To successfully integrate oil analysis into your machine reliability practices, you must first understand how lubricants degrade. That point was made exceedingly clear by lubricant expert Sanya Mathura in a January 2021 webinar for Fluke Reliability.
Mathura, managing director of Trinidad-based Strategic Reliability Solutions, detailed the six ways lubricants degrade and offered advice on handling each scenario to prevent machine damage. Her presentation began with her list of why we use lubricants for machine maintenance:
- Reduce friction
- Minimize wear
- Distribute heat
- Remove contaminants
- Improve efficiency
From the time the lubricant enters a piece of equipment, it starts degrading as part of its function, Mathura says. “The question is: when has it degraded to the point where it can no longer protect the system?”
Also, did the degradation occur normally, or was it exacerbated by the machine operating conditions? To answer this, you must understand what is going on in your machine and the failure modes associated with lubrication degradation.
Identifying lubrication degradation modes
Oil analysis is one of five measurement methods on the P-F curve for determining premature asset failure. It consists of a laboratory analysis of a lubricant’s properties, suspended contaminants, and wear debris. It can provide meaningful and accurate information on lubricant and machine condition when performed as part of predictive maintenance.
Figure 1 illustrates the six primary lubrication degradation modes, according to Mathura. Several have associated environmental triggers. “If you know which mode is occurring,” she says, “then you can prevent [machine] damage and reduce how often it happens.”
Figure 1. Top six lubrication degradation modes
- Oxidation: This is often assumed to be the most common cause of lubrication degradation. Oxidation occurs when oxygen is introduced to the base oil, forming aldehydes, ketones, hydroperoxides, and carboxylic acids–-elements for which oil analysis tests. Oxidation causes varnish, sludge, increased viscosity, base oil breakdown, additive depletion, and finally, loss of the oil’s antifoaming properties.
- Thermal degradation: If system temperatures exceed 200 degrees Celsius, the typical thermal stability point of lubricant, thermal cracking will occur. The excessive heat and cracking involved causes molecule shearing, decreased viscosity, and polymerization. When molecules shear, they can either volatize, leaving no deposits, or condense, causing dehydrogenation and producing lacquer and coke deposits. Decreased viscosity is the primary indicator of thermal degradation, compared to the increased viscosity occurring during oxidation.
- Microdieseling: Also known as compressive heating, this degradation mode is a form of pressure-induced thermal degradation. As entrained air moves from low to high pressures, it can reach over 1,000 degrees Celsius, dramatically reducing the oil’s lifespan. When the bubble interface becomes carbonized, the oil darkens rapidly and produces carbon deposits. With high implosion pressure, microdieseling produces soot, tars, and sludge. With low implosion pressure, look for varnish from carbon insolubles, including coke, tars, and resins.
- Electrostatic spark discharge (ESD): This mode induces temperatures higher than 10,000 degrees Celsius and produces different deposits from other thermal changes. In advanced stages, look for varnish and sludge, as well as elevated fluid degradation and the presence of insoluble materials. “When ESD is happening,” says Mathura, “a lot of people say they can actually hear the static crackling noise.” They also often see burnt membranes on the oil filter where the sparks discharge.
- Additive depletion: This represents situations where additives unexpectedly drop out of the oil and react with other components. Two types of deposits generally occur—(1) organic (rust and oxidation additives drop out and react to form the primary antioxidants that show up in oil analysis) and (2) inorganic (the additives don’t react).
- Contamination: This is included as the sixth mode because any foreign object introduced to the oil can act as a catalyst to different degradation types. The root cause is the foreign object, not oxidation or one of the other modes.
Lab tests per degradation mode
Engineers select different oil analysis lab tests depending on the lubricant degradation scenario. Mathura reviews each test in detail during the webinar. Here’s a summary:
|Oxidation||Acid number, color, FTIR, MPC, Ruler & RPVOT|
|Thermal degradation||Viscosity, color, FTIR|
|Microdieseling||Visual inspection of components, FTIR & QSA|
|ESD||Filter inspection, FTIR & QSA, Ruler, DGA|
|Additive depletion||FTIR & QSA, color, QSA, Ruler & RPVOT|
|Contamination||Color, presence of water/fuel/coolant, or any foreign material|
Start by evaluating the conditions observed: viscosity, color, acid number, presence, water, or fuel. Then try to determine the degradation mode: Oxidation, contamination, thermal degradation, microdieseling, ESD, or additive depletion? Conduct tests to verify the mode, which will, in turn, lead to the root cause.
The degree to which you must pursue root cause analysis depends on asset criticality. Like any other reliability engineering best practice, balance the machine’s criticalness against the effort required to determine the degradation’s root cause.
Mathura also discusses in the webinar what counter-measures to consider per degradation mode.
“The role and importance of lubrication degradation have changed over the last decade, following larger changes in the industry,” she says.
“Our machines have changed a lot, and when machines change, lubrication changes. You have different clearances, different OEM specs, new environmental regulations – all of that means practices have to evolve. We can’t apply procedures for a 20-year-old machine to newer models.”
For more information on the root causes of lubricant degradation and how it affects your machines, watch Mathura’s webinar, “Root causes of lubricant degradation and how to prevent it from harming your machines.” Consult her website for additional resources.
Lubrication Degradation Mechanisms: A Complete Guide by Sanya Mathura