Effective ultrasonic corrosion monitoring

I have been preaching about the future of non-destructive testing (NDT) by comparison with the existing methods. One immediate process deserving examination is how we obtain ultrasonic thickness measurements.

One concern is the evaluation of the competence of the technician conducting the ultrasonic thickness measurements for the monitoring process. The American Petroleum Institute (API) and the American Society for Nondestructive Testing (ASNT) have joined forces to create the Industry Sector Qualification (ISQ) Oil and Gas certification process. This is a process based on the long and successful Chevron qualification method, which requires performance evidence of competence for obtaining accurate ultrasonic thickness measurements on a sufficient array of materials, thicknesses and temperatures. Successful completion of the process will result in the certification of the technician.

This process is still in the initial stage of rollout and has been successfully initiated in the Houston, Texas, location. It will be provided nationally in 2021. It is intended to eliminate assignment of this critical inspection function by contractor inspection companies to technicians with little experience and insufficient training.

The existing method mainly involves ultrasonic inspections, whereby NDT technicians collect thickness measurements using manual ultrasonic instruments at specific locations. The frequency and locations of these measurements are predetermined by plant engineering personnel responsible for the lifetime prediction of plant assets, depending on their product, pressure and material. Condition monitoring locations (CMLs) are referred to as ‘TMLs’ or ‘thickness monitoring locations’.

The number and placement of these TMLs and the frequency of inspections is still an imprecise science. This method has inherent inaccuracies and is subject to cost and frequency concerns, due to the risk versus lifecycle cost of a component.

Manual thickness measurements have issues affecting accuracy, which can impact the corrosion rates and analysis that predict the planned retirement or ongoing maintenance of an asset. Recognition of these limitations may result in over-monitoring, ie too many CMLs and/or too frequent inspections.

Ultrasonic thickness measurement is based on the measurement of the time-of-flight of an ultrasonic pulse in a specific material and adjusted for the temperature. The quality of the manual measurements is affected by the inherent nature of manual measurements, the practices of different inspectors, their methods and techniques, different instrumentation and effective measurements of temperature.

By contrast, permanently installed ultrasonic systems improve the precision of ultrasonic measurement by eliminating many of the inherent errors of manual ultrasonic examination. Accuracy can be improved by determining the actual material velocity, either by measuring it at the time of the ultrasonic inspection or by using a temperature correction coefficient to compensate for the change of material velocity with temperature.

Reference material (ASTM E797) provides a value of –1% per 55°C (100°F) for the temperature compensation factor, although actual measurements can vary, depending on the age and composition of the material. These are limitations that can affect the calculated corrosion rates and analysis of an asset with respect to its retirement or maintenance.

The emergence of NDE 4.0, a confluence of Industry 4.0 technologies and the Industrial Internet of Things, is paving the way for a paradigm shift as described in a blog, published by Inspectioneering, by Sasha Schieke, Manager of Engineering at Molex, and Mark Guisenhoff, Director of Fixed Equipment at Flint Hills resources.

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