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A traditional centrifugal pump is unable to self prime, as the pressure differential between the suction and discharge lines when pumping the air is insufficient to lift the water any appreciable height if the reservoir is below center line.

If however, the pump is constructed such that the impeller is immersed in water, even when the suction line is drained, for example by placing the suction port above the impeller elevation, then the pump will be able to self prime.

enter image description here

Alternatively, one could place an ejector between the suction port and the impeller inlet, and this could be used evacuate the suction line of air. This is the so called jet pump.

enter image description here

My question is, why are jet pumps way more commonly used than the aforementioned immersed centrifugal pump setup? Is the self priming capability in one better than the other?

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Because sometimes your pump is placed above and some distance from the waterline.

but lets look at the startup sequence when the inlet is full drained and the housing is still full of water:

When you start the pump again you pump out the water remaining in the housing creating a small under-pressure inadequate to pull the water through the inlet over any distance, and then ...
that water is gone and you are pumping air again.

all you did here was increase the self-priming range from nothing to ... a foot but you didn't fundamentally solve the self-priming issue.

To self-prime you need to be able to pull a decent vacuum when pumping air.

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  • $\begingroup$ The self priming centrifugal setup without the venturi is in fact used in practice, and is capable of producing the usual suction lift of around 8 meters. See for example michael-smith-engineers.co.uk/products/finish-thompson/… I am not sure if I am understanding you correctly, could you please clarify? $\endgroup$
    – Joeseph123
    Jan 30, 2023 at 9:59
  • $\begingroup$ @Joeseph123 The self-priming pump's impeller and case isn't built like your image depicts. It is call a liquid ring pump in these parts. And we are normally worried about freezing and how to empty the case - mine all have weep holes to ensure the cases don't freeze. But in practice, I still need about 5 gallons of priming water to prime a 3" pump for a 25' suction line and 3' of lift. $\endgroup$
    – Phil Sweet
    Jan 30, 2023 at 11:30

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