CD Industrial Group Inc.
Simulations and Hands-on Learning Challenges

Pressure Compensated Pumps – Basic Functions

Nov, 22 2010

Hydraulic

Pressure Compensated Pumps - Basic Functions

In the next edition of Newsletters that Teach we will look at the topic of timers in the world of PLCs.

In this edition of Newsletters that Teach we will examine some of the operating principles of what is likely the most popular hydraulic pump design in use in both plant and mobile machines today. In the case of many mobile machines, pumps often have more controls than just a pressure compensator. For those machines, we will look only at the pressure compensation (a.k.a. pressure override or pressure cutoff) feature in this tutorial.

Let us start with a quick look at pumps that do not have any displacement or pressure controls.

A gear pump like the one in use on a simple hydraulic machine displaces the same amount of fluid for every rotation of the pump’s shaft. The gear pump shown also has no pressure controls built in. The hydraulic system must contain a relief valve to limit the maximum system pressure. The system of cylinders and hydraulic motors may not always need the full displacement of the pump. To vary the amount of flow to one part of the circuit with a flow control, the pump’s flow will have to be divided. The system relief sometimes serves as the divider. If the relief valve is cracked open to any degree, then the system is obviously near maximum pressure.

Examine the graph to see that the system overall is at full displacement near the maximum pressure. The combination of these two maximums also means that the power requirement (the diagonal vector on the graph) from the prime mover (diesel engine or electric motor) is at it’s maximum as well. A prime mover at maximum power is consuming maximum energy (fuel or electricity). Much of this energy is being used for nothing other than a conversion to heat over the system’s relief valve.

Now let us look at how a system with a pressure compensated pump saves energy. For this tutorial we assume that the reader has basic knowledge of pumping action of the rotating group (pistons and cylinder barrel) in an axial piston pump. The pump is equipped with a pressure activated valve (compensator) that stays closed under spring pressure when the hydraulic system is below the maximum pressure. The maximum system pressure is set by adjusting the compensator. Clockwise rotation of the adjustment screw typically increases the maximum system pressure.

When the pump’s flow is sufficiently restricted or blocked in the system, causing the pressure to rise to maximum levels, the compensator opens directing high pressure fluid into an internal control piston. As this control piston extends, the pump’s displacement is reduced to near zero (off-stroke).

The pump now only comes on-stroke to displace enough fluid to maintain the maximum system pressure. This reduction in displacement conserves input energy and prevents the build up of excess heat (as caused by flow through a spring loaded relief valve). If the flow control setting remains the same the actuator's load has not changed, the pump will go off-stroke again.

Overheating in a system that features a pressure compensated pump may be due to maladjustment. If the system has run at correct temperatures in the past, it is possible that someone may have adjusted the system relief valve. Most systems with a pressure compensated pump still feature a relief valve to control sudden pressure spikes. This relief is typically set a few hundred PSI higher than the pump’s compensator.

If the relief valve is adjusted down to a level below the pump’s compensator setting (or the compensator is set higher than the relief valve), then the energy saving feature of a pump that can reduce it’s own displacement has been removed from the system. At this point, the pressure compensated pump is offering no design benefits beyond that of a fixed displacement pump.

In future editions of Newsletters that Teach we’ll look more closely at the behaviors and the adjustment of a pressure compensated pump.