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Monday, March 23, 2009

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Earlier post "Quick Estimate Flare Tip Pressure Drop" has presented some clues in obtaining flare tip pressure drop for sonic and subsonic tip. Particularly for subsonic flare tip, a simple pressure drop versus flow curve was presented. This graph may be used for quick estimation during conceptual stage so that it minimize unnecessary time lost. However, the graph has limited to 0.5 - 40 kPa with correspondent minimum and maximum flow. Some conceptual studies may have flaring capacity resultant pressure drop more than 40 kPa, the graph may not be useful. In this post, a simple formula will be presented in order to estimate pressure drop for any flow rate for dedicated flare tip.

dP = 10 ^ (a x Log Q + b)

where
dP = tip pressure drop, kPa
Q = Flow at standard condition, dm3/s (note 1)
a & b = coefficient correspondent to tip size as list in following table

Tip Nom. Dia. (mm)
ab
250
1.772663-5.287376
400
1.92372
-6.671917
450
1.948852
-7.034009
500
1.945342
-7.329128
600
1.954552
-7.791866
750
1.968898
-8.24243
900
1.89152
-8.14936
1050
1.722666
-7.781574
1200
1.532001
-7.181798


Note 1 : The standard condition may be different from project to project (read more in "Avoid Confusion In "Standard" Flow Definition". Present standard definition is at 101.325 kPa abs and 15 degC.

Example
Estimate pressure drop of a subsonic flare tip with flow of 30,000 Sm3/h.

Q = 30,000 x 1000 / 3600 =8333.3 dm3/s @ Std

(i) A DN250 tip,
dP = 10 ^ ( 1.772663 x Log Q - 5.287376)
dP = 10 ^ ( 1.772663 x Log 8333.3 - 5.287376)
dP = 46 kPa (outside curve)


(ii) A DN400 tip,
dP = 10 ^ ( 1.92372 x Log Q - 6.671917)
dP = 10 ^ ( 1.92372 x Log 8333.3 - 6.671917)
dP = 7.4 kPa

(iii) A DN450 tip,
dP = 10 ^ ( 1.948852 x Log Q - 7.034009)
dP = 10 ^ ( 1.948852 x Log 8333.3 - 7.034009)
dP = 4.0 kPa

(iv) A DN500 tip,
dP = 10 ^ ( 1.945342 x Log Q - 7.329128)
dP = 10 ^ ( 1.945342 x Log 8333.3 - 7.329128)
dP = 2.0 kPa

You may compare results with chart in "Quick Estimate Flare Tip Pressure Drop".

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posted by Webworm, 11:13 AM

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