Insane Hydraulics
Bold and audacious blog about fluid power
About a month ago I received a Parker PV023 pump to overhaul. It came from a scheduled equipment stop, and was still in working condition, but at the end of its life, so I ended up selling a new pump to the client.
It was equipped with standard MFC control - a single spool Load Sensing with a built-in relief valve - a classic arrangement that works for most LS systems. I like the PV series - these pumps have proven to be exceptional. I know for a fact that this is a very reliable pump, because it outlasted all Danfoss, Rexroth or Eaton open loop pumps that we tried in mining conditions. Don't get me wrong - all of the pumps that I tried-out performed equally great when new and on the surface - but as soon as they went under ground and the operation started throwing blown water coolers at them - all but the PV series have quietly asked to resign...
Note that this comes from a guy who is "not a fan of Parker" to say the least! Getting Parker hydraulics in this corner of the world somehow feels like "biting the bullet" every time I try it, not to mention the fact that getting a decent term from these guys is next to impossible, which is why I'd use Danfoss, or even Rexroth over Parker ten times out of ten, but when longevity in crap conditions becomes the defining factor, PV series is my first choice.
Now you see why I was so much surprised to get a call from the client saying that "nothing worked" after they installed the new pump. In that system the PV pump was connected to a Danfoss PVG32 multi-section directional control valve (further on - DCV), and according to the site mechanic they had no movement on the functions, even when they controlled the spools manually, and the pressure was only 4-5 bar.
The mechanic who worked on recommissioning the machine was very experienced, and immediately checked the simple stuff and attempted adjusting both the directional control valve relief and the new pump pressure compensator, but nothing worked, and so he immediately called me.
I never had a issue with a new PV pump before. So, naturally, I was very interested to see if I could discover what was wrong. In fact, since the pump was the last one in a tandem unit, I started to wonder if I had accidentally left out the splined sleeve when I assembled it.
The mechanic already knew me and how I worked, so when I arrived at the spot - he already had a flow tester at the pump discharge port. Beautiful! I teed another pressure gauge into the LS line, and we began the tests.
Before going into the test itself, I would like to trace the troubleshooting logic chain:
The adjustments, oil level, suction line, prime mover and actuators had been verified, so before anything else I needed to determine if it was the pump or the DCV that was causing the problem.
The easiest way to diagnose a closed center load sensing system lies in reading pressures in the LS and the P lines. The logic is simple - when you move a DCV spool, it should connect the P port to the work port, and the work port to the LS port, which means that pressure in the LS port should rise. Even if the pressure in the P line is low for some reason (like for example an inlet section compensator that's stuck in an open to tank position), you should be able to detect a pressure rise, which would mean that the LS signal distribution works.
Now if your pump is getting a pressure rise in the LS line and does not respond to it by increasing displacement to raise the pressure level in the P line - it's because either it can't (the oil is being vented to tank somewhere) or it won't (there's a problem in the pump itself).
Thus the basic trobleshooting logic can be represented as:
do (activate directional control);
if (pressure in the LS line rose ) then {DCV = good} else {DCV = bad}
In fact, even if you don't have a pressure gauge, you can often check the LS system "country-style" by unplugging the LS hose and seeing if it gushes a stream of oil when you operate a DCV spool. Just point the hose end at your face and use the following logic:
do (activate directional control);
if (I am blind) then {DCV = good} else {DCV = bad}
But, don't do that last one, seriously, don't.
Back to our test now. After the power-pack started I saw a reasonable 14 bar stand by pressure at the pump outlet. Then we operated a spool using manual control lever - and I read 0 bar in the LS line.
The logic charts above would suggest that, apparently, there was a problem in the DCV.
To top it up, since there already was a flow tester mounted at the pump's outlet, and the pump pressure compensator had been set to an unknown pressure - I suggested to pressure test and re-adjust the pump, killing two rabbits with a single stone.
We quickly connected the LS line to the pump outlet and successfully pressure tested the pump and re-set it to 300 bar. Of course the brand new PV pump got through the test like a champ, and I had my proof that the component that I sold and mounted was perfectly fine.
So, yet another proof that problem is in the DCV then? It is clear that the LS pressure is not rising, so we should definitely check out that LS orifice in the inlet section? And probably five years ago I already would be dismounting plugs and hoses and what not to get to the orifice and see if it's clogged...
Luckily I know now that rushing diagnostics is a bad practice. So after I come up to an initial conclusion - I like to take a step back and look again at a system in order to see if I am missing anything and if there is a simple second way to check my current theory.
Plus - something else was bothering me. The mechanic did mention casually that the pressure was only at 5 bar, which was clearly less that the 14 bar that I read at the pump outlet. And when I questioned him about it he said that he did read 5 bar, but he was measuring the pressure at the DCV inlet section. which was actually very easy to do on that particular machine since there already was a pressure test point installed.
I checked the pressure - and it read only 5 bar. Well - this is lower, indeed, but why? Very soon I discovered the reason. This turned to be an old system that previously had worked with a fixed displacement pump, and the pump pressure line had an inline check valve with another line teed upstream. Further investigation (and by investigation I mean shoving my head in between a bunch of hoses and pipes and wondering if my ears will still be "properly aligned" when I pull it back out) revealed that the check valve had a 10 bar spring and the teed line was supplying pilot pressure for another very large directional control valve. The 10 bar check valve was installed to make sure the pilot pressure never dropped below 10 bar, and it just stayed in the line when the system was overhauled with a variable displacement pump.
Aha - so this is why we're getting only 4-5 bar at the inlet! And also the extra line teed to the pressure line meant another possible path for the oil to be escaping to tank, so, surely I would have to blank off that line now and then re-check the system...
And, again, maybe some years ago, the wrenches "would already be flying" and I would be removing the lines "with all my heart", but, once again - I applied the good old "step back and breathe" maneuver:
I thought: Wait a minute here, even if the check valve in the pressure line does explain the 10 bar drop between the pump outlet and the directional valve inlet, it doesn't explain why there's no pressure rise in the LS line when the spools are moved by hand. I can understand that having such a low pressure in the inlet may be not enough to pilot the electrical controls, but no pressure rise at all when I move a spool by hand seemed really strange...
Then I carefully looked at the directional valve, and realized that the work sections used here were the ones that used individual pressure adjustments, which meant that there were individual compensators in each section.
Then I thought: maybe I am just not getting enough pressure to open the compensator bias spring and the oil is simply nor reaching the directional spool at all? Then another thought went through my mind - but PVG 32 is a classic pre-compensated valve, and as I said (and even wrote) before - pre-compensated systems are normally open flow limiters, so, surely the oil should be reaching the spool. But, then I thought again, and then looked at the work section cut view (thank god for smartphones that allow you to have a digital library at the palm of your hand!) and then I realized that the same section uses the compensator spool to perform both the "compensatory function" and the load drop check valve function, which means that the compensator is kept in the closed position by the bias spring. So, if the pressure in the P line is below the bias spring setting, which was exactly the case here, it is simply not enough to shift the compensator and thus the oil can't even reach the spool.
This was a beautiful theory, because it was very easy to test! So, I bumped the stand-by pressure up to 22 bar - and the system immediately "woke up"! No need to say that I was leaving the client's shop with another smiley in my personal file. If you wonder why I didn't simply remove the check valve - I say that it was the client's choice - too many metal pipes would have to be removed for that and there simply wasn't enough time.
The point of this post is not the hydraulic function that I just described. The case is interesting, but highly unlikely to happen, so it's not the point. The point is that stopping for a second and stepping back to ponder for a while about what is happening is better that taking immediate action and rushing in to solve the problem. Human beings are not machines, and may need to take several looks at the trees to notice the forest. So taking a few moments to "think it over" is never a waste of time.
Unfortunately, clients who have their production stopped due to a hydraulic malfunction, often perceive such "action void" moments as a waste of time. "Hey, buddy, you've been looking at your smartphone for the last ten minutes and haven't touched you tools yet! What gives, man?"
Don't bother feeling bad about it, though. Ask them if they would like you to share some information about the problem, maybe? Or just tell them to go and.... no, no, do the first one.
Slow is smooth, smooth is fast!
Important points:
- Flow compensator spool in a pre-compensated DCV can be designed to perform a load drop check valve function, which will introduce a constant pressure drop.
- Knowing how components operate internally makes troubleshooting so much easier!
Don't rush and don't jump to conclusions when you troubleshoot hydraulic systems! Taking a step back and a deep breath is a very good practice every troubleshooter should apply.