Insane Hydraulics

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Check Your Pressure Sensors Regularly (And Some Sentimental Thoughts)

So, that big closed-loop-powered GEHO® slurry pump (combined hydraulic pump displacement of about 2 liters in a two-story installation) started tripping all sorts of alarms this Friday, wreaking havoc on the production of our beloved copper mine. Some of the logged messages appeared to be "hydraulic in nature", and so an opinion of a "not certified but marginally skilled" hydraulic technician was requested (a hidden benefit of being located next to a mine, I reckon).

While some of the alarms could be attributed to operational/adjustment issues - something that I could postpone for a convenient service interval - one of the alarms, that actually happened three times as I was inspecting the system, was indeed extremely alarming (pun intended). It was the dreaded "closed-loop-killer" - the low charge pressure/cavitation alarm!

It's not a good thing when a closed loop loses charge pressure. No Sir! Several-thousand-euro-not-good in "normally sized cases". But this was a million-euro-plus contraption that conditioned the production of a whole mine, so I definitely had to double-check everything before raising any red flags.

The alarm was triggered by the PLC that read the loop pressures with common industrial 4...20 mA sensors (IFM PA3020s), and naturally, before doing anything else, I needed to confirm if the sensor on the "bad" side was providing a correct reading.

How do you do that? You compare the reading from the PLC against the reading of a trusted pressure gauge, of course! Only in that case, there was one tiny (big) problem - the pressure sensor was mounted on a manifold located two stories above the electric cabinet that hosed the PLC and the screen with all the readings...

This could be a serious challenge, but luckily I always carry a couple of wireless pressure gauges with me whenever I go (the very same prototypes that I showcased in the presentation), so the seemingly impossible task became a piece of cake!

I go up the stairs, connect the pressure gauge to the test point, slap the sensor on the HPU's frame with its magnetic base, and then go down the stairs and back to the PLC cabinet to compare the reading on the service screen with that on my smartphone. Easy-peasy! And I say this with all the confidence in the world because I've done similar diagnostics "the hard way" countless times!

In short - the pressure sensor was offset by 10 bar, which was enough to be tripping the cavitation alarm:

Now, what if I didn't have my wireless gadgets with me? I would find a workaround, of course. For example, I could chain several test hoses and run the super-long line down through an opening in the ceiling, or use a WhatsApp call to monitor the gauge upstairs. But none of the one hundred and one solutions I could come up with would offer me the ease of my wireless gauges and the app on my smartphone.

Obviously, the easy and straightforward diagnostic I just described is not the point of this post, the point is the wireless gauges that I built several years ago, and the fact that while I can't imagine my professional life without such a wonderful tool, I get this sad feeling at the back of my mind every single time I use it, because, apparently, nobody else feels the same way.

I've shown the system to many companies, and always, without exceptions, saw zero interest or anything remotely resembling curiosity. Frankly, I don't get it (this is me being sentimental, as promised). The only explanation that I can come up with is the traditional "hydraulic tool acquiring policy" that 99.9% of the Portuguese shops adhere to. And this policy is based on the staple thesis that people responsible for buying the said tools are not the ones who use them, and their decision-making process can be pretty much summed up in the following two questions:

1) Why do you need a digital pressure gauge if you can use an analog one?

2) How much did you say a flow meter costs?!! In your (__insert an adjective ending with "ing" __) dreams!

Strangely, these same people see no problems in dishing out hundreds of euros on 87Vs for the instrumentation departments. Go figure... And don't get me wrong here, I love the Fluke's 87V - one of the best multi-meters you can get. It's just the fact that electrical tools seem to be a much more appreciated investment when compared to the specialized tools required for hydraulic diagnostics!

Anyhow, this experience still provides one valid point:

The PA3020 is a pretty expensive pressure tranmitter. It's supposed to have the max error of 2 bar, last for 100 million cycles, and offset less than 0.05% of the 400 bar span per 6 months. Yet this one "ran away" well beyond its specs in less than a year. So, whenever you work with hydraulic equipment that employs industrial pressure sensors, you should always bear in mind the fact that even quality pressure sensors can go bad for no apparent reason. This is especially important for cases where the sensors trip alarms or provide readings to calculate force (think - drilling equipment). Checking them regularly is not a bad idea. In a properly designed system, such a check should be fast and painless.

My wireless gauge system employs gems' sensors, thin-film, and I check them every 6 months or so. So far they've been surprisingly stable. The PA3020 uses a ceramic measuring cell, by the way. I actually got to keep the bad sensor, so maybe I will still be able to use it for diagnostics with software calibration. Or maybe I'll just cut it open to see what's inside...