I recently installed a 304ss SCh 10 piping system that was a pipe in a pipe jacketed system. The system is used to test sprinkler heads. Water is run through the outer jacket to keep the pipe cool while the fire suppression water runs through the inner pipe. The heating medium used to bring the heat up in the test area is Heptane. After a couple of tests the welds started to pit and fail in the weld zone and the heat affected zone. The pipe was not supported very well and was heated to such extreme tempretures that it started to sag. My question is will the heptane or the high tempretures cause the 304 to fail in general or at the weld joints? The previous system had sch 80 black pipe used and it lasted for almost 10 years.

The welds were tested with air to 125psig after construction. The water jacket comes from either a cisterne (not treaeted) or a pond. The inner water is city water (Chigago). The pipe is actually engulfed in flames. Yellow flames literaly surround the pipe. The system is used to test fire sprinklers. I would have to guess because of the flame color in the 1200-1400F range. The leaks are occuring in either the weld zone or the heat affected zone of the weld with no real patteren. There are leaks on the inside and the outside of the piping jacket.