Electrostatic Discharge in Hydraulic Systems

[FilterSystems]

I know this is a general Mechanical Engineering forum, but there is no Hydraulic forum and this is the nearest trade.

https://www.youtube.com/watch?v=47Dx6TvuJTc

I've been doing a project on electrostatic discharge, and the company I work for (I won't mention the name as I'm not here advertising for it) has done a lot of research into prevention and causes of electrostatic discharge.

I had a customer come in from JCB a few days ago however and I was doing some unrelated tests, and mentioned it and he hadn't even heard of it and was amazed!

So I was wondering how much of electrostatic discharge have people heard of? Is this something that people have come across??

I've uploaded and interesting video that we have done at my company if you're interested (see above).

If people are interested then I can upload more information regarding the subject.

 

[huntoon]

You have my attention! Do you have links to background info on the phenomenon?

Thanks

 

[Pkruse]

I've worked with many different hydraulic systems, and never heard of an ESD concern. What type of application is driving this concern?

 

[huntoon]

I've worked with many different hydraulic systems, and never heard of an ESD concern. What type of application is driving this concern?

Same here.

 

[Danger]

I've also never heard of electrostatic discharge being associated with hydraulics, which I suspect would have arisen during my pilot training if it were of serious consequence. Moreover, I can't immediately think of a particular mechanism by which it would arise. I'm not denying that it exists, but I somehow suspect that there is an advertisement buried here despite the protestations to the contrary.

 

[Bobbywhy]

FilterSystems, Welcome to Physics Forums!

This may be related to those discharges in the video you posted. Years ago I built a Van de Graaf electrostatic generator using oil instead of the normal rubber belt to carry the charge. Here are two references which may be connected to your phenomena:

"Abstract : An electrostatic charge generator for continually providing charge carriers of a given polarity in a flow of dielectric fluid. The charge generator comprises a coaxial tube with an inner conductor wire coaxial with a hollow, cylindrical outer conductor. A D.C. potential is applied between the inner and outer conductors of the coaxial tube and a dielectric liquid, such as oil, is caused to flow therethrough. Since the intensity of the nonuniform field in the conductor tube is highest near the inner conductor, the electrostatic charges of the opposite polarity as the inner conductor are removed from the dielectric fluid faster than charges of the same polarity are removed at the outer conductor. This results inthe dielectric fluid exiting the conductor tube carrying a charge of the remaining polarity."
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADD007846

“With this arrangement, the moving column of a fluid non-conducting medium receives electrical charges as it passes the pointed electrode 66, and the charges are removed and transferred to the electrode as the medium passes between the condenser plates. If desired, the fluid medium may have fine particles suspended therein to act as the charge carriers, but this is not generally necessary as the charges may be applied directly to the non-conducting fluid. The fluid may be a liquid or a gas or vapor, or a mixture."
From Patent 1991236 R.J.Van de Graaff 1935

http://www.lateralscience.co.uk/VDG/vdgpat.html

 

[FilterSystems]

I just typed out a massive post and accidentally pressed the back button and wiped it!

Anyway:

Apologies for the delayed response I'm currently away on a course and unable to get decent internet access.

Electrostatic discharge is a fairly new phenomenon due to oil companies becoming under more and more pressure to make their additive package they use in oil more environmentally friendly, they now advertise their oils as ash-less or zinc-free.

The removal of this zinc causes the conductivity of the fluid to be lowered and so a large difference in "work-function" of the fluid and the filter media which causes electrons to migrate from (and I can't remember off the top of my head so could be the other way round) the fluid to the media (or vice-versa) which of course charges the fluid as it passes through the filter element. This charge then must get discharged somewhere and so either in the fluid or the tank, which causes the rate of oxidation of the fluid to increase and can cause holes to be burnt in the element.

We have taken a lot of graphs with an oscilloscope in various applications if people are interested I can get these sent to me and uploaded? Along with pictures of the element with holes in it.

@PKruse & Hunton - all applications where the conductivity of the fluid is low, and we're talking about picosiemens here.

@Danger - read above post I hope this clears up your doubts and questions.

Again if people have more questions fire away I can answer them or find someone who can!

Thanks

EDIT: Also people asking for further reading, a quick googlesearch won't go amiss ;)

https://www.google.co.uk/search?sug...ie=UTF-8&q=electrostatic+discharge+hydraulics

machinerylubrication is a cracking website.

 

[Danger]

Thanks for the clarification, FS.
I was grounded on a medical in 1976, so my comment about my pilot training is inapplicable.

 

[Pkruse]

So if we do get RED in the system, why do we care? What sort of problem does it cause, and how do we mitigate it?

 

[FilterSystems]

So if we do get RED in the system, why do we care? What sort of problem does it cause, and how do we mitigate it?

It can cause a few problems, obviously depending on the fluid it can be explosive!

Much more common problems are caused by the sparks it gives causing holes in the filter element, which will obviously be larger than the elements micron rating, this allows unwanted contamination of large sizes into the system, which cause component breakdown, or even catastrophic failure of pumps etc.

The other problem the sparks cause is the breakdown of the fluid, it increase rate of oxidation in the oil nd so the oil loses its required properties.

 

[Pkruse]

Dieseling seems to be a much greater explosion hazard, and we already protect our systems from that, so I'd have no concern with ESD causing an explosion.

All my high pressure filters are metal, so I'd have to see some good data before I'd worry about holes in those.

I like to put depth type bypass filters on the low pressure side, so again no concern. I'd have to see data to cause me to worry about the paper surface type filters.

Since standard hoses are conductive, does that also tend to minimize the problem?

One area that probably needs more research are the insulated utility trucks that service live power lines. They use nonconductive parts and fluid everywhere possible.

 

[FilterSystems]

Here are some graphs that I've taken from a presentation we've done in a seminar to several large companies.

These were measured using a stat stick in a normal non "stat-free" element.

It seems like you're covered then Pkruse, it's up to you what you do with your system and how much you feel is necessary to ensure that it's safe. As I said it all depends on the application, same with everything - you wouldn't put a 1 micron element on the return of a steel mill or some other dirty place (although it would be best for the system) it just isn't that necessary as you'll be changing element every ~10 seconds (exaggeration but point valid).

My boss has just put his presentation on the memory stick so once I have time I'll spend a bit of time getting the good information off of it if people are still interested?

(It's a bit hard to gauge if other people are interested when you're passionate about something as you feel that everyone else is as enthusiastic as you!)

 

[Bobbywhy]

As you noticed in my post Number Six, dielectric fluids can transfer charge whereupon it may build up and then discharge. You have said that this can damage your filters.

Here is a related phenomenon: A helicopter can build up a huge electrostatic charge owing to its exhaust interacting with rotor downwash. This can cause problems whenever the aircraft touches down to land or lowers cargo, an anti-submarine sonar unit, or a rescue cable. The method military helicopters use to minimize this buildup is through the use of static discharge wicks. Since the same laws of physics apply inside your hydraulic systems, I suggest that some similar solution to avoid static charge buildup may be possible. Perhaps the strategic installation of "static wicks" would collect this charge, send it to ground, and avoid any dangerous build-up.

EMI and ESD on helicopters
https://www.physicsforums.com/showthread.php?t=176699

http://en.wikipedia.org/wiki/Static_discharger

 

[Pkruse]

Filter systems: I can't say enough good about depth type filters installed in a bypass loop on the low pressure side. Since they handle only a small percentage of the flow, you can put a very good filter in place, and the depth type filters can contain a large amount of particles before you have to change them. This greatly extends the life of your full flow conventional filters, and it keeps the fluid cleaner than you otherwise dreamed possible. Also takes the water out. In the old days, they did not bother with any other filter. Then systems evolved to full flow surface type filters only. Turns out, it is better to have both.

 

[FilterSystems]

Filter systems: I can't say enough good about depth type filters installed in a bypass loop on the low pressure side. Since they handle only a small percentage of the flow, you can put a very good filter in place, and the depth type filters can contain a large amount of particles before you have to change them. This greatly extends the life of your full flow conventional filters, and it keeps the fluid cleaner than you otherwise dreamed possible. Also takes the water out. In the old days, they did not bother with any other filter. Then systems evolved to full flow surface type filters only. Turns out, it is better to have both.

These elements still get affected by electrostatic problems, any element that isn't stat-free that has a hydraulic load applied to it by fluid (l/min /cm^2) will cause electrostatic charge to be built up.

Instead of what filters you use, you should be asking what are the properties of your fluid, we use depth-type filters as you say and find the problems still occur when we run tests with these non stat-free elements.

Our stat-free elements are depth type too!

Also you mentioned on the pressure line you had metal filters? As this the whole element?? We found that when using a conductive filter element (e.g metal tubes or caps) no electrostatic discharge happens in the filter element, but there is an even higher charging of the fluid, so if discharges happen elsewhere it will shorten the lifetime of your oil.

On a sort of side note, I should be doing another video soon, we're cooling some fluid down to -40 °c and testing the conductivity of it, this is to simulate a 'cold-start' in applications where the fluid starts off this cold, we want to see what affects the temperature would have on the fluid. I.e. some of the systems they have in Northern Russia, and Finland etc.