| Hydraulic Accumulator |
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Types Accumulators are widely used in fluid power systems as a means of storing energy. Although weight and spring loaded types are sometimes used, those employing a pressurised gas are preferred because of their compactness and superior performance. There are two main types of pressurised gas filled accumulators these using piston and collapsible bladders.
Figure 1. Accumulators As shown in Figure 1 the fluid is separated from the pressurised gas by either the bladder, Figure 1(a), or by a piston, Figure 1(b). The gas is usually nitrogen that is supplied via the gas valve with a pre-charge pressure that is determined by the pressure range required by the application. Sealing of the piston is obviously important and there can be friction between the piston and the cylinder that can affect the liquid pressure level. This problem does not arise with the bladder types and extra gas can be added by the use of separate storage gas containers. Performance The accumulator is initially charged to a pressure P0 that is set at a level lower than the minimum operating pressure Pr The pressure of the gas will vary with changes in the volume, but the relationship between these parameters will depend on the amount of heat transferred to the surroundings. It is usual to assume a polytropic expansion index for the gas, the value of which depends on the operating times and the duty cycle.
Figure 2. Accumulator Pressure
The values of the polytropic indices cannot be accurately predicted and it is usual to take the value of nj as 1 (isothermal) and n2 as 7 for the gas, where this value is obtained for the expected operating temperature and pressure. Thus equation 6 produces:
This equation gives a conservative value for most applications. In certain cases, e.g. high or low temperatures, it may be necessary to apply a correction factor and, in those situations, information should be obtained from the manufacturer.
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