Incompressible water

[bornedalegend93]

Can someone explain when and under what assumptions can we consider water as an incompressible liquid as oppose to considering it a saturated mixture or saturated vapor/liquid?

 

[zdas04]

Water is never actually incompressible. Water is nearly always practically incompressible. I did a project once where I was pressuring a column of water to 20,000 psig and the change in volume (about 0.06%) was actually significant. That was the only time I've had that problem (even though I've done other projects with higher water pressure, but the change in volume was not significant in those applications). Basically if your calculations will not lead to a different result if you consider compressibility then it doesn't matter. If your calculations lead to a different decision when you consider compressibility than when you don't then it does matter.

Where you are within the liquid region of a phase diagram doesn't make much difference to compressibility.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[25362]

Look at a phase diagram (for example:

http://www.columbia.edu/itc/chemistry/environmental/lectures/week2.pdf)

and you"ll see under what conditions water is not, as you say, a saturated mixture or a saturated vapor/liquid.

 

[willard3]

In and around atmospheric pressure and temperature, water is practically incompressible.

 

[chicopee]

Reading this statement"...can we consider water as an incompressible liquid as oppose to considering it a saturated mixture or saturated vapor/liquid?" there are two separate lines of thoughts, so I am not sure what you are trying to do or think.

 

[racookpe1978]

Well, I ran several years of "solid-solid" pressurized reactor plant operations and testing.

Water ain't solid. Except as an approximation. When you need an approximation. When the "error" inside that assumption of an approximation is less than the accuracy of the approximation of the answer you have decided you can accept within your answer for the safety and operation of your answer.

Thus, in a "solid" reactor piping system, when the sun heats the hull of the submarine which heats the piping in the reactor compartment, the pipe heats up and expands a little bit, the water heats up because the pipe heated up but expands a little bit more, and the pressure of the reactor system goes up. You take a sample of the reactor water, drain a little bit of water from the sample system, the water pressure goes down. You pump a little water into the reactor system with the charging pump to make up for the drained water, the pressure goes up.

Was the water solid? Not really. It has a little bit of a expansion "spring" effect you have to plan for. The pipes and reactor vessel and steam generators also give and expand themselves. That gives you a very, very obvious "spring" effect of relaxation and time delay.

 

[MortenA]

I would say that you would consider water incompressible for the following two cases:
1) Thermal expansion (if you didnt pressure would increase to infinity - but it is more or less a moot point)
2) Water hammer analysis - if you didnt the pressure surge would be equal to the kinetic energy stored in the liquid moving in the pipe - and that would be very high for most systems.

Best regards, Morten

 

[BigInch]

Water is very much (but not entirely) incompressible with pressure but relatively thermally expansive, the two not necessarily being associated with one another, except if the expansion is contained. Normally this will not make any practical difference until pressure (w/o thermal expansion) reaches well over 2000 psi. The bulk modulus property of water describes the effect of its compressiblity, the change in volume one can expect due to pressure.

http://en.wikipedia.org/wiki/Bulk_modulus

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