Due to the fact that pipe flanges rely on bolts, and bolts rely on friction, it seems obvious to me, as a manufacturer of piping, to recommend that bolt torques need to be checked every so often, to ensure piping doesn't leak. I've got a few questions about this, but here is the first one:
How often do you normally check torques on bolts at a plant? In other words, what is a good industry standard preventative maintenance schedule when it comes to threaded fasteners that are loaded below yield? I would prefer that answers provide a reputable source for reference.