NDT Sees What You Can’t
By Carroll Mccormick
Everything man-made wears out and breaks. Maintenance crews want to know when parts will fail so they can change them out before they interrupt production. In the case where equipment does break unexp...
April 1, 2007
By Carroll Mccormick
Everything man-made wears out and breaks. Maintenance crews want to know when parts will fail so they can change them out before they interrupt production. In the case where equipment does break unexpectedly, they often want to know the cause. Non-destructive testing (NDT) techniques, which analyze materials for defects while leaving them completely intact, can help answer both questions.
“Manufacturing companies are having failures that could be prevented if they were using NDT,” says Yvan Couture, the director of commercial business development at Mequaltech Inc. in Montreal. “I think that in general, maintenance guys do not understand NDT and what it can do.”
Mequaltech’s engineers, metallurgists and technicians do understand it well; they have been providing consulting, control and testing for metals and welding technology since 1979.
In addition to helping to avoid nasty surprises, NDT, which is applicable to tiny components and giant hydroelectric turbine-sized parts alike, can help to establish better parts replacement schedules, including enhancing the preventive maintenance schedules that manufacturers propose for their equipment.
“Our technicians are very specialized at detecting where some problems are likely to occur and [can determine] how to solve them,” Couture says. Engineering specialists can also recommend changes to equipment designs — for example, adding plate, supports and extra welds to reduce the frequency of cracks.
The four most common NDT techniques (not counting visual examination, which is in a class of its own) are ultrasonic, liquid penetrant, magnetic particle and X-ray.
Ultrasonic NDT is good for measuring the thickness of metal, say, in process vessels or tanks that contain corrosive liquids. A simple technique, it is the one that a non-specialist without NDT certification can do, Couture says.
“We can train maintenance people how to use it: Put the probe there and get a reading of the thickness.” Ultrasonic measurements done over time can track changes in thickness to determine corrosion or wear rates and help maintenance departments fine-tune their replacement schedules.
Ultrasonic gear costs $2,000 to $5,000, depending on the equipment, and can be cost-effective to own if a plant has boilers, piping, compressors and pressure vessels, Couture suggests.
Liquid penetrant NDT is specifically for non-magnetic metals like stainless steel, glass, ceramics, rubber and plastic, and is only for indications open to the surface. Examples of its use are finding cracks in shafts or sheets of metal, and detecting porosity or pinholes in welding.
Magnetic particle NDT, in which an electromagnet is held to the surface of ferromagnetic materials and a coloured metal powder is puffed onto the surface, detects defects at or near the surface of the material. Uses include revealing cracks around welds and defects in forgings.
X-ray NDT, to put it simply, looks beneath the surface of materials or behind things that cannot be removed, just like X-rays at the hospital or in airport baggage inspection systems. One use of X-ray NDT is in examining piping for a build-up of deposits, or for corrosion or erosion. This can be done at the same time gas is passing through the pipe; production need not be interrupted, except for dense liquids like water or coolant.
NDT technicians in Canada must be certified to Canadian General Standards Board (CGSB) requirements. “CGSB sets the standards that NDT technicians must follow and it evaluates our technicians. We have no choice,” says Couture. “The training is very specific and depends on each kind of NDT. It takes about nine months to reach Level 1, which includes about 80 hours of theoretical training, then months of practice under the supervision of a Level 2 or 3 technician.”
Level 1 certification allows execution of the NDT techniques, but only under supervision. Level 1 technicians do not have the right to evaluate NDT test results. Technicians can take Level 2 exams after another 9-12 months of practice, depending on which NDT techniques are being tested for. Level 2 allows execution of the NDT without supervision, as well as final analysis and interpretation.
Level 3-certified technicians are able to develop new techniques, develop procedures and train people. “This is a very high level and is normally restricted to guys who have 20-30 years experience and know a lot about metallurgy and welding. But Level 3 is usually not required on the market,” Couture notes.
Many NDT techniques, including those mentioned, can be done on-site. Mequaltech, for example, regularly sends its NDT show on the road: It has three vans and two half-ton trucks loaded with equipment — even portable X-ray machines — plus a trailer carrying a full range of equipment, including a darkroom for developing X-ray film.
“Some customers ask us to develop film on the spot. We use the mobile darkroom usually when we have a task far from our office, so we can reshoot if we need to retake images. For projects that last five to six months or longer, we build a darkroom on site. For some projects, for example hydroelectric projects, we can be there for a year,” Couture says.
Many companies hire NDT services for preventive maintenance purposes, but they tend to be those for which the consequences of unexpected equipment failures are especially dire. Mequaltech does a lot of testing for the mining, pulp and paper and food industry, from process equipment and welds to piping to lift equipment, like hoists for steel industry crucibles. Refineries do a lot of NDT and employ four to five of Mequaltech’s technicians full time.
On the other hand, Mequaltech also gets a lot of emergency calls. For example, a pipe might have failed and a company wants to find out if other, similar pipes are also in danger of failing. Finding cracks in a not-yet-failed pipe can help an engineer investigating the cause of a failure to complete his tests.
Post-failure, NDT, chemical, hardness and metallurgic analyses can be done on equipment to determine the cause of failure and the correct way to do repairs. For example, Couture says, “Some companies do not realize that you can repair some shafts. They scrap them, or try to weld them but do it the wrong way.”
Companies like Mequaltech can also advise maintenance departments unfamiliar with NDT techniques about which pieces of equipment should be tested, based partly on a study of equipment drawings and exploded diagrams, visual checks of what kinds of equipment they have, how the equipment is used and how frequently it operates. Statistical analysis is also used to establish the trends of defects.
And the cost? “For a normal company NDT service would cost about $800-$1,000 a day, depending on the type of inspections,” says Couture. “Where you have equipment that has a life cycle between replacements, it can be very profitable to do NDT. You can get more life out of equipment, sometimes, if you can evaluate the condition.”
Carroll McCormick is the senior contributing editor for Machinery & Equipment MRO. He is based in Montreal and can be reached at firstname.lastname@example.org.