Dear padovano,
Flaring is concern in our refinery to. Sometimes is easy to find cause but sometime it is not. It is easy find out if your crude unit send more gas on flare, you should compare how much valve which route gas to flare is open before and after column modification or when you processing heavy and light crude oil.
There are a lot of possibilities for flaring in refinery.
It is also possible that some CV is leaking to flare in closed position; it is easy to check this, simply close valve in front or behind CV.
Safety valves leaking to flare, check one by one,
Hand valve connected to flare, left in open position and forgotten
It is necessary to check all possibilities for flaring.