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.

Particularly useful for applications having a

variable demand of energy during operating

cycle.

Functions-1. Hydraulic shocks suppression and

eliminating.

2. Fluid make up in closed system

3. Holding high pressure

Basic Operating Principle of an Accumulator.

1. The accumulator is empty, and neither gas nor

hydraulic sides are pressurized.

2. The accumulator is precharged.

3. The hydraulic system is pressurized. System pressure

exceeds precharge pressure, and fluid flows into the

accumulator.

4. System pressure peaks. The accumulator is filled with

fluid to its design capacity.

5. Any further increase in hydraulic pressure would be

prevented by a relief valve in the system. Equilibrium is

reached.

6. System pressure falls. Precharge pressure forces fluid

from the accumulator into the system.

7. Minimum system pressure is reached. The accumulator

has discharged its design maximum volume of fluid

back into the system.

Weight-loaded or gravity accumulator

Advantage - it applies a constant pressure on the fluid

throughout its range of motion.

Disadvantage – bigger in size, bulky in weight,

should be vertically mounted,

loss of energy due to friction

Spring-loaded accumulator:

Advantage- compact in size, closed system,

lighter, mounted in any direction

Disadvantage-

- non uniform pressure, small volume

Gas-loaded accumulator:

Also called as Hydro-pneumatic accumulator

The exact shape of the accumulator characteristic curve

depends on pressure–volume relations:

Isothermal (constant temperature): This occurs when the

expansion or compression of the gas is very slow.

The relationship between absolute pressure p and volume V of the

gas is constant:

pV=C

Isentropic (adiabatic processes): This is where there is no flow of

energy into or out of the fluid. The law that the gas obeys is given

by (pV )ϒ = constant,Where ϒ is ratio of specific heat and is approximately equal to 1.4.

Polytropic: This is somewhere between isothermal and isentropic.

This gas change is governed by the law, (pV)n = constant,

where n is somewhere between 1 and 1.4 and is known as the

polytropic coefficient.

Two types of gas-loaded accumulators:A. Non-separator-type accumulator:Advantage-

Simple in construction

Disadvantage-

Gas may be absorbed in oil

Non uniform pressure

Here the oil and gas are not separated. Hence,

they are always placed vertically.

A. Separator-type accumulator:Here the oil and gas are separated by an element.

i.Piston type accumulator:

ii.Diaphragm accumulator

iii.Bladder accumulator:

Piston-type accumulator:

Adv- effective sealing between two phase,

Suitable for low and high temp and longer life

Disadv- pressure is not uniform

Piston-type accumulator:

Diaphragm accumulator:

Advantage- it has no sliding surface that requires

lubrication and can therefore be used with fluids having poor

lubricating qualities,

Quick response time , high operating pressure

Disadvantage- non uniform supply of pressure, lower

volume storage

synthetic rubber diaphragm

Bladder accumulator

Advantage- positive sealing between fluid and gas,

compact design , light weight, less expensive

Disadvantage-unsuitable for high temp oils,

store small volume of fluid

Bladder accumulator-Various Stages of accumulator

Accumulator Selection:

Po= Gas pre charge pressure at room temp (Patm)

P1= Min working pressure

P2= Max working pressure

Vo= Effective gas volume of accumulator,

V1= Gas volume at P1

V2= Gas volume at P2

The gas pre-charge pressure must be as close as possible

to the minimum working pressure p1 to obtain

maximum storage.

For Bladder accumulator- P2 = 4Po

For Diaphragm accumulator- P2 = 8Po

The maximum gas pre-charge pressure is found

from the relationship

Sizing Accumulators for Isothermal Condition-

Sizing Accumulators for Adiabatic Condition

For polytropic expansion, n=1.4

1/n=0.7143

Applications of Accumulators-

Applications where accumulators are used in a

hydraulic system

1. Accumulator as an auxiliary power source

2. Accumulator as a leakage compensator

3. Accumulator as an emergency power source

4. Accumulator as a hydraulic shock absorber

5. Accumulator as a thermal expansion

compensator

1. Accumulator as an auxiliary power source

Store the oil delivered by the pump during a portion of the

work cycle.

When the four-way valve is activated, oil flows from the

accumulator to the blank end of the cylinder. This extends the

piston until it reaches the end of the stroke.

When the cylinder is in its fully extended position, the

accumulator is being charged.

2.Accumulator as a leakage compensator:

An accumulator can be used as a compensator for internal and

external leakage during an extended period in which the system is

pressurized but not in operation.

Based on Electric switch, which control ON/OFF condition of

motor, to charge the accumulator.

3.Accumulator as an emergency power source:

Cylinder be retracted even though the normal supply of

oil pressure is lost due to a pump or electrical power

failures.

If the pump fails due to an electric failure, the solenoid de-

energizes, shifting the valve to its spring offset mode.

Then the oil stored under pressure is forced from the

accumulator to the end of the cylinder. This retracts the

cylinder to its starting position.

The accumulator charges as the cylinder extends.

4. Accumulator as a Hydraulic shock absorber:

Hydraulic shock is caused by the sudden stoppage or declaration

of a hydraulic fluid flowing at relatively high velocity in a pipe line.

The resulting rapid pressure pulsations or high-pressure surges

may cause damage to the hydraulic system components.

If an accumulation is installed near the rapidly closing valve, the

pressure pulsations or high-pressure surges are suppressed.

5. Accumulator as a thermal expansion compensator:

When closed-loop hydraulic systems are subjected to heat

conditions, both the pipe lines and the hydraulic fluid expand

volumetrically.

This condition may cause pressures to exceed the limits of

safety and may damage the system components.

An accumulator of proper capacity pre-charged to the normal

system working pressure is installed. It takes up any increase

in the system fluid volume, thus reducing the system pressure to

its safe limits. The accumulator also feeds the required volume

into the system as thermal contraction takes place.