Need help regarding sprinkler design 1

[fsp22]

Hi everyone. I'd be grateful if anyone can help me. I am a newbie designer working on a residential 8-story building with 3 basements as parking. As you know the residential areas are classified as light hazard and the basements are ordinary hazard group 1.
There will be one fire pump and two risers, one for ground floor thru 8th floor and the other with a PRV installed at its base for basements. I have done hydraulic calculations for upper stories based on residential sprinklers method of NFPA13 and the basements based on density/area method. The calculated demand for upper stories is 230gpm @ 180psi and for basements is 480gpm @ 150psi. The question is if I choose the pump to satisfy 480gpm @ 150psi (and more than 230gpm @ 180psi), do I need to recalculate or adjust the required pressure of upper stories for that higher flow? or a pump satisfying 480gpm@ 180psi will do the job?

 

[UFT12]

Your calculated pressures seem to be too high. Typical working pressures of standard sprinkler systems do not exceed 175 psi and that means over the entire range of a pump, in other words, churn pressures are expected to be up to 175 psi. By satisfying 180 psi you would probably have at least 210 psi churn pressure considering a generic pump of up to 120% rated churn pressure.

If you cannot reduce your pressure requirements (e.g. simply network, increase pipe sizes etc) you could use components rated above 175 psi, at least for the portion of the system that exceed 175.

 

[MatthewJWillis]

Is there not a standpipe in this building? If so, the sprinkler demands do not matter.

Otherwise, you size the pump based on the most demand. I agree with the above. BOTH of your pressures seems way high. Out of curiosity, what size piping are you supplying?

R/
Matt

 

[fsp22]

Thank you for your replies.
Dear Matt, yes I have a combination system (standpipe+ auto sprinkler), but why sprinklers demand doesn't matter? I have calculated sprinklers demand and added 100gpm inside hose allowance according to nfpa13.
Regarding the high pressure you mentioned, the pump room is in basement 3 at (level -10 meters) and I think that's why the pressure is high, but the first branches of 1st riser is in ground floor (level +1 meters) with about 160psi static pressure. And both risers are 2 1/2" (is it wrong?)
I can reduce the pressure requirements but still i'd have lower flow and higher pressure requirement at 8th floor compared to basements requirements. what is confusing me is if I supply that higher flow the required pressure in upper stories will increase because of flow increasement, then how should I calculate the required pressure for that higher flow?

 

[MGXFP]

It doesn't matter because the standpipe system will always be more demanding than the sprinklers in this case. Standpipes are what the fire pump is selected for.

With 8 storeys you could probably get rid of the PRV's if the basements are not stacked [11 storeys overall]. This could be done by tailoring the water supply to the standpipes and fire pump.

 

[UFT12]

fsp22, do we agree that a Standpipe system is NOT hose allowance per NFPA 13?

 

[SprinklerDesigner2]

The standpipe will drive the quantity but each sprinkler system must have it's own set of calculations.

By definition this building is eight floors so you do have eight separate and independent sprinkler systems. Each needs it's own set of calculations even if they are identical floors which might not be on the first and second floor but likely for the floors above.

I agree with UFT12, the pressures seem to high and you might want to look at adding a bit of larger pipe....

Let's assume 15 psi end head pressure and 12' between floors. 12*8=96 feet and 96*.433=42 psi. Without friction loss you need 54 psi which means you are losing more than 100 psi through the sprinkler pipe because loss through the standpipe can't amount to much of anything. My gut feeling is the pipe is much, much to small. I meant to say much two times to make a point.

If the AHJ requires 100 psi at the top outlet with proper sizing you might have to have a 150 to 155 psi pressure at standpipe demand but not the 180 psi you mentioned.

Is this building a "high rise" as defined in the IBC?

 

[fsp22]

Thank you all.

Dear Uft12, you are right, I had misunderstood hose allowance in NFPA 13. It should be used when exceeding the standpipe demand established by NFPA 14.

Dear SprinklerDesigner2, thank you for your help. I think I should calculate standpipe hydraulics first and then size the pipes of each floor's sprinkler system accordingly so that the pressure will be enough. Am I right?

 

[SprinklerDesigner2]

fsp22,

The standpipe and sprinkler systems are unrelated and should be treated as such. I think we can all agree that in the large majority of cases the standpipe will drive both gpm and psi unless one of the floors calls for sprinklers to be designed per Extra Hazard Occupancy which would be exceedingly rare. I have never seen that myself but I suppose in theory it could happen.

What I would do is ignore the sprinkler for right now and design just the standpipe to 100 or 65 psi at the top outlet depending on what the AHJ requires. If the AHJ requires 100 psi at the top of the standpipe you would be safe to size the sprinkler pipe as if you had 100 psi at the point of connection to the standpipe. Not hard to do.

You are relatively new at this so I will tell you something about design technicians that is pretty common. When most of us started the game was to save money and saving money always meant smaller pipes. We all did it, I most certainly did, gloating when I could save money by reducing 100' of 2" pipe to 1 1/4" thinking it impacted the job somehow.

There was a time I would gloat upon my great success of saving $21.62 on a $200,000 job. Silly, isn't it?

It doesn't matter and after 40 years of doing this I learned the best way to do anything is oversize because it really doesn't cost anything when you look at the big picture. An eight story building I would guess sold for at least $100,000 (probably twice that) and does saving $100 per floor by reducing the pipe size really mean anything? You got to look at the whole picture; if I had a little larger pipe where my sprinkler demand was 130 psi at the discharge of the fire pump it is possible I could more than make up for the extra cost in what larger sprinkler pipe cost me just by having a pump with less horsepower. And then there is electric; a 100 hp motor costs a whole lot less to wire than a 200 hp motor and then there is the cost of the motor itself.

And then what happens if you have a transfer switch with generator which is required in some places like Savannah, GA and other areas along the coast?

You might save $1,000 on reducing the pipe and fitting sizes but could easily spend $5,000 on the pump to make it work. A bit silly, is it not?

And by over sizing a bit you'll never have the expense of having to tear something out and replace it because someone found it was to small.

Another thing you will discover is it isn't material that will bust a job but labor. What I try to do is keep sizes uniform so the installers never run out of the proper size fitting. If I have a job that is partially 1 1/2" and 1 1/4" pipe and fittings what I will do is simply keep everything uniform at 1 1/2" because all the guys have to do is be short just one fitting and you'll blow all the money you thought you were saving as they go hand in hand to the supply house spending the afternoon to pick up what they need.

 

[fsp22]

Dear SprinklerDesigner2, Thank you so much for your advice. I really appreciate it.

 

[TravisMack]

I'm a bit concerned you say you are a newbie and doing an 8 story tower with 3 stories of parking. Does this meet high rise criteria. Are you in seismic areas.

As others have said, your residential sprinklers have a limit of 175 psi. Your sprinkler demand is too high. You can't do it all in 3/4" pipe.

Typically, standpipe sets pump capacity. Depending on how you put it together, typically standpipe will also set pressure rating of pump, but if you go a little crazy, then the sprinkler demand could set that pressure.



Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692