Knock-out Drum
Knock-out drums for liquid removal upstream of the flare
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Gravity-based liquid separation upstream of the flare tip
CRA’s Knock-out Drums are engineered to protect flare systems by separating and collecting entrained liquids, such as hydrocarbons, condensate, and water, from relief gases before they reach the flare tip. Installed upstream of the flare stack, these vessels prevent liquid carryover, flame instability, and hazardous “flaming rain.” Designed for high vapour flow rates and harsh industrial service, CRA knock-out drums deliver reliable liquid handling, reduced system upsets, and long-term operational safety.
Function
Liquid-gas separation
Configuration
Horizontal or vertical
Application
Upstream of flare stack
Duty
Emergency relief & blowdown
What is a Knock-out Drum?
A knock-out drum is a pressure vessel installed upstream of the flare stack that separates entrained liquids, such as condensate, hydrocarbons, and water, from relief gas before it reaches the flare tip, preventing liquid carryover, flame instability, and flaming rain.
How it works
Relief gas slows as it expands into the drum, letting gravity pull entrained liquid droplets out of the gas stream before it continues to the flare stack.
Relief gas enters the vessel and slows as it expands into the larger drum volume, giving entrained liquid droplets time to fall out under gravity.
Internal baffles, vane packs, or mesh elements increase capture of finer liquid particles that would not settle out by gravity alone.
Collected liquid accumulates at the bottom of the vessel and drains to closed collection systems, while the dry gas continues to the flare stack.
Horizontal vessels handle high flow rates and large liquid volumes; vertical vessels suit sites with constrained plot space.
Key Benefits
Removes entrained liquids from relief gases to prevent carryover into the flare stack.
Reduces risks of flame extinguishment, irregular combustion, and flaming rain events.
Engineered for large, transient relief flows common in emergency and upset conditions.
Available in horizontal or vertical configurations to suit plot space and flow regimes.
Compatible with liquid seal drums, flare gas recovery units, and downstream flare hardware.
Projects That Push Boundaries
Complete gas conditioning train transforming raw digester gas into clean, usable fuel for a city utility.
The CRA Edge
Engineered to handle high vapour loads, liquid slugs, and transient flare events safely.
Each knock-out drum is sized from actual process data rather than generic templates.
Designed to integrate with flare stacks, liquid seal drums and FGR systems.
Durable construction and reliable liquid drainage give long service life.
Applications
Frequently Asked Questions
What does a knock-out drum do in a flare system?
A knock-out drum removes entrained liquids, such as hydrocarbons, condensate and water, from the relief gas before it reaches the flare tip. That prevents the liquid carry-over that causes flame instability and hazardous flaming rain. It is installed upstream of the flare stack, so the flare tip and downstream equipment are protected from liquid slugs. CRA engineers knock-out drums for high vapour flow rates and harsh industrial service.
Why is liquid carry-over dangerous in a flare system?
Liquid reaching the flare tip causes irregular combustion, flame instability or flame extinguishment, and in severe cases flaming rain, where burning liquid droplets fall back towards grade and the surrounding plant. Liquid slugs also load the stack and burner mechanically. A knock-out drum separates and collects the liquid before it can reach the tip, so the flare tip design does not have to manage liquid on its own. CRA sizes the drum for the liquid slug volumes the relief scenarios can produce.
How is a knock-out drum sized?
CRA sizes each knock-out drum from the actual process data: the expected vapour loads, the liquid slug volumes and the transient relief events the flare header can see. Emergency and upset relief flows are typically much larger and more variable than steady-state flow, so the drum is sized for those cases. Horizontal or vertical configurations are selected to suit the plot space and the flow regime, and the drum includes reliable liquid drainage for long service.
Where does a knock-out drum fit relative to a liquid seal drum or flare gas recovery unit?
A knock-out drum is positioned upstream of the liquid seal drum and of any flare gas recovery unit, so that the liquid is removed first and the downstream equipment is protected from carry-over beyond its design. CRA engineers its knock-out drums for compatibility with liquid seal drums, flare gas recovery hardware and the flare stack as parts of one flare system.
Which flow conditions does a knock-out drum have to handle?
Knock-out drums are engineered for the high vapour loads and liquid slugs typical of emergency relief and blowdown events, and they also have to perform reliably across high turndown and intermittent flare service. Steady operating flow is the easier case. CRA designs for the transient conditions directly, so the drum copes with the largest relief case the header can deliver as well as long periods of low flow.