Quick Estimate Flare Stack Diameter

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A young engineer asked…He is conducting a flare network studies using ASFA / FLARENET in conceptual stage. He need to allocate pressure drop across flare stack and tip. What is the quickest way ?

Flare tip pressure drop generally subject to flare tip design and only the tip vendor would be able to provide. Quickest way is to obtain vendor advice on similar kind of tip. However, in the absence of vendor information, the flare tip may be estimated according to discussion in earlier posts "Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation" and "Quick Estimate Flare Tip Pressure Drop".

Flare stack is made of vertical pipe in onshore plant and incline pipe (some called it boom) in offshore platform. The height is primarily govern by personnel allowable heat radiation at sterile area. As it is a pipe in nature, gas flow through flare stack during emergency and normal operation will result velocity and pressure drop across the flare stack. Thus, the flare stack diameter is primarily determine by Mach no and pressure drop. The recommended Mach no for flare stack is 0.5 during emergency relief (design capacity) and 0.2 during normal flow. Normal flow could be gas discharge from those pressure control valve (PCV) used to avoid pressure spike in process system.

Mach no here will be the Mach number at downstream of flare stack. One shall remember, the pressure drop across flare tip may be taken into account and it potentially reduce the Mach no and hence the flare stack diameter. The flare tip pressure drop could be estimated according discussion in earlier posts "Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation" and "Quick Estimate Flare Tip Pressure Drop".

Flare stack diameter,

d = [3.23 x 10^-5 ( W / ( P2 x Ma2)) x (z T / MW)^0.5] ^ 0.5

where
W = Mass flow (kg/h)
P2 = Outlet pressure (kPa abs)
Ma2 = Outlet Mach no
z = compressibility factor
T = Temperature (K)
MW = Molecular weight

Select a diameter, D which large or equal to minimum diameter (d) calculated above. Once the D is selected, the pressure drop across the flare stack may be estimated using Isothermal flow equation.

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