Hydraulic accumulators are found in almost every industrial plant. Most facilities have several of them, but they often are misunderstood. Accumulators can be the most dangerous hydraulic components in the mill, not because they are inherently dangerous, but because of the lack of understanding. All hydraulic accumulators, regardless of their purpose, store energy and therefore must be treated with some measure of respect.
A charging rig should be
used to pre-charge an accumulator.
Accumulator Functions
A hydraulic accumulator is used for one of two purposes: either to add volume to the system at a very fast rate or to absorb shock. Which function it will perform depends upon its pre-charge. If the accumulator is to be used to add volume to the system, its pre-charge must be somewhat below the maximum system pressure so oil can enter it. If the accumulator will be used to absorb shock, it must be pre-charged close to the maximum system pressure so there will be little or no oil in it.
Pre-charging an Accumulator
An accumulator typically is pre-charged with dry nitrogen. Nitrogen does not react unfavorably with hydraulic oil under pressure, and since it composes nearly 78 percent of the earth’s atmosphere, it is the least expensive gas that can be used safely. The next most plentiful inert gas is argon, which makes up less than 1 percent of the earth’s atmosphere.
Under no circumstances should an accumulator be pre-charged with oxygen or air. If compressed oxygen or air encounters even a small amount of any hydrocarbon, it can react violently, resulting in an explosion, fire, injury to personnel and property damage. An accumulator should bear a safety sticker that warns against pre-charging with any gas but nitrogen. New accumulators come with such stickers, but they often are scratched off or painted over.
A charging rig should be used to pre-charge an accumulator. The pre-charge should be performed with no oil in the accumulator. Release any pressure at the accumulator inlet. Most accumulators have a dump valve that can be opened to drain oil to the tank. Screw the charging rig onto the accumulator’s Schrader valve and turn the gas chuck handle clockwise to depress the pin. The current pre-charge can then be read on the charging rig.
If the pre-charge is too high,
the bladder in a bladder accumulator
may hit the poppet assembly,
resulting in a cut bladder
or excessive wear of
the poppet spring.
If the pre-charge is too high, a bleeder valve on the charging rig can be opened to release nitrogen to the atmosphere until the pre-charge drops to its recommended level. If the pre-charge is too low, the charging rig comes with a hose to connect it to a nitrogen bottle. With the nitrogen bottle connected, crack the valve on the bottle and slowly add nitrogen until the pre-charge reaches the desired level.
The correct pre-charge varies by the application and type of accumulator. Most accumulators are the bladder, piston or diaphragm type. Follow any recommendations from the original equipment manufacturer (OEM), if they are available. If not, the correct pre-charge can be estimated.
When an accumulator is used for volume purposes, such as to apply a brake in the event of a power failure, to supplement the output of a pump, or to maintain a constant system pressure, most manufacturers recommend a bladder accumulator be pre-charged to 80 percent of the minimum acceptable pressure and a piston accumulator to 100 pounds per square inch (psi) below minimum pressure. Unfortunately, in many instances, the OEM pre-charge pressure is not available, and the minimum acceptable pressure is unknown. When this is the case, pre-charging to 50 percent of the maximum system pressure usually yields an acceptable result.
If a bladder accumulator loses its charge,
the bladder may be pushed to the
top of the shell and become
ruptured by the
Schrader valve assembly.
The most common cause of an accumulator failure is too high of a pre-charge. If the pre-charge is higher than it should be, the bladder in a bladder accumulator will hit the poppet assembly during each cycle, causing either a cut bladder or excessive stress wear of the spring in the poppet. In piston accumulators, too high of a pre-charge can damage the piston and prevent it from hitting bottom at each cycle. Too low of a pre-charge (or an increase in system pressure without a compensating increase in the pre-charge) can also lead to operating problems, such as reduced speed and stalls. It may even result in accumulator damage.
In general, it is better to undercharge than overcharge. However, a bladder accumulator that has lost all or most of its charge may have its bladder crushed at the top of the shell and become ruptured by the Schrader valve assembly.
Adding Volume
Volume accumulators have a dump line to release pressure whenever the system is shut down. The pre-charge can be checked without the charging rig by watching the gauge fall when the system is shut down and the dump line is opened. The gauge will drop slowly because the dump line normally is downsized to avoid turbulence in the reservoir. When the gauge reaches the current pre-charge of the accumulator, it will then drop immediately to 0 psi.
This also is a good way to tell if an automatic dump valve has opened as it should. When the system is shut down, if the gauge drops immediately to 0 psi without gradually dropping first, it is likely that the gauge is isolated from the accumulator, and whether the accumulator has dumped will be unknown.
Over time, oil bypasses
the piston seals
in a piston accumulator,
displacing nitrogen.
Shock Absorption
When an accumulator is used for shock absorption, it is not desirable that there be much, if any, oil in the accumulator during operation. The accumulator will respond more rapidly to pressure spikes if the compression process has already begun. For this reason, it usually is recommended that shock accumulators be pre-charged to nearly 100 psi below the maximum load pressure of the actuator it protects.
It is not advisable to use a piston accumulator to absorb shock. Bladder and diaphragm accumulators are more responsive because they don’t have to overcome the static friction of the piston seal, and the mass of the piston need not be accelerated or decelerated.
Mounting a piston accumulator horizontally can
cause the piston seals to wear more rapidly.
The piston accumulator is like a hydraulic cylinder with no rod. It is pre-charged with nitrogen and no oil in the bottom. When the system is pressurized, the nitrogen compresses as the bottom of the accumulator fills with oil. The nitrogen pressure matches the system pressure, so any reduction in system pressure will cause the accumulator to discharge oil to the system. The accumulator thus will supplement the pump during portions of the cycle when the system demands more flow than the pump can provide.
Over time, oil will bypass across the piston seals, displacing nitrogen at the top. The first indication of this is an increase in the pre-charge pressure when no nitrogen has been added. Because of the bypassed oil on top, the stroke of the piston is reduced. System pressure drops, stalls may be noted, and the piston can become damaged by repeatedly hitting the bottom of the accumulator.
Oil can be removed from the top of the accumulator by attaching the charging rig and opening the bleeder valve with the system pressurized, removing all nitrogen and oil. When oil stops coming out, the piston is at the top. The system can then be shut down, the dump valve opened to drain oil from the bottom, and the pre-charge restored, recovering the accumulator’s diminished capacity. If oil never stops coming out of the bleeder valve, the piston is badly worn and must be replaced.
If a bladder accumulator is mounted horizontally,
a cavity may form between the bladder and the shell,
causing fluid to become trapped.
Mounting an Accumulator
It generally is best to mount accumulators in a vertical position with a mounting bracket about two-thirds of the way up the shell. Mounting a piston accumulator horizontally will cause more rapid wear of the piston seals. Bladder accumulators can also be damaged if they are mounted horizontally. In addition to uneven bladder wear, fluid can become trapped away from the outlet if a cavity forms between the bladder and the shell. Diaphragm accumulators usually can be mounted in any position.
Inspecting Accumulators
Hydraulic accumulators should be carefully inspected visually at least once per year, more often in environments unfriendly to steel. Ensure there are no rust spots or cracks in the paint. Look for loose mounting points, worn rubber and any indication of movement during operation. Check all fittings for leaks. At least every five years, the accumulator should be removed from service and hydrotested. Finally, never attempt to repair an accumulator shell. If there are any breaches, the shell should be discarded and replaced.
