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High Mach Number In Expansions On Headers Vent & Relief Systems (U

flarenet relief systems vents mach number high headers expansions

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#1 John Pr1me

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Posted 16 February 2016 - 05:35 PM

Hi Colleges , I´ve been working on a short simulation using Flarenet, and when a 8" header expands to 30" a mach number of 1 is indicated downstream of the 8" pipe. I checked that the problem is only in the expansion point and not along the 8"pipe .
My question is if this velocity means a real problem or it can be avoided?
Probably a software issue?
I would appreciate if someone can refer me to any biography o have any suggestions according to your experience.

Cheers-Johnny



#2 latexman

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Posted 16 February 2016 - 08:09 PM

In piping, Mach 1 usually occurs at an expansion.



#3 Bobby Strain

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Posted 16 February 2016 - 08:51 PM   Best Answer

You can expect unusual results in the situation you describe. But look carefully, as the Mach number in the larger pipe at the smaller pipe exit cannot be 1. Review your system input specifications. Model segments and compare your results.

 

Bobby



#4 shvet

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Posted 16 February 2016 - 11:10 PM

 
My question is if this velocity means a real problem or it can be avoided?
 

 

High Mach number should be avoided.

 

Norsok P-001 5 ed.

"6.5.2 Flare headers and sub-headers
Piping for flare and sub-headers shall be designed for a maximum velocity of Mach 0,6.
6.5.3 Pressure safety valve lines
The upstream and downstream line shall be sized according to requirements in the relevant pressure relieving design code.
Maximum flowing velocity in the lines downstream of the PSVs to the first sub-header, shall in general be less than Mach 0,7. For the PSVs where the outlet velocity is higher, a reducer should be installed as close as possible to the PSV to increase line size and hence limit the velocity to maximum Mach 0,7 downstream of the reducer. Nevertheless, the actual back pressure at the PSV outlet and in the block valve shall be checked to be consistent with back pressure limitations.

...

6.5.5 Depressurisation lines
The maximum flowing velocity in the lines downstream the reducer shall be Mach 0,7.
The pressure loss shall not impose any restrictions on the depressurisation objectives
"

 

For more information see chapter 6 of Norsok P-001

 

ExxonMobil DP XV-C (2004)

"The maximum permissible flow velocity at any point of the PR valve discharge piping is 75% of sonic velocity,
regardless of whether the PR valve discharges to atmosphere or to a closed system. Flow velocity is calculated at
the design capacity of the PR valve.

...

The maximum exit velocity at design capacity shall not exceed 75% of sonic to limit noise problems and avoid
choked flow. If a section of enlarged diameter piping is used to limit the exit velocity, its length shall be at least
10 diameters of the enlarged piping
."

 

 

 
I would appreciate if someone can refer me to any biography o have any suggestions according to your experience.
 

 

 

We have negative experience of high Mach number:

1. A refinery plant gasoil hydrotreating unit. System pressure ~50 barg. Hydrogen circulation compressor. Electric gate valve on compressor discharge 4" from discharge to flare header. Once operator opened gate valve. ~30 m of main plant flare header DN800 was torn away and fell down on the ground.

2. A refinery plant hydrocracking unit System pressure ~170 barg. Emergency depressuring system. After 3-5 emergency depressuring events all pipe bends was shredded. They had to replace them.



#5 John Pr1me

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Posted 18 February 2016 - 08:24 AM

Problem Solved , thanks to all the members who replied my query.  

 

If any one encounters this problem in the future try on checking the software´s specification on the connections between headers and sub headers.

Be careful specially while using FLARENET on specifying  tailpipes and branches .

 

Cheers

-Johnny 



#6 fallah

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Posted 18 February 2016 - 03:14 PM

Hi Colleges , I´ve been working on a short simulation using Flarenet, and when a 8" header expands to 30" a mach number of 1 is indicated downstream of the 8" pipe. I checked that the problem is only in the expansion point and not along the 8"pipe .
My question is if this velocity means a real problem or it can be avoided?
Probably a software issue?

Cheers-Johnny

 

Jonny,

 

In real world there is no 8"*30" reducer and in order to reach from 12" to 30"you have to apply 8"*12"+12"*20"+20"*32" reducers. Then by such application in which the flare gas would expand geadually; the probability of having mach no. near to 1 due to sudden expansion would be removed...



#7 Poe

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Posted 18 February 2016 - 06:49 PM

Hi John pr1me,

 

I'm not clear about "T" specification,  as to say the "T" in Flarenet according to the manual on page 82 comments:
body:
specify the diameter of the body of the "T"
 
Allowable choises are:
 
Run: The Diameter will be That of the inlet pipe
Tail: the diameter will be That of oulet pipe
Branch: The diameter will be That of the branch pipe
Auto: Flare Systems  analyzer will SET the body of the inlet and branch pipe diameters.
 
I understand that by choosing one of these options all the "T" including the entry of the branch is defined.





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