System Demand 2

[ContractorDave]

Situation:
* 1970, 12 story commercial light hazard, scheduled.
* No stand pipe (Hoses have been removed from existing as per AHJ)
* 6" supply avg 65psi thru a 6"DCVA
* 500gpm @ 95psi net fire pump.
How would you determine a system demand?

 

[SprinklerDesigner2]

The only wayto to accuratly determine demand is do the hydraulic calculations but I am pretty sure it would work just fine for sprinklers.

Figuring an average of 90 sq ft. per sprinkler we would have around 16 sprinklers to calculate each discharing 15 gpm.

240 gpm for sprinklers but this would approximate a theoretical minimum so to be on the safe side I would add 25% for 300 gpm.

Friction loss through just the pipe on the top floor might be 40 psi but, seeing how it is pipe schedule, I doubt it's that high but let's use 40 anyway.

7 psi end head pressure.

40 psi friction loss through overhead pipe.

If fed by old 6" standpipe the friction loss would be negligible.

12 stories so let's figure 12' between floors for a max height of 144 feet. Elevation loss would be 63 psi.

Total loss, ore pressure demand, would be 110 psi or less.

95 psi pump and say 50 psi from city we have 145 psi available. I would have to think you would have at least 145 psi @ 500 psi available at pump discharge.

100 gpm added for hose stream and I don't see how total demand would exceed 400 gpm at @ 130 psi.

I don't for the life of me see how this wouldn't work unless something is really wrong but, as I said, we need piping layout and distances to be sure.

 

[ContractorDave]

Thank you SD2. Good response. I have a situation for you. I've been given some forms to fill whilst doing inspections and several questions in the Annual Testing section go something like this: "Backflow prevention assembly forward flow test conducted?" and then,"System demand was achieved through the device?" What are your thoughts? I'm in the Northwest Territories up in Canada and 99% of systems installed here do not have 'system demand' figures available.

 

[LCREP]

Assume it is a pipe schedule system and look at table 11.2.2.1 and section 11.2.2 NFPA 13, 02 pg 99. At least it is a starting point, very conservative, but better then not testing at the required flow. Down side u will need to flow water via the FDC by reversing the check valve to get close to the required flows. A main drain is not going to give u more then 200-300 gpm. OK for light hazard but not much more.