Types of Hydraulic Accumulators & Their Applications
An accumulator is an apparatus by which energy or power can be stored to do useful work. An electric storage battery, for instance accumulates energy from a generator while an air storage tank accumulates pneumatic power.
Hydraulic Accumulators employ gravitational force, the elasticity of a spring or the compressibility of a gas for storing energy in a practically incompressible fluid.
Accumulator Types.
Weight Loaded Type
– This was the earliest form of accumulator and is still used today to operate large batteries of hydraulic presses. They consist of a vertical steel cylinder into which a sliding piston fits. Mounted on top of the piston is a platform holding weights to provide the gravitational force. The size of this type of accumulator is a serious disadvantage, one of 20L capacity weighs about 3,000kg, as opposed to 80kg for a modern pneumatic type.
Spring Loaded Type
– This type of accumulator utilises the strain energy of a spring to load a piston against the systems fluid, the pressure generated is dependent on the initial loading rate of the spring. This type of accumulator is limited in its application as there is not constant pressure acting throughout the piston stroke and because of the restricted cycle life of the spring material.
Pneumatic Accumulators
– Pneumatic accumulators which have been far more successful in their application than the previous types, consist of a non-separator and separator types. This refers to whether or not the gas and fluid are kept apart by some form of interface. Non-separator types are charged with compressed air or nitrogen to the minimum pressure requirement with the unit partially filled with fluid. If the fluid level in the accumulator rises the gas is compressed and its pressure rises and in this way a large volume of oil can be accumulated to do useful work. The great disadvantage of this type of unit is that aeration causes the pre-charge to be reduced. However, where very large volumes (2000 L or more) are required there is no alternative and the air volume is maintained by a compressor operated by a level switch.
Piston Type Accumulator
– The Piston type accumulator is limited by cost and size, its performance as a pulsation damper is severely restricted because of piston inertia and friction. In time, Piston seal wear will allow leakage which will necessitate frequent gas charging. The main advantage of this type of accumulator is its ability to accommodate very high or low fluid temperatures by the use of compatible seals.
Tubular Type
– This unit has been designed primarily as a pulsation and shock eliminator and can be mounted on the discharge of single and multi-cylinder pumps. A typical unit consists of a thick walled outer tube containing a rubber sleeve over an inner perforated metal tube. Gas is compressed into the outer space and when no fluid is present the inner tube prevents collapse of the rubber sleeve. During operation the volume inside the rubber sleeve can alter to accommodate flow transients, the pressure changes being determined by the gas volume and pre-charge pressure.
Bladder Type
– This type of accumulator is similar in function to the piston accumulator except that the oil and gas are separated by a membrane in the form of a bag. The bag is largely unstressed which allows it to cope with large volume changes without affecting the pressure. The bag material can be of nitrile rubber, butyl, neoprene or natural rubber for compatibility with a variety of fluids. When there is no fluid pressure the gas pre-charge expands the bag to fill the accumulator interior. The poppet valve at the fluid entrance closes under this pressure to prevent the bag from being extruded through the port. During operation changes in fluid flow rate cause the bag to expand and contract the pressure being determined by the gas low connecting pressure and volume.
Applications for Accumulators
The hydraulic accumulator can be used to provide high rates of flow for short periods, recharge taking place over a much longer period of time. In this way the hydraulic power pack need only be big enough to cope with the recharge flow rate and not the full flow demand, thus affording considerable economy. This is comparable to the use of the car battery for providing a short period high current during engine starting. When an accumulator is used in this manner the system load factor (meanpower/maxpower) is improved with consequent saving in capital investment and energy consumed. Connecting an accumulator to a jack and isolating the two components from the rest of the circuit with a valve results in a jack of apparently moderate stiffness and allows it to be used as a spring. A typical application of this device may be found in a fork-lift truck. The accumulator can also protect fluid systems from overpressures that would arise from expansion due to rising temperature. By monitoring the level of fluid in the accumulator a pump can be arranged to switch on to maintain this level in order to replace fluid lost from the system due to leakage. A further most important feature of accumulators is their ability to absorb pressure rises that would occur during, say, rapid valve closure. The accumulator would require sufficient capacity such that by taking in fluid the system fluid deceleration rate is suitably reduced. Pressure pulses that occur at the inlet and outlet ports of reciprocating pumps can also be absorbed or reduced by a suitable accumulator.
For more information on accumulators or any hydraulic equipment, please call the sales office on 01452 523352.