Hey Jim nice to see ya. pumps is a big part of my work, we even had to build gear pumps and centrifical pumps on the layth from scratch.
I agreed with your post except for two things (and they were both kinda the same) please do think bad of me as I know computer chats can seam cold but I think you are thinking of another effect that is a result of a centrifical pump not working right. well maby I can explain this with out making anyone mad.
Oh and don't mind my spelling I am a engineer not a english teacher :D :D
Quote:
Originally posted by JimE:
".. the more head ou put on it the harder the motor has to work.. and the more power it draws"
Actually, with centrifugal pumps it is the opposite. At zero head the pump is moving the most amount of water, is working the hardest, and is drawing the most power. As the head increases, the flow decreases so the pump is moving less water and the load decreases which = less amps.
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<font size="2" face="Verdana, Helvetica, sans-serif">not realy, if you look at any centrifical pump flow vs/power vs/ head pressur tables you will find it is the oposat of what you have just stated to a point. as you start raising the outlet pressure yes the flow goes down but the power draw goes up as your impeller is now trying to move in a higher pressur whare the water is resisting the movement. 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.
Quote:
Originally posted by JimE:
Most centrifugal pumps will be "off the curve" at zero or very low heads, try to pump too much water, and overload. Thats why you dont get a 0 head rating.
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<font size="2" face="Verdana, Helvetica, sans-serif">the reason you don't have a 0 head rating is because any pump will pump at 0 head. Hot tub pumps are not rated at 0 head because you will never have a hot tub with that low of a resistance.. with all the nozzels there is a lot of head generated so a 0 head rating would be totaly useless in the hot tub industry. 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.
as you add head to the equasion you add pressure to the inside of the casing which inturn creats the effect of making it harder to pump. when this happens the pump speed slows down and you get less out put. the motor will try to maintain the rated speed but if the head gets higher it can't and does its best at a slower speed, hench the lower output at higher heads. 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.
[ 25 May 2002, 21:53: Message edited by: StirCrazy ]