Re: Do pressures add up?
From: Edward Green (spamspamspam3_at_netzero.com)
Date: 10/24/04
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Date: 23 Oct 2004 21:40:37 -0700
SomeOne <celtic_246@nospam.hotmail.com> wrote in message news:<cldtvp$625$1@news5.zwoll1.ov.home.nl>...
> Hi,
>
> Some time ago I was doing an experiment and got an unexpected result:
>
> In order to clean a pipeline we periodically rinse it with water. To
> increase the rinsing force I introduced, what is called, Bubbling. Next
> to the existing water line used for rinsing I connected an OFA line (Oil
> Free Air). When both water and OFA are used together, the turbulence in
> the pipeline will increase and the rinsing effect is greater.
>
> To prevent water from entering the OFA header or OFA entering the water
> header the pressures must be balanced. I opened the rinsing water valve
> and adjusted the water pressure to 1 bar. Then I closed the rinsing
> water line and opened the OFA line and adjusted that pressure also to 1 bar.
>
> When both water and OFA are used together to rinse the pipeline I
> expected the resulting pressure in the pipeline to be 1 bar, since, I
> assumed, pressures don't add up.
>
> You can imagine my surprise when I noticed that the resulting pressure
> in the pipeline was 1.4 bar. This suggests that the pressures *do* add
> up as the sum of the squares and the resulting pressure is the square
> root of the sum of the squares. Comments and/or explanations are warmly
> welcomed.
A comment then:
Prior posts suggest to me that your answer depends critically on just
how you measured the prior pressures to be 1 bar, and the resultant to
be 1.4 bar. As already implied, you are correct in static situations
-- two tanks at 1 bar allowed to intercommunicate should remain at 1
bar; not 2 bar or 1.4 bar or any other bar. But for two flows at "1
bar" all bets are off -- including bets on just what you meant
operationaly by the prior flows being 1 bar.
Adding tadchem's comments to the mix, probably the best that can be
said given the limited data provided is that you measured some
operational variable to be 1, and another to be 1, and a third to be
1.4, whereas your prior theory predicted the third should be: "who
knows?" If the two fluids you combined were at least the same phase
you might at have lucked out that all the hidden assumptions which
went into considering these readings "pressure" would have remained
roughly invariant in the mixture.
Words of power (Uncle Al style rendition of possibly relevant buzz
words with implication of complete but illusory mastery): static,
dynamic, Bernoulli, entrainment. (No, Bernoulli is not entrainment,
but entrainment exists, sometimes misidentified as "Bernoulli").
You may as well have asked "why not 3" as "why not 1", because you
really have no model which predicts any particular result. Care to
describe in greater detail the position and operation of your gauges,
not to mention the pipe configuration? Give us some room for creative
BS! ;-)
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