3 replies to this topic

[Arsal]

Flare installed at our refinery is 200 ft tall.i have checked the design basis where following formula has been used (Designed in early 70's)

 

Cmax = W/(V*H2*M)    (Reference : Lees' Loss Prevention in the Process Industries: Hazard Identification By Frank Lees)

 

Where,

Cmax = Max ground concentration (% Volume) (admissible limit is 3% by weight)

W = lb/hr

V = Miles per hour

H = Height

M = Molecular Weight.

 

On the basis of above formula the flare height calculated is 186 ft and our flare is 200 ft, it seems good.

 

However, when i have calculated flare height from API-521 formula by Hajek and Ludwig Equation by considering Heat radiation 6,31 (2 000) kW/m2 (Btu/h·ft2) the flare height value is >350 ft which seems very high.however i have checked flare height by different methods as well. In all methods the flare height is more than 300 ft.

At 200 ft flare height radiation level is very beyond the Permissible design level of thermal radiation.

 

My question is that how to encounter this problem ? Is criteria of Cmax is acceptable?

 

Thank you.

Edited by Arsal, 04 October 2016 - 02:54 AM.

[rdcrags]

In the 1970s, my clients ignored the unreasonable calculated heights using RP 521. Several projects calculated to require 350 ft or more. Installed heights were 200 ft or less. My recollection is that API reduced their emissivity factor in their procedure to more closely resemble what was being installed. It sounds like they have made another change toward the unreasonable.

[Arsal]

Thanks for your reply.
I did not get your response completely.Can you please elobrate it more??

[gegio1960]

My 2 cents of experience....

Between 1990 and 2010 I've designed/checked more than 10 flares/flare systems of refineries in Europe, Asia, Africa and Middle East for a major engineering company.

API RP 521 has always been the reference standard, imposed/accepted by all the Ckients, for the radiation calculations and limits.

The dispersion calculations were performed by the flare manufacturers..

The flare height has always been determined by the radiation.

During the radiation calculations, only 2 minor corrections were made to the original API method universally recognized as conservative:

- the values of sun radiation above 200 btu/h/ft2 were disregarded;

- the emissivity parameter was reduced to take into account the "shield effect" of air humidity. 

Good luck!