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12 comments
Anonymous Poster #1

Pressure At An Elevated Point

05/16/2012 1:30 AM

Sir,

In the figure shown, the pressure P1 is 10.5psig in the pressurized tank, the top end

of the pipe is blocked once the liquid reached the end point (Raised becoz of the pressure in the tank, [can be, during the steady flow end point is closed by valve]). I'l calculate the pressure at P2 as = P1-(rho*g*H).

Is it correct? becoz everybody are telling that the pressure will be equal to P1 pressure based on the Paskal's law.

Please correct me.

Thank you.

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Anonymous Poster #2
#1

Re: Pressure at an elevated point

05/16/2012 3:44 AM
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#2

Re: Pressure at an elevated point

05/16/2012 5:49 AM

You're correct up to a certain value of H. Above that, P2 stays at about minus 14.5 psig (full vacuum minus liquid vapour pressure)

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Anonymous Poster #1
#3
In reply to #2

Re: Pressure at an elevated point

05/16/2012 6:25 AM

thank you sir for the reply.

So Can i mean that the maximum pressure at P2 will be the vapour pressure of the liquid at corresponding temperature. Because, i've doubt that what -14.5psig is?

But even below the vapour pressure also system pressure can go but the state will be gas. Please correct me.

THANK YOU

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Anonymous Poster #2
#4
In reply to #3

Re: Pressure at an elevated point

05/16/2012 8:04 AM
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#5
In reply to #3

Re: Pressure at an elevated point

05/16/2012 10:47 AM

In case the link in #3 doesn't cover it.

Can i mean that the maximum pressure at P2 will be the vapour pressure of the liquid at corresponding temperature. No, the maximum pressure at P2 is when H = 0 (assuming H is never -ve) and = P1. You can see this from your formula.

Gauge pressure is relative to atmosphere, so atmospheric pressure = 0 psig = 14.5 psia. Full vacuum = 0 psia = -14.5 psig. You can't get any lower than that, however high H is. In practice the pressure is a little higher due to the liquid vapour pressure.

So the minimum pressure at P2 will be the vapour pressure of the liquid at corresponding temperature.

To confuse things, it's also common to talk of say 10 psi vacuum, which = -10 psig = 4.5 psia. (0 psi vacuum = atmospheric, 14.5 psi vacuum = -14.5 psig = 0 psia. I was using it in this sense in #2 with (full vacuum minus liquid vapour pressure). Vacuum is lower due to vapour pressure, absolute pressure is higher.

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Anonymous Poster #3
#6
In reply to #5

Re: Pressure at an elevated point

05/16/2012 9:57 PM

What would change in that system if the fluid is different from Water?

It is a closed system and shut in after a dynamic process.

Just thinking out loud. I probably should know but school has been a while and thinking becomes harder with the minute...

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#7
In reply to #6

Re: Pressure at an elevated point

05/17/2012 5:57 AM

Just put the appropriate rho (density, kg/m3) in the OP's formula.

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Anonymous Poster #3
#9
In reply to #7

Re: Pressure at an elevated point

05/17/2012 11:21 PM

I was rather thinking about fluids that turn into gas at a lower temperature than water. Would this not change the pressure in the system after you shut it in?

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#11
In reply to #9

Re: Pressure at an elevated point

05/18/2012 8:32 AM

If the vapour pressure at a given temperature changes (as you postulate) or if the whole system were warmed oe cooled, the pressure would change. But the OP's formula P2 = P1-(rho*g*H) still applies.

Max height of water in the tube H' = P1 + Pa - Pv where Pa = atmospheric and Pv = vapour pressure (all measured in length units for simplicity).

P2 as function of H doesn't depend on Pv as long as H < H'. Converting the OP's formula to P2 = P1 - H, when H = 0, P2 = P1. When H = H', P2 = P1 - H' = P1 - (P1 + Pa - Pv) = Pv - Pa (gauge pressure) = Pv (absolute pressure).

When H >= H', P2 = Pv - Pa (gauge), Pv (abs) .

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#8

Re: Pressure At An Elevated Point

05/17/2012 8:52 AM

Seriously?!

It's spelled because not becoz. Learn to use the english language properly or it doesn't matter what math you put down on paper. No one will read it and give you credit.

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#10
In reply to #8

Re: Pressure At An Elevated Point

05/17/2012 11:27 PM

Becoz English is a static language and never changes? I think mobile phone and foreign usage of English puts a lot of pressure on the Oxford English. How long do you estimate will it take until becoz gets an entry there?

Happy ranting cingold.

IS

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#12
In reply to #10

Re: Pressure At An Elevated Point

05/18/2012 8:49 AM

It's not just that. There's other misspelled words too. Seriously, I opened up the article to contribute or get some information, and when I read the first couple sentence, I literally stopped reading.

If he had written a technical paper, it wouldn't matter how sound his technical knowledge and work was.

I've run in to this at work too. When I read an old piece of work, and I see it filled misspelled words and poor grammar, I just assume that the person that did the work couldn't get the math right either. I mean really?! There's a built in spell check!

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