Which statement is true about the pre-charge pressure in an accumulator?

• A. Do not discharge at constant pressure
• B. Pre-charge pressure is optional
• C. Pre-charge pressure is irrelevant
• D. You should always discharge at constant pressure

Answer: (A)

Pre-charge pressure is the gas pressure inside the accumulator before any hydraulic fluid is added. In a gas-charged accumulator with a separating bladder, the gas side does the work by expanding and compressing as fluid moves in and out. When fluid is drawn from the accumulator, the gas expands and its pressure drops; when fluid is added, the gas is compressed and pressure rises. Because the gas volume changes during discharge, the hydraulic pressure you get from the accumulator is not constant. Hence, discharging at a constant pressure is not how an accumulator behaves, making the statement about not discharging at constant pressure the true one.

The pre-charge must be set appropriately for the system—it’s not optional—and it’s not irrelevant. The pressure changes during discharge are a normal part of how the stored hydraulic energy is delivered.