Re: [PipingDesign] Hazardous zone

From: <>
Date: Thu Mar 22 2007 - 14:31:00 EDT


De-rating a hazardous area can potentially get rather complex. In the USA, the codes typically reference or adopt Article 500 of NFPA 70 (the National Electric Code), and NFPA 497 (Classification of Flammable Liquids......), and other NFPA standards that are designed for specific processes or operations. The NFPA standards refer to themselves as "Recommended Practices". I am unaware of any code or standard that specifically addresses de-rating, so one approaches de-rating more by inference and good engineering judgement. Of course "good engineering judgement" can trump pretty much any code or standard in use in many ir not most jurisdictions. The problem becomes one of personal liability. That said.....

With "adequate ventilation", almost any area might be justifiable as non-classified. NFPA 497 defines Adequate Ventilation as "... 6 [fresh] air changes per hour, or 1 cfm per square foot of floor area, or other similar criteria that prevent the accumulation of significant quantities of vapor-air concentrations from exceeding 25% of the LFL." Conservatively, many people assume 2 x 6 changes per hour must be conservative. However, the layout and other local issues must be considered - such as, are there any columns, beams, tanks, etc. that can function as baffles and impede the exchange of fresh air? Also, what MUST be done to assure that the concentration can NEVER exceed 25% of the LFL? Sometimes the characteristics of the particular flammable vapor (or liquid from which it is generated) must be considered. How much could be spilled from a potential rupture or other containment failure? Even with a "proof of operation" and timer on ventilation fans, what modes of failure are possible, and how can they be designed around? A lot of this can come out of a well organized and documented Process Hazards Analysis. A small enclosure (not structural) can be built around the source, and exhaust the source to keep in safe (for personnel exposure and respiration, for instance), and almost everything outside the enclosure is non-classified. And ultimately, the owner of the facility is responsible (legally) for the classification (or lack thereof). Is he (whether an individual or corporation) willing to stand behind the documentation and recommendations? Insurance costs could be higher because the classification is counter to insurance carrier policies.

After all of this, I find that, in the situations where the owner is responsible enough to consider classification, they tend to go way overboard and classify an entire room because of a small potential source 150 feet away, often in a different ventilation zone, and they waste a lot of money on installation capital and on-going maintenance. Also, I find ignorant owners who have just plain been lucky, because they should design in electrical and mechanical systems to control potential hazards, but didn't even know the hazard existed. It's pretty much like hazardous piping design in that regard.

Hope that answered your question.

... Bruce D. Bullough ...
Lead Process Engineer
Foth Production Systems, LLC
8550 Hudson Boulevard North, Suite 100
Lake Elmo, MN 55042
Direct: +651-288-8598 Fax: +651-288-8551

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Jack Osborn <>
Sent by:
03/22/2007 11:45 AM
Please respond to


Re: [PipingDesign] Hazardous zone

Hi Bruce

....I appreciated your answer to Amit....These things are critical and I think you gave him good advice. Can I ask about the strategy of reducing, say a Class 1, Div 2 area, to an unclassified area? understanding is that this can be done if the ventilation rates are 12 air changes per hour or higher. This requires that the Fans have a proof of operation and perhaps alarmed for failure. Is this something you know about and can comment on?

thanks for your answer in advance.....


Jack Osborn, P.E.
Mechanical Engineer
IC Engineering
510 242 - 3094 Received on Thu Mar 22 14:31:00 2007

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