What is Hydraulic accumulator – Types Of Hydraulic Accumulator
Introduction to Accumulators :
A hydraulic accumulator is a device that stores the potential energy of an incompressible fluid held under pressure by an external source against some dynamic force. This dynamic force can come from different sources. The stored potential energy in the accumulator is a quick secondary source of fluid power capable of doing useful work.
There are three basic types of accumulators:
1. Weight-loaded or gravity accumulator:
Schematic diagram of weight loaded accumulator is shown in Fig..It is a vertically mounted cylinder with a large weight. When the hydraulic fluid is pumped into it, the weight is raised. The weight applies a force on the piston that generates a pressure on the fluid side of piston. The advantage of this type of accumulator over other types is that it applies a constant pressure on the fluid throughout its range of motion. The main disadvantage is its extremely large size and heavy weight. This makes it unsuitable for mobile application.
2. Spring-loaded accumulator:
A spring-loaded accumulator stores energy in the form of a compressed spring. A hydraulic fluid is pumped into the accumulator, causing the piston to move up and compress the spring as shown in Fig. . The compressed spring then applies a force on the piston that exerts a pressure on the hydraulic fluid.
This type of accumulator delivers only a small volume of oil at relatively low pressure. Furthermore, the pressure exerted on the oil is not constant as in the dead-weight-type accumulator. As the springs are compressed, the accumulator pressure reaches its peak, and as the springs approach their free lengths, the accumulator pressure drops to a minimum.
3. Gas-loaded accumulator:
A gas-loaded accumulator is popularly used in industries. Here the force is applied to the oil using compressed air. Schematic diagram of a gas loaded accumulator is shown in Fig. A gas accumulator can be very large and is often used with water or high water-based fluids using air as a gas charge. Typical application is on water turbines to absorb pressure surges owing to valve closure and on ram pumps to smooth out the delivery flow.
There are two types of gas-loaded accumulators:
Non-separator-type accumulator: Here the oil and gas are not separated. Hence, they are always placed vertically.
Separator-type accumulator: Here the oil and gas are separated by an element. Based on the type of element used to separate the oil and gas, they are classified as follows:
(a) Piston-type accumulator:
Schematic diagram of a piston type accumulator is shown in Fig..It consists of a cylinder with a freely floating piston with proper seals. Its operation begins by charging the gas chamber with a gas (nitrogen) under a pre-determined pressure. This causes the free sliding piston to move down. Once the accumulator is pre-charged, a hydraulic fluid can be pumped into the hydraulic fluid port. As the fluid enters the accumulator, it causes the piston to slide up, thereby compressing the gas that increases its pressure and this pressure is then applied to the hydraulic fluid through the piston. Because the piston is free sliding, the pressure on the gas and that on the hydraulic fluid are always equal.
(b) Diaphragm accumulator:
In this type, the hydraulic fluid and nitrogen gas are separated by a synthetic rubber diaphragm. Schematic diagram of diaphragm accumulator is shown in Fig. The advantage of a diaphragm accumulator over a piston accumulator is that it has no sliding surface that requires lubrication and can therefore be used with fluids having poor lubricating qualities. It is less sensitive to contamination due to lack of any close-fitting components.
(c) Bladder accumulator:
It functions in the same way as the other two accumulators. Schematic diagram of bladder accumulator is shown in Fig. Here the gas and the hydraulic fluid are separated by a synthetic rubber bladder. The bladder is filled with nitrogen until the designed pre-charge pressure is achieved. The hydraulic fluid is then pumped into the accumulator, thereby compressing the gas and increasing the pressure in the accumulator. The port cover is a small piece of metal that protects the bladder from damage as it expands and contacts the fluid port.
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