In a 6 floor car park, the design area consists of 12 sprinklers in 6th floor. Suppose the flow rate to be 240 g.p.m and the feed main pipe size to be 3". What will the flow rate for which the riser size will be calculated at lower floors? The same 240 g.p.m or more? [This is not a combined system, wet sprinkler , similar floors.]
Said another way, what is the size of this vertical pipe between floors ? Will the size be increased in lower floors, or not. And if it will, how much? Proportional to number of design area sprinklers in every floor or any other way?
You will size it based on calculations on your worst case area. So, whatever pipe sizing you have for the 6th floor, that would be replicated on the 1-5 as well.
You can have a 4" riser feeding a 10 story office without issue, if you have enough pressure. I assume you are not sizing per pipe schedule and are sizing per hydraulic calculations. If the latter, then the calcs drive the size. If you need bigger pipe closer to the bottom to help with friction loss, then so be it, but it is soley determined by calculations.
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