Hydrogen blistering of pressure vessel shells can occur in process environments that cause hydrogen to diffuse through and “charge” the shell material with hydrogen. Process streams that contain acids or a relatively high partial pressure of hydrogen at high temperatures are most likely to cause blistering. Some of the hydrogen diffusing through the vessel shell is “trapped” at nonmetallic inclusions (or stringers) in the shell material. The hydrogen pressure builds up in these microscopic traps, causing them to propagate and link together to form blisters. Propagation of the microscopic traps and growth of the blisters follow the major axis of the stringers, which lies in the plane of the rolled plate. Cracks can also develop, however, in the blisters that propagate towards the surface of the vessel, when the hydrogen pressure inside the blister becomes high enough to cause swelling.
Visual examination (VT) can be used to detect blisters in pressure vessel shells. A flashlight beam directed along the surface of the shell can aid in observing small blisters. Ultrasonic examination (UT) using a longitudinal wave technique can be used to determine the remaining sound shell thickness if cracks have developed in the blisters. Care should be exercised when making an ultrasonic examination to distinguish between possible laminations and inclusions at midwall of the plate, and actual blisters.
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