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wrote on 5/31/2017 8:08 PM:
On Wednesday, May 31, 2017 at 7:35:13 PM UTC-4, whit3rd wrote:
On Sunday, May 28, 2017 at 8:35:39 AM UTC-7, wrote:
On Saturday, May 27, 2017 at 6:57:12 PM UTC-4, Ned Simmons wrote:
On Sat, 27 May 2017 12:20:00 -0700 (PDT), wrote:

On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote:
wrote:
If a centrifugal pump with a maximum pressure of, say, 10 psi is
supplied with water at 80 psi will the water pressure coming out of
the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong?

That's close. A pump on atmospheric air (1 bar) that makes a 10 bar output,
will be different, because the density of inlet air determines the force
on the motor to spin it up. Feeding in 10bar air, you have higher density,
and the motor torque and work done will rise (because an AC motor operates at
constant speed). Water, being nearly incompressible, doesn't create any
such scaling issues with inlet pressure.

That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids.

For liquids, the multistage pumps I've used have pistons, and the low-pressure pistons
stall out when backpressure is high, then the same motor moves smaller high-pressure
pistons to achieve higher fluid pressures with lower volume displacement (but nearly
equal motor power). I suspect there's a differential gear driving all the pistons with
different mechanical advantage, and a reverse-inhibiting clutch on the low pressure pistons.

This is where I have trouble. Assuming these are regular centrifugal turbines, the outlet of the first stage is fed into the axis of the second stage. The pressure from the first-stage outlet is retained at the second-stage inlet, but from there it feeds into the whirling blades of the second stage, the outlet volume of which is LARGER than the inlet volume between any two blades.

Why? Several stages of centrifugal pump in series for a liquid would have same volume flow,
and same work per shaft would deliver the same head, at each stage. So, they'd be the same
size rotors, too. Maybe different shaft seals, though.

The issue I was trying to resolve was the question of whether the pump(s) in series were, each, completely filled. Illustrations often show centrifugal pumps as being less than completely filled. But I realized that they have to be completely filled, especially for a multi-stage pump.

If those spaces aren't completely filled, the outlet pressure from one stage, which would be the inlet pressure of the next stage, wouldn't communicate to the output of that following stage. It would be like pushing on the end of a rope.

But I finally found some good explanations.

All my posts in this thread should be a good read. The impeller of a
centrifugal pump forms compartments inside the tight fitting casing to
trap incoming fluid so that when the impeller spins, it is forcing the
fluid in all the compartments to go in a circular motion. The
centrifugal force from the rotating fluid generates the pressure
radially outward against the outer wall of the pump.

A very basic impeller:
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6b/Impellerrad.jpg/1280px-Impellerrad.jpg