Another ventilation question.

Status
Not open for further replies.
kameele

kameele

Member
Location
NH
I am working on a design involving the installation of a gas chromatograph into a previously unclassified DAC (control) room at a NG metering station. The customer has specified a Div 1 gas detector tied to a Div 2 fan as a method to maintain the unclassified rating. It would also send an alarm to gas control when it turned on the fan. In principle, Section 500.7(K)(2) would seem to allow this, but only if the GC is not considered to be a vapor source. Folks with a lot more gas experience than I have, say that this is the standard way of dealing with this (which obviously doesn?t necessarily make it code compliant)

API RP 500 6.3.1.2 says that ventilation is not an acceptable reason for making an area unclassified if it contains devices handling hydrocarbons. 7.1.2 says that volume, source and rate are to be considered in making an engineering judgment. Based on this, the volume of gas that could come from the tiny tubing (1/8?) supplying the GC, there is very little likelihood of being able to build up enough gas to get anywhere near LEL once the fan was turned on by the GD at 20% LEL.

If anyone feels that this system would be acceptable, would you consider that any new wiring involved should be done as Div 2 or as ordinary? Also, do you have any knowledge of calorimeters or chromatographs being an exception in relevant codes?

Thanks for any input.
 
Personally, I think it takes an excessively broad interpretation of Section 500.7(K)(2) to believe it applies to your situation at all. It was intended to apply to installations under API RP 500, Section 6.5 and not much more. Unless the fan were outside the exhaust stream, I wouldn't be too comfortable operated it knowing an ignitible mixture was at least potentially confirmed to be present.

However, since I still don't know enough details, under conditions such as yours, I tend to recommend reviewing NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals, 2004 Edition. It isn't overly large, but you will often find yourself bouncing around several sections before you come to a final conclusion. Fortunately, it also has some excellent analytical materials and you will probably find yourself very confident with that conclusion.

With a properly installed hood, you may not need to do anything else at all. (See NFPA 45, Section 5.6.2)
 
Bob,

thank you for your response. because of your concern about the fan, i went back and checked its listing. it is actually listed for Class I, Group D, which I believe makes it suitable for both Div 1 and 2 and thus suitable for use in a atmosphere containing NG.

I haven't had a chance to look at NFPA 45 yet, but will take a look later today.
 
You are correct that the motor is suitable for either Division. Very few motors are listed only for Division 2 since they aren't required to be listed for Division 2 at all.
 
Review NFPA 496 (2008), Chapter 9, Pressurized analyzer rooms contianing a flammable source.

Also, Control Magazine, April 1997 had a good article on analyzer buildings. It covers purging and pressurization to protect from outside leak sources, higher levels of air dilution and excess flow valves for your sample lines.

BobG
 
While both NFPA documents are Standards, the reason I preferred NFPA 45 to 496 in this particular case is that it is more specific to the application as I understood it and would likely provide the more cost-effective solution. Had I not been aware of NFPA 45 I propably would have nudged in 496's direction too.
 
I guess I understood the application to be a GC in a natural gas facility. These buildings are typically (from what I have seen), small, one or two room prefab buildings or shacks that house the GC equipment, sometimes a computer and printer, etc. They are not an analytical or QC type lab and do not contain hoods.

These buildings can become a "gray" area because they are typically bought with general purpose electrical equipment and installation methods. Plant process expansions can sometimes put the potential leak sources in close proximity to the building, and sample lines into the building add other potential (small) internal leak sources. Add that to the fact that temperature control of the building may be necessary, so added dilution air may be difficult in cold climates.
 
Theoretically, once the word laboratory applies, so does NFPA 45. However, after the initial Scope statement in Section 1.1.1, it also goes through a pretty long list of self-exclusions in Section 1.1.2. If it does apply, it's pretty broad from analyser sheds to full blown facilities. That said, NFPA 496 is one of its referenced sources and you may very well be lead there eventually. As I said, you may bounce around a lot but once you get through NFPA 45, most folks are pretty confident in their design.

You're definitely right about process expansion "encroachment" and 496 would most likely be my Standard of choice too. Enviornmental treatments can also add dimensions not mentioned so far.
 
bobgorno said:
I guess I understood the application to be a GC in a natural gas facility. These buildings are typically (from what I have seen), small, one or two room prefab buildings or shacks that house the GC equipment, sometimes a computer and printer, etc. They are not an analytical or QC type lab and do not contain hoods.

These buildings can become a "gray" area because they are typically bought with general purpose electrical equipment and installation methods. Plant process expansions can sometimes put the potential leak sources in close proximity to the building, and sample lines into the building add other potential (small) internal leak sources. Add that to the fact that temperature control of the building may be necessary, so added dilution air may be difficult in cold climates.
Click to expand...

Bob--on the nose. and yes, the room needs to be kept warm, but this is not critical, as the sample tubes and cal gas bottle are kept warm with independent heat sources. If the gas detector triggers the fan, nobody will care about some cold air while they figure out where the gas is coming from.

I will also go take a look at 496, which sort of slipped my mind, since I'm not dealing with just a cabinet. and I'm slow--haven't dug into 45 yet. Have to get to that shortly.

Thanks for the further input.
 
Status
Not open for further replies.
Top