Accumulators & Sizing

How do accumulators work and how do you size an accumulator for application?

Take a look at the HS study manual Outcome 3.6.1: Understand the difference between isothermal and adiabatic conditions (pages 3-45 through 3-49).  The outcome focuses on understanding the process of how to size and calculate the usable hydraulic volume from an accumulator.  In addition, the MHM study manual has several outcomes dedicated to hydraulic accumulators: Outcome 1.2.6: Understand the function of accumulators (pages 1-58 through 1-61); Outcome 3.8.1: Check and adjust the charge pressure in an accumulator (pages 3-48  through 3-50); and Outcome 3.8.3: Replace a gas-charged accumulator (Page 3-52); as well as some additional outcomes on troubleshooting and testing a hydraulic accumulator.  These two resources would be a great starting point.

Matthew Riley wrote:
An accumulator stores hydraulic energy by compressing nitrogen gas. When system pressure rises, oil enters and compresses the gas. When pressure drops, the gas pushes the oil back out. A metal tank full of pressure and consequences. Human engineering in one sentence.

To size it, you need:

Required oil volume (ΔV)
Minimum pressure (P1)
Maximum pressure (P2)
Precharge pressure (usually 80-90% of P1)

Basic formula:

𝑉
0
=
Δ
𝑉
(
(
𝑃
0
𝑃
1
)
1
/
𝑛
−
(
𝑃
0
𝑃
2
)
1
/
𝑛
)
V
0
  ​

=
((
P
1
  ​

P
0
  ​

  ​

)
1/n
−(
P
2
  ​

P
0
  ​

  ​

)
1/n
)
ΔV
  ​


Use
𝑛
=
1.4
n=1.4 for fast cycles,
𝑛
=
1.0
n=1.0 for slow ones.

Example: if you need 2 L between 100 and 150 bar with a 90 bar precharge, you need about a 6 L accumulator.