How do you find corrosion under insulation (CUI) and what nondestructive testing (NDT) techniques are best? These are questions conscientious system operators should ask when trying to hold off the costly problem of hidden corrosion in their systems. CUI is one of the most difficult corrosion processes to prevent but adoption of proactive measures, such as the development of an inspection program intended to detect early signs of corrosion and mitigate potential piping failures, is critical.
In fact, corrosion is so costly that the National Association of Corrosion Engineers (NACE) estimates that the global cost of corrosion is approximately $2.5 trillion USD. So, what is the key to limiting the cost of corrosion for a system operator and how do you find corrosion under insulation?
We wrote about the conditions under which corrosion is likely to be found under insulation in a previous blog post. If you’d like to learn about CUI risk factors and where you might find it, you might want to start there. However, if you are wondering how we locate corrosion in an insulated system using nondestructive testing techniques, read on.
Let’s look at the most common methods your NDT consultant may consider or rule out when building your inspection program:
Visual Inspection i.e. Insulation Removal
Visual inspection is one of the most effective NDT methods to locate corrosion but that comes with a couple big caveats. Removal of insulation is the most expensive and labor-intensive way to find corrosion. The logistics of this process can be made even more complicated by the presence of asbestos. What’s more, if your operation is in service during a visual inspection, it’s possible a system may experience process issues.
While extremely effective, visual inspection is ultimately more useful to establish a baseline and a plan of action moving forward for your CUI inspection which will be paired with other NDT methods.
Profile radiography is a simple and effective technique to inspect piping that is vulnerable to corrosion though its ability to detect a reduction in internal wall thickness. This NDT technique has a few distinct advantages that include a high level of portability, the ability to scan piping without removing the insulation as well as relatively inexpensive and quick results.
However, this method can be limiting because it can only verify small areas and can be technically challenging when scanning piping greater than 10 inches in diameter. There also are safety concerns due to radiation which prevent others from working in the surrounding area during inspection. Depending on your system, this can lead to downtime or scheduling delays.
Ultrasonic Thickness Measurement
Ultrasonic thickness measurement is a technique that measures the thickness of a solid element based on the movement of ultrasound waves. This is another effective technique that can measure wall thickness through a variety of linings and coatings. One benefit of this technique is that you only need access to one side of the piping under inspection.
However ultrasonic thickness measurement comes with its own set of limitations which include scans being limited to a small area and the complication of needing to cut small holes into the insulation to perform the technique which can be expensive and may compromise the integrity of the insulation (which can increase the CUI issues already under inspection).
Infrared thermography is a technique that can detect moisture spots within the insulation of the piping. This method works by detecting the difference in temperature between the dry insulation and damp insulation. Where there is wet insulation, there likely (but not always) is corrosion. One benefit of this method is that infrared thermography can work without making direct contact with the piping being scanned which enhances safety as well as saving time and money. This method is generally faster and more sensitive than other moisture gauges.
Neutron backscatter is another technique used to detect moisture within insulation. This method uses a radioactive source which emits high energy neutrons into the insulation which respond to water particles that may be present. These results point to areas where there is a higher probability of corrosion present. This process is relatively quick and accurate. However, methods like neutron backscatter, which point to areas of probable corrosion, can generate false positives because the presence of moisture does not guarantee that corrosion is also present.
This technique, true to its name, provides a real-time look at the pipe’s outside diameter through insulation which produces an image an inspector can see on a TV monitor during inspection. Real-time radiography offers safety and speed enhancements relative to traditional radiography. This method is limited to small structures and requires access to both sides of the pipe under inspection.
Pulsed Eddy Current
Pulsed eddy current is an NDT inspection technique which uses electromagnetic waves to determine the wall thickness of a material which conducts electricity with probes placed on the pipe wall under inspection. While highly accurate, this technique is typically only used as a screening method because it only provides an average wall thickness.
Leverage NDT Methods for Your CUI Inspection Program
As you can tell by the strengths and weaknesses of the typical methods an NDT consultant can use to detect and measure corrosion, a combination of methods are often leveraged to screen a system for areas where corrosion has likely occurred. This initial screening then points to targeted areas of piping that require additional inspection and measurement. While Braun Intertec does not currently offer neutron backscatter, real-time radiography, or pulsed eddy current, it’s important to be aware of the range of options available.
As NDT inspectors with decades of experience between us, we use our expertise and experience examining complex operations to help guide you to the inspection methods best suited for your system, budget and concerns.
Contact Our Nondestructive Testing Consultants
If you’re interested in learning more about how Braun Intertec can help you develop a CUI inspection program or assess your operation for corrosion threats, reach out to us through the contact us button below and we will be in touch.