I want to ask if there's any reference in NFPA talked about the maximum allowed water flow velocity and maximum allowed friction loss in sprinkler pipes?
NFPA 13 does not limit the water flow velocity or friction losses in a sprinkler system. The system only needs to be capable of supplying the minimum demand for the hazard, as predicted by hydraulic calculations.
There is some confusion on this point because NFPA 20 refers to a maximum flow velocity. The minimum pipe sizes for suction pipe in Table 4.26(a) and Table 4.26(b) are based on a maximum velocity of 15 ft/sec (4.57 m/sec). This is intended to limit turbulent flow, which generates air bubbles that adversely affect pump efficiency. This is not a requirement elsewhere in the sprinkler system.
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