Classification of Analyzer Bldg - Group B or Not?

[Bonsall]

I have an Analyzer Bldg that I'm not completely sure is classified correctly. I'm new to area classification and would like a second opinion.

Background Info: The bldg is 8ft wide x 8ft deep x 8ft tall with forced ventilation (standard window unit). It is not pressurized as per NFPA 496. The bldg has two Hydrogen Cylinders (used as a carrier gas) located on the exterior of the building. A flow restrictor in the line where the H2 is entering the building will limit the H2 to approx. 15 L/Hr on the interior of the bldg. Also, the bldg has 2 LEL detectors that continually monitor the LEL level inside the bldg.

Per the building manufactuer, the interior and exterior rating of the building is Class 1 Div. 2 Group C&D. I thought that when Hydrogen was present it becomes a Group B area. The manufactuer states that the 15 L/Hr restrictor and forced ventilation is sufficient to dilute the level of H2 inside the shelter in case there is any leak of H2.

Is there an NFPA standard or some other document that discusses the volume of H2 required to make an area Group B? I did not see it in NFPA 497 (or maybe I'm not interpreting the tables correctly). The two exceptions in the NEC for Group B are for gases other than Hydrogen. If someone could point me in the right direction I would greatly appreciate the help. Thanks,

 

[rbalex]

... Is there an NFPA standard or some other document that discusses the volume of H2 required to make an area Group B? ...

There is no such Standard and the manufacture would have cited it if there were.

Since you mentioned NFPA 497, I assume you have it. Compare the MIE, MIC and MESG of hydrogen with the various Group B "exceptions." You will see hydrogen falls well within both critical values of MIC and MESG. Most of the "exceptions" actually fall outside the ranges but have fairly low MIEs. It is important to recognize most Group B materials were originally classified "by test" as indicated by the superscript "d" in Table 4.4.2 [NFPA 497] and they orginally had no specific MIE, MIC or MESG associated with them. (IMO this was the one of the few positive contributions of recognizing the Zone system of Classification)

In summary, the classifications should be Class I, Division 2, Group B, T1 for the installation you described.

 

[bobgorno]

A process analyzer shelter brings in many considerations not normally encountered. Some of these considerations may be risk based.

Review the following docs to understand what your suppliers justification is, as well as industry practice. Your analyzer system is likely not suitable for Group B. Ovens and components of a GC may not contain a H2 explosion. The H2 is a carrier for your sample, the sample may or may not be flammble in itself. Often the sample itself is not flammable becasue it is a flue gas, loaded with CO2. The sample could be flammable is it contains H2S. The mixture of the H2 and sample gas may become a different group based on LeChatelier. See NFPA 497, Annex C and Google this whitepaper doc (too large for me to attach here):
NEC GROUP CLASSIFICATION OF MIXTURES
Edward M. Briesch
Senior Staff Engineer
Underwriters Laboratories, Inc.
333 Pfingsten Road
Northbrook, Illinois USA
PREPARED FOR PRESENTATION AT
American Institute of Chemical Engineers
34th Loss Prevention Symposium
Atlanta, Georgia USA
March 8, 2000

Review:
NFPA 55 (2005), Chapter 10
API RECOMMENDED PRACTICE 555, Process Analyzers

Key elements are:
400 cu.ft. limits (usally one cylinder). manifolding two cylinders together is a convenience for the operator or technician and can allow a leak to continue too long.
restriction of flow
maxiumum allowable quantities
ventilation, ventilation, ventilation
LEL may not be enough risk mitigation, may need H2 detection (recommended)
May need ambient O2 monitoring if samples or other cylinders can displace oxygen.

Good Luck!!
BobG