MPC varnish potential testing (ASTM D7843) is an essential analytical test to determine the propensity for a lubricant to form varnish deposits. With the probability of varnish-related failures reported to be as high as 100% (GE TIL 1528-3), monthly MPC testing is recommended for all critical turbine installations. The ASTM approved MPC test is straightforward and can be completed as part of your existing lubricant analysis testing program. It can also be performed on-site using a modified test method for rapid assessment of potential varnish-related problems.

MPC Varnish Potential Testing Overview

There are two main parts to the MPC test: filtration and color measurement. During the first part, 50 mL of the lubricant to be tested is diluted with an equal volume of petroleum ether. This mixture is then filtered through a 0.45 μm nitrocellulose patch which is then rinsed with petroleum ether and dried. The intensity and color of the patch are then measured against a control patch using a spectrophotometer that calculates the color difference, or ΔE value. The ΔE value and the corresponding propensity for the formation of lubricant varnish deposits are then assessed according to an MPC scale.

MPC Scale

EPT Clean Oil’s MPC scale is then used as a guideline to help users assess their lubricant’s potential for varnish formation. There is three levels on the varnish potential scale: Good (∆E<15), Elevated (∆E 15-35) and Critical (∆E>35). The higher the MPC value, the higher the amount of varnish deposits and precursors dissolved in the lubricant and the greater the propensity for the lubricant to form harmful varnish deposits.

Operating Target MPC Value

Ideal lubricant operating condition (from a varnish perspective) is achieved when the MPC ΔE value is <15. When the MPC ΔE value is continually maintained at below this value, the lubricant‘s solvency characteristics are maximized, which prevents varnish deposit formation and forces existing deposits back into their less harmful dissolved form.

MPC values increase as dissolved oil breakdown products (varnish precursors) accumulate. These breakdown products are produced by oxidation and begin to accumulate from the first moment that the lubricant goes into service. Each lubricant has a finite capacity (saturation point) to hold these dissolved oxidation products in solution. Once this point is reached, the lubricant becomes saturated and excess breakdown products are forced from the fluid, forming harmful lubricant varnish deposits.

Once formed, lubricant varnish has a natural attraction for metal surfaces because of the polar nature of both the varnish and the metal. As varnish begins to coat metal surfaces, mechanical clearances are reduced, decreasing equipment performance and, ultimately, leading to failure.

Complicating matters, the saturation point varies with lubricant temperature, pressure and flow. These parameters are dynamic in most mechanical systems. For this reason, varnish is often deposited in sensitive mechanical areas prior to appearing in the oil reservoir. It is, therefore, important to select a varnish prevention program that addresses the issue during normal turbine operation when the varnish precursors are dissolved in the oil.

When MPC values are maintained at ΔE <15, the lubricant contains very few varnish precursors and is not prone to deposit formation. Moreover, the lubricant will have high solvency characteristics under all system operating conditions and will actively remove any existing varnish deposits from metal surfaces.

Users often complain that before using Group II base-stock oils, they didn’t have a problem with varnish. As a result, Group II oils have a reputation for being varnish prone, leading users to search for alternatives. The market has responded, with many companies offering low-varnish versions, co-base stock oils, synthetic oils or even after-market additives for the more adventurous.

The truth is, Group II oils do have less capacity to hold breakdown products in solution than Group I. This is because breakdown products are polar in nature and on the spectrum of polarity, Group II base stocks are less polar (more non-polar) than Group I base stocks. Therefore, because “like dissolves like,” polar breakdown products are more likely to precipitate out of solution from more non-polar Group II oils than Group I resulting in varnish deposits.

To understand how these oils breakdown over time, we ran thermo-oxidative breakdown experiments in the lab subjecting each oil sample to peak turbine operating temperature (150°C), air and a copper catalyst. The results presented include acid number, LSV amine and phenol antioxidants and MPC varnish potential (Fig. 1 & 2). 

In the Group I sample, we see a rapid increase in varnish potential achieving a critical value in <5 hours. Acid number remains stable until approximately 80 hours when a rapid increase is observed corresponding with depletion of the amine anti-oxidants. While a Group I user may not change this oil because of MPC value, they would be forced to because of the significant increase in acid number. In the Group II sample, varnish potential values increase gradually, not hitting a critical value until 400 hours. Strikingly, we see no increase in acid number with amine life extended to 700 hours – 10x that of the Group I sample. 

Overall, we see a Group I base-stock oil that is sensitive to oxidation that quickly reaches critical varnish potential values and a Group II oil extremely resistant to oxidation with no acid production over the life of the test, even after the antioxidants are consumed. While some users may discount this lubricant because of the varnish potential numbers observed, in actuality the performance is exceptional. Considering that resin-based varnish removal systems are very effective, the perceived limitations of Group II oils are misunderstood.

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Users often complain that their turbomachinery lubricant is not performing well because the varnish potential (MPC) number is elevated, or their oil is oxidizing when additives are still present. This drives users to search for a different lubricant or after-market additives when the problem lies with the expectation being placed on the oil – namely that little to no maintenance should be required.

The misconception is that only poor quality lubricants oxidize, and that varnish is a result of improperly formulated oils. While lubricant quality is a key criteria, in reality all lubricants break down due to oxidation – even with additives – and increasing varnish potential over time is normal when dissolved oxidation products are allowed to accumulate.

Rather than discussing more effective varnish removal systems, vendors are now moving the discussion toward synthetic oils and after-market additives to extend the operating range of lubricants with high MPC values. While there is a place for advanced lubricant technologies, simple and more cost effective solutions are available using conventional lubricants to permanently remove dissolved oxidation products and maintain ultra-low MPC values.

EPT Clean Oil published a technical paper that discusses varnish formation in depth. Oxidation starts on the very first day a lubricant is put into service. The oxidation process creates a dissolved breakdown product, or varnish precursor, that will accumulate in the oil over time until it becomes saturated. Once a saturation point is achieved, these varnish precursors physically convert from the dissolved form into solid deposits.

The saturation point varies throughout the mechanical system due to differences in temperature and pressure, which dictates where varnish deposits will form first. For this reason, it is important to keep the lubricant in an unsaturated state by removing the varnish precursors. By doing so, you can prevent the lubricant from becoming saturated at any temperature or pressure present in the mechanical system and prevent varnish formation.

The below short video animation, offers further information on the varnish formation cycle.

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