Insane Hydraulics
Bold and audacious blog about fluid power
My last week's post started with a display of my workbench from the previous Friday morning. And today, my friends, I want to show you what my workbench looked like on that very day, only in the evening:
In this picture, I am standing next to a core-drilling rig (DBC Makina S-21) that's been punching 1500+ meter-deep holes over our beloved Somincor mine for the last six months. It's actually pretty close to our house, and I've been seeing it here and there in the fields around the mine every morning as I've been driving to work. I always thought: "I wonder what kind of hydraulics it runs..." You know what they say: be careful what you wonder for - or something like that, right?
Let me finish setting up the scene to complete the mental image. Notice the floor under the makeshift workbench. The manifold I am about to disassemble has many parts that can be categorized as both "tiny" and "irreplaceable," and I call this kind of terrain "you drop it, you lose it," so my concentration is at its highest. To make things a little bit more interesting, it's raining cats and dogs - hence - the poodle on the floor! (I am so very sorry, but I couldn't resist.) In all seriousness, the weather's been pretty bad around here. Winter storms have been punishing the Iberian Peninsula non-stop over the last three weeks, and the situation is still extremely dire in some places, unfortunately. The picture can't show this, but I am trying to do what I can under a thin strip of tarp stretched over my head, and the occasional thunder roaring right above makes me wonder if standing next to a tall steel structure in an open field is a good idea. At least the grounding should be solid, given the several hundred meters of pipe string already in the ground!
This rig had been "acting out" for some time. One of those annoying malfunctions that come and go. It would simply lose the pressure in the main pump circuit (a classic variable displacement LS pump plus a closed-center DCV) and some time later it would mysteriously fix itself. The last time a tech went to see it when it stopped working, he took out a pressure gauge to take a reading, and - boom - it was good as new just like that! So, I arranged with the client that the next time the rig would "hiccup," he would call me immediately, and I would go there and try to catch it "in the act." For some universal reason, once such an arrangement is made, the malfunction is guaranteed to happen on a Friday afternoon, right after the evening plans are made.
So, when I hear the words "intermittent function" on a relatively new machine, I always suspect electrical issues. I was hoping it would be a quick case of uncovering a bad connection to a safety valve, or, maybe, a burnt-out coil, but, unfortunately, this was not the case - the main DCV was, indeed, equipped with an electric open-when-unpowered LS vent valve, located in the inlet module, and when I arrived and confirmed the lack of pressure, the first thing I did was touch it - and the coil was hot, which meant the valve was getting power, so yeah - no quick fixes for me that night.
Here's the main DCV located right under the control panel of the rig:
A beautiful HAWE PSVF 7 equipped with four work sections with manual controls - the most bulletproof solution in the world! Now, I believe we should do a little bit of valve theory before I continue with the diagnostics.
I love the quality of HAWE hydraulic products (especially their small lever-operated directional valves, which are super-reliable), but their catalogs feel... crowded to me. It always seems like there is too much scattered stuff being thrown at you at the same time, with too much detail about things that you don't need to know, and no details on things that you do want to know. Getting from a model code to a function takes a certain effort, and, to make things worse, it seems to me that you should always take their hydraulic schematics with a grain of salt. I may be wrong, of course. HAWE is a competitor's brand for us, so my experience with their stuff is, indeed, limited, and being a self-educated hack makes my critiques biased, but it is what it is. Let me show you what I mean.
The inlet module had a tag that read "PSVF AS1F/350/7 SAE-7," and, of course, the first thing that I did was download the official catalog from the HAWE website and decipher the model code. The catalog states that:
Pretty standard stuff, all except for the damper valve, which is mentioned but whose function is never detailed enough (which is a shame, in my opinion), however, if you look at the hydraulic diagrams, you may get... puzzled. Here is the hydraulic diagram suggested for the PSVF A../..-7, and here is another showing an inlet module with the LS safety vent valve:
Can you explain to me, using these diagrams, how exactly the logic element opens to make the relief function work? (remember - it is supposed to be a pilot-operated relief valve, where a small relief valve controls the opening of a large, normally-closed logic element.) Well... neither can I.
Luckily, there is another diagram in the catalog that shows an example of a complete valve with the PSVF AB1 inlet option, and it is depicted like so:
This one seems to be correct, although now that I've opened the inlet module and discovered how it's built, I would depict the lines a little bit differently to better indicate the correct physical location of the removable filter thingy and the orifices. But hey, at least this one makes sense. See what I meant when I said that we should take their diagrams with a grain of salt? Although, since I know first-hand how hastily technical docs can be made, I understand where such mistakes come from.
Anyhow, for the context of this diagnostic, all we need to know, really, is the fact that the inlet compensator's only function is to be a piloted relief valve and nothing else. This is, actually, true for 99.9% of other similar inlet modules in the world configured for operation with variable displacement pumps.
Let us get back to our diagnostics now. So, the LS coil was getting the current, but maybe the vent valve got stuck open? I teed into the LS line going to the pump to see what was going on and saw that the LS pressure was being correctly communicated to the pump when the functions were summoned, and vented when the levers were in neutral. I also noticed that when active, the LS pressure was almost the same as the pump pressure (the top-most left analog pressure gauge is the pump pressure):
When you see an LS pressure being equal to the pump pressure, it means either pump trouble or a huge bypass to tank. So, I took out my FLIR smartphone to have a look with "seeing eyes." The rig had cooled down, and so telling lines with warmer oil passing through them from lines without flow would be easy, and the first telltale signs became immediately apparent on the FLIR - the pressure and the return lines of the DCV, as well as the inlet module, were glowing. The oil was "escaping" through there, even when the pump was supposedly on standby:
And a flowmeter in the pressure line told the same story:
A closed-center inlet module shamelessly bypassing to tank even when no functions are called! With a piloted relief valve in the inlet, the most probable cause for it not to close (aside from physical damage or an impediment to the logic element, of course) would be either a clogged pilot orifice or an open pilot valve.
A very important detail now: as you can see, the valve is bypassing all right, but the pressure is still relatively high for an open logic element - 28 bar. Even with the pressure drop of the flow meter's restrictor valve, 28 bar is a bit too high for an open logic element with its single-digit bar bias spring. So, my first bet would be on a leaking pilot section, and my second bet would be contamination in the logic element itself.
Since there's no cutaway view in the catalog, I decided to remove the module and find and inspect the logic element, the pilot orifice, and the relief section myself. After I took it apart (mind you, in most precarious conditions!), I found the pilot orifice hiding behind the MDBV plug:
I removed the orifice and I didn't find any particles there, although it did feel like a piece of something dark fell off of it when I pulled it out and laid it over the towel. Then I also removed the logic element and the super tiny pilot relief valve, and then I cleaned the complete pilot passage along with the labyrinth filter thingy. I should have taken more pictures of these parts, but, to be honest, my feet were so cold from standing in cold water in thin rubber boots that all I could think of was getting the job done as fast as I could. When I put the manifold back together and remounted it on the valve, the rig was back from the dead!
I never found obvious contamination, but my current theory is that the malfunction was caused by something preventing the pilot relief from reseating properly. Still, this assistance call is a good example of a diagnostic that "leaves a question open." It seems to me that a completely obstructed orifice would be causing the logic element to open fully and result in maybe a 7 to 10 bar pressure increment at most, and yet the gauge was reading close to 30 bar. I can also tell you that now, after I studied the inlet module and know for a fact that the plug on top of the removable logic element allows you to monitor its closing pressure, this would definitely be the point I would read to see what was going on. My point is - the location of the malfunction was found (the inlet module), an intervention was made (disassembly and cleaning), and the problem, apparently, got fixed (the rig is drilling again). And yet, you are not 100% sure what happened and you can't, obviously, run more tests. All I can tell you, my friends, is that, unfortunately, this is something that you will see more often than you would like to if you get into this business.
So, here are a couple of post-assistance thoughts for you:
P.S.
How about this setup for a Friday evening?:
