Tips on Sizing Accumulators
FLUID POWER  Design Data Sheet 47
Space does not permit a full discussion of accumulators, and
this issue will be limited to a practical method of determining the
minimum capacity of an accumulator when used to supplement the flow
of oil from a pump as in the circuit shown below.
In the basic circuit a closed center 4way valve is used. The
purpose of the accumulator is to store high pressure oil from the
pump during periods when the 4way valve is centered. When stored
pressure reaches a predetermined maximum (usually 3,000 PSI) the
pump is automatically unloaded and allowed to idle for the
remainder of the cycle. Either a special pilotoperated unloading
valve or a pressure switch and solenoid dump valve is used. When
the 4way valve is shifted to start the cylinder, the stored oil
adds to the pump flow to give the cylinder a speed greater than
from pump oil alone.
The big advantage of accumulators on this type system is that a
smaller (and less expensive) pump, electric motor, and control
valve will do the same job as larger and more costly components.
See rule of thumb on percentage duty cycle.
Additional information, plus many accumulator circuits are shown
in publications by Womack Machine Supply Co., including
"Industrial Fluid Power  Volume 1", and
"Fluid Power in Plant and Field".
System PSI. Optimum system pressure for most
accumulators is 3,000 PSI. They have maximum energy storage at
least cost and smallest size when operated at their maximum
pressure rating. Although 5,000 PSI models are available in some
brands, their greatly increased cost offsets their pressure
advantage, so they are less desirable on most systems.
Compressibility. An accumulator adds
compressibility in the oil stream between pump and 4way valve, and
if this is undesirable an accumulator should not be used. However,
on systems using a pressure compensated flow control valve or a
servotype 4way valve, compressibility in the line ahead of the
4way valve is not usually objectionable.
Basic accumulator circuit in
which a low volume pump stores high pressure oil in the
accumulator
while the 4way valve is centered. Pump oil plus accumulator
discharge gives cylinder a rapid stroke.
Minimum
Acceptable
System PSI

Maximum System
PSI With Accumulator Fully Charged 
3,000 
2,750 
2,500 
2,250 
2,000 
2,700 
12 
    
    
    
    
2,600 
17 
    
    
    
    
2,500 
22 
11 
    
    
    
2,400 
27 
16 
    
    
    
2,300 
33 
21 
10 
    
    
2,200 
40 
27 
15 
    
    
2,100 
46 
34 
21 
8 
    
2,000 
55 
41 
27 
14 
    
1,900 
63 
49 
35 
20 
6 
1,800 
73 
58 
43 
27 
12 
1,700 
84 
67 
51 
35 
19 
1,600 
96 
79 
61 
44 
27 
1,500 
109 
91 
73 
55 
36 
1,400 
    
105 
86 
66 
47 
1,300 
    
    
101 
80 
59 
1,200 
Cubic
inches of oil discharged
from a "1" gallon accumulator. 
96 
73 
1,100 
    
89 
Figures in the body of the chart are the number of cubic inches
of oil which can be discharged from a "1gallon" accumulator,
starting with a fully charged pressure shown along the top of the
chart and discharging until the system pressure falls to values
shown in the left column. With larger accumulators, multiply the
figures in the chart times the gallon capacity of the accumulator.
For example, if working with a 10gallon accumulator, multiply
chart figures times 10, etc.
Figures in the chart are about 5% less than if calculated by
Boyle's Law for theoretical discharge. This is to compensate for
loss of capacity during discharge caused by a temperature drop in
the gas when the accumulator is discharged rapidly.
For operating conditions beyond the range of the chart, the
formula in the box below may be used. As explained in the text,
accumulator systems are most often designed for a fully charged
accumulator pressure of 3,000 PSI.
How to Use the
Chart
As oil is allowed to discharge from a piston or bladder type
accumulator, the pressure of the oil drops. For example, looking at
the chart above, in the 3,000 PSI column, when 12 cubic inches of
oil are discharged from a 1gallon size accumulator, the pressure
falls from 3,000 to 2,750 PSI, etc. So, one important factor in
arriving at an adequate size accumulator is to select a size large
enough so this inevitable drop in pressure will not affect normal
operation of the hydraulic circuit. Cylinders and hydraulic motors
must be large enough to produce the required force or torque at the
final pressure remaining at the end of accumulator discharge. In
finding the "gallon" capacity needed in a particular application,
follow these design steps:
Step 1. Calculate or estimate as accurately as
you can, the volume of oil, in cubic inches, which will be needed
from the accumulator on each discharge cycle, to supplement the
volume of oil flowing from the system pump.
Step 2. Considering the size of cylinders or
hydraulic motors being used, calculate to what level the pressure
can drop during the discharge cycle without the output force or
torque falling below an acceptable level.
Step 3. In the chart, use the column headed by
your system pressure when the accumulator is fully charged. In the
left column find the lowest acceptable system pressure at the end
of the discharge cycle. The figure in the chart is the number of
cubic inches of oil which can be recovered from a 1gallon size
accumulator under these pressure conditions.
Example: If your fully charged pressure is
3,000 PSI, and your lowest acceptable pressure is 2,000 PSI, the
chart shows that 55 cubic inches can be recovered from a 1gallon
accumulator. Suppose you calculated that 230 cubic inches will be
needed for your application. The minimum accumulator capacity is:
230 ÷ 55 = 4.18 gallons. The nearest standard accumulator size is 5
gallons  the correct size to use.
While figures in the chart show 5% less volume than calculated,
to take care of loss of capacity which occurs on a quick discharge,
the accumulator, when used on long holding applications, will
eventually give back an additional 5% of oil as the shell, slightly
cooled by the quick discharge, gradually warms back up to normal
temperature.
Mathematical Solution for Accumulator Size
The following is a general formula which will solve for the
cubic inch oil discharge from any size accumulator under any
conditions of precharge and system pressure. It may be used for
operating conditions not covered by the chart.
A lossofcapacity allowance of 5% is built into the formula to
take care of loss of capacity caused by temperature changes as the
gas compresses and expands.
D = [0.95 × P1 × V1 ÷ Ps] 
[0.95 × P1 × V1 ÷ P3]
D is the number of
cubic inches of oil discharge.
P1 is the nitrogen precharge pressure, in
PSI.
P2 is system pressure, in PSI, after volume D
cubic inches has been discharged.
P3 is maximum system pressure, in PSI, with
accumulator fully charged.
V1 is the catalograted gas volume of the
accumulator, in cubic inches.
0.95 gives an allowance of 5% for loss of capacity.
Download a PDF of Fluid
Power Design Data Sheet 47  Tips on Sizing
Accumulators.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this
information.
