Pumps in series theory

[MMM33732]

My return pump is currently a Jebao DC-9000. I have 11' of head height it must pump up. (Its rated max head is 14.1') Its been doing just fine for the last 8 months or so. On 2 occasions lately (that I know of) it was unable to maintain pressure needed to keep pumping up that height. In other words I would notice the display not getting any water but when I went down to check on the pump, it was still trying to pump, it just couldn't make it up anymore for some reason. (pump and impeller were recently cleaned too). Anyways, to prevent this from happening in the future, I'm contemplation a few options.

First is to get a bigger pump. I like the current one a lot (also use it on a closed loop) so I'm considering getting the DC-12000 as its bigger (rating and power wise, not size). Issue is, its head height is only rated for about another 1.5' above my current pump which puts its rating at 15.7' for a max head. Not sure if that's enough to stop this from happening again. Now the issue I see is the current pump already flows about the max GPH I want though my display/overflow/sump. If I upgrade to the larger DC-12000, I could throttle its speed down, however I think that would make it harder to pump up the required 11'. If I throttle down the speed of my current pump, it can no longer pump up to the display. So basically I feel it would add too much flow if I go with a bigger pump.

Another option I'm thinking about is adding another pump about mid way up to in effect reduce the head height of both pumps. In theory at least. Anyone ever do this? And if I were, could I go with a smaller pump such as the DC-6000 or should I stay with 2 of the same sized pumps? Thoughts, comments, concerns?

 

[SGT_York]

The Jebao impellers are not designed to handle back pressure caused by head height. Upgrading to a larger pump won't significantly improve the flow. They are just not designed for what you are trying to implement.

A series pump may work but there are two cautions, primarily is a very high risk of cavitation if not placed properly. secondly and more importantly is it is cheaper to just by a properly designed pump that will handle the head height. It would also be cheaper to just run one AC pump over two DC's. If you can go external Iwaki's would be perfect for your head height.

These DC pumps won't suck water up (they only push it), so if you place it higher and the flow isn't enough you'll destroy the cheap impellers, if you place it too low you won't get much more flow than you currently get. The math is complex and there are better ways to solve your issue.

High head pumps have very small blades but spin rapidly these DC pumps have large impellers that spin slower so more volume per turn perfect for low head operations, very poor for 11 feet.

 

[Wazzel]

Simple thing pumps in series add head, pumps in parallel add flow. In addition to what was already mentioned you may run into an issues with leakage in the second pumps housing. These things are not designed to handle high suction pressure. Getting a properly sized pump would be your best option. Generally speaking smaller the pump the less efficient it is, so one larger pump will burn less electricity and generate less heat than two smaller pumps. If you insist on giving it a try use two of the same pump and have them go from the first one directly into the other. This should limit the issues with feeding the second pump. To not try to place one at some elevation in the system.