Many lubricants residing in standby equipment have only a few hours of service life. These few hours may be only from occasional scheduled restarts, typically circulating the oil at low load, sometimes barely warming it to operating temperature. In other cases, standby, laid-up and peak-load equipment may sit for weeks or even months without use.
Oil degrades differently in machines at rest. Circulating oil gives legs to both additives and contaminants. When oil moves, additives can travel to remote zones in the system where they are needed to replenish those additives consumed during operation.
For instance, in standby mode there may be a need for fresh rust inhibitor in a servovalve where free water has pooled onto the surface of the valve bore. Unless the fluid is circulating, the large repository of available rust inhibitor sitting in the reservoir can’t reach the valve where it is needed.
A similar problem occurs in a diesel engine that is shut off and put into standby service (emergency standby generator or pump, for instance). Acidic combustion gases can glom onto the cylinder wall right after shutdown, risking corrosion if the overbased additives (detergents) in the crankcase can’t be deployed by oil circulation.
The base number (BN) of the oil in the sump could be a peachy 12 mg KOH/gm, whereas the BN of the oil film occluding to combustion chamber surfaces could be a treacherous 1.5 mg KOH/gm. Without circulation, the oil’s chemical and physical properties are not homogenized, leaving certain zones and surfaces dangerously unprotected – the very regions of the machine where additive reinforcements are most needed.
In circulating fluids, contaminants can likewise be carried to filters and separators where they are stripped from the bulk oil before they are allowed to settle. In stagnant fluids, these contaminants (dirt, water, wear metals, glycol, etc.) are allowed to gather in low zones to form a pasty sludge. This is a chemically active environment, more like a witch’s brew.
Even the most robust synthetic base oils and antioxidants in these low sludge zones may not be able to withstand the temptation to oxidize. Soon the process becomes autocatalytic as free radicals form and then transition to chemically instable hydroperoxides. The infection that results will quickly diffuse into nearby bulk oil and soon the entire oil supply may decay to sludge, gums, resins and acidic compounds.
Another consequence of quiescent oil is an imbalance in contamination control. Filters are supposed to remove contaminants at the same rate they enter the oil from ingression (mass balance). When particles don’t have legs (no circulation means no filtration), they grow in population in the bulk oil. The more open to the environment a system is, the higher the risk of contaminant ingression.
Standby and laid-up equipment exposed to large day-night temperature swings tend to have the greatest potential for particle and moisture ingression. As temperature changes, air is inhaled and exhaled through vents and breathers. Incoming air typically brings contaminants into the headspace where it becomes deposited into the oil below.
Generally speaking, circulating oil means warm oil. Too much warmth is a problem that can contribute to premature oxidation and thermal distress. However, moderate heat can offer favorable qualities, such as:
Better additive solubility – Less of a tendency for additives to precipitate, settle or deposit on machine surfaces.
Reduced water problems - Warm oil keeps water soluble and encourages its evaporation out of the oil. Soluble water and evaporated water are less likely to damage the machine than free or emulsified water. Warm oil also encourages water to shed efficiently in settling tanks.
Better varnish control - Warm oil can actually encourage the removal of surface deposits (sludge and varnish). While this recontaminates the oil, it may be a preferred condition to the alternative. Excessive deposits can restrict part movement, interfere with oil flow to frictional surfaces and impair heat transfer.
The question often arises relating to the justification of performing oil analysis from samples drawn from standby and laid-up equipment. After all, such equipment is rarely put under load, making wear metal trend analysis impractical. What would be the purpose of sampling relatively new oil in machines that remain at rest 99 percent of the time?
With standby equipment, some of the objectives of oil analysis change; however, the three classical categories of analysis still remain: fluid properties analysis, fluid contamination analysis and fluid wear debris analysis.
Within these three categories, the questions we want oil analysis to answer will vary or perhaps expand. Where and how you sample may be of even greater importance. In Table 1 are examples of questions oil analysis can answer from standby equipment samples.
What/How to Analyze
Where/What to Sample
|1. Is rust or corrosion occurring within the machine?||Examine particles and sludge looking for rust and evidence of corrosion.||The bulk oil, particles on filters, bottom sediment.|
|2. Is lubricant excessively contaminated?||Test for specific contaminants in question, (water, particles, glycol).||If fluid is circulated occasionally, sample oil just before the oil filter (upstream).|
|3. Have additives precipitated out of the oil or otherwise depleted?||Baseline new oil for additive types and levels. Analyze used oil and BS&W for similar species.||Bottom sample and bulk oil sample.|
|4. Has varnish formed on machine surfaces?||Run patch colorimetry or similar tests to assess varnish potential.||Sample bulk oil. Examine filter endcaps for varnish. Look for sludge on filter media.|
It is often said that machines at rest are less reliable than those running at full throttle and load. I would generally agree with this statement. Therefore, standby equipment in critical applications really does need special attention.
Carefully think through maintenance practices relating to routine PMs, inspections, relubrication, flushing, contamination control, sampling location/procedures, and oil analysis test slate for these machines. Vigilance in each of these areas could make the difference for that one time, more than all others, when you need a machine to start and work reliably.