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Hazardous area classification (HAC) of interstitial spaces.

Question asked by rrwo on Jul 20, 2018
Latest reply on Jul 26, 2018 by safteng

I am involved in the construction of a large plant. The plant utilizes and distributes numerous flammable liquids and/or gasses (the primary ones being ethanol and formic acid). In certain areas this distribution and piping is contained above an accessible ceiling. Additionally there are valves and fittings in the interstitial spaces. We are using the class / division method of classifying these areas. Adequate ventilation has been demonstrated in the interstitial space.


The ethanol piping and valves in the interstitial space  are welded in place but the valves still have collar nuts.


The formic acid uses flanged fittings for valves and pipes.


We are debating whether to leverage the exception in 2012 NFPA 497 5.4.1 (1) that reads 
5.4 Unclassified Locations.
5.4.1 Experience has shown that the release of ignitible mixtures
from some operations and apparatus is so infrequent
that area classification is not necessary. For example, it is not
usually necessary to classify the following locations where combustible
materials are processed, stored, or handled:
(1) Locations that have adequate ventilation, where combustible
materials are contained within suitable, well maintained,
closed piping systems


As described above the piping system is not hermetically sealed. Does the piping system described above meet the requirements of a "closed piping system".