Reheat Furnace Classification

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ThaShocker

Member
Location
Central Texas
I have a question concerning the classification of a natural gas fueled reheat furnace used in the steel industry. This furnace, under normal operating conditions, is basically a large (40 ft x 80 ft) box with combustion air/natural gas burners. We are in the process of installing lighting under the furnace and some believe it to be a Class I area although we have never treated it as such. One can walk under the entire furnace and it is open on all four sides. There are no gas valves or piping under the furnace but obviously we have several which feed the burners within it located above. This furnace operated at approx. 2300 degrees F so under normal operations there is certainly no gas escaping. The burners are all electrically controlled with flame detectors present so if there is a failure to ignite, the valves are then shut. We actually have two of these furnaces built very similarly however on has ventilation fans installed under it (due to heat from a less efficient design). If there are any questions please feel free to ask and yes I do have a code book just looking for a bit of clarification due to the fact that hazardous areas are something I have no experience with. Thanks in advance.
 

Gregg Harris

Senior Member
Location
Virginia
I have a question concerning the classification of a natural gas fueled reheat furnace used in the steel industry. This furnace, under normal operating conditions, is basically a large (40 ft x 80 ft) box with combustion air/natural gas burners. We are in the process of installing lighting under the furnace and some believe it to be a Class I area although we have never treated it as such. One can walk under the entire furnace and it is open on all four sides. There are no gas valves or piping under the furnace but obviously we have several which feed the burners within it located above. This furnace operated at approx. 2300 degrees F so under normal operations there is certainly no gas escaping. The burners are all electrically controlled with flame detectors present so if there is a failure to ignite, the valves are then shut. We actually have two of these furnaces built very similarly however on has ventilation fans installed under it (due to heat from a less efficient design). If there are any questions please feel free to ask and yes I do have a code book just looking for a bit of clarification due to the fact that hazardous areas are something I have no experience with. Thanks in advance.

Why do others consider this to be a Class one? I see nothing in you description to point in that direction.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
You won't find your answer directly in the NEC. You will need to review the appropriate reference standards in NEC Section 500.4(B). The most likely are NFPA 30, 54 and 497.

You may want to start with NFPA 497. Read it all, it's fairly short. Sections 5.4.3 and 5.5.4 may be of particular interest; but again, read and study the entire document for the complete context.
 
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ThaShocker

Member
Location
Central Texas
There is talk of this being a class I due to what some say is a possibility of a gas leak developing and collecting in the area below the furnace. With the amount of area that is open around the furnace I just don't see the gas being in a high enough concentration. Regardless I'm trying to get as much information as I can before we make a decision. Thanks for the reference to the NFPA 497. I will have to try to get my hands on a copy.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
There is talk of this being a class I due to what some say is a possibility of a gas leak developing and collecting in the area below the furnace...
Natural gas has a vapor density of less than one...it does not collect in low areas...it connects in high areas.
 
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