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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?
Thanks,
Eric

IF the volume of water remains constant the pressure coming out
will be
at most 80 psi. if the pump is designed to produce 10 psi. It may
be
lower depending on the size of the housing and the restriction
the
impeller creates. Say your input side is 2" and the pump can
produce 10
psi. at zero head pressure out of a 1.5" outlet.

Feed that pump with an 80 psi head pressure and the pump won't
add any
pressure because it cannot pump faster than the water is already
flowing
through it.

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.

The pumps in series business is confusing the issue. The output of
the
first (centrifugal) pump in the chain is far from an ideal pressure
source. The original question was about a pump with a constant inlet
pressure, either 0 psig or 80 psig. Flow wasn't specified, but as
long
as the flow is constant for both inlet conditions, the delta P
across
the pump will be the same. In other words, the pump will increase
the
pressure by 10 psi in both cases.

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.

Pressure, thus, is converted to velocity. Unless the machine
*compounds* the velocity at each stage, I don't see how it works. And,
in order to compound velocity by a factor of, say, three, either the
shaft driving the stage either has to be turning at (square root of 3)
times that of the first stage, or the the second stage has to have a
completely different scroll design.

But you can carry that only so far. Go to three stages, or four, and
the shaft rotational speeds become outrageous, or the scroll design
does.

Obviously, I'm missing something here, but I haven't yet seen what it
is.

Ed Huntress


Consider connecting the pump to the bottom of a 180 foot tall
(approx
80 psi head) tank. Will the pump be capable of pumping, at the
specified flow, to a height of 23 feet (10 psi head) or 203 feet?

Practical matters of seal design aside.
--
Ned Simmons

===========================
http://net.grundfos.com/doc/webnet/m...p-handbook.pdf
The pressure of a 10 meter head of water is close to one atmosphere.
Section 3.2 describes pumps in series. Fig 3.2.5 and 3.2.6 show how
the pressures add when the pumps are of equal or mismatched sizes.
-jsw