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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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Articles about Thickness Measurement & Monitoring
  • 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 preventing safety incidents caused by critical oversights during the construction phase.

  • January/February 2018 Inspectioneering Journal
    By Bruce A. Pellegrino at Sensor Networks, Inc., and Mark Feydo at Sensor Networks, Inc.

    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 industrial landscape.

  • January/February 2017 Inspectioneering Journal
    By Israel D. Martinez at ORPIC

    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 effective inspection plans.

  • November/December 2016 Inspectioneering Journal
    By Loganatha Pandian at Meridium, Inc.

    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 maintenance.

  • March/April 2016 Inspectioneering Journal
    By Bruce A. Pellegrino at Sensor Networks, Inc., Dr. James N. Barshinger at Sensor Networks, Inc., and Michael Nugent at Equity Engineering Group

    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 pipe or vessel. It is an accurate and relatively low cost non-destructive examination (NDE) method to deploy in most situations.

  • Partner Content

    Properly anticipating and finding the damage in your facility is no small task, and spending millions of dollars on inspection may not be getting you anywhere if it’s not the right inspection processes. PinnacleART can use industry best practice models and corrosion expertise to proactively identify damage types, locations and magnitudes so you can ensure you’re performing the right inspections at the right times. Visit us at to learn more.

  • January/February 2016 Inspectioneering Journal
    By Dr. Jake Davies at Permasense Ltd.

    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 by installing wireless, semi-permanent thickness monitoring sensors.

  • September/October 2015 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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.

  • Blog
    January 6, 2014 By Nick Schmoyer at Inspectioneering

    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; we expect the same trend to continue into 2014.

  • September/October 2013 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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.

  • May/June 2013 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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 failures in petroleum refining.

  • Partner Content

    Training options to meet your needs. E²G contracts with API to deliver training programs for FFS, Risk–Based Inspection, Damage Mechanisms, and Pressure Relieving Systems. ETI and Customized Training offers specialized courses based on industry expertise and client needs.

  • January/February 2013 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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 what constitutes an effective piping thickness monitoring process and will present several practices that may be new to some readers, but these practices have produced beneficial results in other major piping reliability programs.

  • November/December 2012 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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 failures in petroleum refining.

  • September/October 2012 Inspectioneering Journal
    By A.C. Gysbers at The Equity Engineering Group, Inc.

    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 highest percentage of the number of fixed equipment failures in petroleum refining.

  • January/February 2009 Inspectioneering Journal
    By Dan Carnevale at Danatronics Corporation

    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 today. Thickness gages are used in a wide variety of industries including refineries, power plants, process control, oil and gas, transportation, automotive and manufacturing to name a few.

  • July/August 2007 Inspectioneering Journal
    By Sanjoy Das at Bhabha Atomic Research Centre, P.R. Vaidya at BARC, and B.K. Shah at BARC

    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 thickness measurement of insulated and non-insulated pipe is a typical non-destructive evaluation technique in the oil & gas, chemical, petrochemical and nuclear industries. Ultrasonic testing is available for wall thickness measurement, but in some cases, it may not be the preferred technique. For ultrasonic testing, accuracy is dependent on the temperature of pipe, which may carry fluid at high temperature. Hence shutdown of the installation is required. Moreover for insulated pipe, insulation has to be removed before ultrasonic testing. The radiation technique is a complementary testing method which can be carried out without disturbing the installation. In this technique electromagnetic radiation passes through the object of inspection and is finally recorded in a recording medium. The recording medium is either an industrial X-ray film or a radiation detector. This paper is devoted to detection of pipe wall thinning by the radiation technique. Two different methods i.e. radiography and radiometry, are discussed with their relative merits and demerits.

  • Partner Content

    ProSource Radiography Services, a sister company and subsidiary of Pro-Surve is ready to tackle your radiography needs. Should you need a small bore piping survey or other need for computed radiography. ProSource can take conventional film, and convert them to digital files for easy naming and storage. ProSource is ready to be your Sole-Point-Of-Contact (SPOC) for your radiography needs.

  • 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 interrelationships of data by location for trending. Part 1 article areas then included: -UT Data Reporting and Evaluation -Imaging UT Data Evaluating the Quality of Static UT Data -Visual Trending of UT data -Mathematical Trending of UT Data Now, in Part 2, we will cover data quality issue statistics and possible sources of poor quality UT data.

  • 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 tube wall metal wastage. The analysis of tens of thousands of UT readings from black liquor recovery boilers is an intimidating and time-consuming task. Problem areas in the boiler are easily identified and many engineering hours of labor are saved. The graphical prensentation permits the quality (accuracy and consistency) of the UT data to be carefully examined.

  • July/August 1999 Inspectioneering Journal
    By Art Leach at Krautkramer

    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 easy to use and many allow the creation of multiple files. The multiple files are typically used to separate the storage of data from different pieces of equipment or different locations.

  • May/June 1999 Inspectioneering Journal
    By Art Leach at Krautkramer

    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 equipment is common in most plants. A traditional testing method is digital ultrasonic thickness gauging for the measurement of wall thicknes. This one method has become the most widely used method of assuring mechanical integrity of equipment items that are prone to erosion / corrosion.

  • March/April 1999 Inspectioneering Journal
    By Mike Sparago at CorrSolutions

    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 or at the next inspection or the next turnaround. Statistical techniques can help us understand and control the probability of early retirement, allowing us to make better remaining life and re-inspection decisions.

  • Partner Content

    FFS assessment techniques are applicable to a wide range of damage types: LTA's, cracks, creep damage, dents, and more. These are very powerful analytical tools that often allow operators to not only keep the plant running, but to keep it running safely.

  • January/February 1999 Inspectioneering Journal
    By Joseph E. Pascente at Lixi, Inc.

    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
    By Mike Sparago at CorrSolutions

    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 been little advancement in analyzing this valuable data since the early 1980's.

  • September/October 1997 Inspectioneering Journal
    By John Reynolds at Intertek

    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 deals with improving the accuracy of profile radiographic inspection techniques, also called isotope radiography, wall shots, or tangential radiographic inspection.

  • March/April 1997 Inspectioneering Journal
    By John Reynolds at Intertek

    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.

  • January/February 1997 Inspectioneering Journal
    By John Reynolds at Intertek

    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 worse, a catastrophic event, where people can get hurt.

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