When sized and applied correctly, accumulators produce a win-win situation. First, accumulators can reduce the size of the pump needed to power a hydraulic system. And because a smaller pump carries a lower price tag, OEMs can save on initial cost of the power unit. But users can also benefit from higher system efficiency and less electrical use in the long run.
Strategic application of accumulators allows for specifying smaller pumps because accumulators store hydraulic energy. During periods when equipment requires little or no hydraulic power, the pump charges the accumulators with pressurized fluid. Then, when the system demands full flow — which the pump alone cannot accommodate — the accumulators release their stored energy to assist the pump in meeting flow demand. When the system again demands less power, the pump recharges the accumulators to prepare them for the next period of high flow demand.
Higher energy efficiency stems from a smaller motor needed to drive the smaller pump. Furthermore, when the system runs in standby mode, the smaller motor-pump combination consumes less power than a larger unit running in standby. The smaller motor consumes less power during peak flow demands as well, because the accumulators reduce the peak power requirement of the motor.
Learning by example
To demonstrate, assume a system is designed for a maximum operating pressure of 3000 psi. This is the highest fluid pressure with which the accumulator can be charged. Figure 1 shows the minimum system operating pressure to be 2000 psi, which normally is determined by the minimum system pressure requirement of the actuators. Based on these parameters, the first calculation to be made is precharge pressure for the accumulator.
Gas precharge pressure for piston-type accumulators generally is 100 psi less than minimum operating pressure. So for our example, precharge pressure is 1900 psi. This provides the maximum usable fluid discharge from the accumulator, without bottoming out the piston.
Still referring to Figure 1, the machine operates in 30- sec cycles, consisting of 19 total sec of fluid demand by system actuators and 11 total sec of dwell time. This is typical of automated machinery: certain functions of a process require more hydraulic power than others. The variation in hydraulic power demand provides an opportunity to use an accumulator so that the pump can be sized more on the basis of average power demand rather than peak demand. The accumulator recharges whenever pump discharge flow exceeds system flow requirements.
Sequence of events
At zero seconds, system pressure is maximum (3000 psi), and the accumulator has been fully charged. According to Figure 2, it would contain 9.12 gal of fluid. The accumulator actually has a total volume of 25 gal, but nearly 16 gal is occupied by precharge gas. As the actuators demand flow, system pressure gradually drops, and fluid flows from the accumulators.
When system pressure drops to its lowest point in the cycle, 2026 psi, the accumulator contains 1.54 gal of fluid. This means it has provided 7.58 gal in the first 10 sec of the operating cycle to supplement pump flow. After 10 sec, the flow requirement drops to well under 40 gpm (Figure 1), so excess flow from the pump begins to recharge the accumulator. This is evident in Figure 2; fluid volume in the accumulator steadily decreases, but shows a temporary increase from 10 to 12 sec. The peak system flow demand is 100 gpm, but because the accumulator supplements pump flow, a pump rated for 40 gpm can be used. Its flow is represented by the green areas in Figure 1.
Determining the optimum size of accumulator and pump may require several iterations of sizing calculations. Accumulator sizing software, such as Tobul’s Accumulator Sizing and Selection Program, is available from a variety of sources to greatly simplify this process. In sizing accumulators, many factors may have to be considered that affect such concerns as safety, efficiency, redundancy of hydraulic function required, and initial cost of equipment.
When the only objective is to reduce the required size of the pump, the following approach can be used to determine accumulator size. The total system cycle must be examined in detail to determine which portion of the cycle time is used to charge the accumulator, and which portion of the cycle allows the accumulator to supplement pump flow. The total system cycle is the time in which all hydraulic functions are completed and any dwell time before the hydraulic functions start again.
To begin, the accumulator size and pump discharge rate can be selected somewhat arbitrarily. It is only a starting point. Continuing with our example, the peak flow rate of the system is 100 gpm, although only for a few seconds of the cycle. For the first calculation, a 75-gpm pump might be selected.
Assuming a 75-gpm pump will be used, you look at the portion of the system cycle when the accumulator can supplement pump flow. Next, the total amount of fluid to be dispensed from the accumulator is calculated by multiplying the supplemental flow rate by the total time in which this occurs. The next step is to determine the allowable pressure drop, which is the starting pressure minus the minimum pressure required to perform the function. Using the appropriate conversion factors then yields the accumulator size in gallons.
Based on these figures, you would find that a 75-gpm pump would keep the accumulator fully charged during most of the cycle, so it would provide a relatively small portion of the total flow required to perform the work. After completing this initial calculation, it would be evident that a smaller pump and a larger accumulator would be a better combination. The next step might be to try a 60-gpm pump and 15-gal accumulator. In our example, we would again find that supplemental flow from the a c cumul a tor would not be optimum.
Eventually, we would find the 40- gpm pump and 25- gal accumulator to be optimal for this application. The cycle provides just enough time for the pump to recharge the accumulator with hydraulic fluid before the next cycle begins. Specifying a smaller pump would not provide enough time to recharge the accumulator.
With all the repeated calculations that must be performed, it is easy to realize the value of using computers for this task. Tobul’s latest version of its Sizing and Selection Software is Windows-based and addresses 14 different applications useful in optimizing accumulators and systems. The real advantage can be seen when the hydraulic system supplier is able to customize the equipment to the users’ needs, and show concrete examples of performance and initial costs balanced against on-going energy costs.
Determining optimum combination of pump and accumulator size varies according to the unique parameters and design objectives of each application. It is important that a pump-accumulator combination be selected to ensure that the system pressure does not drop below 2000 psi, which is the minimum pressure required to perform the work.
Even higher efficiency
Another way to realize even greater efficiency in this application is to increase system operating pressure. Figure 3 shows maximum operating pressure of 5000 psi. This allows the use of smaller actuators to perform the same work and reduces the flow demand by about 35%. There is also a much greater operating pressure range, from a maximum of 5000 psi to a minimum of 3200 psi. This greater pressure differential provides an excellent opportunity for increased use of an accumulator, and greater efficiencies.
The higher minimum pressure, 3200 psi, is required to exert the same amount of force through the smaller actuators. So the gas precharge pressure is 3100 psi — 100 psi below the lowest operating pressure when using a piston-type accumulator. Cycle times are the same as in the first example.
Peak flow demand is 65 gpm, which is less than in the previous example because smaller actuators can transmit the same power as larger actuators at lower pressure. However, by using 15-gal accumulators, we can get by with a pump rated for only 25 gpm.
To put this into perspective, without the accumulator, this application would require a pump with a hydraulic power capacity of nearly 200 hp. With the accumulator, less than 75 hp is needed. When you see the potential ongoing cost savings that can be realized with an optimized system including accumulators, the equipment supplier can show it does have the user’s best interests in mind — and for the long run.