Inspectioneering Journal

In-Service Measurement of Pipeline Wall Loss Due to Pit Corrosion

By Dr. Nand K. Gupta at Omega International Technology, Inc. This article appears in the March/April 2003 issue of Inspectioneering Journal.

During the past two years, a new High Resolution Gamma Ray Detector Array System to detect and measure small corrosion pits in real-time in the in-service piping in process industries, has been in development. This High Resolution Gamma Ray Detector Array can be substituted in place of the standard detector array in our ThruVU (TVU) system. The first High Resolution detector array has a total width of 1.00" and consists of 76 channels with 0.013" detector pitch. On the other hand, the detector pitch is 0.130" in the standard TVU detector array. So, the new High Resolution detector array can potentially provide a 10x better spatial resolution compared to the TVU standard detector array.

The TVU system with a standard detector array of 0.13" pitch precisely measures the depth of normal size (less than or equal to 0.15" diameter) corrosion areas. The new detector array has proven to measure precise depth of even 0.015" pits. Laboratory measurements (with the new detector array) on a 5" diameter process pipe (from BP’s PTA plant) indicate that there is only slight (<10%) error in depth estimation even for 0.010" corrosion pits.

The Present TVU System

The present TVU detection/measurement system uses a solid state gamma ray detector array and an 192Ir radioisotope gamma ray source mounted on a remotely controlled crawler/creeper to inspect large lengths of pipelines. At the crawler itself, the detector array enclosure incorporates electronics to amplify, integrate and multiplex the detector signals so that the output of all detector channels is sent on a single pair of conductors. The multiplexed analog signal from the detector array is transmitted to a laptop PC computer that also incorporates a 16-bit analog-to- digital converter (ADC) card. The detector array is connected to the laptop computer via a several hundred feet long multiconductor flexible cable. This cable carries the detector signals to the ADC board and also brings power and logic signals to the detector array.

The length (number of detectors) of the array depends upon the cross section (diameter) of the pipeline to be inspected. The 64 channel detector array allows about 8" cross section to be examined simultaneously. While a 128-channel detector array lets us examine a 16" cross section at a time. The detector array collects data from a narrow strip (about 0.02" wide) of the pipeline at a time. With a motion along the length of the pipeline, the detector array continuously measures each new narrow strip of the pipeline. As the crawler travels along the pipeline, a two-dimensional data set is collected to cover the entire pipeline at a rate of about 3 feet per minute.

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