Ventilation

[fifty60]

I've asked questions similar to this in the past, but still have not been able to figure out a confident answer. So I will try rephrasing it in hopes that I express myself better. If I have a non-classified building that has ventilation of 4 exchanges per hour, and then place a process in that building that is Class I Div 2, do I have make any adjustments to the ventilation system.

Is there a normative standard that discusses the building ventilation requirements for building that house classified areas?

 

[rbalex]

NFPA 30 is the "basic" standard for ventilation. Generally, you are trying to achieve whatever it takes to reduce the atmosphere to 25% LEL of the material involved. There are several examples throughout the document. See Chapter 17 for process area considerations and Part 17.11 for enclosed locations. Often, where the source occupies a large part of the enclosed location, the classification just extends to the entire volume, regardless of ventilation.

The Annexes also address various considerations. For example, see Annex F for Fugitive Emissions Calculations when the source is inside an enclosed location.

 

[fifty60]

My main confusion is the physical requirements for the ventilation system. Does the building ventilation need to use CID1 protection methods ie explosion proof or are CID2 methods ie identified for CID2 acceptable? Or since it is the building ventilation band not actually in the classified area are non classified methods acceptable? Is this addresed in NFPA 30…?

 

[rbalex]

Then philosophically, you’re dealing more with a purged/pressurized system from NFPA 497.

Since I don’t know the actual “geometry” of the installation, I would first want to know how the “internal” process was only identified as Division 2 in the first place.

The basic issue is first get the electrical area classification(s) properly identified then select the proper equipment and protection techniques.

 

[fifty60]

I am a lot more competent when it comes to the selection of proper equipment and protection techniques. Although definitely no expert, I have been wrestling with that for more than five years now. I feel like if I know what the area classification is, then I at least know what I don't know when it comes to protection techniques, and know when to go back and study NFPA 70 for direction.

The problem I see more often now is that the process equipment that is being designed as CID2, is the actual source of the hazardous area. Take the case study situation of an automobile manufacturing plant that is introducing flammable refrigerants for the first time. These entire facility may be the size of a small midwestern town, and now this chunk of the assembly line is now charging flammable material.

To me, the most important part of the entire puzzle is the ventilation system. Without "adequate ventilation", then CID2 does not exist. I understand the different approaches to quantifying adequate ventilation as the general "4 exchanges of air" or the more analytical approaches in API RPI 500. But the actual mechanics of the ventilation system is where I struggle.

Does the entire facilities ventilation system come in to play, or just the local ventilation around the process area?

The larger plants ventilation system is what is providing the 4 exchanges of air per hour, and is what is being used to determine "adequate ventilation". If i am using this ventilation system to get adequate ventilation, and from it deem the area Class I Div 2, what are the requirements for the ventilation system motor? CID1 protection methods, CID2, or non classified?

Once I get the ventilation requirements figured out, and can confidently say I have adequate ventilation, then I would apply the Point Release method form API RP 500 to define the extents of the classified area.

 

[rbalex]

The idea that the material in the equipment is what creates the hazardous location is always true - regardless of the ventilation.

 

[kwired]

The problem I see more often now is that the process equipment that is being designed as CID2, is the actual source of the hazardous area. Take the case study situation of an automobile manufacturing plant that is introducing flammable refrigerants for the first time. These entire facility may be the size of a small midwestern town, and now this chunk of the assembly line is now charging flammable material.

.

Only location that gets any classification in such a place would be where the flammable material is transferred from storage tanks to the automobile's cooling system. Once that step is done it is within sealed system and only would release flammable gas at other stages of the process if some sort of unusual condition occurs. The class 2 location doesn't normally have flammable gas present, but has higher risk because that gas is being transferred from one containment to another and therefore warrants some classification. The rest of the facility that gas is not intentionally manipulated in any way.

That don't answer any questions on classification or how ventilation has an impact, just explains where and why a hazardous location exists.

Similar reason why at a gas station the classified locations are out near the fuel dispensers and near the storage tanks, but do not exist throughout the entire facility. Those are the locations where the flammable gas may be present during "normal conditions"

 

[rbalex]

My reference to NFPA 497 in Post #4 was an error; it should have been NFPA 496.

NFPA 497 is relevant though. Both it and API RP500 recognize experience with similar installations as a valid basis for developing electrical classifications.

It should be noted that 4 air changes per hour may not necessarily be adequate, although it probably is.

BTW there is no shame having an HVAC specialist review a location for "dead air" traps or identify potential above grade volatile material collection points. Once it is determined a location has ventilation that is essentially equivalent to "open-air" and/or determining likely locations for explosive atmospheres to collect, regular techniques for electrical area classification can be applied.

It should be further noted that intense analysis rarely yields a safer or more cost-effective installation than simple "brute-force" application of the standard tables or diagrams.

 

[StarCat]

Does the entire facilities ventilation system come in to play, or just the local ventilation around the process area?
This is a very good question. My feeling after working with some of this is that the process area should be in slight negative pressure relative to the whole in general terms and the process should be interlocked with the make-up / exhaust air system.

Star