Dear,
You need to first specify your hazard type, its light/ordinary or extra ordinary... after that you can do it in two ways;
1) You can use the normal schedule pipe sizing of NFPA 13 for given hazard and perform a hydraulic calculation and look on the pipe velocities and your required...
Dear it seems your fire pump capacity is taken considering 2 fire hydrants open at a time. Usually all codes and standards requires 250 gpm per hydrant. As you interested in the fire tank capacity, you must check that this tank is gonna supply only to your hydrants or there are other fire...
