Refinery Flare Stack Seal Selection

Dear Members,

With reference to refinery flare stack seals,

We use either molecular seal or velocity seal just below flare tip or above flare stack, with a purpose to ensure that air should not enter inside the flare system and to create the positive pressure inside the system.

The question is: Which seal is preferred to control the amount of air ingress inside flare stack? Velocity Seal or Molecular Seal? What is the selection criteria?

For eg: Consider the case of a flare package wherein, HP flare header/stack size is 24" and Acid Flare header/stack size is 16". Please clarify which seal to be used for both HP flare case and LP flare case?

The purge gas used in this case is fuel gas.