Ok.....the flare knock-out can be close to the flare stack as long as its outside the flare radiation zone (from engineering) and it has the appropriate overfill protection and flame arrestor. Otherwise it is considered Process equipment and needs to be at least 25m from the flare stack.
All other spacing requirements I mentioned apply. Obviously consult with your piping lead for verification. I personally would be upset as your lead if I listened to some guy on the internet and didn't ask the question to me personally. Just sayin