In the past, low-cost energy, plus a desire to keep the design as simple as possible, meant few accumulators were incorporated into initial system designs. Designers of die casting and injection molding machines were exceptions to this rule. Because of the high flow required in the system for a short time followed by some dwell time to recharge the accumulator, pumps and electric motors were kept to a reasonable size. Unfortunately, this thinking did not carry over to most hydraulic system designs.

With the price of energy continually rising, and design engineers challenged to cut as much cost as possible, accumulators are emerging as a way to help achieve significant energy and cost savings. Here are a few examples.

Supplementing pump flow
In many hydraulic systems where high flow is required for a short duration, followed by a few seconds of dwell time, the size of pumps and electric motors can be significantly reduced by incorporating accumulators into the system. Examples include die-casting, injection molding, and rubber molding machines and flying cutoffs.

Assume an appl icat ion where 2000 psi is required to cycle a cylinder in 8 sec, followed by 8 sec of dwell time at the end of the cycle. In this instance, total flow from the pump would be 1000 in.3, or a flow rate of 32.45 gpm. If a simple hydraulic system were designed for this job, it would require a 41.74-hp motor, a motor starter, approximately a 100-gal reservoir, plus valving and filtration.

If a 15-gal accumulator charged to 3000 psi were used in the same application the design requirements would decrease to a 9.3-gpm pump, a 30-gal reservoir and a 17.9-hp motor. Not only would this revised design reduce the cost of the power unit by up to 60%, the operating cost over one year would be reduced significantly on energy consumption alone.

The installed and operating cost savings potential with and without the use of an accumulator are summarized in Table 1.

Accumulators as rechargeable hydraulic batteries
With today’s high fuel cost, accumulators are finding use as rechargeable hydraulic batteries for energy recovery applications in both mobile and stationary equipment. One typical application where they are used is in excavators. Their lift arms are massive, and high force is generated when they are lowered. By using the resulting high-pressure hydraulic fluid to charge an accumulator, the stored energy in the accumulator can then be used to supplement pump flow when it is time to raise the excavator arms and their load. This energy recovery approach makes it possible to reduce pump size by 25%. In turn, the diesel engine driving the hydraulic system can also be reduced in size by up to 25%. The resulting fuel cost savings can be as high as 30 to 35%.

The same approach to energy recovery is being used in oil fields where a hydraulic power unit drives a single-acting cylinder to lift oil from the well. Stringer rods connected to the cylinder are heavy, and because of the heavy load in pulling the cylinder rod out, the fluid in the rod-end of the cylinder develops high pressure if restricted. Again, by using this high-pressure fluid to charge the accumulator, the accumulator can supplement pump flow on the next cylinder cycle. This energy recovery approach also makes it possible to reduce the size of the power unit’s pump, electric motor, and reservoir. Energy cost savings of 15 to 20% is possible in this application.

Insulating the accumulator and associated piping can further improve system efficiency by capturing heat energy. When gas is compressed, it heats up. When it expands, it cools down. For mobile equipment that sits idle for a few minutes, or until flow is needed, the heat is lost to the surrounding atmosphere. Depending on cycle times in mobile applications where the temperature is running 180° to 220°F, system efficiency can be improved by 10 to 15% with the addition of insulation.

Gas bottles shrink size and cost
In situations where large accumulators are required, using gas bottles in conjunction with the accumulators can significantly reduce the cost of piston accumulators and gas bottles. Standard bladder accumulators are not recommended for use with backup gas bottles. This is because the small restriction in the valve stem assembly in a bladder accumulator presents the potential for the bladder to be extruded up into the gas stem, causing an immediate bladder failure. If gas bottles must be used with a bladder accumulator, a transfer barrier should be used, which provides a tube inside the bladder to prevent the bladder from being extruded into the gas stem.

However, a transfer barrier will restrict the flow rate from the accumulator to about 70 gpm. Capacity is also restricted with a transfer barrier because it can only be filled to 80% of its capacity. Plus, it must retain about 10% of the fluid inside when the system cycles. The result is a less efficient system. For example, a 15-gal transfer barrier can only be filled with about 12 gal of fluid. This would only provide about 10.8 gal of usable fluid from the unit when you consider that only about 90% of that fluid is available for work when cycling the units.

So assume that a 45-gal piston accumulator is required for an application. Further assume that the maximum and minimum system pressures are very close together (much like a die casting machine) where only 4.4-gal of fluid is required from the accumulator. If the working pressure ranges from 1700 to 2000 psi, a 44.48-gal accumulator would be required. The list price of a 45-gal accumulator is approximately $14,000. Using a 15-gal accumulator and two 15-gal gas bottles would reduce the accumulator cost by $5770. The added advantage is that gas bottles very seldom ever require any maintenance.

In addition to allowing the use of smaller accumulators to save money, gas bottles can save space. Gas bottles can be remotely mounted, laid on their side, or mounted in any orientation. So, instead of having to handle and use a very large accumulator that might weigh up to 2100 lb, using gas bottles will permit using a smaller accumulator, weighing less than 700 lb.

Additional cost savings can be gained in seal kits. Seal kits for a 50-gal accumulator can cost $1000 (depending on materials), whereas the seal kit for a 15-gal accumulator costs less than half that amount.