Accumulator Basics

Objectives

In this lesson we will describe the function of an accumulator, contrast bladder and piston style accumulators, list the typical uses of an accumulator and define the terms charge and precharge.

What is an Accumulator?

An accumulator is a device that allows a hydraulic system to store oil, under pressure, for an extended period of time.

Terms

Precharge: The pressurized gas in the accumulator.

Charge: The hydraulic fluid in the accumulator.

Accumulators come in two basic types.

What Are Accumulators Used For?

Energy recovery
Absorb system shocks
Dampen minor fluctuations in system pressure or flow

Provide supplementary flow (e.g. if the pump is undersized)
Maintain system pressure
Store energy (emergency power) in the form of fluid under pressure

Noise elimination
Absorption of thermal expansion

Leak compensation
Compensation for slow pump controller response

Advantages & Shortcomings

Each type of accumulator has its own advantages, and a few shortcomings.

Bladder Type

Advantages

Very responsive to system demands
Precharge gas seldom needs maintaining/topping up
Generally less expensive than piston accumulators
Lighter in weight than the equivalent sized piston model

Shortcomings

Bladder may become brittle and burst
Compressing the bladder too much can damage it
Cannot be fully discharged (cycled right to the bottom) too frequently without causing damage

Piston Type

Advantages

Require less frequent repair
Can be fully discharged frequently without causing damage
Can operate with a near infinite gas compression ratio because there is no risk of the piston bursting

Shortcomings

Slower to respond to shocks
Requires more frequent checks of the precharge pressure due to internal leakage around the piston seal
Heavier than the equivalent sized bladder model

The Bladder Type Accumulator

Let's take a closer look at the inner workings of an accumulator, starting with the bladder type.

Compressed Gas

Bladder

Poppet Valve

Gas Valve

A bladder type accumulator, sometimes known as a hydro-pneumatic accumulator, is a metal tank that contains a rubber bladder filled with compressed gas. There is also a poppet valve in the discharge port and a gas valve used to precharge the bladder.

As the bladder is being precharged, it will expand to the point of forcing the poppet closed. The poppet is there to ensure that the bladder can’t expand out into the discharge port.

Correct precharge will ensure the bladder doesn't quite touch the poppet.

Typically, the accumulator precharge is set to between 60% and 80% of the minimum system pressure. That way the bladder doesn’t expand enough to close the poppet during normal operation.

Accumulators are precharged with dry, inert, nitrogen gas rather than simply using compressed air. Why? Because if the oil and oxygen in the compressed air were to mix, it could start a fire, or even explode!

As the pressure in the hydraulic system increases, oil is forced into the accumulator. This liquid charging is possible when the hydraulic system pressure is greater than the gas precharge pressure.

The incoming oil compresses the bladder and the gas. This compression in turn increases the pressure of the precharge gas.

When the hydraulic system pressure decreases, the bladder expands again, forcing the oil back out.

It is this constant seeking of equilibrium between the pressure of the hydraulic oil and the pressure of the precharge gas that drives the function of the bladder style accumulator.

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