Another one about stored water quantity

[SprinklerDesigner2]

Not in Florida I'm bidding a school where the fire department reports the results of their flow test as 55 psi static, 10 psi residual @ 540 gpm.

All sorts of things wrong with this (an 8 psi pitot?) and the only way I am going to bid it is with a stored water supply and fire pump for inside sprinkler only with the outside hydrants off the public system for outside hose stream.

I am thinking a 200 gpm @ 100 psi pump will do nicely.

The most demanding area I have is the kitchen. Actually laying out heads I need 177 gpm @ 25 psi at the pump suction flange. The kitchen is right next to the pump room.

I know not everyone does it this way but I've always sized the tank to supply 150% of the pumps rated capacity for the minimum time allowed. In this case using a 200 gpm pump in an Ord. Haz. Group II Occupancy I would call for 200*1.5*60=18,000 gallons.

In the past whenever it was up to me to size the pump I always did it this way.

The reality is all I need is 177*60=10,620 gallons.

But doing a demand calc, opening the same 9 sprinklers produces 308.2 gpm @ 63 psi or a 34 minute supply.

But it gets worse.

On a new system the C-Value of new black steel is 130 and at 150% the pump is really turning out 75 psi at not 65 psi. Given these realities the actual discharge would be 332 gpm or but a 32 minute supply for an Ordinary Hazard Occupancy.

But then, as I move the group of open heads closer to the cross main, it gets worse yet with a demand calc indicating less than a 30 minute (about 29 minutes) supply for this Ordinary Hazard Occupancy. Looking at the pump curve a 200 gpm pump will put out close to 400 gpm before it runs out of steam. Can't use it of course but just because I can't use 180% in my calculations doesn't mean it won't.

I always liked my old way of doing it but competition is to intense to be able to afford to give the customer more for his money.

Thoughts anyone?

 

[ContractorDave]

You'd like to give the client a comfortable Buick but someone may convince him the new fangled Smart Car is still going to get the job done. It's hard to bid a job if the playing field is not level, especially if it's you that's tilting it away from yourself. Like in the car analogy, you can always sit with the client and try and sell him on your methodology and why it may be better, but the competition is intense because the money is tight, and it may be a tough sell.

Regards
Dave

 

[KDLLC]

A 150 gpm pump running @ 150% could easily supply your 177 gpm demand. I know many designers & AHJs prefer to size the pump based on the rated capicity, but NFPA 20 allows usage up to 150%. By your prefered method of tank sizing, this would reduce tank size to 13,500 gallon - closer to the 10,620 minimum.

I agree with Dave - since the playing field isn't level, your best bet is to sit down with the owner and discuss his priorites. Maybe a bare-bones quote with an alternate for a larger pump/tank.