The diaphragm-type accumulator consists of a diaphragm secured in a shell and serving as an elastic barrier between the oil and the gas. The cross-sectional view of a diaphragm type accumulator is shown in Figure 7.18.
A shut off button which is secured at the base of the diaphragm, covers the inlet of the Hne connection when the diaphragm is fully stretched. This prevents the diaphragm from being pressed into the opening during the precharge period. On the gas side, the screw plug allows control of the charge pressure and the charging of the accumulator by means of a charging and testing device.
With the help of the following figures (Figures 7.19(a)-(f)), let us now see how exactly a diaphragm-type accumulator works.
Figure 7.19(a) shows the accumulator without the nitrogen charge in it or in other words in a precharged condition. The diaphragm can be seen in a non-pressurized condition.
Figure 7.19(b) shows the accumulator in charged condition. Here nitrogen is charged into the accumulator, to the precharged pressure.
Figure. 7.19(c) shows how the hydrauhc pump delivers oil to the accumulator and how this process leads to the deformation of the diaphragm.
As seen from Figure 7.19(d), when the fluid delivered reaches the maximum required pressure, the gas is compressed. This leads to a decrease in gas volume and subsequent storage of hydraulic energy.
Figure 7.19(e) shows the discharge of the oil back to the system when the system pressure drops, indicating requirement of oil to build back the system pressure.
Figure 7.19(f) shows the accumulator attaining its original precharged pressure condition.
The primary advantage of the diaphragm-type accumulator is the small weight-tovolume ratio, which makes it highly suitable for airborne applications.
One of the most important industrial applications of accumulators is in the elimination or reduction of high-pressure pulsations or hydraulic shocks.
Hydraulic shock (or water hammer, as it is frequently called) is caused by the sudden stoppage or deceleration of a hydraulic fluid flowing at a relatively higher velocity in the pipelines. This hydraulic shock creates a compression wave at the location of the rapidly closing valve. This wave travels along the length of the entire pipe, until its energy is fully dissipated by friction. The resulting high-pressure pulsations or high-pressure surges may end up damaging the hydraulic components.
An accumulator installed near the rapidly closing valve as shown in Figure 7.24 can act as a surge suppressor to reduce these high-pressure pulsations or surges.
In this application (Figure 7.23), the accumulator acts as a compensator, by compensating for losses due to internal or external leakage that might occur during an extended period of time, when the system is pressurized, but not in operation.
The pump charges the accumulator and the system, until the maximum pressure setting on the pressure switch is obtained. When the system is not operating, it is required to maintain the required pressure setting, to accomplish which the accumulator supplies leakage oil to the system during a lengthy period of time. Finally when the system pressure falls below the minimum required pressure setting, the pump starts to automatically recharge the system. This saves electrical power and reduces heat in the system.
The bladder-type accumulator contains an elastic barrier between the oil and gas as shown in the cross-sectional view in (Figure 7.20).
The bladder is fitted to the accumulator by means of a vulcanized gas-valve element that can be installed or removed through the shell opening at the poppet valve. The poppet valve closes the inlet when the bladder is fully expanded. This prevents the bladder from being pressed into the opening. A shock-absorbing device, protects the valve against accidental shocks, during a quick opening.
The greatest advantage with these accumulators is the positive sealing between the gas and oil chambers. The Ughtweight bladder provides a quick pressure response for pressure regulation as well as applications involving pump pulsations and shock dampening.
Figure 7.21 illustrates the functioning of a bladder-type accumulator.
The hydrauhc pump delivers oil to the accumulator and deforms the bladder. As the pressure increases, the volume of gas decreases. This results in the storing of hydraulic energy. Whenever additional oil is required by the system, it is supplied by the accumulator even as the pressure in the system drops by a corresponding amount.
The non-separator type consists of a fully enclosed shell containing an oil port at the bottom and the gas-charging valve at the top. The valve is confined to the top and the oil to the bottom of the shell. There is no physical separator between the gas and oil, and thus the gas pushes directly on the oil.
The main advantage of this type of accumulator is its ability to handle a large volume of oil. However, its disadvantage lies in the fact that the oil tends to absorb gas due to the lack of a separator. A cross-section of a non-separator type accumulator has been illustrated in Figure 7.16.
A gas-loaded accumulator must be installed vertically to keep the gas confined to the top of the accumulator. It is not recommended for use with high-speed pumps as the entrapped gas in the oil may cause cavitation and damage the pump. The absorption of gas in the oil also makes the oil compressible, resulting in spongy operation of the actuators.