1. Flare Stack Height
APPENDIX D
Flare Size Calculations
1.1 Flare stack height, which is usually determined by allowable radiant heat intensities at grade, shall be
calculated in accordance with the procedure given in Item 2, below. Flare stack height so calculated shall
also be checked to ensure that the toxic gas ground level concentration, that is produced from burning
material that can release toxic material (for example, H
S or chlorinated organics), does not exceed local,
state or federal air pollution restrictions. Requirements for flare stack height are as follows:
2
1.1.1 Elevated flare stacks may be guyed, self-supporting or supported by a derrick structure, depending
on space availability and height of the stack. Ladders and platforms are not required on self-supporting
stacks that can be lowered for maintenance.
1.1.2 For a system with a single flare, the height of the flare stack (see Figure D-1) shall be determined.
The maximum allowable radiation intensity levels shall be in accordance with Table D-1.
1.1.3 Continuous gas discharge at rates significantly below maximum emergency relief, but above the
normal continuous leakage and purge rates for pressure relief valves, will result in premature flare tip
burnout. Installation of two flares is recommended: an operating flare for smokeless burning of day-to-day
releases and a separate emergency flare for burning of infrequent emergency releases. The two flares
shall be connected through a water diversion seal, which will allow flow to the emergency flare as
backpressure builds up in the operating flare (see Figure D-1). Pressure drop considerations shall govern
the relative sizes of the tips, but the operating flare tip is generally equal to or slightly smaller than the
emergency tip.
a. When the relative relief rates are established, the sizing procedure given in Section 1 of
Appendix D shall be used for each flare. The radiant heat loads at each point shall be the sum of the
expected heats from both flares.
1.1.4 Flare stack height can also be governed by dispersion.
TABLE D-1
Allowable Radiant Heat Intensities in W/m2(Btu/h/ft2) Excluding Solar Radiation
*Definition: Appropriate clothing is clothing for example, coveralls, long sleeved shirts, pants, hard hat
which can shield all body areas except for the face and hands from exposure to flare radiant heat.
2. Flare Tip
The minimum diameter for pipe tips shall be based on a velocity of 0.5 Mach or as recommended by the
manufacturer. The velocity for proprietary tips shall be recommended by the manufacturer. The equation
for sonic velocity (one Mach) is as follows for an ideal gas:
M = molecular weight
Tips shall be fitted with a flame retention device that is rated by the vendor at 0.5 Mach or higher. Tips shall
also have windshields to reduce flame lick at low flaring rates. Depending on the gas being flared, the tip
and/or windshields may have to be constructed of high-temperature corrosion-resistant alloys, for example,
Inconel 625 or Incoloy 800, to avoid maintenance problems. Normal materials of construction are
heat-resistant stainless steels, for example, Type 309 or Type 310.
FIGURE D-1
Installation of an Operating Flare and an Emergency Flare
