Applications/Functions of an Accumulator
Hydraulic accumulators are used in hydraulic systems as:
Compensation for large flow variation: During operation of hydraulic installation, if requirements of supply of large volumetric flows appear, then these requirements can be supplied by an accumulator.
To smooth out pressure surges:
Pressure surges are caused in a
hydraulic system when fluid flow is suddenly changed. The high inertia
of the moving fluid sets up shock waves in the pipes causing hammer and vibration.
Accumulators will absorb the energy and dampen out these surges thus reducing vibrations and shock.
To provide emergency power source: The fluid energy stored in an accumulator may be sufficient to give an emergency supply in the case of major electrical failure causing the pumps to stop.
Types of Accumulators
There are three main types of accumulators used in hydraulic systems:
1. Weight-loaded Accumulators
2. Spring-loaded Accumulators
3. Gas charged Accumulators
1. Weight-Loaded Accumulator
The weight-loaded accumulator consists of a vertical cylindercontaining fluid connected to the hydraulic system (Refer to Figure 5.12).The cylinder is closed by a piston on which a series of weights are placed that exerts a downward force on the piston and thereby energizes the fluid in the cylinder. The weight may be of some heavy material such as iron,
concrete block, pig or scrap iron, etc. The pressure on the fluid depends on the weight on the piston and the size of the piston, and can be changed by adding or removing weights from the piston.
2. Spring-Loaded Accumulator
A spring-loaded accumulator works on the same principle as the weight-loaded accumulator. A spring-loaded accumulator consists of a cylinder body, a movable piston, and a spring. The spring applies force to the
piston. As fluid is pumped to it, the pressure in the accumulator is determined by the compression rate of the spring. The spring force increases as it is compressed by more fluid entering the chamber.
The advantages of these accumulators are that these are usually less expensive than the dead weight type and mounting is easy. They are built directly into the power unit. The disadvantage is that the spring force and the
resulting pressure range cannot be easily adjusted. This accumulator is also not suitable for applications, which require large volume of fluids.
3. Gas Charged Accumulators
Gas charged or compressed gas accumulators consist of a cylinder with two chambers that are separated by an elastic diaphragm or by a floating piston. One chamber contains hydraulic fluid and is connected to the hydraulic line. The other side contains an inert gas under pressure that provides the compressive force on the hydraulic fluid. Inert gas is used because oxygen and oil can form an explosive mixture when combined under high pressure. As the volume of the compressed gas changes the pressure of the gas, and the pressure on the fluid, changes inversely. Gas charged
accumulators are of two types:
1. Piston type
2. Bladder type
1. Piston type: These have an outer cylinder tube, end caps, a piston element, and sealing system. The cylinder holds fluid pressure and guides the piston, which forms the separating element between gas and fluid. Charging the gas side forces the piston against the end cover at the fluid end. As system pressure exceeds the minimum operating level for the accumulator, the piston moves and compresses gas in the cylinder.
2. Bladder type: It consists of a pressure vessel and an internal elastomeric bladder that contains the gas. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from being ejected with the outflowing fluid. The poppet
valve is sized so that maximum volumetric flow cannot be exceeded. To operate, the bladder is charged with nitrogen to a pressure specified by the manufacturer according to the operating conditions. When system pressure exceeds gas precharge pressure of the accumulator, the poppetvalve opens and hydraulic fluid enters the accumulator. The change in gas volume in the bladder between minimum and maximum operating pressure determines the useful fluid capacity.