Electrostatic Spark Discharge typically occurs when static is built up in an oil at a molecular level, causing it to discharge in the system and create free radicals, which increase the opportunity for varnish to form. This usually occurs at temperatures of around 10,000 °C.
If we were to liken this to an everyday situation, we could think about walking around a carpeted room where the static builds up in our body. When we touch a metallic object (more than likely a door handle), we get a bit of a shock as the built-up static is discharged through us and the door handle.
Inside a lubricant system, static doesn’t just build – it ignites microscopic sparks powerful enough to scar filters and start the chain reaction that leads to varnish.
Similarly, in lubricants, static exists at a molecular level, and in areas of tighter clearances, some molecules are forced to rub against each other, causing a buildup of static. When it accumulates to the point of becoming a full charge, it dissipates at the first opportunity, usually at the filter membrane or some sharp-edged object along the way. These are seen as burnt patches on the filter membrane.
When this spark occurs, it creates a chemical reaction that generates free radicals. Free radicals are highly reactive species that need to engage with other substances. These are the initiators of varnish, and their presence can accelerate reactions, leading to deposits forming in the lubricant. Eventually, this will lead to a system that has experienced both ESD and oxidation.
In this article, we will discuss various identification methods and ways to prevent ESD in modern lubrication systems. We will also spend some time identifying typical root causes for ESD by developing a logic tree as a guide for future investigations.
How to Identify Electrostatic Spark Discharge in Lubrication Systems
Every degradation mechanism produces varying results in the form of deposits or in how these are formed. For ESD, some tell-tale signs alert the user to its occurrence. These include:
- Crackling sounds / buzzing outside of components – This noise is representative of sparks as they discharge on part of the media/asset. Typically, this occurs when the fluid is in motion, allowing it to be heard near the filters when the system is operating.
- Burnt or pinhole filter membrane – The filters usually feel the full effect of ESD, and small burn patches or even pinholes are created when ESD occurs. When changing filters, the membranes should be examined for these patches to determine if ESD is occurring.
Free radicals are produced when ESD occurs. As such, this leads to polymerization of the lubricant, which produces varnish and sludge. This is part of the oxidation process, and the antioxidant levels will begin to decrease. During ESD, certain gases are also released in the oil. Some of the lab tests which can be used for identifying where ESD has occurred include:
- RULER® – Remaining Useful Life Evaluation Routine test, which quantifies the presence of antioxidants in the oil. By trending this over time, one will be able to determine whether the levels of antioxidants are decreasing or not. Typically, this test can be performed twice annually on larger sumps (such as turbines) or the frequency can be increased according to the criticality of the equipment. If the value gets below 25% then this is the critical limit, and methods to regenerate the oil or change it should be explored.
- MPC – Membrane Patch Colorimetry – this measures the potential of the oil to form varnish or deposits. Depending on the equipment, the warning limits will vary, but a good rule of thumb is to treat results below 10 as normal, those above 15 as within the monitor range, and those above 20-25 as the critical range. Be sure to double-check these levels with the OEM of the equipment.
- FTIR – Fourier Transform Infrared Spectroscopy can identify various molecules in the oil. It is likened to identifying the fingerprint of the oil, where each molecule has a specific characteristic spectra representative of that molecule. This test can be used to identify the presence of oxidation or any deposits that may have formed.
- DGA – Dissolved Gas Analysis – this test can be used to identify the presence of particular gases that are released during ESD, such as acetylene, ethylene, and methane.
Those above are just some of the methods that can be used to identify the presence of ESD in a lubrication system.
