accumulators-...2019/09/02 · accumulators- a hydraulic accumulator is a device that stores the...
TRANSCRIPT
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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
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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.
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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
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Spring-loaded accumulator:
Advantage- compact in size, closed system,
lighter, mounted in any direction
Disadvantage-
- non uniform pressure, small volume
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Gas-loaded accumulator:
Also called as Hydro-pneumatic accumulator
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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.
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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:
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Piston-type accumulator:
Adv- effective sealing between two phase,
Suitable for low and high temp and longer life
Disadv- pressure is not uniform
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Piston-type accumulator:
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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
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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
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Bladder accumulator-Various Stages of accumulator
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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
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Sizing Accumulators for Isothermal Condition-
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Sizing Accumulators for Adiabatic Condition
For polytropic expansion, n=1.4
1/n=0.7143
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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
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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.
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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.
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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.
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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.
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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.