gas storage area. Many heads also are capable of being used to charge the accumulator, as they have a hose connection that leads to a nitrogen bottle valve connection. You can find a variety of check/charge heads and kits, click here.

There are other machine-specific indications that an accumulator is not functioning properly, which can include increased cycle times, increased noise, and/or pump unloading valves rapidly cycling. Many times, systems that have accumulators will begin to develop signs that indicate that the accumulator is not functioning as intended. A good hydraulic technician can often determine the integrity of an accumulator in relatively short order by using one of the above techniques. A good maintenance program includes periodic checking and recording the pre-charge pressure in an accumulator, and/or the cycle times of the process.