Telltale Hole (6mm) in Reinforcement Pad Purpose

Do you mean the vent hole that is drilled on the plate to let the entrapped air/gases heated and hence expanded during welding to escape (usually always drilled in case it is seal welded) ?

Generally, the purpose of a telltale hole is 1) Provide a leakage path from an area that is not normally visible to an area that can be inspected. 2) To provide a leakage path to prevent building a "solid system". Clearly both of these functions are concerns that must be taken into consideration to prevent damage to the component. In the case of a system being solid . . . this is sometimes the case in "double sheet" heat exchangers . . . where the space between the inner tube sheet and out tube sheet could become filled with water, or other working fluid. If the telltale hole becomes clogged, or is plugged by accident, and the heat exchanger is raised in temperature . . . the pressure inside void between the tube sheets can become so great as to cause and expolsion. Thus destroying the heat exchanger. I have seen this happen!!!! A heat recovery steam generator on a diesel engine exhaust. The tube sheet telltale was plugged "intentionally" because water was leaking out through the telltale. The mechanic that plugged the telltale did not realize the purpose of the telltale. The heat exchanger exploded throwing chunks of water box almost half a mile (750 meters) from the plant. Also blew out the windows on the side of the building. A large chunk of metal crashed through the windows of the plant control room and damaged control room panels for the other diesels engines causing a shutdown of three additional diesel generators. Telltales are IMPORTANT!!!

As per API 650, telltale hole should be at horizontal center axis of reinforcing. Could you please explain the reason. What will be the effect if the tellhole located at other locations?