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Turbine oil varnish has been a well-documented reliability challenge for power plants. Traditional oil analysis techniques have been ineffective at predicting the onset of deposit problems in turbine oils – often defined as varnish. This has led to the commercialization of several new analytical tests which measures oil degradation products and can be correlated to the potential of deposit formation of turbine oil. The most widely adopted test is referred to as Membrane Patch Colorimetry (MPC). This test isolates oil degradation products on a membrane and measures the color with a spectrophotometer. The total amount of color generated by the deposits on the patch is reported, following the CIE LAB DE scale.
is a newly approved ASTM standard called the
Standard Test Method for the Measurement of Lubricant Generated Insoluble Color Bodies in In-Service Turbine Oils using Membrane Patch Colorimetry
. Standard guides are often used to interpret the MPC results and assess a fluid’s potential to form deposits. These Guidelines will vary between different test labs, based on their customer’s experience. However, they all follow the same general concept that the more deep the color the higher potential the fluid has for deposit formation. Most labs consider MPC results greater than 40 dE as critical.
Like with all oil analysis tests proper interpretation should take into account the application and operating conditions. Only by understanding how the results relate to a specific lubrication system does a facility fully understand the reliability risks posed and direct appropriate corrective actions. Different power plant applications can have varying degrees of sensitivity to lubricant deposits. This means that MPC results of 30 dE could pose a high reliability risk for one power station while the same results could translate into little operational concern for another station. Here are some examples of different power plant applications and their sensitivity to varnish-deposits.