Air Products Home

Ask The Expert Metals

What are the recommended procedures to safely introduce a flammable atmosphere into a continuous belt furnace?

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 or call us at

← Back to Sintering and Powder Metals
An error has occured, please fix the highlighted fields.
To request a quote or for more information about Air Products, please fill out the form below, and an Air Products representative will contact you shortly.
Please use the form below to contact us.

What can we answer for you?

  1. What kind of inquiry?

If you would prefer to contact us by phone, you can call any of our Air Products locations worldwide by visiting Worldwide Locations & Phone Numbers.


This site uses cookies to store information on your computer. Some are essential to make our site work; others help us to better understand our users. By using the site, you consent to the placement of these cookies. Read our Legal Notice to learn more.