In my experience, if the flow to the flare is sufficient to be seen on a mass balance….. you have a big problem! DCU matbal are typically +/-3-5% with the best being 1-2%. If 1% of some flow is going to the flare, you should see it very clearly. What is the flow to the flare system right now, absolute and a % of feed rate?
Flare flows are typically too low relative to pipe diameter to have a reliable flow measurement.