if it is possible to attach a reducer of 20" to any lesser adequate size then Im sure that it will help u...
or if possible to make couple of 90 degree bend in the line so that it will produce enough pressure drop that only a gate valve can handle the 20" flow of fluid...
I guess it all depends upon ur demister pad thickness, if u got one??? cuz in case u've one then the technique might be different as compare to no demister pad(wire mesh pad)...
I will refer u the GPSA standards for thsi purpose separator section, it will give u a rule of thumb to calculate such...
