Height determination for an elevated flare stack is not a precise science. The stack height is selected to reduce the radiant heat intensity at ground level to desired safe limits for personnel and equipment. The Company uses the method outlined in API RP 521, Appendix A. Selecting an acceptable radiant intensity is the key design decision.
The radiant intensity is calculated by assuming uniform spherical radiation from the flame center. Calculation is inexact because of the following conditions.
1. Emissivity must be approximated. Emissivity is that fraction of the thermal energy of the flared gas that is actually radiated as heat. Test emissivities range from 60% to 15% and even lower when steam is introduced into the combustion zone. While steam is used for smoke suppression in most modern flares, steam may not be available during the major emergency that causes the maximum flare release. Therefore, for purposes of calculation assume an emissivity of 40%.
2. The flame center is unknown but is assumed to be located halfway up the flame. This varies significantly for a vertical flame in windless conditions to a nearly horizontal flame in a severe windstorm. Flames from major releases are typically several hundred feet long. Therefore, for purposes of calculation assume a flame length of 100 tip diameters.
A ground level radiation intensity of 1500 Btu/hr/ft2 is the maximum safe level (OSHA Standard) for prolonged exposure. This level is approximately the threshold of pain for bare unprotected skin. Below this level, exposure can be tolerated for virtually unlimited periods. Above this level, skin will blister. This intensity, however, is too low for a realistic flare height design onshore because personnel in the vicinity of the flare will promptly escape to a safe distance as soon as a major release begins. For offshore locations see “Flares at Producing Sites” in Section 1252. The API RP 521 sample calculations assume two different maximum allowable radiation intensities at grade—2000 and 3000 Btu/hr/ft2. Using maximum allowable radiation intensity of 2600 Btu/hr/ft2 at grade in the immediate vicinity of the flare has worked well for the design of recent installations. By using these design criteria, most Company flares are 150 to 175 feet high.