Purge Rates - Husa Compared to CFD

Purge Rates: Husa Compared to CFD Larry D. Berg, Joseph D. Smith, Ph.D. and Ahti Suo-Anttila, Ph.D. Alion Science and Technology Owasso, OK Scot K. Smith, P.E. and Jay B. Modi Zeeco, Inc. Broken Arrow, OK

OUTLINE

• • • • •

Slide 2

Review of Husa Method Review of Purge Practice Husa vs. CFD – High Purge Rate Husa vs. CFD – Range of Rates Conclusion

2006 AFRC International Symposium

Review of Husa Method

• Howard W. Husa – Two papers • 1964 Hydrocarbon Processing • 1977 Fall API Fire Safety Meeting

• Range of rates, gasses and diameters (4” – 48”) • Single Correlation for all experimental results

Slide 3


Review of Husa Method n ⎤ ⎡ ⎛ ⎞ 1 20 . 9 0.65 3.46 ⎟⎟⎥ ∑ C i Q = .07068 D ⎢ ln⎜⎜ e (.065(29 MW )) ⎣ y ⎝ O2 ⎠ ⎦ 1

• • • • • • Slide 4

Q = purge rate (ft3 / hr) D = Internal diameter of flare stack (inches) y = depth down flare stack from exit plane (ft) O2 = Oxygen concentration (mole fraction) Ci = Mole Fraction of Purge Gas type MW = Molecular Weight of Purge Gas Type 2006 AFRC International Symposium

Review of Husa Method • Incorrect MW Functionality

As Published 21

18

15

12 Series1 9

6

3

0 0

0.25

0.5

0.75

1

M W o f Pur g e g as

Slide 5


Review of Husa Method • From data published with paper, deduce actual •

functional form: As Published:

n ⎡ ⎤ ⎛ ⎞ 1 20 . 9 0.65 3.46 (.065 ( 29 MW )) ⎜ ⎟ Q = .07068 D ⎢ ln⎜ e ⎥ ∑ Ci ⎟ ⎣ y ⎝ O2 ⎠ ⎦ 1

• “Deduced” Equation: n ⎡ ⎤ ⎛ ⎞ 1 20 . 9 ⎟⎟⎥ ∑ C i 0.65 6.586 ⋅ e (− .065 Q = .07068 D 3.46 ⎢ ln⎜⎜ ⎣ y ⎝ O2 ⎠ ⎦ 1

Slide 6

MW )


Review of Husa Method • “Deduced” Equation: n ⎤ ⎡ ⎛ ⎞ 1 20 . 9 0.65 3.46 ( − .065 ⎜ ⎟ Q = .07068 D ⎢ ln⎜ C 6 . 586 ⋅ e ⎥∑ i ⎟ ⎣ y ⎝ O2 ⎠ ⎦ 1

MW )

• Husa “K” factors for gas

Q = .07068 D

Slide 7

3.46

⎡ 1 ⎛ 20.9 ⎞⎤ ⎟⎟⎥ K ⎢ ln⎜⎜ ⎣ y ⎝ O2 ⎠⎦


Review of Purge Practice • Husa – Interested in Prevention of Stack Explosions • Simple “pipe” or “utility” flares

Slide 8


Review of Purge Practice • Since Husa (30-40 years

ago) Flares have changed

Slide 9


Review of Purge Practice Multi-Point Ground Flares

Enclosed Flare

Slide 10


Review of Purge Practice Multi-Injection Steam Flares Not only has the equipment changed, but a secondary purpose has been added to explosion prevention:

“Protection of complex combustion Equipment” Slide 11


Review of Purge Practice

• Equipment did not appear overnight • Vendors have developed proprietary purge methods • CFD can provide an independent verification

Slide 12


Husa vs. CFD – High Purge Rate

• Customer wanted to ensure minimal air ingression

• Wanted results 1’-0” from stack exit

• Purge was methane, ethane, propane mix • Steam flares included “cooling” steam

Slide 13



Slide 14

Tip

Husa O2 Concentration

CFD (FLUENT)

58” Utility

15.2%

1.1%

34” Utility

15.3%

0.8%

66” Utility

16.2%

0.2%

56” Steam

15.9%

3.1%

46” Steam

17.5%

3.4%



• Husa greatly over predicts O2

concentration • Husa does not include combustion • Not very interested in 1’-0” from tip exit

Slide 15


Husa vs. CFD – High Purge Rate Temperature ˚C

Temperature Contours

Oxygen Contours

O2 Mole Frac

Wind = 3 m/s

Wind = 8.9 m/s

Wind = 18 m/s

Utility Tip Slide 16


Husa vs. CFD – High Purge Rate Temperature ˚C

O2 Mole Frac

Temperature Contours

Oxygen Contours

Wind = 3 m/s

Wind = 8.9 m/s

Wind = 18 m/s

Multi-Injection Steam Tip Slide 17


Husa vs. CFD – Range of Purge Rates

• • • •

Slide 18

More reasonable to compare: Range of purge rates Variation of depth No combustion



• • • • •

Slide 19

Range of purge rates: 0.05, 0.25, 1.00 ft/s Variation of depth: 1’, 8’, 15’ Methane only No combustion Use steady state and transient CFD for utility tip


Husa vs. CFD – Range of Purge Rates Husa vrs. CFD - 1.0 ft/sec 34" Utility Flare

Oxygen Concentration

15.00% 12.00% Husa

9.00%

CFD - ISIS 6.00%

CFD-FLUENT

3.00% 0.00% 0

5

10

15

Depth from Tip Exit (ft)

Slide 20


Husa vs. CFD – Range of Purge Rates Husa vrs. CFD - .05 ft/sec 34" Utility Flare

21.00%

21.00%

18.00%

18.00%

15.00% Husa

12.00%

CFD - ISIS 9.00%

CFD -FLUENT

6.00% 3.00%

15.00% Husa

12.00%

CFD - ISIS 9.00%

CFD - k-e

6.00% 3.00% 0.00%

0.00% 0

5

10


Slide 21



Husa vrs. CFD - .25 ft/sec 34" Utility Flare

15

0

5

10

15




• At high rates (1.0, 0.25) – Husa, steady

state CFD, and transient CFD reasonable agreement

• At low rate, steady state analysis “OK”, but transient much better

Slide 22


Husa vs. CFD – Range of Purge Rates Movie of transient prediction for 50% O2

Slide 23



Slide 24


Husa vs. CFD Multi-Tube Steam Flares

Typical Multi-Tube (HCSX) Steam Flare Tip Slide 25


Husa vs. CFD Multi-Tube Steam Flares For Husa need: • Velocity • Diameter

Typical Multi-Tube (HCSX) Steam Flare Tip Slide 26


Husa vs. CFD Multi-Tube Steam Flares D

Which Diameter ?

D Slide 27


Husa vs. CFD Multi-Tube Steam Flares Which Velocity ? Vtip

Vriser Slide 28


Husa vs. CFD Multi-Tube Steam Flares 1.0 ft/sec & Center Steam Husa vrs. CFD - 1.00 ft/sec 46" Mult-Tube Steam Flare Riser Diamter / Riser Velocity

21.00%

21.00%

18.00%

18.00%

15.00% 12.00%

Husa CFD - k-e

9.00% 6.00% 3.00% 0.00%



Husa vrs. CFD - 1.00 ft/sec 46" Mult-Tube Steam Flare Measured Diamter / Tip Velocity

15.00% 12.00%

Husa CFD - k-e

9.00% 6.00% 3.00% 0.00%

0

5

10


15

0

5

10

15


Tip Diameter & Tip Velocity have Best Correspondence Slide 29


Husa vs. CFD Multi-Tube Steam Flares 0.05 ft/sec & Center Steam Husa vrs. CFD - .05 ft/sec 46" Mult-Tube Steam Flare Riser Diameter / Riser Velocity

21.00%

21.00%

18.00%

18.00%

15.00% 12.00%

Husa CFD - k-e

9.00% 6.00% 3.00%



Husa vrs. CFD - .05 ft/sec 46" Mult-Tube Steam Flare Tip Diameter / Tip Velocity

15.00% 12.00%

Husa CFD - k-e

9.00% 6.00% 3.00% 0.00%

0.00% 0

5

10


15

0

5

10

15


Riser Diameter & Riser Velocity have Best Correspondence Slide 30


Husa vs. CFD Multi-Tube Steam Flares Suggest: • For stack explosion analysis, use riser velocity and riser diameter • For combustion equipment analysis, use tip velocity and tip diameter

Slide 31


Summary and Conclusions • Husa recommended empirical correlation to estimate • • • • Slide 32

required purge rate for “simple” gas flares Gas flare equipment changed significantly (became more complex) since Husa first analyzed purge requirements Compared Husa purge estimate to estimates based on Fluent and ISIS CFD codes Recommend approach to purge estimate based on what’s needed (i.e., stack combustion analysis vs. tip analysis) Discussion 2006 AFRC International Symposium

Purge Rates - Husa Compared to CFD

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