Reduce Energy Needs With Accumulators

Hydraulic circuit designers in-the-know incorporate hydropneumatic accumulators into their systems to reduce cost and save energy.

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.

Reducing cycle time increases speed and productivity

Accumulators can be added to machines when speed and production must be increased without increasing the size of the power unit. This is easily accomplished if the machine cycle exhibits dwell periods. Accumulator size is determined by the required maximum and minimum pressures and the volume of fluid required to supplement pump flow to reduce the cycle rate. The required dwell time determines the size of the pump. If 308 in.3 of fluid exits the power unit during the machine cycle, the pump must be sized large enough to put the 308 in.3 back into the accumulator during dwell time.

Figure 1. Gas bottles can be used in conjunction with accumulators when large capacity assemblies are required to significantly reduce the cost of the accumulators and gas bottles.

For example, assume a machine has cycle and dwell time rates of 8 sec. A productivity increase is desired, but the dwell time cannot be changed due to machine loading and unloading. Adding an accumulator to the system can decrease the cycle time from 8 to 4 sec. Assuming the system has a 10-gal power unit working at 1000 psi that is capable of 2000 psi, a 2.5- gal accumulator can be incorporated. Increasing the pressure to 1650 psi and reducing the cycle time to 4 sec — while maintaining the 8-sec dwell time — achieves a 33% increase in production rate.

Conserving energy

Figure 2 shows why accumulators are the most efficient solution in systems that need to hold pressure for a long period of time while unloading the pump to conserve energy. Examples include rubber curing presses and on a sluice gate, where the gate is positioned for long time periods.

In power generating plants, where a fail-safe gate or butterfly valves are held closed by a heavy spring, a cylinder is used to keep the spring collapsed and the valve open. The accumulator keeps pressure on the cylinder, holding the spring in the collapsed position while the pump is unloaded to conserve energy and keep the fluid from heating up. This type of arrangement is used between the nuclear reactor — which generates steam — and the steam turbine that drives the generator. The same types of valves are also used in fossil fuel power plants. If the pressure in the cylinder drops to a predetermined level, the pump will come on and charge the accumulator.

Figure 2. Energy can be conserved by unloading pumps at low pressure.

In hydraulic systems where the pump does not unload or a variable displacement pump is not used, the pump runs fully loaded 100% of the time. If the cylinder requires 2000 psi to keep the spring compressed (as in the above example), the power unit would continuously dump pressurized fluid over the relief valve at 2000 psi. This would heat the fluid to the point of requiring a heat exchanger.

Now assume a 5-gpm fixed displacement pump is operating at 2000 psi. We can easily calculate the hydraulic power of this setup:

hp = gpm (psi 1500) hp = 5 2000 1500 This equates to a 6.6 hp pump running continuously. If the pump is unloaded at 60 psi, then: hp = 5 60 1500 0.2 hp would be required in the unloaded condition.

The accumulator can be charged and the pump unloaded at low pressure to prevent wasting energy and overheating the fluid. In most cases, this procedure can be done using a variable-displacement pump. However, a 10-galvariable displacement pump will consume about 2 hp of energy while in a pressure-compensated mode. On the other hand, a fixed-displacement pump, unloaded at 60 psi, will only pull about 12 hp. As a result, considerably less heat will be generated in the system, reducing the need for a heat exchanger in many instances.

Emergency power source
In mobile equipment, a hydraulic accumulator can serve as an auxiliary power source when the main power source fails, such as when the engine shuts down due to a failure or depletion of fuel. The Department of Transportation requires off-highway trucks and wheel loaders to have a secondary method of stopping the vehicle in instances where the engine may die and hydraulic power is otherwise lost. An accumulator sized to supply the pressure necessary to apply the brakes a predetermined number of times is an ideal way to meet this requirement. This also holds true for steering and braking on large off-highway vehicles. The accumulator is sized to be able to negotiate a prescribed road course with the pump off to steer the unit to a safe stop.

A significant application is the use of accumulators to cont rol valves or sluice gates in process, water treatment, sewage treatment plants, and dam projects. When power is lost , say f rom loss of electricity, there must be a way to control the valves or sluice gates to prevent or regulate the flow of water or other medium into or out of the system. Energy stored in an accumulator can be used to power a cylinder or rotator actuary to close or reposition valves that regulate the flow.

An accumulator as the most practical means of providing auxiliary or emergency power source for hydraulic systems in these types of applications. The alternative would be to use a pump driven off a battery power source, or a pump driven by a gas or diesel engine. Both of these solutions are far more expensive, less reliable, and are not practical in most applications.

Summary of benefits
Incorporating accumulators into system design at the beginning not only creates a better design, it provides a more efficient system for most applications. In machines where dwell time exists between different work functions, accumulators usually can reduce the overall cost of the power unit and reduce the operating cost.

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