The Pump Handbook (1976 Ed.), on page 13-12, suggests 30 GPM per 100 brakehorsepower minimum flow for hot water pumps with a recommended maximum water temperature rise of 15 degrees F. For cold water pumps, temperature rises up to 50F or even 100F are considered permissible. This kind of criterion does not address pressure pulsation concerns associated with suction and discharge recirculation in the impeller which may drive minimum recommended flow fractions much higher than thermal heating considerations. As noted by TD2K above, minimum flowrates of the order of 25 to 40% of rated (best efficiency) flow are being recommended by pump manufactures who can now estimate critical flowrates for peak pressure fluctuations due to recirculation. Pressure fluctuation testing of a 125 hp, 3600 RPM, double-suction pump around 1996 found discharge and suction recirculation pressure pulsation peaks at 30 and 40% of rated flow, respectively (see Atkins,R.A. etal, "New Monitoring System Warns of Cavitation and Low-Flow Stabilities", Pump and Systems Magazine, April, 1996, pp. 12-15).