Visual Examination (VT) NDE
VT should be employed to some extent for the in-service inspection of every pressure vessel, and, in this sense, it is the only universal NDE technique. Direct observation identifies many forms of deterioration (such as corrosion, erosion, hydrogen blistering, and the occurrence of large surface cracks), and also can highlight locations where other forms of deterioration may have occurred that will require examination with other NDE techniques. Therefore, VT is usually the first NDE performed after a vessel is opened for an inspection, and it will give a useful impression of the general condition of the vessel. However, as important as VT is, it alone cannot verify the integrity and reliability of a vessel, because some forms of deterioration cannot be seen (such as fine fatigue or stress-corrosion cracks, occurrence and growth of internal cracks, and hydrogen attack).
It is usually necessary to clean pressure vessels internally to facilitate inspection. Steam or chemical cleaning is usually sufficient to remove hydrocarbon liquid films and sludge deposits that might remain inside after shutdown. If corrosion is severe and a relatively thick scale adheres to the vessel shell, hydroblasting or abrasive blasting may be necessary to evaluate the severity of the deterioration. Nozzles and shell surfaces obscured by internals should not be overlooked.
Inspectors frequently use hammers, picks, and scrapers to aid VT for determining the thickness of corrosion scales and the soundness of the shell remaining under the scale. Depth gages can be used to measure the depth of corrosion pits, and calipers are useful for measuring the inside diameter of nozzles to determine corrosion loss.
However, whenever it appears that corrosion could have thinned the shell to less than the minimum required thickness, it is advisable to determine directly the remaining thickness with UT.
VT can be relied on for detecting hydrogen blisters and is very useful for appraising the integrity of corrosion protection claddings. However, UT should be used to determine the remaining shell thickness when blisters or deteriorated claddings are visually detected.
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