Jack: As others have said, the tank is to be sized for the flow and duration of the greatest sprinkler demand, but I want to speak to your question about the pump. I would also suggest you consider using a break tank.
If this is a high-rise (I assume it is, as that's where the ICC tank provision kicks in), then you have to have an automatic standpipe system and the 100psi is a minimum remote outlet performance criterion. If you size a fire pump that takes positive suction head from the city main, you will have a higher discharge pressure and can use a smaller pump than if your calc's utilized just the NPSH from the tank. But if the primary water supply is interrupted, then the pump will be producing less total head - not a problem for the standpipes because that's not the intent of the tank, but a problem for sprinkler demand if you used the higher suction pressure from city supply. Arguably, this is what the FDC is for, but you have an on-paper discrepancy that's not very well addressed in the code and standards. A break tank allows you to have a steady state discharge pressure that will vary only with the height of the water column in the tank. It will be more expensive because of the fill mechanisms required (redundancy required) and the nominally larger pump, but IMHO it's a better setup. Your discharge pressure will be one curve and you can design sprinklers and standpipes to that performance without having to diddle around with high/low static variations. Keep in mind that IBC 403.3.2 requires two city connections to pumps serving high-rise buildings; these can be headed off at the fill lines for the tank. Steve Leyton -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Fairchild, Jack Sent: Thursday, December 22, 2011 5:39 AM To: [email protected] Subject: Pressure Requirements for Secondary Onsite Water Supply To those in urban seismic areas: When required to provide the onsite secondary water supply, is the pump/tank required to provide 100 psi at the top of a combination standpipe or just sprinkler pressure demand? I have a situation where the city/pump combination can meet the standpipe pressure demands at the top, but the tank/pump doesn't. I have two projects with new high rise buildings being constructed. Both are campus style pumping arrangements (no underground pipe). One of the projects has an existing pump/tank combination which was not designed to meet 100 psi at the top based on tank alone. The combination pump/city supply does meet the standpipe pressure demand, but not the pump/tank. The other project has a pump/city supply which I am adding a pump/tank to. The new pump is redundancy only. Adding the tank without increasing the pump size will not provide 100 psi at the top. [F] 903.3.5.2 Secondary water supply. A secondary on-site water supply equal to the hydraulically calculated sprinkler demand, including the hose stream requirement, shall be provided for high-rise buildings assigned to Seismic Design Category C, D, E or F as determined by this code. The secondary water supply shall have a duration of not less than 30 minutes as determined by the occupancy hazard classification in accordance with NFPA 13. Exception: Existing buildings. Jack Fairchild _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum
