On Sunday, May 28, 2017 at 2:17:58 AM UTC-4, HJARTš›†� ï¼*ｉｇｈｔｙ •¬ ï¼·ï½�ｎｎï½�ｂｅ †’š›lBYWJ wrote:
wrote on 5/27/2017 11:08 PM:
On Saturday, May 27, 2017 at 8:28:04 PM UTC-4, dWuVxš›†� ï¼*ｉｇｈｔｙ •¬ ï¼·ï½�ｎｎï½�ｂｅ †’š›DiDrO wrote:
wrote on 5/27/2017 3:20 PM:
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.


Water is actually compressible. The compressed volume doesn't change as
much as gas would.

"The low compressibility of non-gases, and of water in particular, leads
to their often being assumed as incompressible. The low compressibility
of water means that even in the deep oceans at 4 km depth, where
pressures are 40 MPa, there is only a 1.8% decrease in volume."

https://en.wikipedia.org/wiki/Properties_of_water#Compressibility

Thanks. I suspect that most of the people here know that. The pressure/volume relationship, though, isn't in agreement with Boyle's law. Gases approximate it. It's easy to imagine a multi-stage non-positive-displacement compressor that keeps building pressure in a material that obeys Boyle's law.. It's much harder to imagine it with liquids.


The idea is the same. You use pressure to reduce the volume of a fluid.
The gas/water restores it original volume after losing the pressure.

I don't think so. Centrifugal pumps are very lossy machines. They couldn't hold pressure that way.

From a physics point of view, I think the answer lies in sorting out the kinetic aspects of a turbo pump (velocity) and the potential aspects (pressure). A turbine pump that's pumping a liquid must be producing potential energy from kinetic energy.

I'd need to see a good, expert explanation to understand it. I see no way that an ordinary turbine pump can hold the pressure generated by a previous stage, unless the entire thing is kinetic, which we're then measuring as potential energy (pressure).


The graph in the link below shows that 200-bar of pressure (2900 psi)
will compress water by about 0.7% at 4°C (under STP, water is 1g/cc at 4°C):

http://www.engineeringtoolbox.com/fluid-density-temperature-pressure-d_309.html

No turbine pump could hold that 0.7% compression.

--
Ed Huntress