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#1svc

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Posted 11 November 2006 - 02:46 AM

Hilast few days i am working on Flarenet to design a flare header. i wanted to know about the constrain to beconsidered while designing the flare header system. besides that what are the references to work on?

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#2Nirav

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Posted 12 November 2006 - 07:31 PM

QUOTE (svc @ Nov 11 2006, 04:46 PM) <{POST_SNAPBACK}>Hilast few days i am working on Flarenet to design a flare header. i wanted to know about the constrain to beconsidered while designing the flare header system. besides that what are the references to work on?

You must have basic knowledge of various aspect of flare header sizing. Then you will know by yourself about theconstraints or basic data which you must have in order to size flare header. You must refer API RP520 & RP521to know various criteria & constraints including sizing equations for flare header.

In short there are mainly two constraints for flare header sizing. [A] Maximum pressure drop (dP) available toyou and [B] Velocity (mach number). I would suggest at least following as basis.

[1] Flow & properties data for each PSV including various overpressure scenario considered for each of them (inshort, it is termed as "relief load summary" which you must prepare first or have a copy of it).[2] You can define "Maximum Allowable Back Pressure (MAWP)" for each PSV based on type of PSV. Youalso need to consider the "design pressure" of your flare header which could be one of the constraints as far asmaximum back pressure is concern which you can allow on your PSV. There could be criteria over this in your"basis of design" documents, if it is available to you. If not, you need to define it as your basis.[3] You must know "superimposed back pressure" at your plant battery limit (Sometimes, "min &/or max builtup back pressure" in flare header is also defined which could lead to limitations). These are one of theconstraints.

Defining [2] & [3] will give you [A] ...(which i said before as one of the two main constraints).

[4] Mach number limitation. Mach No.must be less than "1.0". At mach no.=1, flow reaches its sonic velocitylimits. So, based on preferences, you can go upto 0.6 to 0.7 of its maximum value during calculations. This isalso one of the constraints. Flarenet will give you this value (and warnings also) when you do calculations. This isdirectly related to velocity.

I hope above helps.Thank you

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Warm regards,

Nirav Kholiya"How you think about a problem is more important than the problem itself."


#3Steve McGahey

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Posted 10 January 2007 - 06:19 AM

Flarenet will check many, many aspects of your system - even things like slugging. I'd suggest you want to lookat

Mach Numbers (keep it less than 0.7)Momentum (keep it less than 100,000 lb/ft.s)Noise (keep it less than 90 dB)

That said, these are somewhat subjective.

- Steve

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#4Steve McGahey

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Posted 23 January 2007 - 10:51 AM

QUOTE (Steve McGahey @ Jan 10 2007, 11:19 AM) <{POST_SNAPBACK}>Flarenet will check many, many aspects of your system - even things like slugging. I'd suggest you want to lookat

Mach Numbers (keep it less than 0.7)Momentum (keep it less than 100,000 lb/ft.s)Noise (keep it less than 90 dB)

That said, these are somewhat subjective.

- Steve

Oh, and don't forget to notice the difference between 1) the pressure at the flare tip and 2) the backpressure on the relief devices (eg the flare network has a pressure differential over it).

This may sound obvious, but I have seen the one pressure taken to mean both of these things. Make sure you'reclear on where your Flare network pressures are being defined.

- Steve

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Steve McGahey

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http://steve.mcgahey.googlepages.com/


#5JoeWong

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Posted 05 February 2007 - 01:11 AM

svc,

Additional personal opinion...

As proposed by Steve, keep the Noise level below 90dB. This may be a bit stringent on emergency relief casee.g. Total plant blowdown, General power failure, etc.

Some time we adopt the following :

i) Normal relief e.g. Pressure dump valve , consider <90 dB (<85 dB with new design)

ii) Emergency relief, relax to < 115 dB

in order to have a reasonable / manageble design.

Apart from what Steve and Nirav mentioned :

a) Maximum Allowable Back Pressure (MAWP) Superimposed back pressure

c) Mach Nod) Momentum

you may also need to look at

e) potential two phase flow and its flow regimeTwo phase flow may occur due to mixing of hot and cold fluid, JT effect, ambient/steel cooling, etc.

f) potential pipe failured due Acosutical Induced VibrationFlarenet do not built in this option. Some (may be most) company / project do not assess AIV.

regards,

JoeWong

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