Yes, your fire pump is sized only for the greatest sprinkler demand. Be careful that your pipe sizing through the pump and around the bypass meet the larger demand. A calculation should really be performed on the manual wet standpipes using the local responding fire departments standard pressure from the pumper truck (usually 150 psi from my experience, more if the building is taller). This is how you should determine your standpipe sizing, or in whatever method is approved by the AHJ. For example, if your sprinkler demand is fairly low in GPM, you might be able to get by with 4" or less of suction, discharge, and bypass piping, but that doesn't mean that 4" is enough for the manual wet standpipe demand. Hope that helps.
Unless I"m misunderstanding you, I must respectfully disagree with this response. In no situation (that I am aware of) would your FDC be upstream of your fire pump suction. Therefore, your fire pump pipe size and bypass size would have no affect on a manual standpipe downstream of the fire pump. You would just need to insure that the FDC, which should be connected downstream of your fire pump check valve, was at minimum, the same size as your Manual Standpipe.
For example, if you have a manual wet standpipe system combined with an automatic wet pipe fire sprinkler system, and a fire pump serving ONLY the fire sprinkler portion of the system, then you could install a 4" fire pump/bypass even if your MANUAL standpipe system piping was required to be 6"....you'd just need to insure that all FDC piping downstream of your fire pump was sized per the highest demand, in this case, the 6" manual standpipe system. When the fire dept pumps through your FDC, flow will not go through the fire pump system piping so the fire pump pipe size is not relevant to the manual standpipe system flow demand.
Let me know if I'm missing something here....
Yeah, I shouldn't have said the suction sizing. I'm talking more if the FDC is tied into the bypass piping. The bypass piping, and anything downstream of the FDC should be sized with the manual wet standpipe in mind. Thanks for the correction.
Yes, you can do that as long as the system is not serving high rise building.
This depends on the building height and your AHJ. If your building is classified as a high rise, then NFPA 14 requires an Automatic standpipe system, however, when retrofitting an existing non-sprinklered high rise building with an automatic sprinkler system, many jurisdictions will allow the standpipe system to remain "manual". In this situation for example, in my jurisdiction, the AHJ allows us to convert the existing dry standpipe system to a manual wet system; the old dry standpipes service the new sprinkler system, but we size the pump only for the fire sprinkler system, and the standpipe demand must be met by the fire department at the FDC.
If your building is NOT a high rise, then the manual system should be fine as this is allowed by NFPA 14...but I'm guessing you're talking about a high rise?
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