Measuring in the field
Many vendors offer specialized equipment and software that can accurately measure equipment dimensions and tolerances and convert that data into computer models of components.
"Capturing dimensions is easy," says Peter Tavis of Edison ESI. "But turning dimensional data into useful information is so difficult that most inspection houses don't offer the service. Most arm their teams of technicians with traditional hand tools—rulers, tape measures, calipers, and plumb-bobs—for capturing 2D data in the field. They then return to their offices to assemble the data into a drawing."
Tape measures may be fine for building a house. But they're woefully inadequate for measuring a slope of a few thousandths of an inch over a 30-foot span, or the complex pattern of an airfoil.
"By using a variety of state-of-the-art digital tools, we typically can replace a team of technicians by two operators," Tavis continues. "What's more, these tools work in 3D and display the results on-screen in real time. So we also know in real time whether we've captured all the data we need."
A popular tool for taking these kinds of measurements is the FARO arm from FARO Technologies Inc. (www.faro.com). The arm is articulated and has a laser or ball-bearing probe on its end that can be moved to various points around a component to take and record measurements.
The recorded data then are converted by computer-aided design software into 3D images of the component for analysis, design, or remanufacturing. A FARO arm has a reach of about 12 feet. For larger drive trains, the tool needs to be repositioned and measurements taken from more than one location. Tavis says that by using overlapping positions, he was able to measure an entire 60-foot turbine train with an accuracy of about 50 mils.
"Typically, the technology used is determined by what needs to be measured," Robert J. Waddell, president of Applied Precision Inc. (www.appliedprecision.ca), explains. "For single-point measurements of large objects that don't need to be so precise, an articulated arm system could be a viable option." For greater precision, he recommends a white light or laser-based system.
When using either a FARO arm or a laser device in the field, care must be taken to ensure accurate results. Bob Siedzik, a senior product engineer at TurboCare's engineering center in Fitchburg, Mass., says that the FARO Laser Tracker can measure up to 125 feet with 1-mil accuracy—but only if the conditions are right.
"The Laser Tracker is one of the most sensitive pieces of portable 3D measurement equipment available," Siedzik says. "But it must be used indoors, or under cover outside. The Laser Tracker also is limited to 'line of sight' measurements, which means that it cannot see between blades."
Vibration, temperature changes, and lighting can reduce the accuracy of both laser and ball-bearing probe measuring devices. It takes experience to realize when measurements are off and to diagnose the cause.
"A smart tech can learn the measuring software in a week," says Tavis. "But understanding what needs to be recorded and how it's done aren't enough. Even a well-thought-out plan can fail to produce sufficiently accurate data if the tool's setup isn't rigid enough."
Tavis recalls seeing a measurement crew arrive at one site with a brand-new Laser Tracker. To keep the tool clean, they put a piece of plywood between it and the floor. Every time someone used the instrument, they had to step on the plywood—which moved and threw off the measurement.
"That's a good example of a poor tool setup and an inexperienced measuring team," he says. "An inspector needs to be able to recognize when his measurements are bad."
Measuring in the shop
It's much easier to make accurate measurements in the shop, where conditions can be controlled. Inside, permanently mounted FARO arms or CMMs (coordinate measuring machines) can provide sub-mil accuracy. TurboCare uses a Mitutoyo/Metris stationary CMM equipped with both a ball probe and laser scanner to measure components such as rotating and stationary turbine blades and turbine carrier rings (Figure 2). "This is the preferred type of tool for designing and building components with maximum precision," says Siedzik. "But because CMMs only work in the shop, they can be used only on freestanding components that have been removed from the turbine."
2. Less than a thou. In the shop, a coordinate measuring machine can measure parts' dimensions with sub-mil accuracy. Courtesy: TurboCare
There are also white light and laser devices that capture the geometry of complex curves such as turbine blades for testing or reverse engineering. If needed, the system can project a grid of measurement points onto the surface. As Waddell explains, Applied Precision uses a Steinbichler white light system from Central Scanning Ltd. (www.central-scanning.co/uk). The system can capture up to 1.3 million measurement points in a single shot. By taking three shots from different locations, it can triangulate the position of those reference points to within 4 mils.
SPG Hydro International Inc. of Sainte-Julie, Quebec (www.spgdata3d.com), offers a multiple-camera setup for rapid 3D modeling. Rolls Royce Canada is using it to inspect turbine blades.
The bottom line is that any company performing turbine retrofits needs a wide array of tools for on-site and in-house measurements to ensure that the new parts not only fit the existing shell but also outperform what they are replacing. "There is no one piece of equipment that does everything," says Waddell.
—Drew Robb is a freelance writer who writes about issues important to the power generation industry. He can be reached at 323-660-4862.
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