Check valve in LS Signal to pump

[pob786]

Hi, I recently came across this system, the pump having been replaced due to damage caused by cavitation (slippers/pistons destroyed) !!!!!
The pump being driven by a diesel engine (800 rpm idle speed and 2000 rpm maximum speed)
The system being an EATON 70423 RBT Pump (45cc displacement/210 Bar/Flow & Pressure compensated) supplying a manifold with two EATON SV13 "normally closed" solenoid valves.
One solenoid valve supplies (via an external flow regulator with a 5mm orifice on the inlet) a Belt motor, and the second solenoid valve supplying (via an external flow regulator) a Water pump motor.
Within the manifold there is a shuttle valve connecting both circuits "downstream" of solenoid valves, supplying "LS Signal" to pump via a 5 psi bias check valve.
My questions being:-
1. What is the purpose/necessity of the check valve?
2. Would the pump be on "High Pressure Standby" when the solenoid valves are de-energised, due to no apparent "bleed off" of "LS Signal" because of check valve installed?
Any information/experience would be greatly appreciated.
Regards,
John P.

 

[Oldhydroman]

First question: was the previous pump the same as the one fitted now? It is possible that some peculiarity of the previous pump required the provision of this check valve. Some pump displacement control systems can be a little unstable (especially if the load sensing line is a little long) and the trick used to be one of strategic placing of check valves and throttles to try to wrestle some stability back into the system. It may be that the previous circuit needed this, that the original designer didn't dare not have it, or that there never arose any opportunity to find out if it wasn't actually needed.

However, from you description I suspect this isn't really a “load sensing” circuit - your pressure signals are coming from between the outlet of the solenoid valve and the inlet of the flow controller. You’re not actually picking up the “load pressure” and I doubt that your solenoid valve has anything other than an on/off function. Let’s imagine then that the “load sensing” signal is just being used to turn the pump from standby (~200 psi) to full pressure whenever a solenoid has been energised. You will probably find that there is sufficient internal leakage inside the pump displacement control valve spring chamber to allow the “load sensing” signal to decay over a period of tens of seconds when both solenoids are off. The pump will gradually drop back to low pressure standby once you've decided to don't want the hydraulic motors to run any more.

This is pure speculation, but the original designer’s thought process might have gone like this: if you had one solenoid energised (one hydraulic motor working) and you subsequently energise the second solenoid then the system pressure will drop for a fraction of a second because the pump is now trying to instantly pressurise the lines out to the second hydraulic motor. The pump displacement will increase in response to the drop in outlet pressure but there is a chance that the “load sensing” signal will depressurise as well and this could confuse the pump (or the designer) a little and lead to a bit of rough running and a greater time needed to re-establish stability in the system. So the check valve holds the “load sensing” signal at an artificially high value for a few seconds and lets the pump behave just as a pressure compensated unit.

Another thought: if there is some possibility of the solenoid valves being jogged (turned on and off rapidly for the purposes of fine position control of the hydraulic motor) then you might not want the pump going to low pressure every time you flick the switch to OFF just to have it come back to full pressure when you flick the switch back to ON half a second later. The check valve in the “load sensing” line will hold the pump on pressure until you've had both solenoids off for a few tens of seconds. Even though the pump could tolerate this level of abuse, it may be the case that the diesel engine doesn't like it and the governor can't respond fast enough to keep the shaft speed constant. [Actually the power demand will still go down when the pump goes to zero displacement (at full pressure) but the power demand will still be higher than the pump being at zero displacement and only 200 psi.]

Sorry, all I have to offer is some guesswork - hope it helps.

DOL