I know the test leak rates c.f. API 6D - but when you do an emission study you have to consider emissions from valves to the flare system - and its common to consider that during plant lifetime ball valves get worn and leaks more. Initial leak rates is (close to) zero but will amount to something over the years.
Maybe you can find something in the German Engineers' Guideline VDI 2440 whose Part 4 is about "Determining emissions"...
You should find the guideline named "Emission control - Mineral oil refineries", dated November 2000 (98 pages, 146 €)

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