VARNISH
As the world is getting more competitive, maintenance becomes the single-largest controllable cost among other production cost factor. Therefore in order to achieve the world-class maintenance operation, industry need to implement integrated solution of management – technology – expertise support. One way to control maintenance cost it is by improving your plant’s reliability. Asset management means to manage any risk that can influence plant critical assets’ performance or availability. We can manage risk by mapping failure mode in each equipment and set the defense task for each failure. This time we are going to talk about how to detect failure modes related with lubrication one type of failure mode, which is VARNISH.
Varnish itself is an end byproduct of lube oil degradation process which we called oxidation. Oxidation itself is caused by heat, presence of oxygen in air and water, also with metal particles as catalysts. Every plant that have its lube oil exposed with high temperature working condition, the lube oil are easily became oxidized and risk to have varnish. Another source of high temperature in equipment is due to microdieseling and Electrostatic Discharge (ESD)
Microdieseling caused by a trapped air bubble in a high pressure zone such as in the pump, or in the oil film of the plain bearing will begin to experience increased temperature that will result in either carbonising of the oil around the air bubble or actual detonation of the oil fumes within the air bubble.
Image 1. Microdieseling (Source: Mastering Lubricant Analysis, KEW Engineering)
Electrostatic Discharge (ESD) occurs when clean, dry oil passes at a high flow rate through tight clearances, internal fluid friction within the oil can generate static electricity. The generated electricity may accumulate to the point where it produces a sudden discharge or spark. These sparks are thought to be between 10,000°C and 20,000°C and typically occur in mechanical filters. ESD can be seen as burn marks on the filter media.
In current market, most common lube oil that been used has API Group II to III type of base oil. This type base oil is very pure and highly saturated base oil due to its refinery process. High saturation makes the oil can dissolve only small portion of the varnish in it (soluble varnish). Varnish that generated above the saturation point of lube oil will become insoluble varnish that looks like a small soft particulate and easily stick to metal parts, especially in high pressure or low temperature condition like bearings, oil cooler, filter, oil tubing, main oil tank. This insoluble varnish sticks in this components, can cause failures such as bearing wear due to lubricant film failure, premature lube oil degradation, etc.
Image 2. Varnish in components
HOW TO DETECT VARNISH
There are several ways to detect varnish, visual inspection and through oil analysis. Shown in image 2, how we can identify varnish when we do overhaul or regular maintenance to our machinery, sticky substance in the surface of the component with yellow, brown and/or black color. Of course this not a good way to monitor varnish in our lubrication system when we need to stop our machinery and open it up, so it is best to use routine oil analysis.
There are 2 techniques (when this article was written) to test varnish with oil analysis,
- Ultra-centrifuge
- Membrane Patch Colorimetry
Ultra-centrifuge is a very high speed centrifuge to push the heavier insoluble varnish out from the lube oil sample and settled at the bottom of the flask. By inspecting how much sediment after the test is finished we can determine the varnish level inside lube oil (image 3).
Image 3. Ultra-centrifuge
Membrane patch colorimetry (MPC) use a very fine membrane filter patch to catch varnish inside lube oil (0.45 µm). This patch later on is tested by spectrophotometer to determine the color and darkness in ΔE. MPC test method also regulated in ASTM D7843 and considered the most reliable test due to the simplicity and good data repeatability. Higher ΔE value means higher varnish risk inside the machinery lubrication system.