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[Fergie]

Dear Folks,

I'am a mechanical engineering student with final project to design 2500 m³ gasoline storage tank according to API 650. 

The design of the roof is cone roof + gas blanket.
Known Data: 

• Tank Capacity : 2500 m³
• Tank Internal Diameter : 17.2 m
• Height : 12 m
• Liquid_in : 286.1 m³/hr
• Flash point : -43 ^oC

What I've done is calculate the capacity of N2 using API 2000 for inbreathing,

• Q_{inbreathing liquid} : 268.9 Nm³/hr
• Q_{inbreathing thermal} : 422.5 Nm³/hr
• Q_total : 691.4 Nm³/hr = 0.19 Nm³/s

After that, using "Rules of Thumb for Chemical Engineers By Carl Branan"

• Suggested Fluid Velocity N2 (Air = 78% N2) : 4000 fpm = 20.32 m/s
• Typical Design Vapor Velocity (Gas or Superheated Vapor 0 to 15 psig) : 50 - 145 ft/s (pick 65 ft/s = 19.8 m/s so < 20.32 m/s)

After that, 

• A = Q / v = 0.19 / 19.8 = 0.0097 m²
• D = SQRT(4 x 0.0097 / PI) = 111.13 mm

Then with SCH 40 that satisfied the D and common in the market is 6" (154.05 mm)

Does my calculation is correct? 
If wrong, can someone please explain the other way to calculate it because after read the API 2000 that is what I thought.
Thanks in advance !!