Knock Out Drum (KOD) and Separator

"Knock out drum" is an informal but historically common term. In a refrigeration system, for instance, it is desirable to "knock out" any overfed or entrained liquid from the vapor stream to a compressor. Other terms include "suction trap", "accumulator", and "separator" or "liquid separator."

The main function of KOD - like as that one installed for a flare system - and from its name is knocking out, i.e to prevent pulsation of flare gases coming downstream of safety valves (like a surge drum), and this shall control and govern a smooth flowing of that gases to avoid and to prevent any interruption and intermittent work of flare tip located at top of flare stack.

I consider the exhaust box of a car as a KOD, and to what extent can you imagine your care without exhaust box?

In addition to that main function of KOD in a flare system, we use it as a vessel to separate the condensates accompanying the flare gases before going into flare tip which can create a fire and/or over flow. Therefore, that vessel must be labeled with KOD, because this is its main function

Like the other commentators the various names are all used somewhat interchangeably but the key point made above is that you tend to use a name that indicates the main function.

So I would use Knock Out Drum when the main function is to remove the liquid

Upstream of a flare stack (if liquid is carried up the flare stack it can cause the flare to go out if in sufficient quantity or fall as burning rain but generally upset the operation)

Compressor suction - liquid droplets impacting on the fast moving impeller causes trouble blade failure huge costs and shutdown

In both these cases a failure to Knock out the liquid has potentially diasastrous consequences so calling the vessel Knock Out drum helps to reinforce that message. Whilst two phase separator would be accurate it doesnt (to my mind) convey that separation has to be 99.999% complete but that is probably because I am so used to seeing KOD in the circumstances described.

I would use X Separator when I am interested in a particular phase

eg The overhead separator on a crude column might be called a Naphtha Separator. You are not particularly interested in the gas phase - typically that goes to fuel gas -but the naptha stream is key to the operation of the refinery

Overall equipment needs a simple name that clearly defines its function and differentiates it from other kit. So in a one stage compression loop you could have the Knock Out Drum upstream of the compressor and typically the Accumulator collecting the condensed vapours rather than having to say Two Phase Separator 1 and Two Phase Separator 2 and no one knowing which one you mean (do you start with compressor suction or the liquid or where in a refrig cycle)

I defer to these other opinions in respect to refinery operations and the like; my own experience is in the area of industrial refrigeration, which seems to go with different terms.

Like asking, "Why do all drilling rig components have animal names. Oil patch mentality, and "Knock out " is part of it. I first heard it 40 years ago, and evidently it still hasn't been refined.