Small relief streams can be burned unobtrusively in a ground flare. The Company now has ground flares at most of its refineries. The ground flare is basically a refractory-lined, multi-burner process furnace without tubes. The stack may be as high as 100 feet. This discussion assumes a low pressure ground flare, although high pressure ground flares also exist.
The capacity of the ground flare is limited: 25,000 and 50,000 Btu/hour are common sizes. Normally, the ground flare is sized for 5 to 10% of the total capacity of the relief system. The ground flare is included to take care of the great majority of the relief incidents, about 95%, that are relatively small.
Ground flares are used to reduce the noise and light of elevated flares. Enclosing the turbulent burning zone eliminates the light, and employing multiple burners greatly reduces the noise. A typical ground flare is at least 15 decibels quieter than an elevated flare.
The elevated flare is reserved for emergencies. The ground flare is not an alternative to an elevated flare. It is an expensive added feature for good public relations. The diversion water seal preferentially routes relief gas to the ground flare up to its maximum capacity, where it is burned quietly, smokelessly, and without visible flame. When the flow exceeds the capacity of the ground flare, the diversion seal automatically diverts the excess relief gas to the elevated flare. See Figure 1200-39.
The relief system is designed so that the ground flare can be shut down for maintenance. All relief streams are bypassed to the elevated flare, and there is no interruption in plant operation.
The ground flare is sized to minimize operation of the elevated flare. This requires a review of the controllable venting activities of the plants and their flow rate and frequency. This review is used to select the capacity of the ground flare from the maker’s standard capacities. Generally, the Company selects either 25,000 or 50,000 Btu/hour capacities.
Most of the Company’s experience has been with two main kinds of ground flares: cylindrical (John Zink), and rectangular (Flaregas). Figure 1200-26 shows a cylindrical ground flare. Figure 1200-27 shows a rectangular ground flare.
Ground flares are not incinerators. They lack the residence time and mixing thoroughness to handle sour gas streams. Sour gas streams must be segregated and not burned in ground flares or there will be odor problems.