4 replies to this topic

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

[Nirav]

Hi
last few days i am working on Flarenet to design a flare header. i wanted to know about the constrain to be considered 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 the constraints or basic data which you must have in order to size flare header. You must refer API RP520 & RP521 to 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 to you 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 (in short, 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. You also need to consider the "design pressure" of your flare header which could be one of the constraints as far as maximum 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 built up back pressure" in flare header is also defined which could lead to limitations). These are one of the constraints.

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 velocity limits. So, based on preferences, you can go upto 0.6 to 0.7 of its maximum value during calculations. This is also one of the constraints. Flarenet will give you this value (and warnings also) when you do calculations. This is directly related to velocity.

I hope above helps.
Thank you

[Steve McGahey]

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

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

[Steve McGahey]

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

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're clear on where your Flare network pressures are being defined.

- Steve

[JoeWong]

svc,

Additional personal opinion...

As proposed by Steve, keep the Noise level below 90dB. This may be a bit stringent on emergency relief case e.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 No
d) Momentum


you may also need to look at

e) potential two phase flow and its flow regime
Two 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 Vibration
Flarenet do not built in this option. Some (may be most) company / project do not assess AIV.



regards,

JoeWong