Thickness Measurement & Monitoring

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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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Articles about Thickness Measurement & Monitoring
July/August 2023 Inspectioneering Journal

Introducing the concept of damage management locations, together with remaining life estimates from CML data, offers a complete view for inspection planning.

Authors: David A. Osage
July/August 2022 Inspectioneering Journal

An investigation was conducted to challenge the paradigm that baseline inspections must be done for all piping and other static mechanical pressure equipment. This article introduces a new approach to baselining practices.

January/February 2022 Inspectioneering Journal

A finite element analysis (FEA) was performed on five different models of varying configurations as an approach to establish best practices in the choice of the most appropriate factor of safety, depending on the type of metal loss.

Authors: Marcos Delgado
November/December 2021 Inspectioneering Journal

This article is Part 2 in a series discussing condition monitoring optimization where statistical inference techniques on the measured data can be utilized to provide reasonable expectations regarding the true extent of damage on the asset.

September/October 2021 Inspectioneering Journal

Condition monitoring optimization goes beyond traditional CML optimization, which is often limited in the breadth of analysis, or can overemphasize a subset of the overall objective.

Partner Content

In today's chemical and refining industries, sustained long term reliability of facilities is critical to profitability and competitiveness. In fact, history shows that differences in operating expenses between best-in-class refineries, and poor...

May/June 2021 Inspectioneering Journal

This case study of a CO2 absorber tower in a remote location investigates the use of a new type of wireless ultrasonic sensor for monitoring the thickness of equipment while on or offline.

Authors: Martyn Cooper
May/June 2021 Inspectioneering Journal

In this article, FFS assessment methods are summarized and practical guidance is offered for qualifying CUI damage on carbon and low-alloy steels.

March/April 2021 Inspectioneering Journal

This article will discuss a statistical analysis method that was developed to evaluate the integrity of pipelines that can identify, in a first phase inspection, whether the nature of the active damage mechanism(s) are uniform (general) or localized.

Authors: Marcos Delgado

This retrospective takes a quick look at two of the most popular topics that were covered in Inspectioneering Journal last year: the accuracy of thickness readings and corrosion under insulation.

Authors: Greg Alvarado
January/February 2021 Inspectioneering Journal

“Condition monitoring location (CML) optimization” is a frequently used term, particularly for piping circuits. Everyone wants their circuits to be “CML optimized,” but what does this truly mean?

Partner Content

The OmniScan™ X3 64 phased array and TFM flaw detector is equipped with enhanced phased capabilities and even faster TFM. The rugged and portable instrument features powerful 64-element probes with a 128-aperture TFM.


Temporarily or permanently installed thickness monitors can help to overcome shortcomings and uncertainties associated with manual ultrasonic inspections and can lead to significantly improved maintenance and asset replacement forecasting.

May/June 2020 Inspectioneering Journal

This article dives into continuous thickness monitoring, which in the opinion of the author is one of the more innovative technologies for corrosion control in the refining industry to come about in the last two decades.

Authors: Spencer Rex

This article discusses two common challenges for inspectors taking thickness readings in high temperature environments and ways to overcome those challenges.

Authors: Calvin Jory
September/October 2019 Inspectioneering Journal

The refining and petrochemical industries will continue to benefit from the evolution of robotics and data management. The biggest hurdle will be to accept this evolution and embrace the new capabilities that come with it.

Authors: Ed Bryner
September/October 2019 Inspectioneering Journal

Thickness readings are critical for today's inspectors to accurately judge the state of their equipment. Recent advancements in sensors, software, and data management tools have made this easier, safer, and more efficient than ever before.

Partner Content

Traditional inspection methods used to detect internal corrosion and corrosion under insulation (CUI) in piping and pipeline assets can be time-consuming and costly. With ART, you’ll discover CUI detection best practices that can help save you up...


Here are 8 tips to help you overcome certain challenges encountered when taking thickness measurements at high temperatures.

September/October 2018 Inspectioneering Journal

The best approach for better inspection confidence and long term asset integrity is to conduct baseline inspections upon receipt of an asset. Baseline inspection can save millions by providing accurate baseline information for later comparisons and...

Authors: Ashfaq Anwer
January/February 2018 Inspectioneering Journal

Over the past few years, network technologies have been developed to eliminate the need for long cables for UT thickness monitoring. Coupled with a software back-end, permanently-installed UT sensors have become much more competitive in the...

January/February 2017 Inspectioneering Journal

Evaluating remaining wall thickness of an asset is not enough to ensure its continued safe operation. Proactive strategies should be implemented to assess the entire environment affecting an asset’s condition, including the development of more...

November/December 2016 Inspectioneering Journal

This article summarizes seven key questions that an organization needs to answer to create a robust mechanical integrity program that is properly designed to monitor corrosion and indicate when issues increase to a level requiring review or...

Partner Content

Lessen downtime by enhancing your plant infrastructure, ensuring mechanical integrity, and extending equipment lifespan. Contact us for on-demand engineering expertise in assessment, problem-solving, specialized testing, and preventative solutions.

March/April 2016 Inspectioneering Journal

While there are many methods for measuring equipment wall thickness, a predominant method used in the O&G and power generation industries is portable ultrasonic equipment. Ultrasonic testing is non-intrusive because it is applied to the outside of a...

January/February 2016 Inspectioneering Journal

Fixed equipment in a refinery can be susceptible to corrosion from the process side, necessitating an inspection strategy to understand the condition of that equipment. Online measurement of pipe and pressure vessel wall thickness is made possible...

Authors: Dr. Jake Davies
September/October 2015 Inspectioneering Journal

This article addresses a debate mechanical integrity professionals in the O&G and Chemical Processing industries periodically have about how thickness data gathered during a thickness monitoring inspection (TMI) should be recorded.

Authors: A.C. Gysbers

In 2013 Inspectioneering published content covering a wide array of topics, from risk-based inspection, to advanced nondestructive testing methods, to pipeline integrity management systems. Last year, we published more content than any year prior;...

Authors: Nick Schmoyer
September/October 2013 Inspectioneering Journal

This article is the fifth of a series of articles that will focus on one critical sub process within a PEIP that is key in managing the integrity of process piping; Thickness Monitoring Programs for Internal Corrosion.

Authors: A.C. Gysbers
Partner Content

Increase the efficiency of your mechanical integrity programs through the design, implementation and optimization of inspection databases.

May/June 2013 Inspectioneering Journal

Piping failures still represent a frustrating and ongoing problem for processing plants. Failures are still commonly reported and contribute to large losses. In the author’s experience, piping represents the highest percentage of fixed equipment...

Authors: A.C. Gysbers
January/February 2013 Inspectioneering Journal

This article is the third of a series of articles that will focus on one critical sub process within a PEIP that is key in managing the integrity of process piping: thickness monitoring programs for internal corrosion. These articles will discuss...

Authors: A.C. Gysbers
November/December 2012 Inspectioneering Journal

Piping failures still represent a frustrating and ongoing problem for processing plants. Failures are still commonly reported and contribute to large losses. In the author’s experience, piping represents the highest percentage of fixed equipment...

Authors: A.C. Gysbers
September/October 2012 Inspectioneering Journal

Piping failures still represent a frustrating and ongoing problem for processing plants (example in Figure 1). Failures are commonly reported and contribute to large losses. As well per the author’s experience, piping still represents the...

Authors: A.C. Gysbers
January/February 2009 Inspectioneering Journal

Ultrasonic thickness gages have progressed a long way since their early development in the 1960's. The first thickness gages were large and bulky although they used the same conventional longitudinal (compressional wave) techniques still in use...

Authors: Dan Carnevale
Partner Content

FER's extensive TA experience, combined with our custom software (RAMI Scope) that tracks all aspects of TA activities, we have the ability to keep your turnaround flowing flawlessly with the methodology to maximize your value of premium time.

July/August 2007 Inspectioneering Journal

Degradation of materials with time during service is a common phenomenon for all engineering components. Hence periodic inspection is required to ensure structural integrity and availability for service. During in-service inspection (ISI), wall...

September/October 2000 Inspectioneering Journal

In part 1 of this article we covered the importance of quality assurance of UT data, that is, understanding for each particular application, the accuracy required of the UT data, and new ways/graphical program to analyze and show the...

July/August 2000 Inspectioneering Journal

The American Paper Institute Recovery Boiler Reference Manual Volume 1, October 1979, indicates that the two main goals of conducting ultrasonic thickness (UT) inspections are to determine (1) the current tube wall thickness and (2) the rate of...

July/August 1999 Inspectioneering Journal

Many digital ultrasonic thickness gauges have internal memory that allows the storage of thousands of thickness readings. Some instruments have "sequential" data loggers that store the thickness values in a numerical series. These data loggers are...

Authors: Art Leach
May/June 1999 Inspectioneering Journal

Keeping critical equipment on-line can be a challenging task. Monitoring the wall thickness of equipment subjected to corrosive chemicals, temperature and operational changes is both a safety and manufacturing concern. Thus, on-line testing of...

Authors: Art Leach
March/April 1999 Inspectioneering Journal

Inspection data analysis tools, like risk-based inspection, help us to focus on quantitative reliability targets. When considering thinning mechanisms, there is a certain probability that a piece of equipment will reach retirement thickness before...

Authors: Mike Sparago
January/February 1999 Inspectioneering Journal

One of the greatest challenges facing many of refining, fossil power, and pulp and paper industries is: How to effectively examine their insulated piping?

January/February 1999 Inspectioneering Journal

Ultrasonic thickness monitoring programs represent one of the most intensive inspection activities in refining and petrochemical facilities. Despite numerous improvements in ultrasonic testing equipment and inspection techniques, however, there has...

Authors: Mike Sparago
September/October 1997 Inspectioneering Journal

This is the fourth in a series of articles on piping inspection that I'm writing for the Journal. One of the previous ones dealt with improving thickness data taking accuracy with digital ultrasonic methods. This article is a "sister article" that...

Authors: John Reynolds
March/April 1997 Inspectioneering Journal

This is the second in a series of articles on piping inspection. In the last article, I enumerated four inspection issues that I believe contribute to inadequate piping mechanical integrity in the hydrocarbon process industry.

Authors: John Reynolds
January/February 1997 Inspectioneering Journal

It's probably more important to those of us who don't have a brain tumor. Unfortunately, it's precisely because piping inspection is not neurosurgery that it's often done poorly, which can lead to significant impacts on process unit reliability, or...

Authors: John Reynolds
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