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Overview of Thickness Measurement & Monitoring

Thickness Monitoring is the process of routinely measuring the material thickness of equipment such as piping, tubing, pressure vessels, or tanks. Thickness measurements are performed by using nondestructive examination techniques (NDE) on condition monitoring locations (CMLs) or other predetermined locations. Thickness monitoring is, generally, the most prevalent type of NDE conducted on CMLs in the oil & gas and petrochemical industries.

Thickness Measurements for Corrosion Rate Calculations

The following is from The 101 Essential Elements in a Pressure Equipment Integrity Management Program by John Reynolds. Download a free copy of the entire book now.

Accurate thickness measurements for corrosion rate calculations are fundamental to fixed equipment mechanical integrity (FEMI), yet it is a subject that is often considered so mundane that it does not receive the appropriate amount of attention. When that happens, the quality of thickness data can vary all over the map. Without accurate data for corrosion rate calculations, much time and money is lost on rework and inspections that are conducted more frequently than necessary, let alone the potential for equipment and piping failing prematurely due to the inaccurate data. An effective FEMI program needs to have appropriate NDE thickness measuring procedures in place to ensure that data will be accurate and reasonably reproducible for corrosion rate calculations.

In my experience, appropriate digital ultrasonic thickness testing (DUTT) procedures with a trained DUTT technician can yield reproducibility routinely within +/- 0.010” and profile radiographic (PRT) thickness data within ~6%. Some round robin tests that I am familiar with indicated that a lack of adequate procedures and training would yield ultrasonic accuracy variability, routinely of 3-4 times these numbers. And these tests included long-experienced inspectors and DUTT technicians. Hence, it is my belief that inspectors/DUTT technicians (company and contract) doing DUTT and PRT thickness measurements need detailed training and procedures in order to provide truly high quality data. And that does not mean simply making sure they are ASNT Level I or II qualified, unless the technicians have been specifically trained and qualified on DUTT. It means that they receive training covering the 8-9 variables that can affect DUTT data quality, including: calibration issues, cleaning, couplant issues, temperature monitoring and correction factors, hot measurement issues, doubling, minimum diameters of piping, effect of placement and rocking the transducer on curved surfaces, taking three readings in each examination point and averaging them, when to use A-scan equipment, dealing with coatings, and gauging through CML marking stickers. For a lot more information on DUTT, I recommend you read section 5.7.1 of API 570(1) and section 10.2 of API RP 574(2), both of which are currently being updated for their 4th editions.

Now that said, I recognize that not all thickness measurements needs to have the accuracy necessary for corrosion rate calculations. And as such, there are alternative methods to DUTT that can suffice under various circumstances including profile radiography, long range UT, guided wave UT, pulsed eddy current, and even the old fashion caliper method. But users of these techniques must recognize that some of these techniques are just screening techniques and understand their limitations in producing accurate UT thickness data.

Do you know if your thickness data accuracy is routinely good enough to allow your inspection data management system (IDMS) to function as well as it can, providing you with accurate corrosion rates, inspection schedules, and projected remaining service life for equipment that is subject to metal loss?

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