Flares smokeless operation

Smokeless operation can generally be achieved, with essentially no noise or luminosity problems, provided that the design gas rate to the flare is not exceeded. However, since the flame is near ground level, dispersion of stack releases is poor and this may result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out. Capital and operating cost and maintenance requirements are high. 

Smokeless operation not needed for emergency flares (only required for continuous flares). 

It is important to achieve smokeless operation for environmental reasons. In order to achieve this goal, the flare design should consider use of the following ... 

Elevated Flares See Flares for a general definition. The elevated flare, by the use of steam injection and effective tip design, operates as a smokeless combustion device. Flaring generally is of low luminosity up to about 20 % of maximum flaring load. Steam injection tends to introduce a source of noise to the operation, and a compromise between smoke elimination and noise is usually necessary. When adequately elevated (by means of a stack) this type of flare displays the best dispersion characteristics for malodorous and toxic combustion products. Visual and noise pollution often creates nuisance problems. Capital and operating costs tend to be high, and an appreciable plant area can be rendered unavailable for plant operations and equipment because of excessive radiant heat. 

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... 

A peak velocity through the flare end (tip) of as much as 0.5 mach is generally considered a peak, short term. A more normal steady state velocity of 0.2 mach is for normal conditions and prevents flare/lift off [58]. Smokeless (with steam injection) flare should be sized for conditions of operating smokelessly, which means vapor flow plus steam flow [33c]. Pressure drops across the tip of the flare have been used satisfactorily up to 2 psi. It is important not to be too low and get flashback (without a molecular seal) or blowoff where the flame blow s off the tip (see Ref. 57), Figure 7-71. 

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