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Used oil analysis on gas engines

The objectives of used oil analysis (UOA) include optimising engine protection for asset investments; increasing overall operating efficiency by reducing total cost of ownership and increasing running times; indicating wear that has occurred to the engine and providing a guide to maintenance teams of the condition of certain components in the engine; and providing a clear guide to the remaining useful life of the oil.

By effectively analysing used oil, an assessment of the condition of both the lubricant and the machinery can be made.

The useful life of the oil can be affected by the type of fuel gas that is being used; the operating load of the engine; the type of lubricating oil and the oil injection rate; and, lubricant sump size and sump temperature.

An engine with a large sump will have a longer oil life than the same engine with a smaller sump, if the other factors are constant.

Original equipment manufacturers (OEMs) all have different methods of determining when the lubricant in a sump should be changed, but all of the different equipment manufactures agree that the vital three key indicators to be checked are:

Total base number (TBN)
* Most OEMs agree that a reduction of between 50-70 per cent from the initial TBN indicates that the oil should be changed
* TBN is a measurement of the capacity of the oil to neutralise the acids formed during combustion. It is an indicator of the oils

Total acid number (TAN)
* An increase in 2-3 milligrams of potassium hydroxide per gram (mg KOH/g) from the initial TAN indicates the oil should be changed
* TAN is used to estimate the amount of acidic contamination and oxidation in the oil

Viscosity of the lubricant
* The OEM recommendations should always be consulted as acceptable increases in viscosity do vary, but an increase of between 20-30 per cent from the initial viscosity is a strong indicator that it is time to change the oil.

Typically, as the oil disperses contaminants produced by combustion, the TBN will reduce. OEMs specify the amount this TBN can reduce by before the oil should be changed. As the oil oxidises, its TAN will increase.

Taking a sample

The sample should be representative and repeatable. Best practice is for the same operator to take the sample from the same place under the same conditions. The sample should be taken from a high flow area, when the engine is operating. Five times the volume of the sample port should be discarded before taking a sample into a clean, labelled bottle.

From a health standpoint, UOA is classed as hazardous and skin contact should be avoided.

Benefits of UOA

For engines with smaller sumps, UOA helps to achieve optimum drain intervals.

Overall operating efficiency improvements are possible when drain intervals coincide with routine maintenance activities. That is, if the oil lasts 2,000 hours and there is a routine maintenance activity at 2,000 hours, overall equipment efficiency will be higher than if the oil has to be drained at
1,000 hours or 1,500 hours.

For gas engines with larger sumps, the drain intervals are typically longer and UOA will identify when the oil needs to be drained, and also help optimise the oil addition rate for engines.

Potential problems in engines can be detected early by identifying various wear metals in the UOA sample:

* Piston and cylinder liner wear: traces and increasing levels of iron, chromium aluminium and molybdenum
* Cooling water leakage: traces of copper, lead tin and aluminium
* Oil cooler corrosion: traces of copper
* Lubricants additive elements: ensures additive elements are at the correct levels to provide the protection for gas engine equipment.

In summary, UOA of a gas engine oil helps to optimise the value of engine oil purchased, and maintain maximum overall equipment efficiency of the engine.

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