Combustion noise is approximately proportional to the square of the mixing velocity. Steam-air injection produces high frequency noise and also raises the intensity of the low frequency combustion roar. Most current flare tips use
multiport steam nozzles to reduce flare noise to some extent. While it is operationally convenient to keep excess steam on the flare, it is undesirable for noise production and energy conservation.
Without steam-air injection, buoyant forces aspirate air into the combustion zone and the laminar flame tends to be noiseless, billowy, and smoky. With steam-air injection, aspiration is thrust controlled, turbulence is greater, mixing is better, and the flame is smaller, stiffer, less radiant, and smokeless. Unfortunately, these conditions produce the combustion roar that is typical of thrust controlled flames.
The expected noise levels from modern flares with Coanda or multiport nozzles are shown in Figure 1200-25. Combustion roar dominates, particularly at higher relief rates. The expected sound level can be scaled by adding or subtracting 6 dB if the distance is halved or doubled.
At high relief rates, high frequency noise is not what really disturbs the neighbors. Rather, they are bothered by the inherent rumble, vibration, and illumination. There is no technological remedy. In an emergency, an elevated flare may disturb the surrounding neighborhood.
It is a poor idea to use an elevated flare to burn off small or frequently vented streams. Alternate means of controlling the pressure of the process should be considered. Any significant use of an elevated flare, particularly at night, may be objectionable to nearby residents.