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.

AFRC-JFRC 2004 Joint International Combustion Symposium October 10-13, 2004. F la re s - N o is e P r e d ic tio n a n d T h e r m o - A c o u s tic E ffic ie n c y Carl-Christian Hantschk and Edwin Schorer Miiller-BBM GmbH, Robert-Koch-Strasse 11, D-82152 Planegg, Germany Phone: [phone], Fax: [phone] e-mail: [e-mail], [e-mail] A b stra c t In m an y in d u stries w here co m b u stib le w a ste gases are o b ta in ed , flares are u sed to burn th ese gases in a controlled m anner. A m o n g other en v iro n m en ta l a sp e c ts th e n oise em issio n s a sso c ia te d w ith flaring are b eco m in g in crea sin g ly im p o rta n t in m an y coun tries as p o p u la tio n d en ­ sity goes up an d resid en tial an d in d u stria l areas m ove closer togeth er. In sta llin g n oise con trol eq u ip m en t on flares is a lm o st im p o ssib le w h ile th e y are in service, since flares are ty p ic a lly a safety related p lan t co m ­ p on en t th a t can o n ly b e tu rn ed off after th e co n n ected plant has b een shut dow n. A ccordingly, p red ictin g th e n oise em issio n s o f flares is crucial as early in th e d esig n p rocess as p o ssib le in order to p la n ap prop riate noise control m easures in tim e and to avoid u n n ecessa ry costs. A con ­ cep t to c a lcu la te flare n oise is to use th e ''T h er m o -A c o u stic E fficien cy" (T A E ) w h ich is th e ratio b etw een th e a co u stica l pow er and th e h eat re­ lease rate o f th e bu rn in g p rocess. V alues for T A E are u su a lly d eterm in ed e x p e r im e n ta lly in th e te st field an d th e n u sed for sim ilar flare design s an d o p era tin g c o n d itio n s. In th e present work n oise levels from field m easu rem en ts on a num ber o f different fu ll-scale in d u stria l flare ty p e s are p u t to g eth er and th e corresp on d in g T A E valu es are com pared. It is seen th a t th e T A E con cep t has sh o rtcom in g s r esu ltin g from oversim p li­ fication o f th e n oise gen era tin g m ech a n ism s o f flares. For im proved flare noise m o d elin g th e various n oise sources co n trib u tin g to th e overall noise em issio n s o f flares m u st b e tak en in to accoun t. T h e present p aper su m ­ m arizes th ese sources and o u tlin es th e in d iv id u a l effects and param eters ha v in g an influence on th e a co u stica l ch aracteristics o f flares. 1