Explosion Proof Design

[MattP]

I am looking at an application for an automated material handling system in an explosion proof environment (flammable substance: mineral spirits) and am trying to find standards, regulations, etc on the design of such equipment. I understand that all motors and electrical components need to be designated as "explosion proof" but what about issues such as metal to metal contact?

Thanks,
Matt

 

[Seabeejam]

Matt,

You need to be looking in the NEC, specifically Article 500 and subsequent articles. The main driver is the classification of the hazardous area, ex: Class I Div II. This will dictate what requirements you need to meet. The hazardous area could also be designated using the newer Zone definitions. Either way, items such as bonding, grounding, hazard proofing, etc are all discussed in Article 500. The NEC Handbook does a real good job expanding on the code text and provides a lot of examples.

Based on the information you provided, you are looking at a Class 1 (flamable liquids and gases) area. The Division takes a little more analysis to define.

Remember that it doesn't have to be explosion proof. It has to be Hazard Proof. Explosion proof is just one method of achieving the Hazard proof requirement. There are numerous methods of making the system safe such as intrinsic systems, purged and pressurized enclosures, hermetically sealed, oil emersion, non-incendive circuit/equipment and explosion proof.

I hope this has been of some help.

Jim Muir
Principal Project Engineer
Innoventor Engineering, Inc.
10 Kimler, Suite A
Maryland Heights, MO 63043

314.692.9998
314.692.9942 (Fax)
jmuir@innoventor.net

 

[MattP]

Thanks for the info, John. I will definitely need to check that out. However, I am looking for standards and regulations regarding mechanical design. For example, can roller chain be used and if so does it need to be stainless steel? How about steel rollers on steel rail or is brass or bronze acceptable? That sort of thing.

 

[peebee]

My guess is that UL drives that sort of thing.

 

[sewerguy]

I recently had to put conveyor system in Class I Div II area, what we did was go with a gas tight conveying system which had a very low n2 pressure/purge applied thus removing one of the three components required for fire/explosion.

I also used to deal with a similar issue all the time with refinery sewer systems. They could potentially have explosive mixtures at any time, and the junction boxes often had a pump and/or mixer installed. We always recommended blanketing the sumps to remove one of the three components of fire triangle, considering any metal on metal contact could have provided the source for ignition. Nonetheless, one of the major oil companies always ignored our recommendations and had the occasional sewer fire but considered that "normal". Go figure.

This may not be the answer you wanted to hear, but as far as I can tell you will always have the possiblity of fuel and a source of ignition (spark from metal on metal), so I would suggest you get rid of the source of oxygen.

 

[kieffer]

Since this is a flammable liquids issue NFPA 30 sets the classification areas and NFPA 70 (National Electrical Code) tells you how to install equipment to meet these issues. There is not a lot of information on issues like metal to metal contact. Perhaps NFPA 77 on static might be of some help.

James D. Kieffer, PE
Kieffer & Associates Inc.

http://www.kiefferassociates.com