Working, Purposes, Dimensions, and Design of a Knockout Drum

Knockout Drum

A knockout drum is a particular pressure vessel built into the flare header system for removing and accumulating any liquids from the flare gases. In some applications, it is known as the knockout pot, KO drum, flash drum, or flare knockout drum. It is the primary component in the pressure-relief arrangement system in processing industries.

Here are the functions, operations and design of a KOD.

Uses of a Knockout Drum

The knockout drum primarily functions as a holder for liquid discharge. It also limits the number of droplets entering the flare burner or the liquid seal drum. The drum can remove oil or water droplets from the industrial flare gases. The liquid collected in the vent system cannot be used because;

  • It causes irregular smoking and combustion
  • It can put out the flame
  • It could be a safety hazard because flaring could generate a burning chemical spray that reaches the ground.

The KO drum must remove any liquid to prevent condensation in the transfer pipes and headers. Also, by removing the liquids from the gas, the end product becomes efficient.

Dimensions and Sizes of a Flare Knockout Drum

Dimensions of a knockout drum are arrived at after a series of trial and error. The process of sizing these drums relies on mechanical references and fluid dynamics concerning the gravity-settling theory. This theory states that liquid droplets settle from gravitational force. The size of the knockout vessel is a determinant of the expected amount of liquid and vapour flow. The diameter of the flash drum is estimated by the maximum vapour velocity the system allows.

Knockout drums must follow a specific length-to-diameter ratio of 2:4. This size maintains a low vapour velocity of settling liquids. Designing the KO drum requires expertise, with many thumb rules to follow. Considering the multiple trial-and-error procedures, only experienced process engineers must design these drums.

KO Drum Design Consideration

Knockout drums are available in six design configurations. The variety of configurations depends on how the vapour is dispensed from the process.

  • Horizontal drum with vapour penetrating one side and leaving on the other

This is the simplest design and applies in systems with high vapour flow that require high storage capacity for liquids. This model does not need internal buffing because gas enters via one side of the drum to exit on the other side. Water droplets condense at the bottom of the drum to exit through the drain.

  • Vertical drum with radial inlet nozzle and vertical outlet nozzle

This setup has a vertical knockout drum where the inlet gas enters the chamber horizontally. It has baffles built-in to ensure gas inflow proceeds downwards. Water droplets are gathered at the bottom of the drum, whereas the gas flows towards the flare header as it goes out.

  • Vertical drum with tangential nozzle

A tangential nozzle is placed slightly inclined in the vertical drum. The tangential inlet pipes pass gas into the drum with a centre centrifugal separator. As the separator spins, it sends down the inlet gases to the bottom. After the liquid undergoes coalescence, it settles down at the bottom. After gases reach the tank’s bottom, the centrifugal tube will change its flow direction.

  • Horizontal drum with 2 horizontal inlet pipes and 1 central outlet

This simple design allows gases to enter the KO drum in two directions. The liquid droplets settle at the bottom of the drum with the gases flowing out through the central outlet.

  • Horizontal drum with a central inlet and two horizontal outlets

This design is the opposite of the type above because it passes the gas mix through the central inlet of the KO drum. After the liquid droplets settle, gas will flow out through any end of the drum.

Knockout Drum Location

When installing a KOD, a heavy engineering equipment manufacturer will ask you to consider the following;

  • Liquid droplet agglomeration and vapour condensation in setups with an extension between the KOD and flare stuck.
  • Accessibility for maintenance and service
  • Thermal radiation effects that could cause damages

A flaring system is compulsory in petroleum, oil and gas refinery units. You must invest in a well-built knockout drum for quality refining processes. A knockout drum ensures the safety of people working these plants while fulfilling product recovery.


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