Hydrogen Blisters Repair
Hydrogen blisters generally do not significantly compromise the integrity or reliability of a pressure vessel. The separation causing the blister to appear is parallel to primary membrane stress, and therefore, does not reduce the load bearing strength of the shell.
Drilling a small vent hole into the blister will relieve the hydrogen pressure that builds up inside the blister (see Section 738) and prevent it from swelling enough that cracks develop and propagate towards the surface. Only if cracks have already propagated to the surface will it be necessary to evaluate the blister further. Cracked blisters will effectively reduce the thickness of the vessel shell. The remaining sound shell thickness can be determined by ultrasonic examination (UT) using a longitudinal wave procedure from the uncracked side of the shell. The remaining shell thickness can then be evaluated (see Section 750) for repair, replacement, or rerating for a lower pressure.
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