Assuming:

P0 = Pre-Charge pressure
P1 = Min working pressure
P2 = Maximum working pressure

Pre-charged with Nitrogen (P0) to a predetermined pressure, the bladder and the accumulator shell have the same volume. (Fig 2)

It is not until the system pressure overcomes the pre-charge pressure that the fluid can enter the accumulator. As the system pressure increases, the fluid compresses the gas contained in the bladder until gas and system pressure become balanced at P2, which is typically the relief setting of the hydraulic system or maximum working pressure of the hydraulic system. See Fig 3

On demand, the circuit pressure falls as the stored energy is released to operate a function in the hydraulic system; the bladder expands to release the stored volume and the pressure drops to P1 (typical minimum working pressure to operate the actuator).
See Fig 4.

 

Correct pre-charge settings

P0 = Pre-charge pressure
P1 = Min working pressure
P2 = Maximum working pressure

Storage application P0 = P1*90%
Pulsation damping P0 = P2*60%
Surge protection P0*60%

Note: consider the gas temperature when initially pre-charging the vessel and compare it with the actual working gas temperature in the hydraulic system. P0 should be set in accordance with P1 based on the normal system’s operating temperature.

T1 = Typical gas temperature when pre-charging the accumulator
T2 = Gas temperature during normal operation of the accumulator unit.

Example

P1 = 100 Bar
T1 = 20 Deg C
T2 = 40 Deg C

If P0 = 90 Bar at T2 (40 Deg C) then P0 @ 20 Deg C = 84 Bar

In this example, when initially pre-charging the unit at 20 deg C, the pre-charge pressure would be 84 bar so that when the unit is connected to the hydraulic and reaches the operation temperature, P0 becomes 90 Bar @ T2.

When checking the pre-charge pressure once the accumulator is fitted to the hydraulic system and reaches its working temperature, the pressure P0 should be at 90 Bar.

Based on
New pressure after temperature change P’0 = P0*

Reference: Boyle & Mariotte law for Ideal gas PV/T

• P0, set to a lower pressure than the minimum operating pressure of the hydraulic system P1, will help maintain a longer bladder life and good operation of the hydraulic system.

This will prevent the bladder from extending to the bottom of the shell, which could result in a bladder extrusion or the bladder caught on the poppet leading to:-

1. Damaged bladder.
2. Failed bladder
3. Broken poppet valve
4. Reduced stored capacity of the accumulator (If pre-charge P0 is above minimum working pressure P1).

Note: always use dry Nitrogen and pre-charge in accordance with the manufacturers ‘ instructions.