Lube-Tech
One final factor that is becoming an increasing risk, is electrostatic discharge (ESD) – this is a phenomenon where discharge sparking occurs within a system; this typically occurs in the filter housing. The temperature of the discharge spark is greater than 10,000°C. These sparks cause the oil molecule to “crack”, forming free radicals. These radicals continue to react with other oil molecules, oxygen and other radicals. The resulting oxidation products can then conglomerate to form the sludge-like material that we call varnish.
Figure 1: ESD graphic
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.122 page 2
this merely prevents the sparking from occurring at the filter. The charge continues to build up and can then discharge in more safety critical areas, such as the tank, where there is a risk of igniting the oil mist vapour and a resulting fire/explosion.
Oil conductivity plays a large role in the risk of ESD occurring. High conductivity oils, typically greater than 500pS/m, are less prone to ESD due to having more electron flow pathways. These oils tend to be the outdated Group I Base Oils, being more conductive due to the presence of heavy earth metals, such as Zinc. Low conductivity oils, typically less than 500pS/m, are more susceptible to ESD. These are usually the more heavily refined Group II and III Base Oils. As a result of this, measuring the conductivity of the oil is the initial parameter when identifying any risk of ESD.
There are several additional risk indicators, one being the workload at the filter; if the low conductivity accompanies a workload greater than 0.1L•
min-1 cm-2 Electrostatic Discharge (ESD)
What causes ESD? ESD occurs when the oil and filter become electrostatically charged. This happens due to a transfer of electrons between the materials; the reason for this transfer can be observed in the triboelectric series, identifying the electron affinity of the filter media and of oil. Filter media is typically glass-fibre, which will prefer to donate electrons and become positively charged, whereas oil will be more willing to receive electrons and become negatively charged. As these materials become separated, there is an increased voltage; once this exceeds 3kV/mm, discharge sparking can occur.
As well as contributing to varnish formation, these sparks can cause holes to be burnt into the filtration layers, this can effectively turn your 3µm filter into a 200µm filter. One possible solution considered to combat ESD is to make the filter conductive, but
the chance of sparking greatly increases. Secondly, colder conditions can be problematic as conductivity positively correlates with temperature, so it may be that during cold start-ups the conductivity is significantly lower than the recommended 500pS/m and as a result the charge build up can occur. Furthermore, the oil condition can be a great pointer that ESD has already occurred. The main aspect of this article is looking at oxidation products and it’s already been explained that ESD is a cause of varnish formation; therefore an increased level of varnish is a potential indicator for ESD. We can also look and listen for physical signs; when ESD is occurring, there can be some clear indicators that the sparking is taking place. For one, if you can get near the filter housing, you may actually hear the sound of the electrical discharges continually occurring. Additionally, the filter element itself will display signs of burning damage; a process which can be easily handled by a laboratory with the experience of analysing used filter elements.
LUBE MAGAZINE NO.151 JUNE 2019 23
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