Dolphin Ampmasters

[StirCrazy]

positive displacements slow down all right when they is a restriction.. then they shut down :D seen some one shut a pump dischagre by accadent and it wasn't pretty :D :D
I did type that wrong was mixing stuff up hehe when I talk about a centrifical pump slowing down, I should have said it is a small amount and it usaly is right back up to speed. as the amount of shear inside the pump will increase lowering your output. it is not lowered by the motor slowing down.. sorry for the confusion..

"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."

yup it also occures inbetween the impeller and the viens when the pump is reaching its stall out speed or when you are trying to pump a fluid that is to viscus for the style of centrifical pump you are using.

"The heat produced in a pump at low or no flow is from friction not cavitation."

yes the heat is from friction of the water shearing (I didn't stat this to clear) but in adition to the heat from friction you can also end up with cavatation inside the pump on the low pressur sides of the stationary viens which accelerates the wear inside the pump.

you stated that a centrifical pump will trip out at 0 head. you have me totaly baffeled with this statment
I have one in my fresh water tank and it never trips out.. I don't understand why you think it would.. I am sure lots of people have power heads going in there tanks constantly with out them tripping out all the time.. heck my power filter is 250 gph it is running at 0 head all the time and it never trips out.

unless maby we are having some semantics about the term "0 head".. of course 0 head would be impossable as you have outlet piping of the pump its self, but when I use the term 0 head it is to refer to a state that you would get if you put the pump submerged in a tank of water so it draws water directly in its intake and discharges directly back into the tank with out having to lift the water

"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 : )"

I know what you are saying but if you dead head you will start to generate heat.. aat a point the water inside the casing will start to boil off lowering the resistance of the water, this will make more heat and destroy the pump if the motor doesen't trip first. "untill it either burns itself out" was refering to the pump.. sorry I didn't make that clearer [img]smile.gif[/img]

I thik we are pretty much on track (some quick typing on my part with out reading it over first) but there is still confusion on this 0 head thing, I have had lots of power heads over they yease and they all ran in the tank fine.. so unless it is a confusion like I stated above you have me confused on that one..

Steve