Originally Posted by huhh
1 decent size bubble and you can kiss you pump goodbye, you'll lose flow on the inlet. The OP's diagram is the res with the tube going up and over then back down to the pump, a very bad way of running a gravity fed pump.
Ok ... So first it was, the pump would not work at all because centrifugal pumps "don't have any suction" ... then, it would stop working if the pump was stopped and then started again ... now a bubble will stop flow on the inlet and destroy the pump. None of that is true.
If the impeller cavity is allowed to be completely flooded with air it will stop pumping .... exactly the same as if you allow the reservoir to run dry even if it was directly connected to the pump. Large bubbles travel through centrifugal pumps all the time without any problem.
In the OP's diagram the pump is supplied by a syphon and is gravity fed. As long as the res is not allowed to run completely dry it will function just fine. Bubbles travel through siphons all the time too. More importantly bubbles don't just magically appear in tubes running from reservoirs to pumps anyway.
Originally Posted by Namkab
Could you explain exactly how it damages the pump? I would like it explained to me because I dont know. To me it seem like an impeller pump wouldnt be harmed because its not relying on the liguid to lubricate the pump, the lubrication and bearings are behind seals seperated from the cooling loop. I dont see how the impeller spinning in air would damage anything. Everyone acts like pumps will melt if they see air, why is that? The only thing I can think of is exceeding the RPM range of the pump when its free spinning in air.
The Laing / Xylem D5 and 355 type water pumps both work in the same way. They have a magnetically driven impeller with a single graphite bearing that sits on a ceramic ball on a post that forms the motor shaft. The bearings are indeed lubricated by the water flowing through them. The Graphite and ceramic combination is very long lasting when lubricated and cooled by water but if run dry the ceramic bearing very quickly starts to eat into the graphite cup bearing. The graphite only has to wear down a very small amount before the impeller will start to touch the wall of impeller housing and will eventually stop turning. It takes more than a few seconds though.
I have D5's that have run dry for very short periods on many occasions while filling loops and there is no sign of any excess wear on their bearings. Its obviously something that should always be avoided as much as possible but its also not like they explode if any air goes through them either.
Its quite possible to run these pumps with no reservoir at all or just a small T line.Edited by Jakusonfire - 7/6/13 at 1:51pm