You're pretty much on target in considering API 520. If you look at §5.8.1.4 and 5.8.1.5 of API 520 (8th edition from December 2008), you'll find a fairly explicit method for sizing relief valves for viscous liquids. A worked example problem follows immediately. That ought to step you through the procedure with minimal pain (at least, one would hope so). Speak up if I can help further.
I have noticed API proposes an iterative procedure to determine the required orifice area. But in my case, i cannot find a solution. Each time i increment to a higher standard orifice size, the Reynolds number decrease and so does the Kv coefficient. As a result, i always need a bigger orifice size....until i reach the point where there is no higher standard size available.
Another point where i am lost is that the Reynolds number decrease as the orifice size increase...strange?
You must go to a DIERS approach, which resolves the Kv factor for Reynolds numbers less than 400. The API 520 Kv table/chart is useless at low Reynolds Numbers.