|NDT.net - October 1999, Vol. 4 No. 10|
|TABLE OF CONTENTS|
A real-time density-measuring system that gauges the wall thickness of pipe without contact, the portable x-ray or isotope-sourced Profiler produces an output signal that is delivered to a computer for image analysis. The measuring system is designed to identify problem areas of a piping run, pinpointing where additional testing should take place.
The Profiler provides a repeatable measurement accuracy of 2% using gamma absorption from a low-level isotope. Lixi president Joe Pascente says the unit looks at a tangential angle to an insulated pipe to detect corrosion under the insulation. "Its output is not an image, but rather a series of pulse communications to a computer, which converts this information into wall or blockage thickness," he says.
Pascente points out that piping degradation caused by corrosion and erosion "are by far the most prevalent failure mechanisms in various process piping systems." Pipe degradation is usually caused by external corrosion under the insulation, internal corrosion caused by a variety of mechanisms, or internal erosion from the flowing product.
He adds that both of these nondestructive methods are also location dependent; tangential x-rays provide only one "snapshot" of the pipe per location, and ultrasonic thickness tests require that either insulation be removed or inspection ports be drilled in the insulation. "As a result, performing a comprehensive examination for all of a facility¹s piping is cost-prohibitive," Pascente concludes.
The magnitude of the testing problem can be appreciated by understanding that refineries, chemical plants, and electrical-power plants use thousands of miles of pipes that have been insulated to prevent heat loss or heat absorption. The insulation often comprises several materials, with a calcium-based material being the densest. It is usually wrapped with a layer of aluminum or stainless steel.
Verification of wall thickness of these pipes is accomplished by first removing the insulation and then performing an ultrasound inspection or by taking x-rays through the insulation at an angle tangential to the edge of the pipe. The time required to obtain data with either method is measured in hours per meter. Furthermore, the ultrasound method requires that the insulation be replugged after inspection. The insulation surface also must be cleaned or the resulting data will not be accurate.
"The most common and straightforward way to inspect for corrosion under the insulation is to cut plugs in the insulation that can be removed to allow for ultrasonic testing," Pascente explains. However, the plugs can be a source of moisture leakage. "The main problem with this technique is that corrosion under the insulation tends to be localized, and, unless the inspection plug is positioned in the precise spot, corrosion sites will be missed," he adds.
The tangential x-ray method also presents some procedural problems. It sees only surface corrosion but not wall thickness, which is the key test parameter. Using tangential x-ray sources also requires that the area being inspected be roped off for radiation safety.
The portable version of the Lixi Profiler has undergone several iterations since its initial release. The latest modification can incorporate one of two sources: an x-ray source from the X-Ray Technology Group of Oxford Instruments plc (Scotts Valley, CA) or a Gd153 isotope source provided by MDS Nordion (Kanata, Ontario, Canada). Pascente says the Nordion source was chosen for integration into the Profiler because that particular isotope matches the energy of steel with thicknesses up to 3 in. and because of a ten-year working relationship with the company.
Both sources produce ionizing radiation that is detected by the MCP x-ray detector. The Gd153 isotope source is more appropriate for use when measurement portability is essential. The x-ray source is more suitable for fixed, in-line inspection measurements.
Lixi custom-designed the microprocessor-controlled MCP x-ray detector controller and scintillator, which function as the x-ray and isotope detector. The company also developed the Profiler application software, which converts the detected emissions to thickness values.
The notebook computer that comes with the system also presents users with a choice depending on the application requirements. One possibility is a Libretto notebook from Toshiba America Information Systems Inc. (Irvine, CA) for use in situations where substantial data storage is needed. Another selection is a Cassiopeia palmtop computer which can be belt-mounted for quick readings but only provides minimum data storage for portable inspections.
The inspection can be performed by two technicians: one manipulates the Profiler while the other monitors the laptop-computer display. However, for some test situations, a single technician can handle both the Profiler and the palmtop computer. Test locations that show a material loss greater than a designated value are then marked for further evaluation by another nondestructive-test method.
Using the Profiler as a scanning tool, the technicians can quickly evaluate all logical problem spots. These include turbulent sections such as elbows and tees; points of flow restriction; regions around chemical-injection points; areas adjacent to superheated nozzles in steam piping; or other points of concern identified through either external visual inspection or system service history.
A Profiler has been evaluated in the field by Engen Petroleum Ltd. , a major oil refinery with "many kilometers of insulated piping that needs inspection for underinsulation corrosion," says Jimmy Groves, a pressurized-equipment inspector at Engen. An 8-in.-diameter pipe with a wall thickness of 0.3 in. was used to confirm the system¹s ability to measure total wall thickness. Calibration data were acquired at thicknesses of 0.3, 0.4, and 0.6 in. At least three values are needed to convert the standard logarithmic-output response to a linear response for subsequent values. Three sets of data were taken for each thickness value to provide higher repeatable accuracy.
The Profiler was inserted into one end of a pipe to look at one wall and then at a pipe location that contains a 0.034-in. groove (see Figure). Next, a 0.022-in. shim was added to the 0.3-in. wall thickness. The Profiler was subsequently removed from the pipe and positioned around the pipe with the shim in place; measurement data were taken. The Profiler was then dropped below the shim to the two walls for more data and then placed at the location that had the 0.034-in. groove for additional data.
All the data were stored and then downloaded to the computer for image analysis. Specific data for any point on the graph were obtained by pointing the screen cursor to that part of the graph. The data-acquisition time for this experiment was one second per data point. The time was variable and was determined by the accuracy and the isotope activity level.
Based on field-testing results, Groves says the Lixi scanner has been proven as a fast-acting measurement and monitoring tool for detecting wall-thickness loss on process piping. He has now been using the Profiler for nearly six months.
In one test, a furnace tube section was scanned with and without an internal "coke" deposit. The Profiler clearly detected the coke deposit by a change in wall-thickness readings.
Groves says that the unit¹s U-shaped structural positioning arm can be manipulated around pipe fittings, such as bleeders, elbows, supports, and thermowells. "To get a good scan, the center of the radiation source has to be directed at a center point of the pipe," he adds. And to get higher accuracy, two scans need to be done at 90° apart.
Groves verifies that the Profiler can do global inspections on hundreds of meters of insulated or uninsulated piping. Any areas of concern warrant a crosscheck with a visual inspection and other nondestructive methods.
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