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HVAC Conflict of Logic

HVAC Conflict of Logic

Hi all, I have a situation where I have a switchgear building which is located within the reach of a gas cloud. We are not able to locate the building away. The HVAC system intake damper will close in the event of 30% LEL to prevent gases from being introduced into the building. On the other hand, the building requires pressurization to be maintained which requires air intake. Hence, there is control conflict. In this case, I am hoping anyone had the same situation and was able to mitigate by engineering controls and share them. Worst case we want to avoid is to shutdown the electrical building. Thank you in advance.

RE: HVAC Conflict of Logic

Take fresh air in a different place and deliver it to the switch-gear in a duct. You already have the control sequence.

RE: HVAC Conflict of Logic

Howdy Punisher,
It is common to take the make-up air from a safe location. In the case of heavier-than-air flammable gas I have seen the intake air duct elevated 20ft above grade. I also believe, but cannot confirm, that in these locations failure of the pressurization-fan results in an immediate alarm and the eventual de-energization of the main power feed to the bldg. In these cases there are redundant pressurization-fans.
I believe that API-505 has something to say about this topic.

ps It is always preferable to locate an electrical bldg (or room) away from any classified areas.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: HVAC Conflict of Logic

Since you reference LEL, are there people in the building?

If so, I don't see what the conflict is. Doesn't personnel safety trump everything? Your "Worst case" should be to avoid injury or loss of life.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
FAQ731-376: Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list

RE: HVAC Conflict of Logic


LEL = Lower Explosive Limit. It's not a personnel-related abbreviation in this context.

I work in a facility which, in the wrong combination of circumstances, has the potential to level the whole district. Pretty much every design decision I make has the potential to affect personnel safety as well as protecting a very high capital value asset. I have similar problems to ThePunisher, and few solutions as it is a legacy site. High integrity detection of flammable material and remote tripping of the supply are the order of the day.

RE: HVAC Conflict of Logic

Sorry, was thinking Lethal Exposure Limit. If it's the former, then possibility of explosion harming civilians would still dictate a shutdown, unless they address the intake issue.

One possibility, barring moving the intake might be to run the intake into an air scrubber, but that just postpones the problem. At some point, the desire to avoid a shutdown still gets trumped by safety.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
FAQ731-376: Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list

RE: HVAC Conflict of Logic

Based on my understanding of your post, the positive pressure is for cleanliness which is good for equipment cleanliness and longevity. The damper closure is for an emergency situation that would prevent an explosion, which should not routinely occur. The latter should trump the former. If there is a risk of explosion, the negative pressure concern becomes negligible during that condition.

RE: HVAC Conflict of Logic

Hi IRstuff,

The scrubber option was considered. Based on your comments that "...but just postpones the problem." Do you mean this is just a temporary mitigation?

To all,

Thank you for all your comments and opinions.

RE: HVAC Conflict of Logic

There isn't a control conflict, only a conflict of desires.

You don't want to shut down the electrical supply (fair enough), but you locate the building within reach of a gas cloud - conflict
You don't want to increase costs by transporting pressurising air from a safe area - conflict

other option is maybe have a supply of compressed (clean) air available to pressurise the building for X minutes to safely ESD the plant?

If you've got a gas cloud wandering around I think that's a good enough reason to trip the plant...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: HVAC Conflict of Logic

Hey Punisher,
Where is the Site located? If NA, then API-505 (or 500) will be the recommended practice for this type of installation.

API RP 505 (R2013) - RP for Classification of Locations for Elec Install at Petro Facilities (Class I, Zone 0, and Zone 2)

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

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