mohtogh-
In addition to the above situations, a removable spool is preferred for equipment isolation for safe entry. A 3 foot spool piece at a vessle nozzle is convenient because you simply remove all but one of the lower bolts and swing the spool down. That connection is now isolated without needing a blind. A less convenient but still preferred alternative is if you have an elbow immediately off the nozzle, put another flange pair on the vertical run and a lifting lug on the elbow for rigging out the piece.
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