Hydraulic pump remain continuously on load even though no valve is operated

[Usman Abdullah]

i am using pressure compensated Axial Piston pump by Rexroth the problem i am facing is my pump remain on load even though no valves are operated on manifold block the gauge right next to pump show 1800psi - 2000psi pressure when pump is running, can anyone explain if it is normal or pump need to be neutral and show 0-psi pressure when no valves are operated

 

[3DDave]

Pumps don't generate pressure, restriction and the resistance to flow does. It seems like there is 1800 to 2000 psi worth of resistance to flow out of the pump. Without any information about the rest of the circuit it isn't possible to determine if this is the expected result.

 

[BrianPetersen]

The compensating circuit shifts the (variable-displacement) displacement of the pump in an attempt to match the pump displacement to the demand while producing the regulated pressure.

If it is connected properly and is operating properly, the *flow* will have modulated down to match demand (perhaps almost zero, perhaps near maximum rated pump displacement), but the *pressure* will still be there. If something changes in the demand part of the circuit that demands more flow, that would cause the pressure to momentarily drop, at which point the compensating circuit will shift the pump to a bigger displacement until the new flow rate matches the new demand at the regulated pressure.

 

[Compositepro]

There are many ways to design hydraulic circuits, so we cannot know if yours is working as designed. It is not uncommon for there to always be pressure supplied by the pump and returned to the reservoir though a pressure relief valve. It is not energy efficient but there is always pressure available for any valve to open and cause something to happen. If your pump is pressure compensated, then it is operating at minimum flow and will provide more flow, as required when some downstream valve opens.

It is also not uncommon for fixed displacement pumps to return all the flow to the reservoir through a relief valve when there is no need for it anywhere else. Such systems always run at full power consumption. It is a trade-off between energy cost and capital cost and complexity.

 

[hydtools]

It sounds like the pump is doing what it is supposed to do. The pressure compensator causes the swahplate to go towards minimum flow while maintaining the set compensator pressure.

On the other hand a closed-center system with a fixed displacement will have a valve to unload the pump to low pressure when all directional valves are closed.

Ted

 

[akkamaan]

A pressure-compensated axial piston pump is a constant pressure pump
There are two main types of Constant Pressure, CP, systems
1. CP
2. CPU, Constant Pressure Unloaded

CP systems always run with max pump pressure and strive to keep that pressure as long as the pump can discharge enough flow through the metering orifice. When the pressure reaches the main compensator setting the swash plate destroke the pump and the only flow that gets discharged is for the internal leakage.

A CPU system senses when the directional valve is in neutral and a secondary low-pressure compensator will destroke the pump. This low pressure is commonly called "stand-by" pressure and is usually somewhere around 35 bar (can vary between 25 bar and 50 bar depending on application). This stand-by pressure will take the mechanical load of the pump when "idling" which then will reduce wear and unnecessary heat losses. If max pressure is 200 bar and the load pressure is 80 bar there will be 200 bar - 80 bar = 120 bar in heat losses over the metering orifice in the directional valve.

There is a third system very similar to the CPU system called Load Sensing system, LS. This system has a variable max pressure determined by the actuator load. A pressure signal is sent to the low-pressure compensator when the directional valve is "touched" this pressure signal will then be added to to the stand-by pressure to make the current max pressure. If load pressure is 120 bar it will be added to the stand-by pressure and make 80 bar + 40 bar = 120 bar, and only 40 bar will be heat losses over the metering orifice in the directional valve.

So now back to your pump issue. We need to know which type of system your pressure-compensated pump is working in.
If it is a CP system you have, and an unnormal larger internal leakage flow will put extra load (power output) on the pump with max pressure.
If it is a CPU system we need to know if the stand-by pressure is signaled with (positive) pressure or (negative) non-pressure. A complete schematic will show which.
The LS system is more complex but always works with a positive pressure signal. Some LS systems have a max pressure load valve which by-passes the LS function to make the system a temporary CP system. If the loading valve i "hung up" it could cause your symptom too.