Accumulators are an extraordinary tool in the Snap On chest of hydraulic apparatuses. They can provide the answer at least half a dozen questions including, “how do I create more peak flow” or “how can I provide emergency flow during a power failure,” et al. Maybe in a future blog I’ll discuss how an accumulator can dampen pressure spikes or provide leakage compensation, but in this issue I want to discuss the use of accumulators to provide or supplement flow.
Accumulators can be precarious if not applied correctly; accumulators can be mystifying even when applied correctly. An accumulator is charged on the dry side with compressed nitrogen. Because gas can compress to many thousand times the pressure of our atmosphere and then spring back to its relaxed state, we can use this spring-like quality of gas to store energy.
A charged accumulator provides nothing until it is filled with hydraulic fluid at a pressure higher than the pneumatic precharge. The accumulator is typically filled by the pump during low-duty or non-operating periods of the machine cycle. An accumulator charged to 1000 PSI could see hydraulic pressure up to 3000 PSI before the pump unloads or turns off altogether. In this circumstance, the pneumatic side will have compressed to a third of its original volume, providing the remaining portion of its original volume to be used for hydraulics.
A misconception about accumulators is that because they’re used in hydrostatic systems, that they provide flow the same way a hydraulic pump would. Accumulators are a hydropneumatic hybrid, exhibiting characteristics of pneumatic and hydraulic systems. In a hydraulic system using a fixed displacement pump, fluid being pumped has to go somewhere or the pump or other component will blow apart. However, with an accumulator, flow of hydraulic fluid is limited to difference between the hydraulic pressure it was filled at and the pressure immediately downstream of the fluid port (“fluid port” is obviously a generalization, since both gases and liquids are fluids).
You can close a ball valve at the fluid port of an accumulator when it is charged and filled, with no damage to the accumulator or any other component; the energy just remains trapped. You obviously would not want to close a ball valve at the outlet of a fixed pump during regular operation. The accumulator's trapped-energy trait mirrors a compressed air system, where you can plug the port on an air tank with no negative consequence.
Because a filled accumulator can just stop providing flow when its fluid port is blocked, consider what happens when downstream of that port is restricted or is pushing back. The energy in an accumulator will flow out at a rate dictated by the pressure differential between the stored pressure and the downstream pressure, while factoring the restrictions caused by valves and plumbing.
This means that if the accumulator is charged to 1000 PSI, filled to 3000 PSI, and your work pressure is 1500 PSI, you have 1500 PSI of potential to create pressure drop (based on the upstream accumulator pressure compared to the downstream work pressure). If for example, you had an 0.080” orifice downstream of the accumulator, it would flow exactly 6 GPM based on pushing 1500 PSI through that orifice.
The problem now is that because you’re flowing energy from the accumulator, you’re losing potential to create flow. Pressure drops in the accumulator, lowering the differential you can use to create that flow. If after a moment, pressure drops to 2500 PSI, you now have only 1000 PSI of differential. Flow now drops to just below 5 GPM of accumulator output. When accumulator pressure drops to 2000 PSI and work pressure remains at 1500 PSI, we now have just 500 PSI of potential to flow through that 80 thou’ orifice, and now we have just 3.5 GPM of flow.
Eventually, accumulator flow will slow down further until accumulator pressure is equal to load pressure, and the accumulator then becomes static; it is no longer flowing in or out, but equalized with load pressure.
There lies the difficulty in understanding accumulators … they are dynamic devices most useful when being filled or discharged, but provide supplemental flow contingent upon the state of downstream conditions. It is useful to understand this principle when both applying and troubleshooting with accumulators. They simply do not provide constant flow such as a pump would.