Double Danfoss PVG32 valve groups with one variable displacement pump problem 1

[Huolto]

Hello,

Original ciruit was operating one crane with Danfoss PVG32 valve group and variable displacement pump, that worked fine.
We added second crane with PVG32 valve group and can't get enough pressure on either of the cranes.

LS line pressure is ok when measured from either of valve group but when testing max pressure we get only 60-70 bars. Raising max pressure from pump is not helping, raising LS line pressure from pump is not helping, raising pressure limit from valve groups not helping. When lines for new crane are closed with ball valves, old crane operates normally.

What could cause this problem? As attachment a raw schematic of hydraulic circuit.

 

[BrianPetersen]

Insufficient pump capacity - insufficient delivery volume. Possibly incorrect (open center) spool leading to excessive flow demand.

 

[Stoolpe]

Are you certain that your new PVG32 valve is the correct specification?
It should be a closed center with load sensing.

Check with your supplyer. Or if you have data that you can share with us.

And be sure to check the setting of the main pressure relief valve / valves in the system.

 

[Jacc]

Yes, first check the inlet section relief valve on the new valve since it's easy to do. Just turn the adjuster and see if the it responds, if not reset it to the original position.

Then check if you have by mistake gotten your new valve configured for a fixed displacement pump, I think this sounds more likely.

Some info on variable pump vs fixed pump LS (or Closed vs Open inlet if you will):
LS-valves, such as the Danfoss PVG32, are primarily intended to work with variable displacement pumps of LS type. The LS-signal is taken from the workport side of the spool and is guided back to the pump regulator in small hose. The pump regulator uses this pressure and the internal pump pressure to keep the delta P over the spool constant regardless of how open or closed the spool is. The pump swash plate angle will basically follow the spool opening and the flow out of the valve will be consistent and not affected by varying load.
However, most LS-valves also have the option of being configured to work with a fixed displacement pump. This is accomplished by letting the LS-signal control a 3-port bypass valve in the inlet section, so that only the required oil is supplied to the spools and the rest goes in bypass to tank. In other words, the 3-port flow controller in the inlet section mimics a variable LS-pump.
The advantage with such a fixed pump LS-system is that you will get the same good speed control characteristics that you get with a variable LS-pump but to the cost of a fixed pump. The drawback is that you don't get the low heat generation of a variable LS pump, you will get the same heat generation you always get with a fixed pump system.


The Danfoss PVG32 can be easily changed from one configuration to the other by changing some small parts in the inlet section. These parts are available as a kit, part no: 155L5152.
The kit looks like this:

https://www.holsteinhydraulik.de/en...or-open-center-and-closed-center-pvp-32-inlet

Note that the second image is actually instructions, sort of.
This kit can be bought anywhere in the world where Danfoss is sold.

 

[Jacc]

I just thought of something else, on your sketch the LS-lines are just T'ed together.
They must be connected with a shuttle valve. I assume they already are but you just didn't draw that on the sketch?

 

[Huolto]

Hello all,

Thank you guys for input!

Indeed originally, pump side module of the PVG 32 valve was supposed to be for variable pump according the order code. After opening LS line and checking the plugs inside, plugs was for fixed pump

So my guess is that company which delivered crane and PVG 32 valve had been testing crane with fixed pump and forgot the plugs.
Now plugs are correct and still pressure is max 80.

I'm currently waiting for shuttle valve for Jacc's idea. We'll see how it goes when it is in place. Fingers crossed

 

[Huolto]

Hello all,

Good news, shuttle valve did it. Now I have pressure and both cranes operating! This is good news.
Thank you everyone for help

 

[Jacc]

Glad to hear it works!