Flare stacks heat intensity

The height of a flare is fixed on the basis of the heat generated and the resulting potential damage to equipment and humans. The usual design criterion is that the heat intensity at the base of the stack is not to exceed 1500 Btu/hr/ft2. The effects of thermal radiation are shown in the following table ... 

Determine the stack height required to give a heat intensity of 1500 Btu/hr/ft2 at a distance of 410 ft from the base of the flare. The flare diameter is 4 ft, the flare load is 970,000 lb/hr, and the molecular weight of the vapor is 44. 

With a heat intensity of 2,000 Btu/hr./sq.ft. (six times the intensity of solar radiation) the pain threshold is 8 seconds. Therefore, to allow a man time to run to safety, he should not be subjected to a heat intensity higher than approximately 1,500 Btu/hr./sq.ft. in the event of a major refinery or chemical plant failure which sends large amounts of combustible gas to the flare stack. A stack of sufficient height can be selected to satisfy this condition. But if this is impractical, adequate protective measures should be taken to ensure his safety. 

Figure 6-3. Circles of 1,500 and 3,000 Btu/hr. sq. ft. heat intensity for one 48-inch flare stack height, 200 feet flare load. 970,000 Ib./hr. molecular weight. 44.

Nomographs can be used to solve typical stack height and heat intensity problems. To find the heat intensity q in Btu per hour square foot if y is known, first obtain X from Figure 6-5. Then with the value of X and the values of the flare load (in pounds per hour) and molecular weight, read q in Figure 6-6a. 

The project team shall make a survey of current day and night noise level data in the surrounding facilities. This will help to assess whether permitted noise levels will be achievable, even with the new equipment, at reasonable cost. Otherwise, alternative site selection is required. Heat radiation is another consideration. New facilities shall be installed at places sufficiently far away from heat radiation sources such as flare and flue gas stacks. Heat radiation calculations shall be performed to ensure that the new site is located well within the permitted heat radiation intensity limits. Such limits are given in API 521 (2014). Luminosity is another important consideration for new plant siting this may require taller flare stacks. 

Spacing of elevated flares from process equipment or facility boundary depends on the flare stack height, flare load, and the allowable radiant heat intensity for personnel, public, and equipment. 

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