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An accumulator is a storage unit that uses pressure to store energy. Accumulators are used to control the flow and power of liquids and gases through the machine to which they are attached. The main types of accumulators are hydraulic and water-air combination.
A hydraulic accumulator, or pressure accumulator, is used to absorb shock, maintain pressure, dispense fluids, and provide reserve power to the main pump. One of the main differences in accumulators is how gas and fluid are stored and converted. The part of the unit that keeps the fluid and gas separate is called a separator. There are three types of separators: bladder, diaphragm, and piston.
In a bladder style, an internal tank called a bladder is filled with nitrogen. As the bladder fills, the pressure increases until it is over the preset limit. When the pressure exceeds the limit, a valve in the top opens and hydraulic fluid enters and flows to the area of the machine that requires lubrication.
With a diaphragm style, a cylinder is welded into the center of the accumulator and gas is inserted through the diaphragm. As the pressure builds, the cylinder fills and when the pressure reaches the maximum setting, a valve opens and fluid enters. The main difference between the bladder and diaphragm styles is the amount of pressure that can be withstood. The bladder style has a pressure ration of 4:1 where the diaphragm style has a pressure ratio of 10:1. The higher the ratio the more fluid the system can hold which makes it more efficient.
In a piston style, the piston is mounted inside the unit on top of the cylinder that holds the gas. As the gas is forced into the cylinder, the piston moves to maintain the pressure. When the pressure reaches it maximum, the piston opens to release the gas and allows the hydraulic fluid to flow in. This type of accumulator is often used in large-scale devices such as heavy tractor equipment. The piston type, however, is designed for more continuous use so it typically cannot hold pressurized gas for long periods of time.
A water accumulator works in a similar way to the hydraulic accumulator. The main difference is that instead of using gas to build the pressure, water is used. As the air fills the accumulator tank, the valve opens and forces the water out. This technology is how most office water coolers work. When someone turns the valve to release water, the air pushes the water level down toward the release valve.
There are several factors to be considered when choosing between bladder and piston accumulators.
First, it depends on the what fluid the system uses. Bladder accumulators are better for water systems since it can resist corrosion better than a piston accumulator. On the other hand, you need piston accumulators if you feel that a certain failure in the system could compromise safety (think brake systems).
Lastly, response time to variations in the system pressures is also a factor. Bladder accumulators are best if you need something that responds more quickly. Usually, bladders are used if you need a response time of 25ms or less. Go with piston types if response time needed is 25ms or higher.
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