NACE Resource Center

Portable x-ray inspection

For the detection of pipe flaws

A major inspection challenge facing the factory process industries is to examine insulated piping corrosion hidden under insulation. The common methods for locating problems in piping involve tangential x-ray or ultrasonic thickness tests. Both tests produce accurate results for pipe-thickness measurements, but most process facilities contain miles of piping, and knowing where to look for degradation is a monumental problem. Both of these NDT 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 piping is cost-prohibitive.

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.

A pipe-scanning inspection tool, called the Profiler, was developed which can quickly and easily scan an insulated pipe while the pipe is in service. 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. 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. The software converts the radiation-intensity output into linear-equivalent thickness data, which are presented on the computer monitor. The output of a collimated gadolinium-153 (Gd153) isotope source is directed to a special scintillator. The scintillator electronics contain the equivalent of a low-level x-ray CCD camera.

In turn, the scintillator is coupled to a photomultiplier tube whose electronics are matched to the scintillator output. 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 or a Gd153 isotope source. 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, Internet reference 27)