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.

xFor 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.

xIn 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.

For more information, e-mail egodin@parker.com or mschubert@parker.com or visit www.parker.com.