Strategic Oil Analysis: Systems, tools and tactics

Tribology & Lubrication Technology, Apr 2009 by Johnson, Mike, Spurlock, Matt

Cycle time From incipient Failure to loss oF machine Function. Having data regarding the potential time to catastrophic failure helps determine the optimum oil sample interval. While most equipment can be served effectively with quarterly oil sample intervals, some instances may require shorter intervals. Additionally, a hybrid approach may be taken by which a quarterly sampling is conducted until evidence of a problem surfaces and the interval is shortened. Either way, the time to failure after initial detection is an important piece of information in fine tuning the oil analysis program.

SAMPLE COLLECTION METHODS

Sampling technique is an important part of any type of testing activity, whether machines are involved or not. Sampling machine conditions, regardless of the type of testing involved, demands that the sample process itself be dependable.

Dependable sampling technique requires that the sample process itself does not influence the quality of the information collected. The sample must be an accurate representation of the actual conditions and must be repeatable (consistent). Accomplishing these two considerations with oil sampling is not inherently difficult but can be made so if the machine is not prepared for accurate sample collection.

Sample collection from an oil drain line and/or from the top of the reservoir with a suction tube produces poor quality and unrepresentative data, even if these are common methods. To overcome the variability and lack of quality created with these methods, the sampled machines need to be retrofitted with fixed sample ports, and the methods for collection should be very clearly defined.

Sample collection from a circulated oil system is prone to similar data error. Laminar flow conditions can have appreciable impact on the distribution of material collected when samples are taken from non-turbulent flow conditions.

TEST SLATE AND ALARM SETTINGS

Once the appropriate equipment has been selected and a dependable routine for sample collection defined, a determination must be made as to what properties should be monitored. With a properly designed test slate, oil analysis is capable of diagnosing and addressing changes in three core areas: machine health, lubricant health and contaminant level.

The primary and secondary slate flows from understanding prevailing failure modes for the selected machine types and component surface interactions. Once the component failure modes are identified, the tests that best reveal the presence of a failure mode are adopted. We'll discuss test slate selection in future articles in this series.

Each test measures for an increase or decrease of a property. Alarm settings are used to bring attention to a rise or fall of specific criterion. An oil health criterion such as viscosity represents a potential failure mode if it is either too high or too low, so viscosity has alarm settings for both rising and falling numbers. Wear debris represents a potential failure mode only when it is rising. Consequently, wear debris alarm settings apply to numbers that are increasing.