By Jeanna Van Rensselar
An acceptable oil analysis program includes:
- Standard processes for the establishment of normal ranges
- The identification and revision of limits
- A reliable system for recognizing failure modes
An oil analysis report may contain findings on up to 40 parameters, identifying each acceptable range. Various elements factor into the results: the type of oil, its formulation, how it should be maintained and the operating conditions. The type of equipment and its operating environment is also critical.
Most labs make recommendations in the form of green, yellow, and red flags. Yellow and red flags mean action is required; green flags are monitored for future trends.
There is not a universal standard for setting flag, or alarm, parameters. Also, some lab report stats are not critical to that specific piece of equipment. So not every oil analysis parameter needs an alarm limit (see Figure 1)1.
Because there are so many considerations involved in oil analysis, most labs do not publish limit information. This can leave end-users wondering who to believe—formulator, OEM, lab, industry, and/or association such as ASTM.2
Absolute and trend limit-setting methods
There are four traditional absolute methods for determining oil analysis ranges and flagging limits. According to Ray Garvey,3 these methods are:
- Industry standards. These are generic limits placed on machines that are grouped according to working pressure or type (e.g., gearboxes or hydraulic systems). These standards are generally a starting point, while limits set by industries and associations often involve equipment with strict safety and reliability requirements.
- Statistical alarms. These are based on common distribution functions that are usually built into software—either the cumulative distribution function or the Gaussian distribution.
- Trend-based or rate-of-change limits. These involve mathematical methods for defining unacceptable departures from trending levels.
- No predefined limits. These are judgment-based, relying on the expertise of analysts familiar with the type of machinery being monitored.
The Criticality Index: How to choose which machinery to monitor5
The Criticality Index determines the amount of condition monitoring required for a piece of equipment. It considers such factors as:
- The importance of the machine’s function
- Can another piece of equipment take over the function if that piece of machinery fails?
- The overall impact of downtime
- The projected repair cost
This index assigns a category to all machines:
- Critical machinery. The rest of the operating environment cannot function without these machines (i.e., power plant turbines). Equipment in this category requires complete online and (where possible) inline condition monitoring—regardless of cost.
- Essential machinery. While key to the operating environment, the failure of this equipment does not cripple operations. Experts recommend testing in order to prevent costly repairs and inconvenience.
- General purpose machinery. The balance of operations equipment falls into this category. Informal and periodic monitoring is usually sufficient.
The four absolute methods described above are most effective when:
- Operators know the failure modes and root causes or
- oil property requirements are closely defined and a change can be directly connected to operating conditions.
These static absolute limits are acceptable in most circumstances. But OEMs also set dynamic trend limits that also take operating hours or distances into account. This is due to the trend toward longer oil drain intervals and the increasing predictive maintenance practices. To determine the trend limit, the OEM will identify the acceptable change of a parameter over time.
A combination of both methods is often the best approach.
From the formulator’s perspective
Oil companies tend to develop flagging limits that focus on the oil’s condition and when the oil’s useful life is over. Phillips 66 Lubricants’ formulation experience and familiarity with the chemicals involved help them understand flagging limit settings for a particular lubricant family.
Flagging limits are always monitored and adjusted with industry upgrades. A good example is when the diesel category upgraded to API CJ-4 and mandated lower sulfated ash content. This industry change resulted in lower starting base number (BN) in the lubricant.
According to STLE-member Shawn Ewing, technical coordinator commercial lubricants for Phillips 66 Lubricants, typically flagging limits utilized by commercial oil analysis labs have specific purpose.
“As a manufacturer of lubricants, Phillips 66 is concerned with the properties of the fluid itself,” he explains. “Viscosity, oxidation, nitration and contamination, for example, are indicators of how well the lubricant is doing in service.”
The lab’s perspective
“Laboratories generate lots of data; a sophisticated process for analyzing the data is required to turn it into relevant and actionable information,” Bryan Debshaw of POLARIS Laboratories® says. “A reputable laboratory is able to separate data by equipment manufacturer and model, lubricant product and viscosity grade and the operating environment, among other factors and considerations. They are able to offer a more precise maintenance recommendation by having different sets of flagging limits based on these and other factors. Laboratories can even set progressive flagging limits with multiple tiers so operators can make their own decisions on extending fluid life or performing preventive maintenance.”
Debshaw advises that instead of focusing on the flagging limit for an individual test, the focus should be on the maintenance recommendations in the oil analysis reports (see Figure 2).
He explains that in order to maximize the value of fluid analysis, it is essential to discern when minor corrective action can prevent major repairs. The most basic level of oil analysis catches wear before component failure, but progressive flagging limits allow maintenance to mitigate the damage caused by wear metals.
Having a multitier system identifies:
- when data is normal
- when the data trends should be monitored more closely
- when secondary diagnostic tools and minor corrective maintenance is appropriate
- when severe conditions indicate component failure is eminent
STLE-member David Doyle, general manager of ALS Tribology, explains that ALS encourages clients to discuss test results when they have a question. “Many times, the value of the information provided on a test report can be enhanced tenfold when there is a conversation between the client and the laboratory,” he says. “This benefits both parties.”
Common flaws in setting limits4
Common errors that can result in the misuse and misinterpretation of limits include:
- Too many limit levels. Some condition monitoring programs include so many limit levels that recommendations at each level are blurred. Most labs use at least the following three levels: abnormal, warning, and critical.
- Poorly defined actions at each limit level. If end-users don’t have clear actions to take when the limit is exceeded, serious conditions can be missed and benign conditions can be exaggerated.
- A confusing set of parameters. An overly complex list of measurements may mean users don’t have a clear understanding of what is required, so they often do nothing.
- Unknown limit justification. Limits set at the beginning of a machine’s life can become meaningless or irrelevant due to the evolution of measurement techniques or the operating environment.
Debshaw concludes, “Open, collaborative communication between end-users, equipment OEMs, lubricant formulators and independent laboratories is essential for maximizing equipment uptime and savings.”
This article represents only highlights from a larger feature piece – find additional insights, such as the OEM’s and end-user’s perspectives in the full Tribology & Lubrication Technology article: Unraveling the mystery of oil analysis flagging limits.
References
- POLARIS disagrees with this, commenting, “Some parameters are more important than others, and while every parameter may not be understood by the end-user, a competent lab should be able to explain and support the alarms for each parameter.”
- ASTM D7720-11
- Machinery Lubrication: 4 Easy Ways to Set Smart Oil Analysis Alarms
- Questioning How Limits Are Set, by Andrew Becker, Paul Marsden, Sylvester Abanteriba and David Forrester. [PDF].
- Van Rensselar, J. (2011), “Vibration analysis: The other half of the equation,” TLT, 67 (8), pp. 38-48.
About the Author:
Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.
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What are the parameters for oxidation results. My engines show 18 and 20 respectively. At what number does the test indicate a move to “caution”