The pump to boost the suction pressure
Remember that NPSH and cavitation are two different issues. When there is head falloff because of insufficient NPSH, the cavitation is so advanced that it blocks the flow passages. Damage can occur well below stated NPSH values.So, to get the pump operating "out on the curve" you may well need a charging pump in series with the pump to boost the suction pressure sufficiently. Increase the capacity of the charging pump enough, and the head can go negative.If you need to run out on the curve for brief periods of time and have sufficient NPSH, ask the pump manufacturer for advice. I can't think of any case where you would want to order a pump where your normal duty point was way out on the curve. Better to pick another pump.
From my earlier post where I mentioned you could (not a good idea) drive the head negative. When you did this, the flow would be very high in the positive direction. You would be using the running pump as an orifice. If you have a copy of the classic "Centrifugal and Axial Flow Pumps" by Stepanoff, there are some multi-quadrant pump curve examples that would show this.Don't get me wrong, operating here wouldn't make a lot of sense. But pump companies test the pumps under wierd conditions to assess pump behaviors, etc. during motor trips, surges, etc. When the pumps get very large, such information can become crucial.
An optimum H-Q BEP should be Split case pump 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.
2011-08-22
