Insane Hydraulics

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Closed Loop Failure Due To Line Elasticity and How To Avoid It

Believe it or not, but this hydraulic tale had its origin at a spit-roasted pork fest. Say what? Yep, you heard me right! It all started as I was savoring succulent rosemary-tempered slices of pork meat served over freshly baked hand-kneaded Alentejo bread in best Portuguese tradition!

Let me connect this to oil hydraulics now. A well established industrial company was hosting a spit-roasted pork fest - an "open doors" event of sorts - and I was lucky to be invited. If you're into red meat - roasted pork prepared the way they do it here in Portugal is out of this world!

Anyhow, here I stand, casually "mingling" with fellow techs, as I catch a glimpse of these large power-packs casually "parked" in the background. Naturally, I couldn't resist the temptation and went there to have a peek. What I saw was a pair of very professionally built high power closed-loop units. Just as one would expect - a solid build from a solid firm. So, I admired the units for a moment and then went back to appreciating traditional Portuguese cuisine. It was an afternoon well spent.

A couple of months later I got a call from a friend of mine, who worked at that company, asking for a technical opinion about a "mysterious break-down". He said - "remember the power-packs from the pork fest?" - and I said - "You're going to have to fill me in on the power-pack details, man, all I remember is the meat was good" - then he went - "Well, I know you'll like to hear about this one! Two of them failed in less than a month after the commission, and nobody can understand why! Here's the deal: ..."

The gist of the following lengthy explanation would be - the closed-loop units were driving industrial shredders (I am not sure now what it was that they were shredding, but that's not important). The installation, commission, and operation monitoring had been done by a very experienced team. High pressure and charge pressure levels were well within the recommended specs, with mechanical pressure switches installed to sound the respective alarms. The oil particle count tested at NAS 5 and all the standard flushing procedures had been performed before starting the system. If someone asked my opinion - I'd say I wouldn't be able to perform the commission better, if anything - I'd call it an overkill (but then again when you toy about expensive hydraulics you're way better safe then sorry). And yet, even with all these cares, and without tripping any of the alarms, the main pumps of two units failed but a couple of weeks after the commission.

Allow me to run you through my "questionnaire":

Type of transmission? - Closed-loop

What type of work does the closed-loop transmission perform? - Shredding (OK - so we have "harsh load conditions" here, nothing I haven't seen with raise boring rigs before).

High Pressure? - Normal

Oil type, working conditions? - The best you can find, NAS 5, 50 C.

Is the charge pressure stable? - Yet, it is.

How are you monitoring it? - With a mechanical pressure switch.

Well, not ideal (that's me starting to think what could be wrong here) - so I say - "well, maybe the accumulators got accidentally charged to an incorrect value or something. This has happened to me before, you know, asking for a pre-charge of 10.0 and getting one with 100".

And then I get the key answer, after which I am almost certain about the cause of the failures - "Well... What accumulator?.."

So I go - " know, the charge pressure one, surely your system has one, doesn't it? I mean - the hose accumulator effect in closed loop transmissions and all.."

And I get - "Actually, it doesn't - do you think it needs one? A hose accumulator effect you say - care to elaborate?"

"Not only I think that you need one, but I am almost sure that the lack of charge pressure accumulator is what killed your pump. You would need to log the pressure in the low side of the loop near the pump with a high-speed logger, that is - if you need to have solid proof - but from my experience I can tell you that when you have a large closed-loop coupled to long transmission lines and "spiky" loads - having an accumulator in the charge pressure circuit is not an extra - it's a must!"

In the end - the pumps got rebuilt, the charge pressure accumulators got installed - and the systems never failed again - and it was the first time I got in touch with the "real life" proof that loop line elasticity can be a very bad thing for a closed-loop transmission that is subjected to suddenly changing loads. I've read about this phenomenon, but I have never come across it before.

So, what is this thing that can destroy a perfectly good closed-loop transmission in a matter of days?

It is the shock load conditions (i.e. very uneven load with abrupt torque demands, which is typical for crushing and boring equipment) coupled to elastic and relatively long loop lines. What happens is that when the load spikes, the hydraulic motor is slowed down, because in order to build the pressure necessary to satisfy the sudden torque demand a certain amount of oil needs to be injected into the line due to its elasticity, and during this, let us call it, "stall" moment, the amount of oil returning from the motor into the loop can (for a very short time) become much lower than the amount of oil that the pump is currently trying to "pump through". When the difference between the pump discharge flow and the return flow becomes greater than the charge pump flow - a very nasty pressure dip, and, consecutively, cavitation-enabling conditions develop in the low side of the loop, again, for a very brief moment of time, not long enough to trip a mechanical pressure switch, especially when it's installed at the charge pump outlet, but nonetheless strong enough to damage the pump if such a situation is repeated enough times.

The best solution to combat such a situation is to install a high flow-rate capable accumulator in the charge oil supply line (normally pre-charged to about 40% of the normal charge pressure level) - which will provide the additional short-term charge flow bursts when necessary.

Some systems may even use two accumulators, especially when large hydraulic motors (like the Hagglunds CAs, for example) are equipped with anti peak and anti-cavitation manifolds. In such cases, you can have one accumulator in the HPU and another one feeding the motor manifold anti-cavitation gallery (with the pressure to keep the accumulator charged supplied, for example, from the high side of the loop through a reducing valve, or a separate line coming from the HPU). See the possible examples here.

When I was asked - but how the hell did you know about it - my answer was - I read about it in a book. And this is why I am writing this post. Those technicians who visit this blog from time to time will now be able to say - I read about it in the insane-hydrauics blog!

Conclusion - if you set up a simple closed loop with short lines and normal loads - you can leave the charge circuit as is. But when you build a large transmission that will run something that will destroy something (like a crusher or a borer) - make sure you have that charge pressure accumulator in place and verify it regularly - the pump will thank you!