C1.D2 locationg - type z pressurization system design

[jmotyka]

Under the nfpa 496 type z pressurization umbrella what are your thoughts on this design / theory. Use nitrogen to perform the initial enclosure purge and pressurization, this allows a pressure switch to close which in turn closes a contactor that makes the electronics energized. This enables an enclosure mounted fan to turn on and a damper to open, then the fan takes over the pressurization of the enclosure and enables the nitrogen to be turned off (the fan is pulling air through ductwork from a non-classified area -I know it's not ideal to have negative pressure in the ductwork but lets say it is sealed and a combustable gas sensor is also monitoring the ducted air).
The reasoning for this design is to avoid having c1.d2 rated pressurization fan and explosion proof (NEMA 7) boxes for contactors and batteries while also limiting the amount of nitrogen consumption (only used during system start-up). Thanks in advance for your comments.
Regards,
Jason

 

[rbalex]

While I don't know the full extent of your design, there is sufficent evidence that you are not too familiar with NFPA 496 in the first place. Which chapters are you attempting to apply?

 

[jmotyka]

section 2-8, Type Z Pressurization. This in conjunction with API 14F

 

[rbalex]

My previous answer was harsher than I intended.

I believe you left too many open questions. Are you intending to pressurize or purge? (the difference is subtle but significant.) What was the basis of area classification in the first place? (?Type Z? implies you are starting with a Division 2 application.) Motors are not ?rated? for Class I, Division 2 in the first place. The gas detectors in the duct is not a recognized application under NEC Section 500.7(K).

 

[rbalex]

The API 14F helps a bit, but you tend to see more Division 1 applications for offshore work.

Edit add: Your reference to Section 2-8, indicates you aren't using the latest Edition. (Too bad - that was when I was on the Tech Commitee ) In any case, I'm actually asking which chapters between say 4 to 8 in the latest edition)

 

[jmotyka]

Sorry I don't yet have the latest edition, still stuck in 1998 but have requested it and should have it soon - any significant changes?

 

[bobgorno]

There are concerns with your OP. Batteries in XP enclosures? Where does the gas generated during charging cycles go? There are other ways to handle that. Nitrogen for purging and pressurizing an enclosure is OK when needed if you have no instrument quality air, and you have adequate protection from N2 Asphyxiation. A fan to pressurize an enclosure seems extreme.

Sounds like you have a some work and discovery ahead of you.

Good luck and keep the questions coming....

 

[jmotyka]

One of my replies got lost in space there - hopefully it doesn't show up after I post this. I intend to purge and pressurize both are required to start up a system in a Class 1 Division 2 location (see nfpa 496 3-4, yes in the 1998 edition). As for the motors - you know what I mean, by C1.D2. rated -I think - not actually listed but appropriate for use in this location, explosion proof or brushless/non-sparking. I'm not sure I follow you Bob on your nec ref. 500.7(K)(2) it seems like it is acceptable??
As for batteries in XP enclosures, when I have seen it done there is actually a suitable breather vent used in the XP box. Using nitrogen or a fan for purging and pressurization is not new, check out PEPPERL+FUCHS, they bought out BEBCO who makes these types of systems.

 

[rbalex]

“Purging” implies the source of potential ignitable materials is internal to the enclosed volume and it is being diluted to an acceptable level. That’s why there is a “new” Chapter 8. “Pressurizing” implies you are keeping ignitable materials from an external source from entering the enclosed volume.

Virtually any three-phase induction motor is acceptable in Class I, Division 2 unless it has some additional protection elements that require switches, etc. See the last sentence Section 501.125(B).

If 500.7.(K)(2) validly applies either you don’t need to pressurize or, if you pressurize, you don’t need the gas detector.

In general, battery cells themselves, installed per Art 480, do not create classified locations and, if they are installed in a location that would have been classified Division 2 anyway, don’t need any special attention. Auxiliary equipment such as chargers or disconnecting means need to be suitable of course.

 

[jmotyka]

I agree with your definition of purge and pressurize. If you have an enclosure (non xp) that has been sitting idle in a classified location the interior of that enclosure must be considered classified until purged, once purged you can maintain the non-classified status by maintaining pressure, I will read the "new" 496 once I get it. The API specs. I believe require both pressurizing and gas detection. If a waft of classified air inadvertantly get blown over to you "fresh air" intake and you suck in some ignitable air you want to know about it - Personally I thing it is a good idea for my specific application (off-shore platform). I don't want to really go on about all these fine details but would rather bring this discussion back to my OP if possible, thanks

 

[rbalex]

In the interest of full disclosure – I acknowledge I am in the minority but I am opposed to the general use of gas detectors as a classified location protection technique until NFPA or some other consensus body develops an ANSI sanctioned application Standard for them similar to NFPA 496 for purged/pressurized systems.

I sort of lead the opposition to them during the 2002 NEC cycle when I was on CMP14 and they were introduced as a protection technique.. I won’t go into all the details; you can read my comments in the 2002 ROC. My opposition was not because they are historically unreliable. The fact is they have an excellent record when installed, maintained and calibrated properly. (500.7(K)FPN 3)

The limitations listed in 500.7(K) were a direct result of my position. Note API RP14F is not one of the Standards listed in the FPNs. Note also the two application standards listed in the FPNs are ANSI sanctioned, API RP14F is not. (FPNs 1 & 2) The significance of this is that in civil suits ANSI standards carry greater weight as minimum “standards of care” than non-ANSI documents. .

My opposition to gas detection systems is because their mere presence implies the location is STILL at least Division 2, no matter what other protection techniques are applied. Once a gas detector detects the presence of gas – the gas is already there and it will usually reach LEL before simply turning the power off will do any good. The only reasonable alternative is to apply other Division 2 protection techniques. This argument was persuasive enough to get CMP 14 to place the current limitations on gas detection systems.

Edit add: Completed the "(500.7(K)FPN 3)" reference