Let's assume that our largest fire water demand needs the following:
- 4 hose streams (946 lpm x 4 = 3,784 lpm)
- 2 fire water monitors (1893 lpm x 2 = 3,786 lpm
The total flow rate is 7,570 lpm. These fire fighting equipment are typically rated to operate at 7 Barg. With this information, we can calculate the required discharge head of the fire pump using hazen-williams equation. For more complex fire water grid networks a computer software is needed for hydraulic calculation.
As a wild guess, let's assume that the friction loss from the fire pump to the furthest point of the grid should not exceed 1 Barg (assuming the same elevation). Thus, the fire pump discharge head shall be at least 8 Barg. With another assumption of 0 suction pressure, the required differential pressure is 8 Barg when the fire pump is flowing 7,570 lpm.
Per NFPA 20, the fire pumps are able to run from 0 to 150% of its rated capacity.
Considering a future expansion of at least 20% increase in fire water demand, we need at leat 9,084 lpm @ 8 Bar. Therefore, solving for the rated capacity, the fire pump shall be 9084 lpm / 1.5 = 6,056 lpm
Based on NFPA 20 the nearest capacity that is higher than 6,056 lpm is 7,570 lpm. Therefore, the rated capacity we choose is 7,570 lpm. To estimate the differential pressure at this rated capacity, let's take the performance limits of NFPA 20. Whereas, at 150% flow the differential head shall not be less than 65% of the rated differential pressure.
If our estimated differential pressure at 150% flow is 8 Bar, the rated differential pressure is 8 Bar / 0.65
= 12.3 Bar. This value shall be confirmed once the fire pump vendor catalog with performance curve is available.
