Predictive Maintenance That Works

This is the sixth in a series of predictive maintenance (PdM) articles that began in the April 2011 “Focus on O&M” in which the essentials of PdM were introduced. In this occasional segment, we have explored specific PdM techniques, such as motor-current signature analysis, oil analysis and thermographic analysis and their routine use, and ultrasonic and vibration analysis. In this issue we look at lubricating oil wear-particle analysis.

Wear-Particle Analysis

Wear-particle analysis can be used to determine the type, location, and severity of component wear occurring within the lubrication or hydraulic system of a machine. This information can be used to troubleshoot suspected machine problems and project an estimated time to failure. The technique allows equipment wear problems to be identified prior to serious deterioration or failure so that repairs can be made on a planned basis during scheduled downtime. Wear-particle analysis can be used on machines that have a circulating oil system, including steam and gas turbines, generators, hydraulic systems, diesel and gasoline engines, gearboxes, and compressors. For the most accurate results, samples should be taken from an active, low-pressure line, ahead of any filtration devices. For consistent results and accurate trending, samples should be taken from the same place in the system each time; using a permanently installed sample valve is highly recommended.

Most independent labs supply sample containers, labels, and mailing cartons. If the wear-particle analysis is to be done by a lab, all that is required is to take the sample; fill in information about the machine number, machine type, and sample date; and send it to the lab. Results are normally available within 24 to 48 hours of receipt of the sample at the lab. If the analysis is to be done onsite, analytical equipment must be purchased, installed, and standardized. Once a sample is taken, information about it is entered on the form, it is taken to the onsite lab, and the results are available as soon as the analysis can be scheduled.

If an in-line measurement device is used, a permanent sample port or monitor must be installed. Wear-particle concentrations then can be monitored continuously and the results of the analysis made immediately available. Some instruments allow data to be downloaded directly to a computer for trending and further analysis.

Wear-particle analysis can be used to determine the number, size, composition, and shape of ferrous and some nonferrous wear particles. Some techniques also permit visual examination of the collected particles, which can include additional nonferrous materials such as seal or gasket material. The presence of large ferrous wear particles can indicate severe wear; and the presence of large quantities of small wear particles may be important if there are close-tolerance valves whose surfaces can become eroded. One analysis technique calculates the ratio of small to large particles, noting that when this ratio changes significantly, a severe wear process has begun. Another calculates the total weight of wear particles present in the sample and trends that value as an indicator of the severity of the wear process. A third technique measures the concentration of ferrous wear particles, noting that an increase in concentration is often indicative of the start of a severe wear process.

Spot-checking is used primarily when someone suspects that the equipment has a severe wear problem. The resultant data can be compared with data from similar machines that have previously been identified as having a wear problem and those that are operating normally. If the suspected machine has a sufficient number of large or severe wear particles, further examination and corrective action should be taken immediately. Continuing to operate equipment with high levels of wear particles can substantially increase component wear and contribute to a severe failure.

Periodic analysis can provide a more subtle indication of gradual component wear and the transition from normal wear to a critical or severe wear process. This advance notice of potentially serious wear problems means that repairs or replacements can be planned in advance and be made at a convenient time.

Monitor Machine Wear Trends

Monitoring oil condition over six or eight sample periods can indicate improper maintenance or repair practices. These can include: improper installation or removal of bearings, inadequate lubrication, the failure to properly flush out a system after repair, improper installation or repair of seals and packings, improper fluid- or lubricant-handling procedures that introduce water or dirt contamination, or improper filter-handling or replacement techniques. Consistently excessive wear in the same machine or machine type can also indicate a poor design. This can include bearings or gears that are not adequately sized for the speed or load, inadequate lubrication, or inadequate filtration.

Wear-particle analysis is limited by the consistency and timeliness of the sample. The value of trended information diminishes quickly if samples are not taken from the same place on the machine each time, and the longer a sample sits before it is shipped and analyzed, the less significant the data will be. These concerns can be diminished by using an onsite lab or a diagnostic system with a permanently mounted sample port or sensor. For critical or expensive equipment, the added hardware and training cost can be a very cost-effective investment.

More Coming

In the next segment of “Predictive Maintenance That Works,” we’ll continue our discussion of specific nondestructive testing condition-monitoring techniques used at power plants and why each should be a part of your PdM program.

— Dr. Robert Peltier, PE is POWER’s editor-in-chief.