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By: Stephen H Date: Thursday, January 26, 2006 Time: 12:00:00 AM
I've seen two different uses. On storage tanks, it is a hole in the repad of a nozzle. If the nozzle welds (or plate) start leaking, it will show up at the tell-tale hole. I guess this avoids trapping product when you try to work on a tank as well. The same hole is typically used to air-pressure-test the repad for leaks. If a nozzle is stress-relieved, it would also serve to vent the area between the repad and shell. In the design of a repad, it is assumed that the shell has pressure against the inside of it. If the area between the repad and shell is allowed to pressurize the same as inside the tank, the tank wall in that area no longer has a net pressure against it. It would theoretically try to flatten into a chord, transferring more force into the repad. At the ASME Section VIII class a while back, they referred to tell-tale holes as small holes drilled almost through the walls of a pressure vessel. The idea being that if uniform internal corrosion exceeds a certain amount, it will start leaking at the holes, and give an indication, rather than progressing on to a possible catastophe. I don't know how commonly these are used.