Page 567 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Applied Process Design 533

Table 7-32 where H = height of flare stack, ft, Figure 7-73
Recommended Design Flare Radiation Levels Including L = height of flame (length of flame from top of
Solar Radiation stack Lo flame tip), fl
D = X = radial distance from flame core (center) to
Permissible Design Level (K) grade, fl
Kilowatts R = y = radial distance from base of stack, ft, Lo grade
British Thermal per intersection with D
Units per Hour Square tc = time interval for escape, sec
per Square Foot Meter Conditions Note: For safety, personnel and equipment
�����--���������������-
5000 15.77 Heat intensity on structures and should be outside the "y" distance.
in areas where operators are not R = distance from flame center to point X on
likely to be performing duties ground (see Figure 7-77)
and where sheller from radiant
heat is available (for example, This has been shown to be quite accurate for distances
behind equipment) as close to the flame as one flame length [61].
3000 9.46 Value of Kat design flare
release at any location to which
people have access (for exam- Emissivity Values [621
ple, al grade below the flare or
a service platform of a nearby Carbon Monoxide 0.075
tower); exposure should be lim- Hydrogen 0.075
ited to a few seconds, sufficient Hydrogen Sulfide 0.070
for escape only Ammonia 0.070
2000 6.31 Heal intensity in areas where Methane 0.10
emergency actions lasting up to Propane 0.11
1 minute may be required by Butane 0.12
personnel without shielding but Ethylene 0.12
with appropriate clothing Propylene 0.13
1500 4.73 Heal intensity in areas where Maximum 0.13
emergency actions lasting sever-
al minutes may be required by
personnel without shielding but Length of Flame [62] See Figure 7-77
with appropriate clothing
500 l.58 Value of Kat design flare
release at any location where Lr= 10 (D) (�P,/55) 1/ 2 (7-89)
personnel are continuously
exposed
where Le = length of flame, ft
Note: D = flare tip diameter, in.
On towers or other elevated structures where rapid escape is not possi- �P, = pressure drop at the tip, in. of water
ble, ladders must be provided on the side away from the flare, so the
structure can provide some shielding when K is greater than 2000 British
thermal units per hour per square foot (6.31 kilowatts per square meter). This gives flame length for conditions other than max-
Reprinted by permission, AP! RP-521, Guide for Pressure Relieving and Depres- imum flow.
surrng Systems, 3rd E.d., Nov. 1990, American Petroleum Institute [33].
The center of the flame is assumed to be located a dis-
tance of one-third the length of the flame from the tip,
Height of stack for still air [60]: Lr/3 [62]. The flame angle is the vector addition of the
wind velocity and the gas exit velocity.

(7 - 87)
V cxn ' gas exit velocity= 550 � �p / 55 , ft/sec (7-90)
The shortest stack exists when qM = 3,300 BTU /hr sq ft From Figure 7-77
(Figure 7-76). The limiting radial distance from the
flame, allowing for speed of escape of 20 fl/sec is [60]:
Xe = (Lr/3) (sin 8)

y = 20 t 0 = [x 2 - H (H + L)]o.s (7-88) Ye= (Lr/3) (cos 8)