Hazardous Location Classification Question - Class I Div 2

[dpc]

I've gotten involved in dealing with classification of a biogas facility. In the equipment building, all of the gas is contained in piping running to some engines. Based on my recollection of the NEC, this room would require a Class I, Div 2 classification, since the gas is not normally present, but could be if there was a leak or other failure. They also have a substantial ventilation system for the building. The equipment supplier is saying that the building is "Unclassified" because the ventilation system will keep any leaking gas below a concentration that could explode. My understanding was that mechanical ventilation can reduce a Div 1 location to a Div 2, but it cannot be justification for considering the space to be unclassified.

So my thought was that it would be Class I, Div 2 regardless. BUT.... in reviewing Article 500.5, I see a third definition of Class I, Div 1 that seems to say a location that would normally be Class I, Div 2 would be considered Class 1 Div 1 if it contains electrical equipment and if a leak could directly cause this equipment to become a source of ignition.

So... based on this definition, it could be considered Class I, Div 1, but reduced to Div 2 by ventilation. In which case they will need some type of alarm if the ventilation system fails, I think.

It's been a while since I've dealt with hazardous location issue, so any feedback is appreciated.

dpc

 

[rbalex]

Unless an application is within the scopes of Articles 511 - 516, it is virtually impossible to classify a location from the NEC alone. You need to refer to one of the Standards in the Fine Print or Informational Notes (FPN/IN) listed in Section 500.4(B); particularly FPN/IN No.4. The most general Standards for gases and vapors is NFPA 497 and API RP 500. Both will mention that closed piping without valves and flanges can be "unclassified". This does not necessarily mean piping with valves and flanges must be classified, but rather it needs more evaluation.

Without detailed knowledge of the processes involved and their environment, a full area classification determination can't be made from the description given so far; however, it is certainly conceivable that the equipment supplier is correct.

 

[wroggent]

I find API RP 500 to be more explanatory. I think one big difference between NFPA 497 and API 500 are how they treat the inside of buildings. In the back of NFPA 497 are sample diagrams for area classification - looking at them you'll notice that providing ventilation in a building would allow reduction of, for example, the entire building being classified as Division 2 (with some Division 1 areas around the source), to instead a region defined by specific radii around the potential source (with no division 1). See Figure 5.9.1(C) versus Figure 5.9.1(f). To summarize, NFPA 497 is showing that an area (read-some areas) can be changed from classified to unclassified if adequate ventilation is provided. API on the other hand has 6.3.1.2 which says "Providing ventilation to allow the reclassification of an enclosed area from classified to unclassified is not allowed in enclosed areas containing devices handling hydro-carbons..." API 500 somewhat expands on that in 7.3 where it says "... If adequate ventilation is provided, mechanically or naturally, many enclosed locations may be classified Division 2 instead of Division 1..."

Pick your poison I guess

 

[mls1]

An example where ventilation can be used to avoid classifying a space is a battery room. Certainly, there will be a risk of H2 buildup with vented lead acid cells but IEEE 484 specifically states that a battery area meeting the ventilation requirements of paragraph 5.4 of the standard should not be considered a classified (hazardous) location. In addition to referencing NFPA 497, this document provides a pretty good overview of applicable references and classification methodology:

http://12.33.72.15/pdfs/technical/practical guidelines for ctg power plants.pdf

 

[dpc]

OK, thanks for the comments. If I actually have to do something on this, I'll need to get a current copy of NFPA 497 and API RP 500 for further education. Since I posted this question, the situation has changed a bit and I think I may politely pass on this "opportunity".

I appreciate the feedback.

dpc

 

[Strogoff]

Hello, DPC.
The famous Edith Clarke books are now available in pdf here:

http://babel.hathitrust.org/cgi/pt?id=uc1.b4164139;q1=edith clarke circuit analysis#view=1up;seq=11

 

[Hoxton]

Over here in the UK, because there were no siutable codes or guidelines the Landfill industry got together and produced an "Industry Code of Practice". This avoided going down the route as viewing it purely as process plant, refinery etc.

This (ICoP2) and other codes are available here

http://www.esauk.org/reports_press_releases/esa_reports/dsear_guidance.html

Please remember that compliance with this, will not satisfy your codes, but you may find the way they arrived at their document useful.

The basic points are::

Full document is to be studied and assessed by a Competent Person. The following is a bullet point summary:
The ICoP covers extraction etc as well as power generation. Only the sections dealing with power generation are summarised here:
6.1 Overview of power generation
6.11 Engine enclosure
Control room to be separate from engine compartment
Cables passing through dividing walls are sealed
Ventilation inlet not to be within a Zone 2 created by other sources.
Ventilation for control room is independent of engine hall ventilation
Engine housing fitted with explosive gas detection.

6.11.4 Zoning (of engine hall) should be considered but boiler rooms are not zoned (BS 5885).
6.11.5 Ventilation – 12 changes /hour (subject to risk assessment)
6.11.6 Further Guidance
Risk is likely just after maintenance – procedures to ensure suitable ventilation at all times, including maintenance.

The other point is that if this is biogas from a digester, then please do not forget H2S - hydrogen sulphide.

Not only a poisonous gas, it desensitises the nose, so the danger is still present after the smell of rotten eggs has gone!

H2S also corrodes copper wiring!

In general, the practice in the UK is to put the generation outdoors in containers, so any leaks from external pipes etc cannot accumulate indoors.