Multijet flares

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

This chapter discusses some of the criteria for selecting, designing and spacing elevated, burning-pit, and multijet flares. The design of safety valve and flare headers was covered in an earlier chapter, as well as discussions concerning associated blowdown drums, water disengaging drums, etc. 

Normally an overcapacity line to an elevated flare is provided to handle the excess flow when the flaring rate exceeds the capacity of the multijet flare. The overcapacity flare is usually not equipped with steam injection, and smoke formation is accepted during infrequent operations. The overcapacity line and flare is designed to handle the entire maximum flow so that it can spare the multijet flare when the latter is shut down for maintenance. 

Clearance from structures higher than the flare stack 60 m. In addition, no structure where personnel access may be required while the multijet flare is in operation shall exceed in height of a projected diagonal line from the base of the flare stack to the top of the stack wall diametrically opposite. 

Clearance from the elevated overcapacity flare must comply with radiant heat spacing requirements for elevated flares, considering persoimel exposure when maintenance work is being performed on the multijet flare and the... 

Burner Design and Back Pressure for Multijet Flares... 

Air Flow - The capacity of a multijet flare to induce air flow must be calculated, to make sure that it is adequate to meet the maximum air flow requirement for smokeless combustion. (W, of Equation 4 below must be > W, of Equation 5). The term air flow capacity refers to the primary air flow rate which will be induced around each jet, and may be estimated from the following equation ... 

The value of W calculated from the gas composition or from Equation 5 should be considered to be a minimum requirement. A multijet flare should be designed with a calculated air capacity as high as possible, as limited by practical limitations of economics and geometry. 

Critical Burner Dimensions - The position of the flameholder s and burner lines relative to the bottom or the stack is critical for efficient operation. For example, the multijet flare has a turndown ratio of 10 1 when the flameholder centerline is 125 mm below the bottom of the stack but only 2 1 when it is 150 mm above the bottom of the stack. 

Steam Injection - While the multijet flare will achieve a significant reduction in the smoke produced, it does not provide true smokeless combustion over its full operating range. This is particularly true with the heavier (C4+) and unsamrated gases. Steam injection at a rate of about 0.5 kg steam per kg of gas will provide an additional reduction in smoke for most gases. Steam should be... 

Figure 6-8. This schematic layout of the multijet flare shows the two stages for blowdown control. Notice how the seal dams prevent burner back pressure from varying seal level.

The multijet flare developed by Esso Research and Engineering Company bums with no smoke, noise or visible flame. It costs about twice as much as a flare with no steam but half as much as an elevated flare with steam if off-site steam facilities are included. 

Other designs of ground flares suitable for refinery application are available. In some of these cases, noise is appreciable, in comparison with the multijet type, but their compact size, low space requirement, simplicity, and hence low cost, may give an overall advantage. 

Figure 4. Typical flare seal drum arrangement. For use on multijet or staged elevated flares.

Disposal of effluent water from multijet ground flare seal drums should comply with paragraphs (1), (2), and (3) above, except that ... 

Figure 6-8 is a simplified flow diagram for a typical multijet. The flare uses two burners. A small burner handles normal leakage and small blows, while both burners operate at higher flaring rates. This staging is controlled by two water-seal drums set to release at different pressure levels. 

A third emergency release is provided in the center of the stack, bypassing the multijet burners. The water seal to this release will blow at flaring rates higher than the design capacity of the flare. When the over-capacity seal has been blown, the flare is both luminous and smoky. But the unit is usually sized so that an overcapacity blow would be a rare occurrence. The over-capacity line may also discharge to an elevated flare rather than to the center of the multijet stack. 

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