13 replies to this topic

[mohammadhosseinzadeh]

would you please advise me regarding the following question?

How we can calculate required gas flow rate for sweeping of flare system?
or
What is important parameter which affect on sweeping flow rate?

[ankur2061]

Dear Mohammad,

For pipe type flares the purge or sweeping gas rate is a function of the minimum velocity for purge gas and the diameter of the flare tip. You can calculate the purge gas rate for flares with modern labyrinth or internal gas seals considering a conservative velocity of 0.04 ft/s. Multiplying this with the flare cross-sectional area in ft2 will give you the volume flow rate in ft3/s.

Alternatively you may use the following equation to calculate the purge gas rate for pipe type flares with diameter 457 to 914 mm.

Purge gas flow rate = 1.2E-5*d^3/M^0.565

Where,
purge gas rate is in m3/h
d = diameter of flare tip in mm (applicable to diameter range of 457 to 914 mm)
M = molecular weight of purge gas (purge gas could be inert gas such as N2 or fuel gas)

Hope this information is useful.

Regards,
Ankur.

[JoeWong]

Mohammad,

I guest you have dropped me an email related to similar topic and i am yet to response to you. I take this opportunity to response to you...It is great that you raise similar question in order to get more inputs...

Flare sweeping gas flow determination...
There are many factors affecting it :

i) Flare tip design (multi-tube, pipe flare, tulip, etc)
ii) Purge gas type (combustible like fuel gas or non-combustible like nitrogen)
iii) Molecular weight
iv) gas composition
v) component
vi) Flare tip diameter
vii) External wind velocity
viii) wind shield
.
.
.
Others CheJedi, please add...

Before determine flare purge, first, you have to consider if purge reduction seal (i.e velocity seal, etc) is installed. With this seal, it significant reduce the purge rate (possibly 10 times or more).

The first method proposed by ankur is the experience approach where a velocity based on "rule of thumb" is used. However, this figure is different if seal is present. Secondly, sad to say that flare tip vendor will have different figures (large different)

The second method as proposed by Ankur is the Tan method (if not mistaken) which only take into account of molecular weight and tip diameter, have been used by many companies (well known) for sometime. However, there are other method like Husa, Herbert, Corbett, etc which is having different consideration and limitation. Lately API has recommended HUSA method.

Advise base on above discuss factors, looks at the limitation of the method with your particular case to select a proper method and last ensure your purge rate is not lower than rate requested by flare tip vendor.

Good luck.

[mohammadhosseinzadeh]

your actions are highly appreciated.

JoeWong:
Would you please give more information about HUSA method or others method(Adress)?
API (section and Edition)

[JoeWong]

Please refer to API Std 521 - ISO 23251 - Ed 5 - Jan 2007 - Pressure-relieving and Depressuring Systems, Section 7.3.3.3.3.

Please also read section 6.4.3.6.2.

[mohammadhosseinzadeh]

Please refer to API Std 521 - ISO 23251 - Ed 5 - Jan 2007 - Pressure-relieving and Depressuring Systems, Section 7.3.3.3.3.

Please also read section 6.4.3.6.2.

JoeWong:
Thank you for your advise,
unfortunathly i do not have access to this edition,I have access to API RP 521 1997

[djack77494]

Maybe it doesn't apply in too many cases, but for a large flare system with an efficient seal, you might want to check for cooling of the (gas) contents of the flare system. Any volume reductions resulting from cooling will have to be met with an inflow of sweep gas. I suppose this is not often an important factor, but it could be at times.

[JoeWong]

Mohammad,
The HUSA equation is included in 2007 edition.

Are you still using 1997 edition ? I strongly encourage you to get a latest edition. There are several concepts changed, or i would prefer to say improvement in this revision (which i don't want to detail here). You should use the latest revision.

[ankur2061]

Dear Mohammad,

I am attaching the Husa correlation as an excel file I prepared. This should help you till the time you procure API STD 521.

Regards,
Ankur

[mohammadhosseinzadeh]

Thank U all friends, especially ankur which attached very useful exceel sheet.

[mohammadhosseinzadeh]

Thank U all friends, especially ankur which attached very useful exceel sheet.

Dear friends:

As U know there is sweeping and purge gas that i tried to clear my purpose in the attached drawing.
please refer to attached drawing to find better my purpose.
Is this purge gas (husa calculation or others methods)included both purge & sweeping rate(as per my drawing) or there is diffrent method for sweeping rate calculation?

Attached Files

•  flare_system.ppt   26.5KB   721 downloads

[JoeWong]

In my opinion. purge and sweeping are same although many "philosopher" differentiating them...

[ankur2061]

Dear Mohammad,

I am attaching the Husa correlation as an excel file I prepared. This should help you till the time you procure API STD 521.

Regards,
Ankur

Dear Forum Members,

The attached excel file is a revision of the earlier file which was referenced from BP stds. The new file is revised based on API STD 521, 5th edition, 2007.

Regards,
Ankur.

Attached Files

•  Min_Purge_Gas_Rate_Rev1.xls   25KB   1113 downloads

[Qalander (Chem)]

In my opinion. purge and sweeping are same although many "philosopher" differentiating them...

Dear mohammadhosseinzadeh,
I am also very clear to accept in principle JoeWong's statement;
with the slight modification that

Purging and Sweeping gas flows are equivalent term in connection with flare systems.

Where such gases perform a sort of Carrier/ Vehicle function(not necessarily Flammable or Combustible)
This helps in maintaining induced flow towards flare tip flame front and adds to the system's safety.
Best regards
Qalander