Three Inspection Methods Every Process Engineer Should Know

Let me start with a disclaimer that I am not an inspection expert. However, process engineers work closely with inspection personnel, and a working knowledge of the most common types of inspection is very useful. This article provides a very brief introduction to three common types of NDE (Nondestructive Evaluation). Yes, there are types of destructive evaluations, one of which is Scanning Electron Microscopy (SEM). Here is a nice article in Nature about localized corrosion that uses SEM. However, the three inspection methods described below are all nondestructive. I included links to a few videos because of the shortcomings of my written descriptions.

1) UT

If you work in a refinery, rarely will you hear the word 'Ultrasonic' in it's entirety. What you will hear technicians and inspection experts discussing are 'UT's'. This type of measurement is relatively straightforward. Ultrasonic waves are emitted and the time it takes for them to return is used to calculate thickness of the material. UT's taken at the same location of a pipe or vessel over time can help in the monitoring of corrosion rates. Accurate corrosion data helps in plant reliability and prediction of when pipes and vessels need to be replaced.

UT Inpspection Demonstration (Video)

2) HIC

HIC stands for Hydrogen Induced Cracking. As a gross oversimplification, HIC occurs when Hydrogen protons diffuse into the carbon steel, and then meet other hydrogen protons, which combine to form hydrogen gas, which becomes trapped in the vessel/pipe wall and can accumulate over time. HIC is most likely to occur near welds, and is the reason welds in many services need to be 'stress-relieved'. Stresses from welding and the presence of wet H2S can lead to HIC. Standard UT thickness tests will not be effective in detecting HIC because subsurface cracks can occur without bulk loss of wall thickness. Generally, the potential for HIC increases with the concentration of H2S. Wet H2S environments like amine regenerators are particularly susceptible to HIC. Ultrasonic can be used to detect subsurface Hydrogen Induced cracks.

HIC Explained (Video) - First 90 seconds.

3) Wet Mag

As opposed to subsurface hydrogen induced cracks, defects on the surface can be detected using "wet mag". Basically, the test requires inducing a magnetic field on the metal surface that is being inspected (commonly inside a pressure vessel). Then, a suspension containing magnetic particles is applied to the surface. If there are any cracks, the suspended magnetic particles will accumulate inside of them. Viewing the inspected area under an ultraviolet light facilitates the identification of surface cracks. Since this is ultimately a visual test, watching a few videos, such as this one will really help in understanding this inspection method.

Wet Mag Demonstration

Thanks for taking the time to read through this article. Leave any questions in the comments. Don't hesitate in reaching out if you have any questions about refineries or working as a process engineer.