Thanks for the tip! I wasn't aware there was a "new" standard for flares. I don't get Hydrocarbon Processing but since your post, I was able to get a quick look the API Standard 537, First Edition, Sept-2003.
From the scope section, the Standard is intended to supplement API RP-521 and includes a description of the mechanical design, operation and maintenance of 3 types of flares: elevated flares, multi-burner staged flares, and enclosed flares. Also included is a number of data sheets developed by API to be used as a means of recording and communicating design information.