A Pipeline Mechanical Integrity (MI) program is a critical component of a Process Safety Management (PSM) system under OSHA’s Process Safety Management (PSM) standard — specifically addressed in 29 CFR 1910.119(j) [1]. A well-structured MI program is designed to continuously monitor and assess the condition of pipelines and associated equipment. By regularly inspecting, testing, and maintaining components such as valves, pressure vessels, and piping, the program helps to identify deterioration, corrosion, or other potential defects before they can lead to a catastrophic failure. This proactive approach minimizes the risk of leaks or ruptures that could result in severe accidents or environmental disasters. This article explores the important non-destructive testing techniques commonly used PSM mechanical integrity programs for corrosion under insulation.
Corrosion Under Insulation (CUI)
Corrosion under insulation (CUI) is a potential threat to carbon steel, and to a lesser extent, stainless steel. CUI on carbon steel pipe is a particularly aggressive and dangerous form of localized corrosion. It occurs when water infiltrates the insulation system, becomes trapped, and stays in contact with the pipe’s outer surface for extended periods. CUI is most aggressive on carbon steel pipelines in the 25°C to 120°C (77°F to 248°F). This range often coincides with process lines in refineries, chemical plants, or steam systems — making them highly susceptible. Carbon steel OD corrosion is a combination of generalized corrosion and pitting corrosion.
Stainless steel piping systems corrode differently under insulation: typically via chloride-induced stress corrosion cracking (SCC) or pitting. This is the primary corrosion mechanism under insulation for austenitic stainless steels (like 304 or 316). Stainless steels under insulation are particularly vulnerable in moderate-to-high temperature applications (generally 60°C–150°C). At these temperatures:
Corrosion Under Insulation (CUI) - Profile Radiography
Non-destructive evaluation with profile radiography is a reliable method for assessing wall thickness loss, but the inspection process is complex and requires significant manpower. Its use is also limited by pipe size, as it is typically only suitable for pipes with diameters less than eight inches, restricting its broader application in non-destructive testing (NDT) [2] . Combining profile radiography with real-time radiography can enhance inspection accuracy. Computed radiography relies on a specialized photostimulable phosphor plate to capture radiation, which is then converted into a digital signal through a photomultiplier. This method delivers high-resolution images and offers efficient data storage and analysis capabilities. Digital detector arrays are also noted for producing excellent image quality while requiring lower radiation doses.
PCET technology was originally developed and patented by for applications in the oil and gas industry, All PCET technology are based on the same principles. The technology measures the differences between the conductivity and permeability of different metals, and the quantity of those metals in comparative readings. The usual means of conducting a test is to inspect the insulated component, identify a consistent area of thicker metal, and place the reference point (RP) in the middle of that area. The customer then makes this area accessible for by removing the insulation so that a corresponding UTT measurement and used as a reference point for normalization. The software then normalizes all data points, and compares them to the RP thickness, and converts them all from a percentage to an average wall thickness in inches. It is critical that the ultrasonic thickness testing (UTT) be taken at exactly the RP.
Pulsed eddy current measures wall thickness loss of piping by distinguishing variations in electromagnetic signals. Pulsed eddy current incorporates coil sensors and magnetic field sensors to locate CUI. By emitting an eddy current from an excitation coil towards the as-specified objects, a time-varying magnetic field signal can be reflected and further collected by the receiver coil. The magnetic field will change if volumetric loss is present.
A voltage decay curve as shown below is used to estimate volumetric wall loss in a corroded pipe. The shape of the voltage decay curve is influenced by the electrical conductivity and thickness of the material. In a uniform, uncorroded area, the eddy currents decay in a predictable way. When corrosion is present (such as thinning from CUI), the eddy currents behave differently because the material is thinner and possibly less conductive in that area. This results in a faster decay of the eddy currents and results in a steeper voltage decay curve.
A qualitative analysis is shown below comparing the voltage decay measured in a 10 mm and 5 mm carbon steel pipe. After the initial pulse is emitted, the decaying magnetic field and the associated eddy current decay overtime. In the case the voltage develops across the receive coil is measured in microseconds (us).
Guided Wave Ultrasonic Testing of Process Pipelines
Guided Wave Ultrasonic Testing (GWUT) has been developed over the past 30+ years, with foundational work by institutions like Penn State, Imperial College, and the Southwest Research Institute [5-7]. Today, it’s a trusted method for inspecting hard-to-access pipelines, including those that are underground, elevated, or insulated. Its main advantage is efficiently evaluating long sections of pipe from a single test point, minimizing the need for insulation removal, scaffolding, or boom lifts.
Compared to non-destructive techniques like profile radiography and pulsed eddy current testing, GWUT stands out for its range. However, while GWUT can identify potential problem areas, it doesn't provide precise wall thickness measurements—secondary testing (e.g., UTT) or profile radiography may be required.
GWUT data acquisition is similar to conventional ultrasonic testing, using A-scans (amplitude scans). Guided waves are sent both upstream and downstream from the transducer, which is configured to focus waves in one direction at a time by altering the pulse sequence.
The resulting data plot centers around the probe’s location (0 feet), with reflections displayed to both sides. The vertical axis shows signal amplitude, which is correlated to percent cross-sectional area (CSA) loss. Sensitivity is established using known features like welds, and calibration techniques like time-corrected gain (TCG) or distance amplitude correction (DAC) are used, just as in standard UT.
In the example below example on a 12” OD carbon steel pipe, reflections from welds are visible, as well as smaller signals between 10 and 14 feet that suggest potential corrosion. These areas were flagged for follow-up with direct ultrasonic thickness testing and visual inspection to determine actual wall loss and predict pipeline life.
Summary of Non-destructive Testing Techniques for Corrosion Under Insulation
Profile radiography, pulsed eddy current, and guided wave testing have inherent technology based advantages and disadvantages. The technology specification should be well understand prior it non-destructive testing method selection. Table 1 summarizes some qualitative factors for consideration.
Table 1: Comparison of profile radiography, pulsed eddy current, and guided wave testing for corrosion under insulation
References
Process Safety Management of Highly Hazardous Chemicals standard (29 CFR 1910.119)
Cao, Q.; Pojtanabuntoeng, T.; Esmaily, M.; Thomas, S.; Brameld, M.; Amer, A.; Birbilis, N. A Review of Corrosion under Insulation: A Critical Issue in the Oil and Gas Industry. Metals 2022, 12, 561. https://doi.org/10.3390/met12040561
Sophian, A.; Fan, M. Pulsed eddy current non-destructive testing and evaluation: A review. Chinese J. Mech. Eng. 2017, 30, 1474.
Cheng, W.; Komura, I. Pulsed eddy current testing of a carbon steel pipe’s wall-thinning through insulation and cladding. J. Nondestruct. Eval. 2012, 31, 215–224.
Barshinger, J., Rose, J. L. and Avioli, M. J. Jr., 2002, 'Guided Wave Resonance Tuning For Pipe Inspection', Journal of Pressure Vessel Technology 124, pp. 303 – 310
M.J.S. Lowe, D.N. Alleyne, P. Cawley, Defect detection in pipes using guided waves, Ultrasonics Volume 36, Issues 1–5, February 1998, Pages 147-154
H.Kwan, C.Dynes, “Long-range Guided Wave Inspection of Pipe Using the Magnetostrictive Sensor Technology – Feasibility of Defect Characterization”, Nondestructive evaluation of utilities and Pipelines II, International Society for Optical Engineering, SPIE, Vol.3400, 1998, pp.326-337
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