Hydraulic accumulators: Storing pressure energy

A hydraulic accumulator is a component in hydraulic systems in which pressure can be stored. This utilizes the fact that gases, unlike liquids, are compressible. The more a gas is compressed, the greater its counter-pressure becomes. Through additional measures, such as external heating, the gas pressure can be increased even further.

Hydraulic accumulators and their operating principles

A hydraulic accumulator consists of a container with two chambers. The chambers are separated from each other by a diaphragm. Gas is filled into one chamber until it reaches a defined tension. The other half is filled with hydraulic fluid. After filling the hydraulic chamber, the hydraulic accumulator is ready for operation. The connection to the hydraulic system is closed with a valve. This ensures that the pressure from the hydraulic accumulator remains available on demand.
As an alternative to the two-chamber system, a hydraulic accumulator can also be equipped with a piston.

Hydraulic accumulators and their designs

In practice, the following designs of hydraulic accumulators are used:

• Diaphragm accumulators
• Bladder accumulators
• Piston accumulators

The difference between diaphragm and bladder accumulators lies in the shape of the element that holds the gas and the resulting design of the hydraulic accumulator.

Diaphragm accumulators have a flat rubber diaphragm that deforms under pressure. The design of these hydraulic accumulators is therefore usually cylindrical.

In a bladder accumulator, the gas pressure is generated in a bladder made of rubber or GRP. It expands in a spherical shape. Accordingly, the containers are also spherical in shape.

Piston accumulators, on the other hand, are always cylindrical, as they must allow for the linear movement path of the piston.

Areas of application for hydraulic accumulators

Hydraulic accumulators play a major role in complex hydraulic systems. They keep energy ready “on demand.” Since they can be connected in series quite easily, the total amount of available energy can be considerable. They are very important for emergency and safety circuits in hydraulic systems. With their help, important processes can be completed in a controlled manner in the event of a power failure. Typical examples are cranes and excavators, which can lower loads in a controlled manner with the help of hydraulic accumulators, or the device can move into a defined idle state. In the case of locks and doors, they can be used as emergency backups for openers or closers. However, they usually only allow for a single action. Once the pressure is released, the hydraulic accumulator is non-functional.

Another important function of hydraulic accumulators is their ability to dampen vibrations. If there is a strong pressure change on the hydraulic side, the hydraulic accumulator can automatically dampen this pressure change. One of the most famous examples of spherical accumulators as vibration dampers is found in the hydropneumatic suspension of vehicles. The hydraulic accumulators are the core of the “Citroen DS,” which enabled a driving experience similar to flying thanks to its particularly smooth handling.

Hybrid systems with hydraulic accumulators

Several manufacturers of construction vehicles already offer equipment today that generates significant efficiency advantages through the use of hydraulic accumulators. These so-called “hybrid systems” use the lowering of a load in excavators to pre-charge a hydraulic accumulator. After the load is deposited, the previously stored energy is used to raise the now empty boom again. Other systems support the slewing movement of an excavator using hydraulic accumulators.

What sounds good in theory can cause some problems in practice. For example, in excavators with supported lifting systems, hot weather can cause the boom to rise by itself. In this case, the machines must be equipped with appropriate safety systems.