[JimE]

Hi Steve. No offence taken but I think you should check your theory. What you are saying is more applicable to positive displacement pumps (gear, vane, diaphragm, piston) but not centrifugals.

"BUT as you reach the limit of the pump you will get a increase of shear between the moving water and the stationary water which will cause cavatation and can airlock your pump (in extream cases) in any case this will also cause heat and if it goes on to long it can destroy your pump."

Cavitation occurs when the pressure at the inlet of the pump is reduced to the vapor pressure of the fluid being pumped and the fluid "boils". This occurs at low heads/high flows or when there is a restriction in the inlet piping. It doesnt generally occur at low flow/high head. Check the npshr vs flow on a pump curve. The heat produced in a pump at low or no flow is from friction not cavitation.

"If we go y your statment any power head in a tank should trip out.. this doesent happen.. because ) head is the easyest state to pump in."

Umm, yes it does happen.

"the part you are right about is that when you reach a point of high head pressure that the impeller in the pump isn't rated for your shear streeses insidde the pump become so great you generat slip. this causes the impeller to overspeed and if the motor is equiped with a overspeed trip it will trip, if not it will spin fast with no output and overheat untill it either burns itself out or the motors thermal overloads cause it to shut down."

No, at high head/no flow the motor could run forever. What happens is the water overheats from recirculating in the pump head and eventually melts (if its plastic) and siezes - then the motor quits : ) Also, if you follow your line of reasoning, a motor by itself (no pump-no load) will over speed. Well, ac induction motors cant really over speed - rpm is a function of number of motor poles and frequency

Jim

[ 25 May 2002, 23:18: Message edited by: JimE ]