Is important a seal drum -knock-out drum with FLARE ?

I presume this is a verical mainly gas flare. One of the main functions of a flare KO is to remove liquids which otherwise burn and fall back to earth. You always have these when you flare lots of different things from different sources. If you have any light hydrocarbons they should be vaporised before being sent to the flare.
Dont know anything about liquid flares.

Even if designed for all vapor releases into a flare header there is a possibility of condensation. At the very least there should be facilities (level measurement and drains) at the base of the stack to remove liquids. If you cannot ensure two phase or liquids will not be discharge into the flare, you must design for it with a ko drum.