MAINTENANCE FEATURE

There are dozens of predictive maintenance technologies, and some have become standards in many industries. Those "standard" technologies include vibration analysis, ultrasound, oil analysis, wear-particle analysis, and thermography. Following are descriptions of the ways maintenance professionals have traditionally used these predictive technologies as well as applications you may not have considered.

Vibration analysis is used primarily with rotating equipment to find problems such as misalignment, out-of-balance conditions, and bearing defects. Prior to using vibration analysis, maintenance technicians had to wait until a bearing failed to realize there was a problem. Using vibration analysis, however, periodic readings are taken and recorded. Maintenance personnel then compare these readings to a baseline. When wear reaches a certain level, the bearing is scheduled for replacement, before it fails. This reduces the amount of reactive maintenance and ensures the replacement occurs with minimum impact on the production or facility schedule.

Other applications include extending vibration analysis to other kinds of mechanical components. For example, some maintenance departments have used vibration analysis to help in isolating problems with belts and sheaves. They also have used the technology on gear drives to help find defects or rapid wear problems. And they have even used vibration analysis to monitor non-mechanical problems in fluid power systems, for example.

While ultrasound is the most common way to detect cavitation problems in hydraulic pumps, reading the vibration signature of a pump can detect the problem, too. Also, restrictions or disturbances in a fluid handling system creates turbulence and unique vibration signatures that can help identify the problem.

Ultrasound functions primarily for leak detection, particularly for steam and air leaks. These leaks can be expensive and many companies let them go unnoticed.

The principle of ultrasound is simple. Most leakage problems produce a range of sounds. The sounds, when properly detected and measured, provide the user with the location and severity of the leak. Common applications for ultrasound include leak detection for pneumatic and other gas systems, vacuum systems, gaskets and seals, and steam traps. Ultrasound also detects valve blow-through.

Since many small leaks are difficult to find simply by listening for the leak, the ultrasound technique helps technicians discover the many small leaks that add up to significant losses over time.

Other applications include the detection of electrical problems. Loose connections in junction boxes and bus bars can be monitored for the sounds of arcing. This technique is useful in power distribution centers and motor control rooms.

Ultrasound also serves for inspections of electrical switchgear and overhead transmission lines, where routine inspection is time consuming and hazardous. These areas are monitored for corona discharge. When the instruments "hear" the discharge, technicians can quickly find the problem with little time wasted. Thus, technicians find small problems before they become critical and cause equipment failure.

Oil and Wear-Particle Analysis. Some people equate oil analysis and wear-particle analysis. Actually, they are two very different technologies. Oil analysis determines the condition of a lubricant. Wear-particle analysis determines the condition of equipment based on the concentration of wear particles in the lubricant.

If, for example, a technician suspects that the lubricant in a gear case contains water perhaps from being left open during an equipment wash down an oil sample could be drawn and checked for water. The decision could then be made to change the lubricant or not, based on the results of the test, rather than than speculation. This ensures that the correct maintenance is conducted not too much (resulting in high maintenance costs) and not too little (resulting in unnecessary breakdowns).

Some companies do not concern themselves with minute quantities of water. Some test for water simply by heating a flat surface to between 200°C and 250°C and sprinkling a small quantity of oil on the surface. If it bubbles and spits, they know that it has too much water in it. This is not as accurate as performing the analysis, since even as little water content as 0.02% can create abnormal wear and rapid deterioration of the equipment. Testing oil samples for water content is not a luxury; it is a cost-effective practice.

To cite another example, consider a gear case that is showing signs of abnormal wear, e.g., noise or overheating. An oil sample could be checked for wear particles. Considering the types and condition of particles found, it is possible to isolate a number of possible problems and their causes, e.g., operating the equipment beyond design speed or capacity or filter failure. Once the problem has been identified, the appropriate maintenance action can be scheduled, again with minimum impact on operations or the facility.

Other applications will entail analysis of a lubricant itself or the wear-particles in the lubricant but there are, nevertheless, some unique applications that employ these tests. For example, wear particles can show when there is insufficient lubrication. "Insufficient lubrication" does not necessarily mean the absence of a lubricant in a system. The lubrication system on an enclosed drive, for example, could have a clogged spray nozzle, preventing proper lubrication from reaching a hard to-inspect area. While the visible part of the drive may be getting proper lubrication, the one area that is lacking lubrication would produce wear particles that indicate that condition. The samples can also indicate conditions such as additive failure, lubricant contamination or excessive loading that exceeds the rating of the lubricant.

Thermography serves primarily to find electrical components that are hotter than normal. Such a condition usually indicates wear or looseness. Thus, thermography allows technicians to perform maintenance on only the electrical components that need attention without requiring that all components get the same level of attention.

In utilities, for example, the correct torque is essential on electrical components to ensure that no heat is generated from a loose connection. Before thermography, it was necessary for each connection in a control panel to be checked manually for correct torque. Using thermography only the connections that are hot receive attention. This reduces the staff necessary to perform preventive maintenance on the connections.

Other applications include the monitoring of outdoor wiring such as overhead transmission lines, which wear due to environmental conditions. Thermography also serves to measure transformer temperatures to find problems indicated when certain areas are hotter than others.  In addition, thermography supports maintenance in industries that have high temperature processes. The technology helps pinpoint areas where refractory material is wearing and allows repairs prior to catastrophic failures.

Another less-used application for thermography is checking coupling alignment without major shutdowns of the equipment. As a misaligned coupling rotates, it generates heat. The greater the temperature difference, the greater the misalignment. Using thermography, maintenance personnel can observe the temperature rise across a coupling. Some companies have used this technique long enough to have developed profiles of the temperature rise for each type of coupling. Using this profile, they can determine the amount of misalignment (not what plane it is in). Then, the technicians can proactively schedule the coupling for realignment.

Making decisions
In some cases, production or operations personnel protest when the maintenance department runs a predictive analysis on a critical piece of equipment and wants to take it down for a proactive repair. When this happens, why not get a second opinion?

Many predictive techniques can be used to check for the same problem. For example, a problem indicated by an oil sample could also be checked by vibration analysis or thermography. An electrical problem indicated by ultrasound can also be checked by thermography. By crosschecking using two predictive technologies, all departments can be satisfied that they will make the right decision.

Engineer’s Digest February 1997