Sizing Hydraulic Accumulators for Various Applications

Bob Wojcik, Hydraulic Engineer

Properly sizing an accumulator depends upon several system conditions that must be fully understood before actually sizing the accumulator for the application.

To understand accumulators, first identify the various applications where accumulators can be beneficial for hydraulic systems and the system’s inherent application energy conservation issues or concerns.

Secondly, explore the critical concerns and system circuit aspects that are required to properly size the accumulators.

Proper application and sizing of accumulators requires extensive information. Therefore this article will cover only the first of 10 accumulator applications. Quality Hydraulics & Pneumatics will publish subsequent articles to cover the other nine applications!

There are 10 principal applications for hydraulic accumulators:

1. Auxiliary Power Supply. An accumulator is used as a source of energy/work in combination with a hydraulic system pump to provide auxiliary fluid flow during high demand requirements.
2. Leakage Compensation. A hydraulic accumulator can be placed in a hydraulic circuit to provide makeup fluid if no other source of flow and pressure is available for this purpose. This can also be an energy conservative solution.
3. Thermal Expansion. CompensationSystem pressure trapped and subject to temperature changes from low to high and/or fluid expansion under high heat conditions can cause expansion and raise pressures to unsafe levels. An accumulator can protect the hydraulic system from these pressure variations.
4. Emergency Power Source. In the case of a power loss, the accumulator can operate the necessary functions to bring the equipment into a safe state by providing stored fluid and energy.
5. Fluid Make Up Device. In a closed hydraulic system, an accumulator can make up the difference in fluid volume between the rod end and blind end of a hydraulic cylinder.
6. Pulsation Dampening and Hydraulic Shock Absorption. When a pump’s ripple effect and/or compensator reaction time are critical to system operation, the accumulator will compensate for the ripple effect and respond to circuit demands faster than a pump will react. The accumulator will also dampen hydraulic line shock conditions.
7. Power Source in Dual Pressure Circuits. When a dual flow or pressure circuit is used, the accumulator could provide higher flow rates for the high pressure portion of the cycle and thereby reduce the overall system horsepower requirement. Thus the circuit is more energy conservative.
8. Holding Devices. If a circuit requires holding pressure on a function for perhaps many hours, the accumulator can save the day. If the pump were running for all those hours, the system would be very energy inefficient. However, holding the pressure with an accumulator sized specifically for this function saves lots of costly energy!
9. Transfer Barrier. An accumulator can allow for two different fluids being pressurized to the same pressure while using one as the source of pressure and the second develops the same pressure.
10. Fluid Dispenser. Liquids and lubricants can be stored in an accumulator and then be dispensed to a number of machine bearings exactly when needed, under controlled pressure.

Hydraulic Accumulators operate on the principles of Boyle’s Law of Gases!

The basic relationship between the pressure and the volume of gas is expressed by the equation: P1V1n= P2V2n, where P1 and P2 are the initial and final gas pressures and V1 and V2 are the corresponding gas volumes.

The next consideration in sizing accumulators is to understand the rate at which the gas will expand in the application. Will the gas expand rapidly or slowly as compared to the corresponding flow requirement? The rate of gas expansion can affect the operation and performance of the accumulator in the application and therefore correct formula data must be provided in the equations for correct sizing of the accumulator.

The two types or conditions of gas expansion rate are called isothermal and adiabatic. An isothermal rate condition is when the compression and expansion of the gas takes place slowly, allowing sufficient time for the heat generated to be dissipated. In isothermal expansions the n factor in the equation is equal to one (1).

In the case of an adiabatic rate condition, the compression and expansion of the gas takes place rapidly. This affects the specific heat of the gas and the n factor in the equation changes to 1.4. Generally, if compression or expansion of the gas occurs in less than one minute the application is an adiabatic rate condition. Otherwise it is isothermal.

First Application:

Here is an example of one of the more common applications for an accumulator. It corresponds to #8 “Holding Devices” in the application listing above.

This application uses an accumulator to hold a circuit pressurized for prolonged periods of time. An example might be the hours when a machine is operating a “curing process”.

This application would be considered Isothermal as it will have little or no actual compression or expansion time to be considered. One thing to consider in these “Holding Devices” is that there could be leakage occurring in the associated components of this circuit. Therefore, some volume at pressure must be considered to account for the leakage. Please refer to catalog information about each circuit component to estimate the leakage compensation needed.

If, for example, the system requires 300 in3 of fluid to make up for leakage and provide the holding for the required curing cycle:

Since we have established that this application is Isothremal and we know that the ‘n’ factor is
equal to ‘1’, we will ignore the ‘n’ factor in the equations below!
The maximum operating pressure is 3000 PSI,
this drops to a minimum of 1500 PSI for the required holding force, and
assuming a GAS (nitrogen) charge of 1000 PSI:

The known factors for the solution:
V1 = ? (size of accumulator) in cubic inches – the unknown
P1= 1000 PSIA
P2 = 3000 PSIA
P3 = 1500 PSIA
Vx = 300 cubic inches 

The next larger standard size accumulator is 5 gallons.

Other examples of accumulator applications will be published in subsequent application articles.

For immediate assistance with your specific accumulator application, please contact a Quality Hydraulics & Pneumatics Certified Fluid Power Specialist or technical manager for assistance.