I followed this discussion with some interest - not least the reference to flame arrestors. I assume the reference is to storage cabinets in NFPA 30 (the term flammable cabinets is not used in NFPA 30). NFPA does not require such cabinets to be vented (9.5.4). but if they are to be vented (126.96.36.199) to be vented directly to a safe location outdoors or to a treatment device to mitigate the formation of VOC’s or ignitable vapours to the satisfaction of the AHJ (I paraphrase). Annex A makes it clear why (A 9.5.4) venting is not recommended for fire protection but that it may be required for other reasons (health and safety). In such cases the ventilation should not affect the fire performance. If vented, it recommends venting from the bottom with makeup air supplied at the top (mechanically assisted in accordance with NFPA 91) and the vent should not be combined with other ducts or vents from other storage cabinets. Any vent should be to a safe location free of source of ignition, consequently there should be no reason for flame arrestors (NFPA 30 does not refer to flame arrestors).
I come from the world of process analyzers in the chemical and petrochemical industries where there is frequent reference to the use of flame arrestors (between the analysis equipment and also at the point of discharge post analysis). However as a few correspondents have pointed out flame arrestors seldom add reliability as they are prone to fouling and corrosion. In the best of circumstances they need be sized for the ignition characteristics of the ignitable vapour to be present - and I would respectfully suggest that such information is unlikely to be known or anticipated in the case of storage cabinets. Also flame arrestors are designed not only to prevent propagation of flame through the barrier but also to contain the much higher explosion pressures that can arise from a supersonic blast wave travelling ahead of the flame front (pressure piling) which leads to a supercharging of the explosive gas air mixture and much higher detonation pressures. Consequently flame arrestors are designed for the line size to which they are to be fitted and increase dramatically in both volume and weight with duct diameter - I respectfully suggest that ventilation ducts are unlikely to match the pressure rating requirements of the arrestor.
All to say if a ventilation system is to be used, presumably it is to mitigate the toxic risk, it would require dilution of vapours to a concentration far below that which could be flammable (typically by a factor of >100:1). If such were the case - it would be better to have a monitor on the ventilation flow rate.
James Osprey P. Eng. C. Phys
Tel: (514) 339-5374 ext. 208
If there is not ducted ventilation through the cabinet, the bungs (both of them), should remain in place. This will likely improve the temperature barrier properties of the unducted cabinet. Unless, of course, the authority having jurisdiction requires otherwise. Then you want those flame arrestors to remain in good condition.
If an existing cabinet is exhausted from the high bung rather than the low bung and you can accomplish it without undue difficulty or expense, I would correct the error.
Peter Zavon, CIH
Thanks in advance and pardon the cross-posting.
NFPA 30 states that IF flammable cabinets are ducted to the ventilation system that the supply be provided at the top and the exhaust be connected to the bottom. Presumably, this creates a downward directed cross draft that ensures removal of vapors with high vapor density from the bottom of the cabinet.
I have two questions:
1) if there is not a ducted supply does that supply bung stay open or closed?
2) if an existing cabinet is vented with the exhaust from the top should it be changed to vent from the bottom?
Christopher E. Kohler, MS, LPG, CCHO
Laboratory Safety Manager, EHS
Laboratory Safety Instructor, ACS
Adjunct Faculty, SPEA
Environmental Health and Safety
1514 E Third Street
Bloomington, IN 47405
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