Pressure compensator enables precise pressure transmission

The pressure compensator is a device in hydraulic systems that regulates and maintains pressure at a very specific level.

From a technical perspective, different approaches are available for this purpose. The pressure compensator is therefore only rarely used as a separate component, but mostly as an integrated function in a hydraulic device.

Pressure compensator: the need to define pressures

The need to define and maintain pressures precisely is primarily due to the nature of hydraulic systems. Hydraulics is the transmission of forces by means of fluids. Unlike gases, fluids cannot be compressed. This means that any pressure that arises must be absorbed somewhere in the hydraulic system. If pressure peaks occur for which there is no compensation, internal damage to the system is almost inevitable.

The pressure compensator is a simple way to control the flow rate with all connected actuators in accordance with the design of the hydraulic system. It is significantly easier to regulate pressure via a compensator than to constantly change the speed of the hydraulic pump. In addition, pressure compensators can regulate pressure at a precisely defined point. If pressure were always controlled via the hydraulic pump, the pressure fluctuations would affect the entire system evenly.

Technical approaches to pressure compensation

From a technical perspective, various options are available for integrating a pressure compensator module. Three approaches are common:

• Pressure relief valves
• Hydropneumatic elements
• Variable displacement pump with controller

Pressure relief valves are primarily used to limit overpressure. Once the pressure reaches a certain value, part of the flow returns to the tank via the pressure relief valve. These technically very simple systems usually consist of a ball valve with an adjustable spring. Depending on how strongly the spring is pressed onto the ball, the set back pressure relative to the internal pressure of the hydraulic system changes. If the spring back pressure is reached or exceeded, the ball is pushed back and part of the flow can drain off.

A classic hydropneumatic element as a pressure compensator can be found, for example, in the famous hydropneumatic suspension in Citroën automobiles. Here, spheres with pre-charged gas and a rubber membrane are connected to the hydraulic damping system. When the vehicle drives over an uneven surface, the shock absorber plunges into the hydraulic system. The resulting overpressure is then compensated by the compressible, pre-charged gas in the sphere dampers. The result is a very precisely balanced internal pressure in the hydraulic damping system—and a very comfortable ride over uneven ground.

The variable displacement pump with controller is the most sophisticated form of pressure compensator. However, it has a decisive advantage over the other two approaches: it can also compensate for negative pressure. The variable displacement pump consists of a module that maintains an existing pressure at a specific level. If the pressure is too low, the pump opens slightly or increases its output so that the desired pressure is reached. If it is too high, the pump reduces its output again. This technology is very sophisticated and prone to wear, but it reliably results in constant pressure that can be quickly adapted to changing requirements. However, reliable operation of the variable displacement pump with pressure compensator requires sensors (pressure gauges) and actuators (adjustment motors on the control valve or a speed controller for the pump drive).