Accumulators that are charged with gas are typically seen on modern hydraulic systems. They serve various purposes, including energy storage and reserve, leakage and temperature correction, shock absorption, and energy recovery.
Accumulators, although they offer a variety of benefits to the functioning of hydraulic systems and are capable of delivering many years of trouble-free service, are items that require maintenance. For example, the necessary gas pre-charge pressure needs to be maintained to ensure correct operation and the longest possible life of the product. Additionally, frequent inspection, testing, and certification may be needed by law because, after all, accumulators are considered to be pressure vessels.
Choosing the Right Type of Accumulator
The bladder, piston, and diaphragm are the three types of gas-charged accumulators you may come into when working with hydraulic systems.
The bladder kind is the most common. Bladder accumulators have a quick response time (less than 25 milliseconds), a maximum gas compression ratio of approximately 4:1, and a maximum flow rate of 15 litres (4 gallons) per second. However, “high-flow” versions can achieve flow rates of up to 38 litres (10 gallons) per second. In addition, bladder accumulators have a high resistance to dirt and are mostly unaffected by the presence of particles in the hydraulic fluid that may be contaminated.
On the other hand, piston accumulators can manage far larger gas compression ratios (up to 10:1) and flow rates of up to 215 litres (57 gallons) per second. Piston accumulators can be installed in any orientation, unlike bladder accumulators, which must be installed with their bladders in a vertical orientation to eliminate the risk of fluid becoming trapped between them and the bladder and the shell. Piston accumulators require more fluid cleanliness than bladder units, have slower response times (more than 25 milliseconds), and exhibit hysteresis. On the other hand, bladder units have a higher level of fluid cleanliness than piston accumulators. This is because the piston seal has static friction that needs to be overcome, in addition to the fact that the piston mass needs to accelerate and decelerate at the appropriate times.
Accumulators with a diaphragm provide many of the same benefits as bladder-type machines, with the added ability to manage gas compression ratios of up to 8:1. They can only hold a limited volume, and their performance is occasionally hindered by gas passing through the diaphragm, which is a drawback in itself.
Considerations for Upkeep
Nitrogen gas should never be allowed in ‘quickly’ while charging the gas end of a bladder or diaphragm accumulator; rather, it should be allowed in ‘extremely slowly’ at all times. If the high-pressure nitrogen is allowed to expand as it enters the bladder rapidly, it has the potential to cool the polymeric material of the bladder to the point that it breaks immediately after becoming brittle. In addition, rapid pre-charging has the potential to push the bladder below the poppet at the oil end, which will result in the poppet being severed. The performance of the accumulator may suffer, and it is possible that it could be damaged if the pre-charge pressure is too high or if the minimum system pressure is lowered without a commensurate drop in the pre-charge pressure.
An excessive amount of pre-charge on a bladder accumulator can cause the bladder to be driven into the poppet assembly during the discharge process, damaging either the poppet assembly or the bladder. This is a prevalent factor that contributes to bladder incontinence.
An insufficient amount of pre-charge, or none at all, can potentially have devastating effects on bladder accumulators. In this situation, the system pressure can cause the bladder to be compressed into the top of the shell. Because of this, the bladder may rupture due to being forced into or pierced by the gas valve. In this case, only one of these cycles is necessary to eradicate the bladder.
Similarly, a piston accumulator’s extremely high or low pre-charge can cause the piston to bottom out after its stroke, which in turn can cause damage to both the piston and its seal. The good news is that if this occurs, a warning will be produced that may be heard. Piston accumulators are far more resistant to damage caused by incorrect charging than bladder accumulators, although a faulty charge can still cause harm to piston accumulators.
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