Based on my experience at least, electrostatic discharge in hydraulic systems is not a widespread problem. But for reasons I’ll explain in a moment, it may be on the increase. So it’s definitely something you need to be aware of.
As you probably remember from school science experiments, electrostatic charge is generated whenever there is friction between two bodies moving relative to each other. No real surprise then, that electrostatic charge generation occurs in hydraulic systems as a result of friction between the fluid and system components.
A common symptom of electrostatic discharge in a hydraulic system is an audible clicking noise as charge repeatedly increases and then discharges to a surface of lower voltage through sparking. And this often occurs in a filter – resulting in burn marks and other damage to the hydraulic filter element.
While you may yet to have come across a ‘clicking’ hydraulic filter assembly, there are a couple of reasons why this problem may be becoming more common. The first is a growing trend towards the use of hydraulic oils with non-metallic additives. Hydraulic oils with zinc-based anti-wear additives have relatively high conductivity.
Hydraulic oils with good conductivity assist the dissipation of electrostatic charge as it moves around the system. Studies have shown that hydraulic oils with zinc-based, anti-wear additives rarely accumulate enough charge for harmful discharge to occur. Synthetic hydraulic oils and those with non-metallic anti-wear additives on the other hand, have much lower conductivity. This increases the potential for electrostatic charge accumulation and therefore, the likelihood of discharge.
The second reason the incidence of electrostatic discharge may be on the increase, is the change in materials used to manufacture filter elements. Hydraulic filter elements are being made so they are more easily disposed of in an environmentally acceptable way. This has led to the increased use of non-metallic materials in filter element construction and designs where the metal core is part of the housing and not the element. The combined effect of these changes has been to lower the conductivity and increase the capacitance of hydraulic filter elements.
That said, this is not a problem that’s being ignored by hydraulic filter manufacturers. All are aware of the issue, and most are researching the problem and developing ways to minimize or eliminate it.
But what can you do if you come across an instance of electrostatic discharge in the meantime?
Well, while grounding of the hydraulic system reservoir, piping and filter housings prevents arcing to nearby conductors, it does not prevent electrostatic charging of the fluid or filter media, nor does it accelerate the decay of the charge. However, the charge generated in hydraulic filter elements can be reduced by increasing filter size. This reduces flow density through the filter and therefore the amount of charge generated. And increasing the volume of oil in the hydraulic system, by increasing tank size for example, extends the time between successive charge generations and also increases the available time for the charge to decay.
As you can see, electrostatic charge and its discharge is another reason why skimping on tank size and filter capacity can be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.
I’ve been a long time suscriber and reader of your Hydraulics Pro Club and your emails. These are both excellent and highly recommended. I appreciate that you are willing to work apart from the accepted industry knowledge base. A good example of this is the removal of suction strainers.
When I see a recommendation of this type, I start to look at the available documentation and science behind the claims.
This issue with ESD in filters seems to only be supported by filter manufacturers whose articles end in a sales pitch for their particular solution to the problem. Are there any independent studies that you are aware of to support this? There seems to be conflicting information about whether non metallic additives cause the issue. I’m having problems with the idea of fluid running nearly exclusively in steel conductors building up a static charge.
Yes indeed, as I said at the start of my post, in my own experience, ESD is not a major issue in hydraulic systems. A google search reveals a number of papers and case studies, most of which are not specific to hydraulic systems. All that said however, I do know of instances where this has occurred in a hydraulic system. So it is something that all hydraulics users should be aware of. But they should be alert not alarmed.