Year of publication: 2009


In turbines and large oil circulation systems, oil fillings of several thousand liters are usually in use for years. During this time, aging products may form or contaminants may enter the system that cannot be simulated during oil development. Without careful monitoring through trend analysis, sticky reaction products or sludge-forming soft impurities go undetected. Varnish-like deposits may reduce the oil flow at control valves and plain bearings, and oil sludge settles in oil-bearing lines and tanks. The reliable functioning of the entire circulation system is threatened. With the new MPC test (Membrane Patch Colorimetry), OELCHECK now also detects this risk.


Problematic deposits have formed, especially recently, in gas turbine lubrication and control systems. Their extremely high operating temperatures with reduced oil volumes cause problems for turbine oils. Oxidation-preventing additives are consumed especially at the "hot spots" of the system. However, the high oil circulation temperatures also accelerate oil aging.

Table of contents

  1. During turbine oil analysis, indications of possible oil problems are found:
  2. The MPC test is particularly recommended:
  3. Test description
  4. MPC index – warning values and measures

During turbine oil analysis, indications of possible oil problems are found:

  • The FT-IR spectrum shows oxidation.
  • The acid number (AN or NZ) of the oil increases.
  • The color number shows: The oil becomes darker.
  • The air release behavior worsens.
  • The viscosity increases.
  • The RULER values for oxidation inhibitors decrease.
  • The cleanliness class for particles > 4 μm increases.
  • The solid impurities on a 0.45 μm filter increase.

All values are an indication that increasingly soft particles, mostly of higher molecular weight, are suspended in the oil. When they conglomerate with each other, they easily form deposits on all oil-wetted components in the system. Thus, they can stick together valves and complicate the oil-based control of the turbine. However, the deposits can also cause damage in the filters. Not only is it necessary to change the relatively expensive, fine filter cartridges frequently. Often the impurities are also the reason for electrostatic charging. The following discharge (ESD) causes sparking and local burning of the filter material. Because of the holes caused in the process, the filter loses its effectiveness.

Until now, it has not been possible to predict the risk of deposit formation based on standard tests alone. For several years, the QSA test has been experimented with in the U.S., where a spectrometer is used in an in-house test of an oil laboratory to evaluate filter discoloration. The interpretation of QSA values is very subjective.

OELCHECK therefore began working on the development of an independent standard years ago as a member of an ASTM turbine oil working group. This has the aim to objectively display a forecast of the further available oil application time. OELCHECK is currently the only European service laboratory to offer the MPC test as an additional special test. This test is the only examination method in the world that not only detects the problem-causing insoluble oil residues, but also quantitatively evaluates them.

The MPC test is particularly recommended:

  • For systems with large oil fillings of more than 2,500 liters.
  • For an extremely long oil life of more than 10,000 hours.
  • For turbine, hydraulic and gear oils. For gas and steam turbines, turbo compressors and blowers.
  • For oil circulation systems in the paper industry, steel and rolling mills. For hydraulics and planetary gears (capacities from 500 liters).
  • If oil analyses provide evidence of residue formation.
  • When first indications arise in practice.

ASTM (American Society for Testing and Materials), which is comparable to DIN, has submitted a draft standard for the MPC test. The ASTM standard does not yet specify how the results or the measured value should be presented.

OELCHECK therefore gives the maximum possible information for the MPC test:

  • MPC index: Numerical value between 0 and 100, for turbine oil max. approx. 60. The higher the MPC index, the greater the risk of sludge and varnish-like deposits formation (see limit values table). * Luminance – L: the blackness or whiteness of the residue remaining on the membrane. The lower the L-value, the higher the proportion of black particles in the oil. These may indicate a diesel effect due to too much air in the oil or soot particles generated at "hot spots."
  • Red value – a: is influenced by corrosive wear and degraded EP agents. The higher the a-value, the greater the risk from sludge-forming corrosion particles or degraded EP agents.
  • Yellow value – b: is influenced by degraded or ineffective oxidation inhibitors and by oxidized base oil components. The higher the b-value, the more susceptible the oil is to sticky deposits.
  • Solid contaminants: Weight increase of the filter by filtering off 50 ml of oil. The higher the weight gain, the dirtier the oil.

Obviously, each MPC lab report includes an image of the membrane under study. In addition, the course of the L-value and the a- and b-values in the color spectrum are mapped. Trend monitoring of color values can be used, for example, to assess the effectiveness of a filtration method in removing specific contaminants. For example, if an electrostatic filter method removes mainly the polar oxidation particles, the b-value becomes lower, but the L-value and the a-value do not improve. Additional measures may then become necessary.

Test description

  • Ship a quantity of oil of 100 ml, better 500 ml, immediately after sampling.
  • Shake the sample by hand for 30 s after a rest period of at least 5 h.
  • Mix by slow stirring of 50 ml oil and 50 ml filtered heptane.
  • Weigh the new, dried filter membrane with a 0.45μm pore size.
  • Filter the mixture across the membrane under vacuum at 710 mBar.
  • Dry the membrane in a vacuum, then at 80 °C for 3 h in a drying oven.
  • Weigh the membrane with the retained residues.
  • Calculate the percentage increase in weight of the filter due to the buildup of deposits.
  • Evaluate the discoloration caused by the coating using an i-Lab colorimeter.

In colorimetric evaluation with the i-Lab device, the residues on the membrane are analyzed with a spectral sensor. A light source consisting of three LEDs in the device emits light in the spectral range onto the membrane, with the deposits absorbing or reflecting all or part of the light. A detector measures the intensity of the "reflected" light at different light wave ranges, and the electronics evaluate the difference between transmitted and reflected light. The differences in reflection and color intensity at the respective spectral ranges allow the calculation of an MPC factor. The level of this factor is directly related to the potential of the oil to form deposits.

The MPC test is more time-consuming and personnel-intensive to perform than other test procedures. The sample can be examined at the earliest the day after its receipt in the OELCHECK laboratory, because the conglomerates often form only in the unshaken, cooled sample. Despite all the aids, filtration requires an experienced laboratory technician, because the conglomerates should not be destroyed.

MPC index – warning values and measures

MPC index Meaning Possible reason Practical action
0 – 15 Normal Typical area of common oil aging with formation of soft particles that do not form varnish-like deposits. Next oil analysis at the usual interval after 4,000 h or 6 months.
16 – 30 Note The limit value above which varnish-like deposits can form, especially at low oil temperatures, will soon be reached. Oil analyses after half of the usual interval. Checking antioxidants (RULER test) and purity. Monitor or replace filter elements.
31 – 40 Attention A very large number of soft particles created by oil degradation begin to be deposited on plain bearings or at cooler points in the lubrication system. Valve sticking can occur especially after standstill. Oil analyses after half of the usual interval. Check filter cartridges and oil system (tank) for deposits. Observe bearings with regard to temperature rise. Inspect microfilters in front of hydraulic valves. Improve filtration or check for suitability.
41 – 45 Critical Additive degradation and oil oxidation as well as too long use of the oil or too high oil temperature led to the formation of particles that formed deposits in bearings, valves or tanks. Oil analyses after max. 500 h or monthly. Improve filtration processes. Clean electrostatically if necessary. Check bearings and gears for deposits. Consider oil change and system cleaning.
46 – 50 Oil change Additive degradation and oil oxidation are very advanced. Deposits have formed in bearings, valves and tanks.

Further use of oil and additional measures such as filtration are no longer useful. The oil should be changed and the system thoroughly flushed. After the oil change, check the success by oil analysis.

> 51 System cleaning The oil is no longer usable due to contamination with particles created by the influence of temperature and oxidation. Deposits have formed all over the system. Flushing in connection with an oil change is no longer sufficient. An oil change with thorough system cleaning, including tank and pipe cleaning is necessary. Analyze oil at short intervals during cleaning and after refilling.