While conducting a routine inspection with an ultrasonic instrument at a South Carolina glass manufacturing plant, Larry Wilcox detected a problem with a 20-year-old piece of switch gear. By the manufacturer's standards, it had already outlived its life expectancy.
"In a best case scenario, if the malfunctioning equipment had caused even one circuit to burn out, the plant would have had to shut down to repair the equipment," said Wilcox, President of Predictive Maintenance Group (PMG) in Johnson City, Tennessee. "When the cost of lost production time is added to the cost of damaged equipment and repairs, the company could have lost about $100,000. Fortunately, predictive maintenance was at work, and the customer replaced the part at its convenience. Repairs cost the company barely $1,000!"
But the worst case scenario, according to Wilcox, is when something blows which can cost a company from $100,000 to a million dollars. The damage can even be catastrophic.
An Integrated Approach to Predictive Maintenance
Wilcox believes in using all available technologies to diagnose and detect mechanical problems. Thus combining ultrasound and infrared thermography has become PMG's signature approach to predictive maintenance.
"It seems lackadaisical for a predictive maintenance specialist to rely on just one technology," Wilcox said. "When a mechanic works on your car, he has more than one wrench in his tool box; it's filled with many different tools he needs to get the job done. We do the same with electrical inspections. With ultrasonics and infrared thermography, an inspector has a greater chance of detecting a problem before it escalates and gets out of control."
In the case of the glass manufacturer, there are several 13.2 kilovolt (kv) transformers that drop voltage down to a power level that is more practical for the plant. Each transformer has three high-voltage hot leads coming into the switch gear. These leads connect or disconnect the system's electricity. Wilcox discovered that high voltage had started to cause corona and had built up ozone around the high-voltage wires. Ozone had ionized the surrounding air and deteriorated the unit.
Infrared Thermography, One Line of Defense
To diagnose the situation, Wilcox turned first to infrared thermography which involves a complicated and expensive portable non-contact thermometer that works like a camera to provide real-time live pictures. PMG employs the technology on inspections to penetrate the skins of cabinets, load centers and motor control centers. He locates high-resistance connections by the increased amount of heat present, a sure indication of a problem. Every time a circuit goes on it warms up a metal clip attached to the top and bottom of a fuse. Repeated warming up and cooling down periods cause the clips to lose their tension. This allows corrosion to creep in between the two conduct surfaces. As this builds up it forms a resistance that impedes the flow of current. As the current starts to back-up, heat is generated.
It becomes a vicious cycle," Wilcox explained. "The more heat produced, the more resistance that develops. And electrical problems never cure themselves on their own; they get increasingly worse. Sooner or later the problem must be addressed."
Ultrasound is Like a Crystal Ball
However, while an infrared instrument detects heat, it is not sensitive to the three most destructive things that can happen in an electrical system -- corona, arcing and tracking (a miniature arc looking for a place to become a full grown arc). Infrared thermography will not detect these problems in their early stages, first because the heat generated is non-existent or minimal, and second because the technology is blind to what is going on behind a sealed cabinet. Infrared is a valuable tool only when corona, arcing and tracking go undetected and the condition exacerbates causing a galloping increase in temperature.
An ultrasonic instrument (such as UE Systems, Inc.'s Ultraprobe 2000), on the other hand, picks up the sound of corona, arcing and tracking even above the ambient noise of the manufacturing plant and has the ability to "see" through walls. This makes it an ideal tool in early detection of potentially destructive electrical distrubances. It pinpoints problems while they are still in their infancy, when repairs are easy to make and inexpensive.
An ultrasonic instrument detects the high-frequency noise produced by electrical discharges and translates it, via heterodyning, down into audible ranges. An inspector listens to the specific sound quality of each type of emission over headphones while he observes the intensity of the signal on the instrument's meter. Normally, electrical signals should be silent, although some may produce a constant 60 cycle hum or some steady mechanical noises. These should not be confused with the erratic, sizzling frying, uneven and popping sounds of an electrical discharge.
"As corona builds up it deteriorates the insulation of the cable, an extruded strand shield whose function it is to contain the voltage within the cables," Wilcox explained. "Eventually you have a high-voltage cable with insufficient insulation and a grounded cable nearby. As long as the two cables don't touch it's fine. But when a high voltage cable makes contact with a ground cable it results in arcing which either shuts down the equipment or causes a circuit to burn out."
Left undetected, the situation might have become catastrophic. Wilcox explained. "Thirteen thousand volts can jump to ground and cause an explosion. When corona occurs adjacent to two or more pieces of cable also carrying 13,000 volts, it could blow to one side or the other and affect one or both cables. If three cables are affected and go to a good ground things can get really bad," Wilcox continued. "With that kind of voltage, the arc or electrical spark that's created, is estimated to be between 20,000 - 50,000 degrees Fahrenheit. The steel cabinets that house these connections would be completely vaporized. It can burn up a $50,000 transformer or blow out the back of a building. Worse still, it has the potential to take human life."
At the same glass manufacturing facility, PMG used the ultrasonic instrument to uncover three additional problems: corona inside the stress cone; corona outside the high-voltage cable leading into the stress cone; and corona on another transformer (a dry one that would have cost $30,000 if it had to be replaced).
Inspecting Bearings for Wear
Although Wilcox's focus is primarily centered on electrical systems, he occasionally uses ultrasonics and infrared thermography to monitor bearing wear. "The only accurate way to check a bearing is to listen to it go through its cycle," said Wilcox. "The ultrasonic instrument tells me if the bearing is under lubricated or over lubricated. If it's undergreased, the bearing wears unevenly and, over a period of time, generates a good deal of heat. Infrared thermography confirms my diagnosis."
According to Wilcox, master engineers have found that there are three distinct cycles of a piece of rotating equipment as bearings start to deteriorate. First, as a bearing shows signs of wear it gives off an ultrasonic signal. As it worsens, the ultrasonic signal becomes louder. In phase two, the equipment begins to vibrate. As that condition exacerbates, heat begins to build, which is detected with an infrared instrument.
Unfortunately, if a bearing is allowed to reach this stage, it already has damaged the shaft.
Using both ultrasonics and infrared technologies, PMG recently helped another manufacturer save thousands of dollars in wasted energy because of faulty bearings. "In one section of the plant, the company had been spending $147,000 a year in electricity and fuel. Once we identified the problem bearings and repairs were made," Wilcox concluded, "the cost of energy dropped to $69,000." ET