Control Room Pressurization - Air Lock?

[CobyRupert]

I'm having a argument w/ myself and need a 3rd opinion.
If control room doors have an "airlock configuration" (i.e. 2 doors in series, with a vestibule area between the control room and Cl.1 Div 2 area), do you have to test air flow with both doors open?

We have a control room surrounded by a Cl. 1 Div 2 hazardous area. We will use a HVAC unit to supply Type Z pressurization to the Control Room to make it unclassified.

As per NFPA 496, Chapter 7.4 I understand the requirements to be:
(1) Maintain 25 Pa (.1" H2O) in control room with opening closed.
(2) Provide air flow of 60 ft/min "through all openings capable of being opened. The velocity shall be measured with all these openings simultaneously open, and a drop in pressure below the 25 Pa (.1 " H2O) specified in (1) shall be permitted while meeting this requirement."

The owner has an expectation that the control room will be entered through an airlock (but not really a "lock", i.e. no interlocks prevent both set of doors to be opened simultaneously. More like 2 doors in series with each other with a vestibule chamber area in between the doors)
Questions:
1. When we do the airflow 60ft/min test, as per requirement (2) do we have to have both set of airlock doors open?

- On one hand one could expect the answer to be "yes" because:

1. There is no interlock lock between the doors, i.e. both could be open at the same time? , and
2. How would one get 60ft/min flow into the airlock vestibule if the second door is closed?

- On the other hand, would the answer still be "yes" if the "airlock chamber" was 20' long? (30 ft long? a 50' corridor, 4500 sq. ft? ...etc...? See where I'm going? i.e. where's the cutoff line?).
-Also when strictly reading item (2), isn't the control room "opening" only the first set of doors into the airlock, whereas the second set of doors (between the air lock and Cl. I Div 2 area) is not technically an opening for the pressurized area (control room)?

Perhaps the real question is: Is there any point to have an air lock? (Other than the HVAC doesn't have to cycle as much (i.e the room isn't "depressurized" as much when the control room door is opened.))

 

[petersonra]

If you open one door but not the other one how do you get any air flow at all?

I am not sure what the airlock does for you.

 

[CobyRupert]

I think the biggest benefit to an air lock would be to reduce the heating/cooling load (like vestibules do at other "normal" unclassified locations (stores, offices, etc...) Though they would add the inconvenience of personnel having to go through 2 doors.

I don't think the airlock does much to reduce the HVAC fan size needed to get the required air velocity (60 ft/min). (That is, it seems reasonable you'd have to test the 60 ft/min requirement with both doors open; though it also seems reasonable that if you have an airlock, the requirement for the 60ft/min velocity test would (should?) go away. Basically the code (NFPA 496) doesn't address this situation.

 

[petersonra]

all the pressurized control rooms I have been in (I can count them on one hand so it is not like it is all that many) had such a vestibule.

it has been long enough since I have been in one I don't recall all that much about them other than that the vestibules themselves seemed to be pressurized. but it has been long enough that I might be remembering wrong.

 

[gadfly56]

Although for a standard 6'8" door you only need ~1,000 cfm to get 60ft/min average across the opening you'd probably not want to stand in a constant stream of 1ft/sec air, and as mentioned there is the HVAC load to consider, 'cause you know no one will ever leave the door chocked open!

As for testing, I vote both doors open. Even with an interlock, interlocks can fail and then you're back to both doors possibly open, no matter how long the vestibule. And if the vestibule isn't itself airtight, it's volume might have to be taken into consideration for the pressurized volume.

 

[rbalex]

Since the air lock appears to be used for routine ingress/egress, Section 7.4.1 Exception doesn't apply. This is especially true since there are no interlocks. Both doors "open" simultaneously must be the consideration. The length of the air lock is not a consideration since both doors can be open simultaneously. NFPA 496 doesn't require observing an otherwise bad design.

Note: Where all available openings are open during the positive airflow test, the operational minimum pressure is not required to be maintained.​