By Teresa Hansen, Associate Editor
Predictive maintenance (PdM) involves monitoring equipment over time for conditions that indicate impending failure, determining whether corrective action is required and, if necessary, taking action before equipment failure. PdM is especially important to power generation facilities because so many are operating beyond their original design lives.
PdM technicians identify critical components and systems, determine how often they need to be monitored, set up inspection routes and schedules and regularly measure key indicators. They then compare those measurements over time, looking for changes in operating conditions that indicate potential breakdowns.
An effective PdM program can include several monitoring and measuring methods, including infrared temperature measurement, vibration analysis, oil analysis, ultrasonic testing, electrical measurement, power quality measurement and insulation resistance.
Infrared thermography, or thermal imaging, is another effective PdM technology. A July 2005 study conducted by Acuity Market Research Inc. for Fluke Corp. found that more than 60 percent of engineers and managers working in the U.S. industrial maintenance field used infrared thermography as a PdM tool. This was a 57 percent increase in use during the five years leading up to the study. Fluke Corp. expects this trend to continue, especially in the power generation industry where electric utilities are the largest users of thermal imaging.
Most power plants have been using infrared thermography in their PdM programs for some time. Engineers and maintenance personnel are familiar with annual infrared thermography surveys as part of PdM. Now, however, they are finding they can use the technology year-round to inspect components and measure key indicators.
Power distribution equipment is inspected with a Fluke Ti30 Thermal Imager prior to a scheduled maintenance outage.
This use of infrared thermography as a PdM tool is being driven by a new generation of easier-to-use, more powerful and more affordable infrared cameras. A mid-range thermal imager with enough pixel count, accuracy and temperature range to meet various application criteria can be used by power plant personnel to troubleshoot problems, track critical equipment more closely and follow up repairs after the annual survey is performed. Infrared thermography can be an effective PdM tool for several important power plant components and systems, including:
Power distribution. Consistent, reliable electricity delivery is one of a power generator’s highest priorities and infrared thermography is a viable method of regularly monitoring power distribution equipment. For example, one particular power plant inspects 2,300 V and 4,160 V breakers and transformers with a thermal imager, identifying problems prior to scheduled maintenance outages.
Switchyard. At many power plants, switchyard inspections are performed during periods of maximum load, even though local conditions at that time of day can mask serious problems. At the power plant mentioned above, switchyard inspections are normally done before dawn to avoid solar reflections and wind effects. The load is lighter but the air is usually calm, so any observed problems are almost certain to be significant.
Boilers, pipes, traps and valves. While efficient electricity delivery is essential, efficient production is equally important. In a thermal power plant, when steam valves leak or fail, high energy content steam or water blows through to the condenser. This represents money down the drain. A thermal imager can be used by plant maintenance staff to regularly scan pipes, valves and traps to identify problems early on and thus control costs.
Because energy losses are not limited to steam lines, infrared thermography can also be used for boiler inspection to identify areas of insulation breakdown. Hot areas on boiler walls indicate areas of worn insulation and energy losses. Infrared thermography helps identify these areas so they can be repaired during the next maintenance outage.
Switchyard inspections using thermal imaging can identify transformer hot spots.
Motors. At most plants with in-house imagers, nearly all infrared analysis is qualitative and comparative-that is, similar pieces of equipment are examined under similar load. In advanced PdM systems, various system components/equipment may have their own monitoring program. One example is pulverizer motor inspection. Consider a power plant that has 27, 400- to 500-horsepower motors driving pulverizers that feed the boilers. The plant has a motor casing monitoring program and regularly examines the case temperature for each motor. The National Electrical Manufacturers Association temperature ratings are posted on the motors’ nameplates, indicating baseline operating temperatures. The normal temperature is approximately 120 F to 140 F, depending on ambient conditions. A 40-degree temperature rise usually indicates the need to clean the filters. When the rise exceeds 40 degrees, it indicates that the motor needs to be cleaned and reconditioned. Assuming the motors are all about the same size and operate under similar loads, it’s a fairly simple matter to use in-house imagers to identify “hot” motors and take corrective actions.
Infrared thermography helps identify maintenance needs, but prioritizing the problems requires evaluating many factors. The most significant problem may not be the one with the hottest apparent temperature. Other factors include equipment criticality, total repair/replacement cost, safety concerns and lost production costs.
Basic vs. Advanced Thermography
Much of a thermal power plant’s equipment can be inspected using comparative infrared analysis. In many cases, a power plant will hire an outside company to conduct its annual survey so that it has professional thermal images of all critical equipment with which to compare during the year.
Bringing advanced and intuitive thermography capabilities to the power plant level involves challenges, primarily in interpreting, identifying, locating and correlating details within individual infrared images produced during the course of routine maintenance surveys. While issues like emissivity are minimized by dirty metal surfaces, other issues like reflections, convective losses due to wind and other conditions can lead to erroneous conclusions. More advanced infrared thermography involves learning the principles of heat transfer, reflectance (mirrors), emittance (walls) and transmission (windows). Once these principles are mastered, effective analysis makes the technology and the images it produces useful for accurate and timely PdM.