The 2007 Edition of NFPA 86 (Standard for Ovens and Furnaces) defines a Class C Furnace as one that “has a potential hazard due to a flammable or other special atmosphere being used for treatment of material in process.” Purging itself is defined in this document as “the replacement of a flammable, indeterminate or high-oxygen-bearing atmosphere with another gas that, when complete, results in a nonflammable final state.”
There are specific instructions and requirements within the NFPA standard for both the introduction and removal of a special atmosphere by purging (these should be reviewed for the specific type of furnace being used). Typically, these requirements are that prior to introduction of a special atmosphere, an inert atmosphere be introduced. Once purging has commenced, flow continues until the purge is completed per the timed-flow method (see below) or until two consecutive analyses of all chambers indicate that the oxygen content is less than 1%. For removal of a special atmosphere once the purging has begun, the flow shall continue until the purge is complete per the timed-method or until two consecutive analyses of all chambers indicate that the atmosphere is below 50% of the lower explosive limit (LEL).”
For example, portable combination oxygen/hydrogen analyzers are in common use with hydrogen-bearing atmospheres to determine both the percent oxygen present during initial purge (prior to combustible gas introduction) and the percentage hydrogen present during combustible gas removal. As an alternative, a vacuum purge is acceptable if the initial room air is pumped out with a mechanical pump to a vacuum generally in the range of 100 microns or 1 x 10-1 torr (13.3 Pa). Remember that if the pump is used during removal of a special atmosphere, a flow of inert gas must be introduced into the mechanical pump chamber, and gas ballast values shall be piped to a source of inert gas.
The timed-purge method relies on confirmation of the adequacy and effectiveness of a timed-flow purge by actual trial at the time of commissioning or initial startup and periodically thereafter. Verification testing is considered acceptable if, after five volume changes of flow (Figure 1), two consecutive gas analyses of the effluent gas indicate the proper readings as mentioned above.
Despite the use of automatic systems many older furnaces use a timed-purge sequence reliant solely on the five volume changes in an hour rule. These older systems should be updated to current standards.