Centrifugal pump test

[petertha]

I drew up & 3D printed a bench test centrifugal pump. I was expecting anemic flow based on TLAR design #1, but it is apparently worse than that. Before mounting to my stationary motor, I gave it a spin-up in in the sink with my cordless drill. Even trying to pull 1" hydrostatic head & priming best & pinching outlet hose on & off, it just did not want to flow. Like at all. Some specs: inlet diameter = 6mm. Flares internally to 8mm rotor eye diameter. Throat maximum is 6mm wide x 4mm high, 8 rotor vanes are 6mm high (~0.2mm clearance to volute), rotor vanes are 1mm sec thick, outlet diverges from rectangular throat to round section by about 20% more section area. The minimum cutwater distance to rotor is about 0.6-0.8mm. The 2 pump halves are not sealed with a gasket, just lightly tightened face to face using M2 bolts you see. Any ideas? I can scale the volute down if its too aggressive for (not sure really - 'cordless drill' speed).

[petertha]

PS - the pump insides are MUCH smoother than what you see on the outside which is rough as a result of print orientation & tree supports. I forgot to take a pics of the insides but will.

[Dave Otto]

When you say 1" hydrostatic head, you are also saying that the inlet to the pump was being supplied with the fluid source 1" above the pump? So it should not need priming, just trying to understand.

Dave

[CI]

I have used drill-powered pumps like that to transfer liquids, and the worked well, and were self-priming.
I suspect the pumps I used had a tight rubber impeller, which pulled enough vacuum to allow them to be self priming.
That is a nice pump design.
.

[Vixen]

Hello Peter,

Can you post a full cross section, including the inlet. Then we can see exactly what you have designed. We can go from there.

Mike

[petertha]

Sorry for poor descriptions. When I mention 1" hydrostatic head, I meant that the inlet is only 1" above water surface in bucket & it failed to draw any water even though the pump & inlet tube was relatively primed.

Some better annotations.

[petertha]

a section midway between throat plane & outlet plane transitioning from rectangular to circular

[petertha]

I see my bearings are choking the shaft, maybe that's the problem Haha. I seem to be in between configurations but the pump / volute / rotor is the same as shown

[Dave Otto]

I have never seen a self priming centrifugal pump, not saying they don't exist I just don't recall ever seeing one.

Dave

[petertha]

I tend to agree Dave & that might be part of the issue. I tried to get the pump cavity relatively full before spinning the shaft, but its an ungainly affair. I hold the drill with one hand & hold the pump body with the other hand  & hold the intake hose into the bucket with the third hand.... But I will try & ensure its fully flooded before rotating with split plane up so the inlet hose can dip into the water bucket. If I just pour water from the tap into the inlet with tube bent up, it free flows through the pump so I'm not really testing anything. I was hoping it would suck a moderate head, even 1"

I now have the motor mount & pump mount printed & ready to go on a board, but need to make a shaft coupler. And need to keep the liquid off the electrons.

[Dave Otto]

There is a reason that they make foot valves.
https://automationforum.co/what-is-a-foot-valve-and-what-are-its-uses/
I think that you should design your test set up so that it gravity feeds the pump.

Dave

[petertha]

That's certainly the hydrostatic orientation of the intended end result. I need to revisit my links. I thought I saw a model pump pulling water (level slightly below inlet) but maybe I'm mistaken.

[Vixen]

Hello Peter,

The first thing to remember is water in NOT compressible.

Parallel (equal width) impeller blade will not work in a liquid. The outward flow will simple slow down to a crawl (or stop). The impeller blades need to be narrower at the tip compared with the inlet. The inlet area should equal the outlet area, so the impeller blade width must reduce as the diameter increases.

This is the coolant pump from my W165. The reduced impeller width at the tip is very noticeable. You can also see the same taper on your BMW's pump



You will also notice the inlet pipe has a wider trumpet and the impeller has a bullet hub to streamline the water flow and avoid rapid changes of direction. Again this feature is on the BMW's pump





The outer volute should increase in cross section area progressively without any sudden step changes or throat/diffuser, where the outlet pipe joins the volute.

I prefer straight impeller blades to the curved blades, however your curved blades should still work well.

This type of pump will NOT self-prime, so submerge the pump in the test tank with the motor above water.

Cheers

Mike

[petertha]

Thanks Mike. If I follow your example, the rotor blades should be tapered in section view like so? Shorter width at tip, longer width at inlet area. I'm showing as straight lines to simplify. Obviously the inlet housing half conforms to that rotor shape with small clearance?

Some further keyboarding is leading me to understand that a fully primed / filled pump is a need to have, not a nice to have. What Dave & you are confirming. Supposedly a well designed pump can draw some hydrostatic head, but if it has any entrained air, which mine surely did, is just sloshes about inside the pump which is probably what was happening. I'll set up my next test accordingly. Yes submerging entire pump inlet facing down & shaft sticking out to my drill would have been a smarter approach. Onto test #2.

I should mention in the background I was using Gemini The Fibber to help with some dimensional guidelines, building a spreadsheet & confirming intermediate results as we progressed step by step. That process can lead to some very strange rabbit holes depending on how one asks questions & what it 'thinks'. For example it was quite persnickety about things like the throat section aspect ratio (width:height above rotor), the offset distance at water cut, blade curvature radius, diverging outlet etc. I'll keep that out of the discussion for now, first things first, make water flow!

[john mills]

the main thing to get the pump to shift water will be to food the inlet  timers in the fluid the comet pump on the repco valve grinder is immersed and the blades are just parallel no great velute shape or close clearances but runs fast
pumps a good flow  impeller is only may be 1" dia
i have seen a much larger pump 12" impeller with just 3  parallel curved chanels in the impeller
i have an open faced 5" dia impeller pump the blades are the same hight to the od
rebuilding lots of automotive pumps lots have very simple parallel blades the all shift water or coolant
the shape of the blades forward facing  straight or curved backwards produces the flow characteristics
basic pumps can be very basic but the wont lift unless compleatly flooded to start
 try immersing the pump in the fluid when starting and run as fast as reasonable
pressure is determined by the dia and speed rpm of the impeller