·Table of Contents
·Methods and Instrumentation
New Portable Pipe Wall Thickness Measuring TechniqueJOSEPH E. PASCENTE
1438 BROOK DR.
DOWNERS GROVE, IL 60515
Conventional methods of inspection are tangential radiography, probed UT and visual. Newer methods like pulsed eddy current standing wave ultrasound have limitations to standardize on these technologies. The objective for this use of this new technology is to very quickly scan long distance of insulated pipe to locate the area while still inservice that is suspect because of wall loss. Very quickly is 20" (50 cm)/second.
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 facility`s 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 ends 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.
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. Also developed by Lixi is the Profiler application software, which converts the detected emissions to thickness values.
The computer that comes with the system also presents users with a choice depending on the application requirements. I can be a notebook, belt or palm computer. The storage requirements need to fit the application for either instant information or storing for bench mark of a new facility.
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. The cable can be greater than 300 meter or a wireless adapter can be employed for line of sight communications.
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. Blockages includes coke, buildup and clams. Some oil lines will have sludge on the bottom of a pipe which can not be measured with UT on the bottom of the pipe. This problem is now eliminated by the use of the Profiler. Now, socket welds can be fully measured. This includes the total of both pipes and the distance between the inside pipe and where it seats inside of the outer pipe.
A Profiler has been evaluated in the field by Engen Petroleum Ltd. (Durban, South Africa), 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 system`s 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. 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.
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 unit`s 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. To Mounted on the other end of the pedestal arm, the x-ray converter includes a scintillator, which is used with a microchannel-plate x-ray detector to produce electrons that are amplified by a factor of 1 million.The discriminator and level detector function as a gray-scale detector, selecting signal pulses based on amplitude to match the region of interest for the pipe material being inspected. The notebook computer calibrates and converts the pulses into data for image presentations on the computer screen as wall-thickness measurements.
|Fig 2: Computer monitor shows the signal pulse readout of pipe-wall thicknesses in an oil refinery, as measured by the Profiler inspection system.|
Through proper calibration of the detected radiation, the Profiler provides, as an output, a double-wall thickness measurement of the pipe, which is used to locate changes along the pipe length to indicate the existence of a blockage or corrosion.
Lixi, Inc. has created a unique portable pipe inspection instrument based on the Gad Scope technology. The Profiler was developed by Lixi as a very fast measuring tool to determine total wall thickness in pipes. For versatility in pipe inspection, these pipes can either have process fluid inside, with insulation outside or have process fluid, with no insulation on the pipe.
Pipe Inspection Problems
Refineries, chemical plants, electrical power plants and paper/pulp manufacturing facilities have MANY miles of pipe that are insulated to prevent heat loss or heat absorption. This insulation is often made up of several materials, with calcium based material being the most dense. The insulating material is usually wrapped with an aluminum or stainless steel outer wrap.The current method of determining wall thickness is radiography and ultrasound. Both pipe inspectiuon methods require lengthly time to acquire data, which all needs to be recorded manually with respect to pipe location for future analysis.
The drawbacks of the ultrasound method.
The drawbacks of the x-ray method
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