Knockout Drum

Many pipes in refineries contain a combination of flowing liquid and vapor. This is known as "two-phase flow". The "Knockout Drum" or "Knockout Pot" is a simple solution for separating the liquid and vapor. The knockout pot is a wide spot in a line that allows liquid droplets to settle and accumulate (thanks gravity) and the vapor to go overhead.

Many knockout drums have a plate or sheet at the inlet for the two-phase flow to impact. This can help liquid droplets to settle. The plate forces the two-phase flow to redirect up and down. The redirection of flow uses the momentum of the liquid droplets to help separation. I have heard the plate called a "diverter plate", "deflector plate", "deflector baffle", and a few more nicknames. I'm not sure what the correct name is, but I can usually pick up by context when this topic is brought up.

The other common internal of knockout drums is the demister pad, which the vapor must pass through to go to the overhead line. The demister pad is composed of thin mesh wire layered several inches thick. The demister pad provides a location for liquid to collect and then drop down. It helps to minimize the amount of remaining liquid in the vapor flow.

Here are two reasons for using a knockout drum:

1) The overhead vapor is used as fuel for combustion
=> Fired heaters and boilers that are designed for fuel gas will not respond well to liquid in the fuel.

2) The overhead vapor is sent to a compressor
=> Liquid in the feed to a compressor is a well-proven method to cause irreversible damage.

The diagram is a simplified overview of a knockout pot. It's sufficient to understand the general arrangement and purpose.

About: I write to provide snapshots of oil refinery operations and technology. My goal is to write in plain English, without jargon, and to provide an entry-point to the downstream oil industry.