Air Sampling Systems Transport Time 3

[desnov74]

Hello all,

NFPA 72 states that 5.7.3.3.2 that the maximum transport time for smoke from the most remote port in an air sampling systen should be 120sec. According to section 8.5.3.2.1.4 of NFPA 76 (2005 & 2009), the transport time is 90sec.

Should environments that have a high degree of air changes or challenging air flows to Telecom facilities use the 90 second rule?

 

[stookeyfpe]

I would apply the value in NFPA 76, especially if you're dealing with a telecommunication or data center. I agree that it appears the NFPA 76 technical committee has the same concerns you do in that these rooms have high air exchange and velocity rates because of the need to remove all the heat.

 

[rbulsara]

Not commenting on the NFPA time, but the concept of cooling in telecom or data centers. The air is NOT changed at all in those applications. The "heat" is removed but the same air keeps churning in the data centers. The is practically no or little makeup air. So no smoke will go undetected, even though a lot of air is churned (circuitlated through) the air handling units.

 

[stookeyfpe]

Thanks Rbulusara for the clarification. With this information am I correct to assume that because of the air volume being moved, this could reduce the volume of measureable combustion products that may be measured by the detection system?

 

[rbulsara]

I am not sure who it affects if at all, whatever it is it has to be very mininmal, that is the advantage of very senstive sensing. In fact moving air allows (in data centers) placing the sampling ports only in say on the ceiling yet detect smoke below the raised floor as the air beneath the floor quickly comes up in the room.

For such claims, some tests needs to be done. NFPA standars are usually based on some tests so I have no issues following their recommendaitons. Such system being not "required" by code or even insurance writers views and acceptance of practices varies greatly among users and specifiers.


My views are based on having extensive experience in data centers (large ones) and having seen only occassional use of air sampling smoke detection systems.

Either 90 or 120 sec seems an eternity for any reasonable air sampling system, I know they act much faster than that, unless you have a very poorly designed system.

 

[stookeyfpe]

Rbulsara,

Thanks for your guidance. The original poster needs to defer to your answers as my data center experience is clearly not as strong as yours.

 

[desnov74]

Thanks for the response guys,

I absolutely agree with you rbulsara and see your point. However the point of a sampling air system is incipient smoke detection, as I'm sure you know. But in situations in 76 under the raised floor where there is a large volume and velocity of air being "churned", detector sensitivity is comprimized. Hence, I believe thats where the reduction in time to 90 seconds.

I would think that an environment that did have high air changes, and did not churn the same air would be even less sensitive. So my thinking would be to use the 90secs. time.

I have posed this question to the NFPA forum and to some researches who are doing some work on air-sampling applications. Once I get some responses I'll let you know what their opinion was.

Thanks again for the responses.

 

[desnov74]

I posed the question to the nfpa and got an official response to the question:

After consulting with Ralph Transue, the former Chair of the Technical Committee on Telecommunications, I have an answer for you:

The reason that NFPA 76 requires automatic detection systems with better performance than the minimum that NFPA 72 permits is the goal of telecommunications service continuity by protecting network equipment and cables.
a. The prescriptive approach of NFPA 76 chapter 6 achieves equipment protection by
i. Fire resistive equipment
ii. Very early warning fire detection
iii. Rigorous alarm response
iv. Manual intervention
b. Chapter 6 does not require automatic fire suppression provided that the telecommunications equipment is fire resistive.
c. Alternatively, NFPA 75 does require automatic fire suppression and NFPA 76 also requires automatic fire suppression if the equipment being protected is not fire resistive.
d. Therefore, because NFPA 76 chapter 6 relies upon manual intervention initiated by very early warning detection and rigorous alarm response, it is vital that the very early warning detection system operates very early.

Bell Core, AT&T and Bell Canada ran several full-scale tests of detection systems for the purpose of minimizing damage to telecommunications equipment to maintain service continuity. The requirements in chapter 8 (the tool box chapter) of NFPA 76 for very early warning detection performance is based on the results of the full-scale testing. The technical committee could not merely refer to NFPA 72 for very early warning detection systems because NFPA 72 compliant systems do not achieve the performance required.

If you need anything else regarding this, please feel free to contact me.

Please note the authority having jurisdiction determines compliance with the Code.

This response does not represent a Formal Interpretation as noted below.

Timothy A. Hawthorne
Fire Protection Specialist
NFPA – Quincy, MA USA

IMPORTANT NOTICE: This correspondence is not a Formal Interpretation issued pursuant to NFPA Regulations. Any opinion expressed is the personal opinion of the author, and does not necessarily represent the official position of the NFPA or its Technical Committees. In addition, this correspondence is neither intended, nor should be relied upon, to provide professional consultation or services.

 

[rbulsara]

Desnov74:

Thank you for sharing NFPA's opinion.

stookeyfpe:
You are welcome, glad to share my experience. Telcom industry are much more regular users of VESDA systems than say data centers.

 

[desnov74]

rbulsara:

Thanks for the puple star!

I agree we don't do much telcom stuff though.

We've been specifying a lot over the past year using air-sampling in data center work. The trend, at least here seems to be towards as over conventional detectors. Especially when we have a raised floor and plenums.

Thanks again for the input.