Centrifugal pumps minimum flow through

Pressure drop. Good control requires a substantial pressure drop through the valve. For pumped systems, the drop through the valve should be at least 1/3 of the pressure drop in the system, with a minimum of IS psi. When the expected variation in flow is small, this rule can be relaxed. In long liquid transportation lines, for instance, a fully open control valve may absorb less than 1% of the system pressure drop. In systems with centrifugal pumps, the variation of head with capacity must be taken into account when sizing the valve. Example 7.2, for instance, illustrates how the valve drop may vary with flow in such a system. 

A centrifugal pump handles 220°F (377 K) water and has a shutoff head (with closed discharge valve) of 3200 ft (975 m). At shutoff, the pump efficiency is 17 percent and the input brake horsepower is 210. What is the minimum safe flow through this pump to prevent overheating at shutoff Determine... 

Centrifugal boiler-feed pumps usually have a maximum allowable temperature rise of 15°F. The minimum allowable flow through the pump to prevent the water temperature from rising more than 15°F is 30 gal/min for each 100 bhp input at shutoff. 

Design a centrifugal pump and control valve system so that a maximum flow rale ol 75 gpm and a minimum flow rate of 25 gpm are achievable with the control valve at 100 percent and 10 percent open, respectively. Liquid is pumped from a tank whose pressure can vary from 50 to 75 psia. The material is pumped through a heat exchangei (which takes 30-psi pressure drop at 50 gpm) and a control valve into a tank whose pressure can vary from 250 to 300 psia. Assume a flat pump curve. 

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