Flare line connection 1

The Code will not tell you if you can tie-in to a flanged connection or not. Many Owners will have their own specification not allowing flanged connections to a flare header - the reason being - to avoid ingress of air/oxygen to form a combustible mixture with the vented/flared gas. Of course, there is usually a purge gas in the flare header to maintain a positive pressure to keep away the air ingress but things never run ideal in a plant.
The second reason is the probability of accidently putting a blind or a leaky flanged joint.
Welding is the best option, if you can. Flanging will have some risk.
Check with your process if it will vent toxic gases and the location of tie- in.
BTW, is your current flare system have adequate capacity to handle additional capacity? Are any study done?

@mohamed
The question is definetly unclear. Describe what is an issue and our concerns.

Pls ask a process engineer familiar with hydraulics in this flare network - there are many variables that will affect choice of location where this tie in should be made. You wont find this guidance in B31.3.
By the way, how many levels of flare do you have in your plant - LLP, LP, HP etc?