An optimum H-Q BEP should be sought at a mid-point of conditions.  The duty point will shift back and forth along the curve as conditions change.  The NPSHa is usually stable in a Water Treatment Plant not fluctuating more than a foot.  As demand increases, more treatment is taking place.  However, some cities like to fluctuate the level of their water towers to cycle its contents (keep it from becoming stagnant) so I would suggest targeting the mid point of the water tower when calculating the differential head and finding a pump with a BEP.When the tank is at its lowest point, there will be less head required giving a higher flow rate from the pump.  As the tank fills, the head increases decreasing the flow rate. 
 

Operate over BEP to more than 120% you have:1. More Vacuum pumps power consumption.2. More NPSHr with cavitation failure risk.3. Erosion in high velocity zones, Cut water, suction, discharge vanes zone, etc.3. More radial load with bearing and shaft failure risk.4. Global vibration levels increase dramatically.5. Reliability going down to 30%.6. Other unstable hydraulics issues.

2011-08-24

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