Post-compensation vs. Pre-compensation

When is post-compensation an advantage?

Pressure compensation is the control of flow by compensating for the changes in load pressure. Most hydraulic systems today use pre-compensation as a means of maintaining consistent flow from an orifice or spool. However, there are applications when post-compensation has advantages over pre-compensation.

The fundamental difference is that with pre-compensation, the pressure drop across the orifice or spools is determined by the compensator. With post-compensation, the pressure drop is determined by the load sense (LS) spring inside the pump.

In post-compensated systems with multiple functions, the pump flow is divided at a fixed ratio. If flow settings exceed the pump output capability, the flow is reduced to each function at a fixed ratio. This is why post-compensation is sometimes referred to as “flow sharing”.

In post-compensated circuits, the pressure drop across each valve is determined by the load sense spring in the pump and all valves or orifices will have the same pressure drop. The load sense differential, sometimes referred to as standby, decreases when the pump cannot satisfy the total demand. All pressure compensators reference the highest load of the various functions.

The benefits include high efficiency under partial load and/or partial speed conditions and all functions slow down together at a fixed ratio when the pump cannot fully satisfy demand.

In the example below, the pump differential, or standby, is 200 PSI. The load sense pump will develop enough pressure to overcome the load and maintain a 200 PSI differential. The pressure drop across the valve or orifice remains fixed and is calculated by: system pressure minus the highest load pressure minus the compensator spring value.

The circuit below is an example of the flow sharing aspect. When another function is operated and the pump cannot fully satisfy the flow demand, the differential decreases. The pressure drop across each valve or orifice is reduced at the same fixed ratio, so the flow is divided, or shared, equally. In this example, each valve is fully open so total pump flow is shared equally between the functions.

So what happens when the functions require different flows and the pump cannot fully satisfy the total flow demand? The pump flow will be divided into the ratio of each function to total flow available. In the example below, the theoretical total flow demand is 42 GPM. The ratio of the function flow demand to total theoretical flow demand multiplied by the maximum pump flow is the resulting actual flow from each valve.                

Post-compensation will increase stability and control in systems where demand can exceed the pump’s flow output. Because of its increased efficiency under partial load conditions, the compensator saves horsepower and reduces heat. It will also make the initial movement of actuators more predictable and provide better operator control.