Hot vs Cold Varnish

Did you know there are 3 types of varnish? Cold varnish, hot varnish, and sludge.  Varnish is a “catch all” industry term used to describe many different types of oil deposits. Unfortunately, the widely used term can confuse users into trying to mitigate one type of varnish with an incorrect remedy. In this discussion, I will address cold and hot varnish. 

  Cold Varnish, or "true varnish," is a common suffering point in gas turbines or in any system where the lubricating oil is also used in the hydraulic control system, governor, or any tight tolerance servo valves. These particles are submicron and are not visible in warm oil. They are also invisible to routine oil analysis testing. As the varnish particles increase and saturate the oil, they will come out of solution in cooler, low-flow areas. A visible inspection of pencil filters, last chance filters, and servo spools is recommended on a regular schedule as part of the varnish prediction program. Cold varnish may also be trended with advanced lubricant analysis including MPC, RULER, UC, TAN, and others. When a system is effected by cold varnish, varnish mitigation strategies and flushing practices recommended by Mobil ServSM Lubrication Engineers specific to your system should be considered. 

  Hot varnish, or “high-speed bearing deposits,” are usually visible in the load zones where oil and/or additives may degrade and adhere to the hot bearing surfaces. Journal bearings and tilt pad bearings have relatively large clearances that can be forgiving, however, the bearing faces may become insulated with varnish and not dissipate heat as quickly. This hot varnish is more likely found in turbo-machinery where bearings are above 180F and are more likely with increasing speed. It is a mechanical issue that is manifested in the oil and the initial layer of varnish can cause higher temperatures creating even more hot varnish. The elevated bearing temperatures may trend upward in a saw tooth pattern and send turbo-machinery into alarm or even trip as the bearing temperatures approach Babbit creep at ~270F. The best way to monitor hot varnish is with temperature probes (at the API 670 location), vibration, and advanced oil analysis (especially Ultra Centrifuge test). Hot varnish mitigation is best done by changing bearing design or oil flow to lower bearing temperatures. Online mitigation options exist by increasing the solubility of the oil if done with the help of Mobil Serv Lubrication Engineers. 

Have you had any experience with cold or hot varnish? 

Anonymous