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# Combining a loadsensing pump with a fixed displacement pump

## Combining a loadsensing pump with a fixed displacement pump

(OP)
Good Afternoon people

My question is is it possible to combine two pump-sets with a configuration one variable displacement pump with a load sense control and a fix-displacement pump. These pumps are connected to a system with two users. One user needs 40 l/min and the other user needs 20 l/min. According my friend this is not possible, but I think it’s possible. ( I am not a hydraulic specialist.)

I think this will happen:

The fixed displacement pump will always be main pump and will deliver at maximum displacement against maximal the relief valve settings. ( There is no option to variate the displacement.)
For example a scenario, suppose the proportional valve is set on a fixed position that allows [14 liters/min] through the valve with a pressure drop of 10 bar. If the valve is on a fixed position the proportional valve can been seen as an fixed orifice.

Using the Bernoulli equation gives:

∆p(Q)=1/2.ρoil.(Q/Avalve)^2

Where:
Q = flow through the valve [m^3/sec]
Avalve = Cross Area off the valve [ m^2]
Δp = Pressure drop [N/m^2]
Ρoil = Density of the oil [kg/m^3]

In the annex you can see on the graph, that at 20 a 21 liter the pressure drop will get larger than 20 bar. When the pressure drops gets larger than 20 bar the load sensing control will reduce the displacement. In this case, the fixed displacement pump delivers 14 l/min and load sensing pomp 8 liters per minute. The work is done by the fixed displacement pump. ( not max benefit of the load sensing pump)

By opening the proportional valve more the pressure drop and the load-sens pump can help the fixed displacement pump more. My advice would be to select a smaller fixed displacement pump, so that the load sensing pump can do more work.
My question is it possible to combine these pumps and is my thought good about the system. I enclosed a hydraulic diagram and a excelsheet.

John

### RE: Combining a loadsensing pump with a fixed displacement pump

The control of the variable pump will not be affected by the additional fixed pump. However, if the system pressure hits 210 BAR, both pumps will see it and while the fixed pump will blow over the relief valve, the variable pump will back off to min flow.

The variable pump will only ever see the pressure drop across the valves and it will respond to the highest delta P.

I see no reason why this setup will not work under most load conditions, although, as there are no compensators in the valves, there will be no flow sharing.

### RE: Combining a loadsensing pump with a fixed displacement pump

You could replace your 210bar pressure valve by a sequenced valve at 25bar and connect the drain to the LS pipe (eg SUN RSBCLBN). Thus you will have a better regulation of the flows in the proportional valves, a better efficiency, less heating of oil, less wear of the pump 10cm3 and less noise

### RE: Combining a loadsensing pump with a fixed displacement pump

To limit the maximum pressure of the small pump it is necessary to connect the drain of the valve of sequence to the signal LS after the jet in the big pump.
Why do you have 2 pumps?

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