Sizing The Pumps And The Roof Tank

PUMP CAPACITY
Pump capacity can be determined from the total number of fixtures to be served. The GPM rating shown in the table below apply to each pump of a duplex or triplex unit. If abnormal water-demand fixtures (such as swimming pools, laundries or process work) are present in the building, add extra capacity to the pumps. 

PUMP HEAD (TDH)
The Total Dynamic Head of the pump includes the height from the pump to the top of the roof tank (static head), plus friction loss through the pump discharge pipe and fittings, minus the minimum available street pressure.

Example:
Static Head.......................................................................................................................120 ft.

Plus friction loss (can be estimated at 25% of static head)......................................................30 ft.
                                                                                                                                            150 ft.

Less street pressure available (varies by time of day and season of year; use minimum)............58 ft.

Pump head (TDH)...............................................................................................................92 ft.

ROOF TANK SIZE
Total roof tank capacity should be thirty times the GPM rating of the pump, plus required fire standpipe reserve.

Example:
Pump capacity (GPM)..........................................................................................................200
x30

Tank capacity (gallons).....................................................................................................6000

Plus fire standpipe reserve (gallons)...................................... ............................................3000

Total tank capacity (gallons) .............................................................................................9000