NFPA 13 - Sprinkler Temperature Ratings vs Max Ceiling Temperatures

[ziptron]

Hi Everyone,

Does anyone have any understanding or experience with why the NFPA choose such a large temperature gap between the max ceiling temperature allowed and what the "ordinary" sprinkler head can provide? Same goes for all sprinkler classifications and their MAX ceiling temperature.

When you look at the "ordinary" classification, it is a sprinkler head that ranges between 135F to 170F, however it can only be used in a place where max ceiling temperature is 100F. If you have a sprinkler head on the higher end of that "ordinary" scale, you're basically 70F above the max ceiling temperature allowed..

Is there some research out there that backs this up and explains the rational? Are sprinkler heads not all that precise and have wide range of temperatures at which they will discharge below their rating even though they may be very good at discharging above their rating.. and thus, is the NFPA and the general industry afraid of false/early discharges?

Maybe we should be allowing 170F sprinkler heads to be installed where ceiling temperatures max out at 150F?

Any insight/discussion would be much appreciated.

Thanks!

 

[Sdpaddler50]

I will take a shot at this, because no one else has.

Disclaimer, this response is my personal opinion and I did not research my answer to verify it is 100 percent correct. If someone has info to the contrary, then I would like to hear it.

the key point here is temp the link must be heated to, not the air temp around said link. IE, a 165F rated head will pop when the temp of the link reaches that threshold. The heat soak, ie air temp. to achieve that, will probably be much greater than 165F.

There are other key factors involved like RTI, QR vs. SR, but that is the fundamental explanation.

 

[ziptron]

Yeah I understand the mechanism of it. However that 165F link you mentioned cannot (according to NFPA) be used in a ceiling where temperatures may exceed 100F. Just seems like too big of a safety factor for me.

 

[Sdpaddler50]

Ok. I guess I misunderstood your question. Personally, I don't have an issue with the SF. I did a loss investigation once where a couple heads popped in a hot
attic. Created quite a mess. But, I will let others chime in now.

 

[LCREP]

25 guys got into a room and threw numbers against the wall until they all came to a majority vote. Welcome to the reality of a consensus standard! Move onto more important things to worry about say global warming.......

I say this as a committee member of a NFPA standard. If you want to see how the sausage is made attend a committee meeting and see what happens. The general public can attend any meeting by sitting in the back of the room and watch the fun.

 

[cdafd]

The other aspect is do you want a sprinkler head for say temperature ratings at ten degree increments ????

Say a 100, 110, 120,130,140,150,160,180,190????

Be a lot of heads to decide among and which one to use???

 

[stookeyfpe]

I'm going to approach this from a different perspective and that is a problem occurring? The temperature limits established in NFPA 13 probably haven't changed in the 25 years I've practiced fire protection engineering provided the maximum temperature limits are followed. As Sir Sdpaddler stated, the UL and FM standards for sprinkler listing require that it be subjected to the plunge test in which the sprinkler is plunged into a heated air stream flowing at a given velocity to evaluate the sprinkler's fusible element. This is done to determine it's operating temperature and response time index.

I've maybe experienced one incident in a warehouse where the owner did an exceptionally crappy job of maintaining the insulating and a 165F sprinkler activated after 30+ days of 100F+ ambient temperatures. My anecdotal data indicates to me that this isn't an issue when the NFPA 13 rules are followed.

 

[ziptron]

Thanks everyone for your input, we're getting somewhere for sure!

I realize that the NFPA regulations are not really an issue for anyone because anytime you place a large safety factor on something, that one thing will likely work.

Through my discussions with sprinkler manufacturers, I am continuously assured that the sprinkler will go off at pretty much exactly its rated temperature. If I am to believe this, the large safety factor put in place by NFPA seems strange. I know what we have now works, but I'm asking theoretical questions regarding why is it that we have what we have. It COULD be that its a number that was threw out by 25 people and it stuck to the wall, but you'd think those 25 people would haven chosen something higher, something closer to the sprinkler's actual rating.

Perhaps my main question lies with trying to understand the chances that a sprinkler head will go off at a lower temperature than what it was in fact rated for. Is this possible? Is there any research that shows this? Does this happen? Also, it would be interesting to know if there are there any other mechanisms that elevated temperature can bring forward and cause the sprinkler to eventually fail? Any research on creep for example would be interesting.

Stookeyfpe, how did for example the 165F sprinkler activate in 100F ambient temperature? By all logic, 100F is less than 165F and it should not activate...

 

[stookeyfpe]

Simple:

The building insulation was not maintained so in the same building I attached a data logging thermocouple on a truss near the metal seam roof and left it there for a week. This was a particularly nasty South Texas summer where we experienced 67 days where it was over 100F. I found that on days where the ambient outdoor temperature was 104F or greater we logged temperatures of 135-150F at the roof deck. The building was a 65+ year old cotton bale warehouse. I concluded the lack of any insulation plus 65 years of less than stellar maintenance, plus the age of the sprinklers resulted in enough expansion and contraction of the fusible links that a premature failure could be a reasonable failure mode.

That was the only credible solution I could reasonably substantiate without going through a whole bunch of testing. The owner soon sold the building and it was torn down.