Types of Hydraulic Accumulators | Working, Applications

Accumulators are simple devices that store energy in the form of fluid under pressure.
The purpose of an accumulator is to store hydraulic energy in the form of pressurized fluid, provided by the pump, and later provide it to the system whenever needed. Because of their ability to store excess energy and release it when needed, accumulators are useful tools in developing an efficient hydraulic system.
Accumulators are found in numerous applications, they are used in conjugation with the hydraulic system on large hydraulic presses, construction equipment, farm machinery, power brakes, automotive suspensions, hatch covers on ships, landing gear mechanism on airplanes, etc.

Types of Accumulators

Accumulators are generally classified by means of the use of energy storage. Accumulators are classified as,
  1. Weight loaded accumulators
  2. Spring-loaded accumulators 
  3. Gas loaded accumulators
Gas loaded accumulators are further divided as the non-separator type and separator type.
Separator type gas loaded accumulators consist of

  • Bladder type
  • Diaphragm type
  • Piston type.
Types of Hydraulic Accumulators
Types of Hydraulic Accumulators
Let us discuss each type of accumulator in brief.

Weight Loaded Accumulator

A weight loaded accumulator is shown in the figure.
Weight Loaded Accumulator
Weight Loaded Accumulator
Weight loaded accumulator consists of a ram loaded with dead weight. The mass of dead weight may be concrete steel or other heavy material. The piston rod is much bigger in size to avoid buckling failure due to heavy load. The accumulator pressure is given by,

Where, 
d = Diameter of piston
W = Deadweight
w = Piston and Piston rod weight
And volumetric Capacity of weight loaded accumulator is given by,
Vacc = π/4 x (d2) x L
Where,
L = Piston stroke length

Advantages of Weight Loaded Accumulator

  • Pressure remains constant for the full stroke.
  • It can supply a large volume of fluid under high pressure.
  • Limitations of weight loaded accumulator
  • They are bigger in size and much bulky as compared to others of the same capacity. They consume more space and need heavy structures to support them.

Applications of Weight Loaded Accumulator

Due to their size and bulk, their applications are limited to stationary equipment such as heavy-duty presses, where large volumes are required.

Spring Loaded Accumulator

A spring-loaded accumulator is shown in the figure below.
Spring-loaded Accumulator
Spring-loaded Accumulator
Spring-loaded accumulator consists of a cylinder containing a spring-loaded piston, with fluid entering on another side of the cylinder. The fluid is stored under pressure by the spring force. As the spring is compressed completely, the accumulator pressure reaches its peak and as the spring approaches its free length, the accumulator pressure drops to its minimum. The design is mostly multi-spring, to reduce the size of the accumulator for the same pressure.

Advantages of Spring-loaded accumulator

  • With the use of multiple springs, a compact size accumulator can be developed for the same pressure as compared to the deadweight type accumulator.
  • All parts are enclosed in a casing and no moving parts protrude out.

Limitations of Spring Loaded Accumulator

  • The pressure exerted on the fluid is not constant as in the deadweight type. As the springs are fully compressed, the accumulator pressure reaches its peak and as the spring approaches their free length, the accumulator pressure drops to a minimum.
  • Due to the presence of springs in the upper part of the cylinder, the useful piston stroke is limited. Hence accumulator cannot provide higher volumes.

Applications of Spring Loaded Accumulator

Spring-loaded accumulators are used where space limitation is the primary criterion and considerably smaller volumes are required. For example, automotive and mobile hydraulic applications.

Non-separator type Gas Loaded Accumulator

A gas-charged accumulator pressurizes the fluid by the pressure of the gas. Gas loaded accumulators are also called as hydro-pneumatic accumulators.
When there is a separator placed between hydraulic fluid and gas, it is called separator type and when there is a barrier placed between hydraulic fluid and gas, it is called gas loaded accumulator with separator.
Non-separator type gas loaded Accumulator
Non-separator type gas loaded Accumulator
As the fluid is charged in the shell, the fluid level rises, resulting in compression of the gas. The compression of gas induces pressure in it and this pressure exerts a force on the fluid surface.

Advantages of Non-separator type gas loaded Accumulator

  • No mechanical elements to apply force on hydraulic fluid, hence simple in construction.
  • It can absorb shocks and fluctuations since gas is easily compressible.
  • Low operating cost.

Limitations of Non-separator type gas loaded Accumulator

  • Due to a lack of physical separator, the gas may be absorbed in the oil. This makes oil compressible leading to the jerky motion of actuators.
  • It must be always kept in a vertical position to confine gas at the top.
  • The pressure on the fluid goes on decreasing as the volume of gas goes on increasing.

Applications of Non-separator type gas loaded Accumulator

Non-separator type gas loaded accumulator is used in applications where the hydraulic fluid used in the system has high sir separate-ability, and the slight missing of gas in fluid doesn't create any functional problems.

Bladder type accumulator

Bladder type accumulator employes a bladder, as an elastic separator between the hydraulic fluid and the compressible gas, normally nitrogen.
Bladder Type Accumulator
Bladder type Accumulator
The bladder is fastened inside the steel shell by means of a vulcanized gas valve assembly. The bladder can be recharged, or replaced by removing it. The task of oil-valve assembly is to close the inlet opening when the bladder has expanded fully, thus preventing the bladder from being forced into the opening.
Working of Bladder Type Accumulator
Working of Bladder Type Accumulator
The above figure shows the working of the bladder type accumulator. As shown in the figure, in a normal position, the bladder is fully expanded to the rated volume V0 and pressure of gas P0. As the accumulator is getting charged, the bladder goes on contracting due to compression of gas by hydraulic fluid. Accumulator gets charged until maximum working pressure P2 is reached and corresponding volume V2. Hence, the maximum charged volume of hydraulic fluid is equal to, ΔV=V1-V2.
Later, whenever the system needs the fluid it is discharged by the expansion of the bladder.

Advantages of Bladder Type Accumulator

  • Bladder provides positive sealing between the gas and the oil chamber.
  • It is compact and light in weight.
  • Since the bladder is a rubber element and has small inertia, it provides quick pressure response even to minute changes.
  • They have very few functional parts and are less expensive.

Limitations of Bladder Type Accumulator

  • The pressure exerted on the fluid is not constant and goes on reducing as the bladder goes on expanding.
  • Comparatively smaller volumes are accommodated.
  • The bladder is required to be replaced after a certain period of service.
  • They cannot handle high-temperature fluids.

Applications of Bladder Type Accumulator

Due to the quick-responding nature, the bladder type accumulator is widely used in the wheel suspension system of automotive vehicles. Further, it is found applicable where smaller volumes at comparatively higher pressure are required.

Diaphragm Type Accumulator

Diaphragm type accumulator uses a diaphragm as the elastic separator between the hydraulic fluid and the gas, normally nitrogen.
Diaphragm Accumulator,Diaphragm type Accumulator
Diaphragm type Accumulator
A diaphragm is clamped between the walls of the pressure vessel. A shutoff button is fixed on the base of the diaphragm. This button serves to block the fluid inlet opening when the diaphragm is fully expanded, thus preventing the diaphragm from being forced into the opening.
The functioning of the diaphragm type accumulator is shown in the figure.
Working of Diaphragm Type Accumulator
Working of Diaphragm Type Accumulator
In the normal position, the diaphragm is fully expanded to the rated volume V0 and the pressure of gas P0. As the accumulator starts getting charged, the diaphragm is pushed up, causing the compression of the gas.
When the maximum working pressure P2 is reached, the accumulator is said to be fully charged. This volume is equal to, ΔV=V1-V2
Afterward, whenever the system needs the fluid, it is discharged due to the expansion of the gas contained part of the vessel.

Advantages of Diaphragm Type Accumulator

  • The diaphragm provides positive sealing between the gas and the oil chamber.
  • It is compact and lighter in weight as well there are no mechanically moving parts.
  • It provides quick response to even minute changes in pressure.
  • Higher operating pressure than bladder type accumulator.

Limitations of Diaphragm Type Accumulator

  • The pressure exerted on the fluid is not constant and goes on reducing as the diaphragm goes on expanding.
  • Comparatively smaller volumes are accommodated.
  • The diaphragm is required to be replaced after a certain period of time.
  • It cannot handle high-temperature fluid.

Applications of Diaphragm Type Accumulator

Due to the quick-responding nature, it is widely used in the wheel suspension system of automotive vehicles. Further, it is found applicable where smaller volumes at comparatively higher pressure are required.

Piston Type Gas Charged Accumulator

In this type of accumulator, a piston is a barrier between the gas chamber and the oil chamber. The construction of a piston-type gas-charged accumulator is the same as that of a spring-loaded accumulator except the spring is replaced by the pressurized gas.
Piston Type Gas Charged Accumulator
Piston Type Gas Charged Accumulator

Advantages of Piston Type Gas Charged Accumulator

  • It can handle high or low-temperature fluids.
  • It can be mounted in any position, but vertical mounting is preferred.
  • The piston provides a positive seal between gas and oil.
  • Its working life is longer.

Limitations of Piston Type Gas Charged Accumulator

  • Comparatively expensive to manufacture.
  • The pressure is a function of compression of gas and varies with the volume of oil in the chamber.
  • Comparatively slower response to pressure changes hence cannot be used as pressure pulsation damper.

Applications of Piston Type Gas Charged Accumulator

Due to expensive nature, their application is limited to high-temperature systems with smaller volume requirements.

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