PDM (predictive maintenance) helps us improve production," explains Peter Chalifoux, maintenance superintendent of Allied-Signal's Chesterfield, VA, plant. "Most of our processes are continuous. An upset anywhere in the process shuts down the line and we not only lose production, but the quality of our product is affected. That's why predicting when the equipment will go down and coordinating with the production and maintenance people enables us to adjust our process and minimize downtime. It lets us make repairs without affecting the quality and quantity of product we put through our plant."

Mike Barkle, Chesterfield's engineer for reliability services, expresses similar views: "I can't see how a business can operate, budget or even plan its manpower needs without some method of predicting what will be needed. I understand there are still some places that don't rely on PDM. They'd rather use preventive maintenance (PM), although I don't see how that can allow them to plan properly."

Nylon fiber is the main product at the plant. The facility also produces nylon pellets, which are either converted into engineering plastics by extrusion compounding or sold to converters for extrusion. The plant, employing about 2000 people, operates seven days a week, around the clock. Predictive maintenance is of the utmost importance in maintaining Allied-Signal's reputation for quality and delivery.

In recent years, Allied-Signal's Reliability Services group has searched out new techniques to extend or improve existing methods. Airborne ultrasonic scanning, which came to its attention last spring, is one of these new approaches.  The plant uses a portable, hand-held, ultrasonic instrument with frequency tuning capability.

"We felt it would complement many segments of our work," says Linda Benson, reliability engineer. "We have, for example, used ultrasonic testing as a backup when checking heat-exchanger tubes for leaks. We've used it to find tube sheet leaks checking where we had previously used a bubble solution without success. Ultrasonic scanning is a quicker, easier and much surer way of finding these leaks. We've used it to find leaking pneumatic controls, and our electronic department has used it to detect arcing. We're still exploring its uses and constantly finding new ones."

Sound and ultrasound
The theory of ultrasonic detection is relatively simple. Working machinery has constant ultrasound patterns. Changes in these "sonic signatures" can be recognized as components wear. An ultrasonic detector senses subtle shifts in the "signature" of a component and pinpoints potential failure before it causes costly damage.

Under normal conditions, sound waves travel around objects smaller than one wavelength. This makes the audible sound source hard to locate, as the sound will vibrate solid surfaces several wavelengths long, rendering walls and large objects "transparent" to it. Ultrasonic frequencies with their short wavelengths can be considered directional and will not readily travel through more than one medium. These properties allow the ultrasound sources to be located. Ultrasound is also easy to separate from the general run of plant noise.

How ultrasonic detectors work
By definition, ultrasonics are above the range of normal hearing, so a sophisticated detector must transpose the ultrasonic signals from the range beyond human hearing to the range within it. The translated sound can be heard through earphones and the intensity observed on a meter.

An ultrasonic detector is used either in scanning (non-contact) or in contact mode. In the non-contact mode, the operator scans an area listening for airborne ultrasound usually associated with a pressurized gas or vacuum leak. In the contact mode, a metal probe supplied with the detector is stimulated by ultrasound and transmits the waves when touched against equipment surfaces.

Heat exchangers
"Pinpointing leaks in heat-exchanger tubes and tube sheets is the most important use to which we've put the ultrasonic tester so far," says Wayne McNeese, supervisor of reliability engineering. "We can't use the soap and water method because the tubes are too long and there is no way of getting in with the liquid."

In the past, if Reliability Services suspected a hole in a heat exchanger tube, operators would fill the shell side with some liquid, usually water, and pressurize it. They would then watch for water running out the tube.

This method, although it works, is laborious. It can take a long time for a larger heat exchanger to fill, and occasionally cooling water seals a leak. Ultrasonic scanning simplifies the process, making it faster and more accurate.

"Heat-exchanger tubes probably provide the best illustration of the usefulness of ultrasonic scanning to our program," Benson says. "In one case, some of our maintenance people suspected a leak, and we worked for about two days trying to find it by conventional techniques without any success. Then we sent some people down with the ultrasonic scanner, and they detected it in 15 minutes. Without it, there's no telling how much longer it would have taken—probably several days."

There are two off-line techniques of leak detection: tone generation and pressure. In tone generation, ultrasonic transmitters are placed; at the inlet and outlet ports and "flood" the shell side with intense ultrasound. Ultrasound lacks the energy to penetrate the solid tube walls, will penetrate a hole and carry through its length. A quick scan of the tube sheet discloses the leaks and tests the integrity of the sheet. By the pressure method, the heat exchanger is pressurized with air, and the rushing ultrasound of escaping air reveals the source of the leak.

Arcing
The plant has hundreds of inverters in one area to drive its equipment at different speeds. So that they can be swapped around, these inverters have plugs similar to but larger than those that fit into a home outlet. Moving them from position to position loosens their internal connections. As there are literally hundreds of inverters plugged into each panel at any one time, it is very difficult to detect any that are arcing.

"Sometimes the arcing was severe enough to set a plug on fire," Chalifoux explains. "Now, we can pinpoint arcing plugs early on with our ultrasonic detector."

The scanner allows operators to hear arcing inside the plug, warning them of a future failure due to the connection overheating. Because of this early warning, the plugs were scheduled for repair without upsetting the process.

Infrared (IR) scanning is an alternate method for detecting arcing. It works because the arcing generates infrared heat. Allied-Signal also maintains infrared capability at the Chesterfield plant.

"The ultrasonic system is certainly a lot cheaper and easier to use than IR," says Barkle. "Our ultrasonic scanner indicates problems, and then, if we need to, we return later and confirm them by IR."

Ultrasonic detection allows the integrity of electrical systems to be tested by sweeping the detector, in its scanning mode, over the test area. In this way it will reveal loose connections and other openings that force a current to jump a gap. Because arcs or coronas make a crackling or frying sound, arcing at switches and corona discharge is easily heard and faulty components readily identified.

Basic to PDM
"Going by my experience, an ultrasonic scanner is an excellent basic tool," Mike Barkle says. "It has many uses, unlike much of our more expensive and specialized equipment. The ultrasonic system can be used to detect problems in electrical, mechanical and pneumatic systems. It covers a very broad range, and it doesn't take much training to use it. That is why I think it fundamental to good predictive and preventive maintenance programs."

Linda Benson adds: "One can almost use it out of the box. Fifteen minutes of instruction and most operators can do a pretty good job of finding small leaks and many other faults."