Based on knowledge of potential heat sources, why would not consider the potential for hot surface ignition from the glass pipe?

There is heat transfer from the flame to the material inside the pipe and knowledge of heat transfer (NFPA 1033) is required knowledge. The phase change of solid to liquid provides evidence of the heat transfer so the glass surface must be heated in the process. So you have evidence to formulate a valid hypothesis of hot surface ignition. This hypothesis can then be tested by the application of the SM. The testing of this hypothesis through analysis of the surface temperature of the glass and the threshold for hot surface ignition for a specific material configuration can be performed, so it is a "testable hypothesis."

With that said, hot surface ignition has a much lower propensity for ignition than for piloted flames (due to chemistry), so I would agree that on a relative basis, it is more unlikely than other sources.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com