Two hydraulic flow control with a load sens line between them ?

[Samuel.]

Hi everyone,

I've ran into some questions. Imagine two hydraulic flow control installed in series with a load sensing line in between them thats connects to the LS port of a variable displacement piston pump. The first one 1 is set to 5 gpm and the second is set to 2 gpm. What would happen to the pressure at the pump ?

After thinking about it I thought that since the pump couldn't meet the demande of the first flow control (5 gpm) because of the second one (2 gpm, the pressure would juste increased until reaching the PC setted control, which would bring the pump's swashplate to a 0 degre condition, which means no flow. In that case the design is all wrong.

It's been a mental twisting challenge can someone explain how the system would react ?

 

[hydtools]

Why have flow controls in series?

Pressure depends on load being driven.

Ted

 

[Jacc]

"After thinking about it I thought that since the pump couldn't meet the demande of the first flow control (5 gpm) because of the second one (2 gpm, the pressure would juste increased until reaching the PC setted control..."

I agree with this part.

"...which would bring the pump's swashplate to a 0 degre condition, which means no flow. In that case the design is all wrong"

It doesn't swivel to 0 degree, it swivels to 2 gpm while keeping the pressure at PC setting.




As a thought experiment (a theoretical one), imagine the first flow controller being 1.9 gpm and the second one being 2 gpm.
The LS-pump will swivel to find it's standard LS dP over the first flow controller and the first flow controller will settle at 1.9 gpm. The 1.9 gpm will then go straight through the second flow controller since 1.9 is lower than 2 gpm.
The pump will stabilize at a pressure equal to the load pressure + LS dP and a flow of 1.9 gpm.

Now imagine the first flow controller being 2.1 gpm and the second one still 2 gpm.
The first flow controller is looking to get a maximum of 2.1 gpm through by closing until it gets there. However, the second flow controller will not allow more than 2 gpm so the first flow controller will just remain fully open. That means the LS-signal line is basically connected straight to the P port of the pump making it impossible for the pump to reach a LS dP.
The pump will stabilize at a pressure equal to PC setting and a flow of 2 gpm.

So only a very small change in the flow setting of the second flow controller will flip the pump from working in harmony with load pressure to going all in with CP setting.


Of course this is a theoretical thought experiment where everything is very accurate and very "square".
In real life that squareness is very much rounded off because of various marginal effects, flow controllers may be nonlinear, not accurate, have hysteresis etc. etc..
Nevertheless it shows how bad idea it is to reduce the flow of an LS-system by installing a flow controller downstream of the LS-signal line, it will just put the pump at maximum pressure for no reason and generate heat and noise. The flow should be controlled within the LS-loop, not downstream of it.

To summarize:
On a constant pressure systems it makes PERFECT SENSE to reduce the flow with a flow controller downstream of a valve.
On a LS-system it makes NO SENSE to reduce the flow with a flow controller downstream of a valve (or rather downstream of the LS-signal).


(PS
On a constant flow (fixed pump) system you must never use a 2-port flow controller, always use a 3-port. Or even better, match your actuator to the pump flow.)

 

[hydtools]

https://youtu.be/PbXB2VnhpXw

Video on load sensing pump.

Ted

 

[Samuel.]

Jacc,

Thank you for your great answer. Makes total sense.

We're revisiting the design avoid such waste of energy.

 

[Jacc]

Edit:
If the LS-pump supplies a single valve, an orifice or a needle valve is enough to get flow control as it will act as a flow controller with the deltaP of the LS-pump.

If several valves are supplied by the same LS-pump then a flow control valve is a must.
If several valves are supplied by the same LS-pump you should also consider a complete LS-valve such Danfoss PVG32, especially if you want to be able to vary the flow during operation.