What Is a KO Drum? Knockout Drum Design
Quick Answer: A knockout drum (KO drum) is a pressure vessel designed to separate liquid droplets from a gas stream by gravity and/or impingement. KO drums protect downstream equipment—compressors, flare tips, gas turbines, and flow meters—from liquid carryover damage.
Types of Knockout Drums
| Type | Orientation | Separation Method | Typical Application |
|---|---|---|---|
| Gravity KO drum | Horizontal or vertical | Gravity settling; reduced gas velocity allows droplets to fall | Compressor suction, fuel gas systems |
| Mesh pad KO drum | Vertical (typically) | Wire mesh demister pad captures fine droplets | Flare KO drums, gas dehydration |
| Vane-type KO drum | Vertical or horizontal | Vane pack forces gas to change direction, coalescing droplets | High-efficiency separation upstream of turbines |
| Flare KO drum | Horizontal (usually) | Gravity + residence time for large liquid slugs | Flare header systems (API 521) |
How a Knockout Drum Works
Gas enters the drum through an inlet nozzle, typically fitted with an inlet device (deflector plate, half-pipe, or inlet vane distributor) that spreads the flow and reduces velocity. Inside the drum, the gas velocity drops below the terminal settling velocity of the liquid droplets. Droplets fall to the bottom by gravity and collect in a liquid sump, which drains through a level-controlled outlet.
For fine mist removal, internal mesh pads (typically 150 mm thick, density 144 kg/m^3) or vane packs capture droplets as small as 8-10 microns. The Souders-Brown equation is used to calculate the maximum allowable gas velocity:
V_max = K x SQRT[(rho_L - rho_G) / rho_G]
Where K is an empirical constant (typically 0.035-0.107 m/s depending on internals and service).
Sizing Criteria
| Parameter | Guideline |
|---|---|
| Gas velocity | Must be below Souders-Brown limit for the selected K factor |
| Liquid residence time | Minimum 3-5 minutes (API 521 for flare KO drums: 20-30 minutes for slug flow) |
| L/D ratio | Horizontal: 3:1 to 5:1; Vertical: 2:1 to 4:1 |
| Mesh pad area | Sized for 100% of design gas flow at maximum operating pressure |
| Liquid sump volume | Based on drain rate and slug volume; includes high-level alarm and shutdown |
| Design code | ASME Section VIII, Division 1 or 2 |
| Nozzle sizing | Inlet: momentum limited; outlet: velocity limited |
Flare Knockout Drums (API 521)
Flare KO drums are governed by API 521 (Pressure-Relieving and Depressuring Systems). They sit upstream of the flare stack and must handle the maximum relief rate during emergency depressurization. Key requirements:
- Liquid residence time: sufficient to handle the largest foreseeable liquid slug
- High-level shutdown to prevent liquid carryover to the flare tip
- Design for full vacuum (if steam-out or cooling is possible)
- Drain system sized to empty the drum within the required turnaround time
Materials
KO drums are typically fabricated from carbon steel (ASTM A516 Gr. 70) for standard service, or stainless steel / alloy for corrosive fluids. Internals (mesh pads, vane packs) are commonly SS 304 or SS 316. The vessel is designed per ASME B31.3 for connected piping and ASME VIII for the shell.
KO drum performance directly affects the accuracy of downstream instrumentation and the mechanical integrity of rotating equipment.
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