Flares - noise prediction and thermo-acoustic efficiency

In many industries where combustible waste gases are obtained, flares are used to burn these gases in a controlled manner. Among other environmental aspects the noise emissions associated with flaring are becoming increasingly important in many countries as population density goes up and residential and industrial areas move closer together. Installing noise control equipment on flares is almost impossible while they are in service, since flares are typically a safety related plant component that can only be turned off after the connected plant has been shut down. Accordingly, predicting the noise emissions of flares is crucial as early in the design process as possible in order to plan appropriate noise control measures in time and to avoid unnecessary costs. A concept to calculate flare noise is to use the "Thermo-Acoustic Efficiency" (TAE) which is the ratio between the acoustical power and the heat release rate of the burning process. Values for TAE are usually determined experimentally in the test field and then used for similar flare designs and operating conditions. In the present work noise levels from field measurements on a number of different full-scale industrial flare types are put together and the corresponding TAE values are compared. It is seen that the TAE concept has shortcomings resulting from oversimplification of the noise generating mechanisms of flares. For improved flare noise modeling the various noise sources contributing to the overall noise emissions of flares must be taken into account. The present paper summarizes these sources and outlines the individual effects and parameters having and influence on the acoustical characteristics of flares.