Hi,
The best method for arriving at noise levels of flares IMO is the German standard VDI 3732, which is based on measured noise levels from many elevated and ground flares, mainly in Germany. The standard is bilingual (German and English). It can be downloaded with a credit card for 56.40...
For refinery flares, according to API RP 521, the exit velocity for a stable flame may be as high as Mach 0.5 for intermittent service. Flares for hydrocarbon gases have been tested with a velocity as high as Mach 0.8.
Flare tips require a material resistant to high temperatures, at least for the top part of the tip.
310 and 800H both meet this requirement, 316 does not, and 625 is marginal. 800H has the highest temperature resistance, but in my experience 310 will give the longest tip life in arduous...