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. Another good reason for hydraulic system designers not to be stingy about the size of the hydraulic tank.
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