8 replies to this topic

[Yogesh Bhatt]

Dear All

 

Per below attached snapshot of the simulation I am facing an abnormal situation of vacuum pressure in the pipe segment S130-16"-406-2. The back pressure here is 0.97551 bar abs whereas the flare header pressure is 1.02869 bar abs. Pls see the attachment.

Please advice how this is ever possible.

 

Thanks in advance.

Attached Files

•  fire case.JPG   53.42KB   35 downloads

[katmar]

2 clues for you:

  i)  Bernoulli's Equation

  ii) Velocity Head

[latexman]

This is less likely, but are you sure the solution converged?

[Dacs]

I think we need to differentiate the difference between static and dynamic pressure.

 

Try to confirm which of those two is being shown in the Flarenet result.

[thorium90]

I concur with latexman. It is obvious that you have cropped out the panel at the bottom that shows the error messages from your picture. Perhaps if you analyzed your error messages, you might be able to solve your problem. Attached a picture of how your screen should have looked like with the panel windows below.

Attached Files

•  real screenprint.jpg   87.4KB   11 downloads

Edited by thorium90, 06 May 2013 - 02:02 AM.

[Dacs]

One segment is 16" and other segment is 32"

 

Assuming the same flow for both, I think velocity head has to be accounted for, hence determining if the pressure reported is static or dynamic (total) pressure.

[Yogesh Bhatt]

Dacs

 

Its the static pressure shown in flarenet.

[Yogesh Bhatt]

Yes the simulation converged with 1 error saying velocity violation for 16"-406-2 pipe segment. The velocity in the line is 142 m/s.

But as I am ok with mach no. 0.7, why is the velocity of 142 m/s an error.

 

Please advise.

 

Thanks

[Yogesh Bhatt]

Dear All

 

As I have learnt from Mr. Brian in one of his blogs that only static pressure is to be considered for backpressure determination for a PSV.

Quotes from Mr. Brian are as below :

 

"The reason you do not consider total pressure is that the kinetic portion of the pressure

acts only in the direction of travel of the gas whereas static pressure acts in all directions .

An anology to help you on this .. you are trying to get out of a room into a corridor that is

full of people..the mass of people there is the staic pressure part. if they are moving slowly

forward then they have a small kinetic energy( pressure) but the mass/number people per

m2 is still the same, if they walk faster then static is same but total increases.

this is how i used to teach Flarenet , think of people in corridors rooms etc squashing

together through doors, into small corridors and then accelerating in wider corridors. It

goes a long way to assist in visualising hydraulics."

 

Now in my simulation file, if I check total pressure then everything is fine but if I see static pressure there comes the problem of lower backpressure in upstream node than the downstream node..

 

Experts please share your thoughts on this.

 

Thanks a lot in advance..