What are the recommended procedures to safely introduce a flammable atmosphere into a continuous belt furnace?
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||Tom Philips |
Sr. Principal Applications Engineer
For sintering and brazing atmospheres in a continuous belt type furnace with open ends, you must follow NFPA 86 Standard for Ovens and Furnaces. Typically, atmospheres containing greater than 4% hydrogen in nitrogen are considered flammable. In fact, any mixed atmosphere—even if it contains less than 4% hydrogen—is considered “indeterminate” and must be treated as if it were flammable.
NFPA 86 recommends you satisfy the following conditions before introducing any flammable or indeterminate atmosphere is into the furnace:
- At least one zone of the furnace must be hotter than 1400ºF.
- The furnace must be purged with an inert gas until the atmosphere analysis indicates it’s below 50% of its LEL (lower explosive limit). General recommendation is to use five volume changes of inert gas flow.
- There must be visible indication of purge flow. Plus, purge piping should have normally open solenoid valves.
- The atmosphere system should be designed with interlocks so the flammable gases are shut off using normally closed solenoid valves in the event of power failure, a temperature drop below 1400ºF, or insufficient flow of the main atmosphere component.
For a copy of our paper about the impact of temperature on flammability limits and furnace safety, visit www.airproducts.com/limits2 or call us at