The sampling pipe left the furnace through a flange located next to a large valve assembly that regulated the flow of exhaust gases from the furnace to a heat exchanger used to raise steam for use elsewhere. The valve was biased to fail-safe by means of a powerful spring, which meant that if a fault occurred anywhere in its control system it would automatically close to the safe position.During the firefight that had taken place in that part of Pittsburgh, a stray bullet had smashed the pin that secured one end of the pivot arm attached to the spring, causing the arm to snap back toward the sampling pipe. In doing so it sheared off the head of one of the bolts that held the flange and was finally brought to a stop hard against the pipe itself. Thus there was only the second, overstrained bolt and the thin material of the pipe wall, softened by the heat inside it, to oppose the fierce pull of the spring against the pivot arm.The sampling pipe snapped at the moment when Private Dringham of the damage inspection party was drifting past on his way to check some nearby high-voltage insulators.
