What is causing the arcing in our hydraulic flowmeter?

[tomc5]

I think my situation below can only be explained by a physicist, so I hope I came to the right place!

On our hydraulic test bench we use a glass tube, float type flowmeter (also called a rotameter?); fluid is hydraulic fluid and non-flammable; the float is anodized aluminum. We occasionally get a report of arcing in this flowmeter, but reports and descriptions of the event are non-specific except that "an arc (or spark )was seen inside the tube". I am told in one instance that the glass tube shattered.

My initial conclusion was we simply had a bad metal-to-metal contact between the flowmeter metal base and the back panel, but all surfaces were clean. We then also added a ground wire between the flowmeter metal base and an Earth ground with no apparent effect on preventing the arc.

1. what is the possible cause of this charge buildup?
2. where is the charge being stored? On the Glass? or on the anodized float?
3. what else can I do to prevent the arc?

I have called several flowmeter companies but rec'd no help in explaining the phenomenon.
Thanks for any explanations you can offer.
Tom

 

[Integral]

If charge is being stored it will be on the glass tube, so grounding the frame will not help.

Is your hydraulic fluid conductive? If so I do not see how a charge will develop. However, if it is non conductive then the possibly of a charge build up approaches certainty. One way to prevent this may be to insert a grounded metallic section in the flow path just before the glass tube. If that does not work perhaps (This is just a thought on my part ) perhaps you could connect the metallic section of tube to a "de Ionizer" this would consist of a low voltage AC signal applied to the tube. The idea would be to induce positive and negative charges which would prevent any significant build up of charge hopefully maintaining a neutral charge on all non conductive surfaces.

This is all wild speculation on my part, have you tried contacting the manufacture of the hydraulic fluid? They may have some helpful information.


EDIT:
A bit of googling shows that static build up is a relatively common issue. It appears that there are filters designed to prevent this. There are also static disapative lines available. Again, contact the manufacturer of your fluid, they should be able to help.

 

[tomc5]

Sigma-
Thanks for your quick reply.
I am certain the hydraulic fluid we are using is non-conductive although I cannot “quantify” how non-conductive it is. It is a synthetic oil (polyalphaolephin-based oil).

If charge is building up on the inside surface of the glass tube, I imagine it builds to a value which allows it to arc over to some grounded part of the flowmeter, i.e., the metal base which supports and encloses the tube. There is contact between the glass tube and the base, so why can’t the charge just drain off to the base rather than arcing?

Is it possible charge is building up on the anodized aluminum float and then arcing over to the glass as it finds ground through the metal base, or just arcing from float to base?

Thanks again,
Tom

 

[Integral]

The trouble is, since the glass tube is an insulator the charges are not mobile and cannot all drain off at the base. Your best bet may be one of those de ionizing filters or static dissipative hoses.

 

[tomc5]

I've been web searching your idea of adding a deionizer in the form of filter as you suggested. I will also have to study how these devices will affect my flow and system pressure requirements (very hi pressure = 3500 psi and flow is 25-30 gpm). Your original idea of an active deionizer is also interesting to me.

However, I am not clear on what the theory would be in my application; would the deionizing process pretreat the fluid to some charged state before it reaches the glass tube so that flow within the glass tube gets neutralized somehow? I'm way out of my league here, so your help is greatly appreciated.

Thanks again,
Tom

 

[FredGarvin]

We have had certain hydraulic systems experience the same phenomena. However our situations involved the hydraulic lines themselves arcing and eventually splitting. I can not remember the particular hose type went to after the problems arose. I would have to look that up come Monday.