“Accumulators in
Hydraulic Systems”
Submitted By: Ahmed Sleem Abdel-Hameed
Accumulators in Hydraulic Systems
Accumulators usually are installed in hydraulic systems to store energy and to smooth out pulsations.
Accumulators will cushion hydraulic hammer, reducing shocks caused by rapid operation or sudden starting
and stopping of power cylinders in a hydraulic circuit. The two most common types of accumulators are:
➢ Diaphragm (or bladder) type.
➢ Hydro-pneumatic piston type.
Simple Schematic of Bladder Type Accumulator Simple Schematic of Piston Type Accumulator
“The image shown above are under the courtesy of Machinery Lubrication magazine”
Bladder accumulators have an advantage over piston-type accumulators because they respond more
quickly to system pressure variations than do piston types. the response time (less than 25 milliseconds). And
this is for two reasons:
1. Rubber bladders do not have to overcome the static friction which a piston seal has.
2. In the case of piston-type accumulators the piston mass to be accelerated and decelerated.
Another benefit that gives bladder accumulators an edge is that they have good dirt tolerance; they are mostly
unaffected by particle contamination in the hydraulic fluid.
On the other hand, Piston accumulators can handle much higher gas compression ratios (up to 10:1)
and flow rates as high as 215 liters (57 gallons) per second. And another advantage is that piston accumulators
can be mounted in any position unlike ladder accumulators, whose preferred mounting position is vertical to
prevent the possibility of fluid getting trapped between the bladder and the shell.
Functions of Accumulators:
1. Energy Storage:
The pre-charged (Compressed) gas in the accumulator can be compressed into small volumes at high
pressures. Potential energy is stored in the compressed gas to be released upon demand.
For example, in the piston type accumulator, the energy in the compressed gas exerts pressure against the
piston separating the gas and hydraulic fluid. The piston in turn, forces the fluid from the cylinder into the
system and to the location where useful work will be accomplished.
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Submitted By: Ahmed Sleem Abdel-Hameed
2. Pressure Pulsation Absorption:
Pumps generate the required power to be used or stored in a hydraulic system. Many pumps deliver this
power in a pulsating flow. For example, the axial piston pump can produce pulsations detrimental to a high-
pressure system. An accumulator properly located in the system (downstream of the pump) will substantially
cushion these pressure variations.
3. Shock Cushioning:
In many fluid power applications, the actuators of the hydraulic system stop suddenly, creating a
pressure wave that travels back through the system. This shock wave can develop peak pressures several times
greater than normal working pressures. When accumulators are properly located in the system, they will
minimize these shock waves.
4. Supplementing Pump Flow:
The pump stores potential energy in the accumulator during idle periods of the work cycle. The
accumulator transfers this reserve power back to the system when the cycle requires emergency.
Pre-charging of Accumulators:
➢ Bladder type accumulators pre-charge is set to between 60% and 80% of the minimum system
pressure.
➢ That way the bladder doesn't expand enough to close the poppet during normal operation.
➢ Accumulators are pre-charged with dry, inert, nitrogen gas rather than simply using compressed air.
➢ When charging the gas end of a bladder or diaphragm accumulator, the nitrogen gas should always be
admitted very slowly.
➢ If the high-pressure nitrogen is allowed to expand rapidly as it enters the bladder, it can chill the
bladder's polymeric material to the point where immediate brittle failure occurs.
Bladder Type Accumulator in an actual hydraulic system
Pre-charge Pressure = 80% of Minimum Pressure
➢ In the case of piston accumulator, excessively high or low pre-charge can cause the piston to bottom
out at the end of its stroke, resulting in damage to the piston and its seals.
➢ Piston type accumulators are more tolerant of improper pre-charge than bladder accumulators.
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Submitted By: Ahmed Sleem Abdel-Hameed
“The figure above shows the internal components for typical piston-type accumulator”
“The figure above shows the internal components for typical bladder type accumulator”
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Submitted By: Ahmed Sleem Abdel-Hameed
