An Acid Gas Flare Intermediate KO Drum (intermediate knockout drum) is a gas–liquid separation vessel installed within an acid gas flare system to remove entrained liquids and mist from relief/vent gas before the gas enters downstream flare headers and the flare stack. It acts as a protective barrier that prevents liquid carryover to the flare tip and minimizes operational hazards associated with wet sour service.

The term “intermediate” typically indicates that this KO drum is positioned between a local collection header (or a specific unit’s relief network) and the main flare header, providing staged liquid removal—especially valuable where condensation or liquid slugs are likely.

1. What Is an Acid Gas Flare Intermediate KO Drum?

An intermediate KO drum is a separation vessel designed to “knock out” liquid droplets and slugs from relief/vent gas streams before they enter the flare header and flare stack. In acid gas service (commonly containing H₂S/CO₂ and water), the KO drum must be engineered with appropriate materials, corrosion allowance, and safeguards to manage sour and potentially condensable conditions.

Depending on plant configuration, the intermediate KO drum may complement (not necessarily replace) a main flare KO drum by providing local liquid removal upstream, reducing liquid loading into the main flare network.

2. Why an Intermediate KO Drum Is Required in Acid Gas Flare Service

Acid gas flare service often involves H₂S/CO₂, water vapor, and possible hydrocarbon or solvent carryover. Even if an upstream stream is nominally “gas-only,” liquids can form or be entrained due to upsets, pressure drops, JT cooling, ambient cooling in long flare lines, or upstream equipment carryover.

• Prevent liquid carryover to the flare tip: Liquids can cause flame instability, smoking/sooting, and potential damage to flare components.
• Protect flare headers and downstream equipment: Slugs can create severe erosion, corrosion, vibration, and hydraulic disturbances in piping.
• Reduce flare system upset risk: Liquid accumulation can increase backpressure, create partial blockages, or trigger slugging events.
• Improve safety and compliance: Captured sour liquids are routed to a closed handling system, reducing H₂S exposure and fugitive emissions.

3. Working Principle

The intermediate KO drum works primarily through velocity reduction and gravity separation, often enhanced by internals:

• Inlet momentum reduction: An inlet diverter or impingement plate reduces momentum and distributes flow.
• Droplet separation: Larger droplets settle by gravity into the liquid section.
• Mist capture (if required): A demister pad or vane pack removes fine droplets and reduces carryover.
• Liquid collection and discharge: Liquids accumulate in a sump/boot and drain to a closed drain/sour water system.
• Clean gas outlet: Separated gas exits from the top outlet to the downstream flare header/flare stack.

Because flare conditions can be highly transient, the KO drum should be able to handle both continuous condensate formationand intermittent liquid slugs.

4. Typical Mechanical Configuration

4.1 Horizontal vs. Vertical Arrangement

• Horizontal KO drums: Often preferred for large flow rates and better slug handling due to larger liquid holdup volume.
• Vertical KO drums: Suitable where plot space is limited or for smaller systems, though slug holdup can be more constrained.

4.2 Common Internals

• Inlet diverter / baffle: Reduces inlet velocity and prevents direct impingement on mist eliminators.
• Demister pad or vane pack: Improves fine mist removal (selection depends on fouling risk and allowable pressure drop).
• Flow calming baffles: Enhance separation stability and reduce re-entrainment.

4.3 Drain and Liquid Handling

In acid gas service, liquids are typically sour and hazardous. Bottom arrangements commonly include:

• Sump/boot holdup: to buffer slugs and provide stable level control.
• Controlled drainage: manual valves or level-controlled valves based on site philosophy.
• Closed drain tie-in: avoids open drainage due to H₂S toxicity and emission concerns.

5. Key Design and Sizing Considerations

5.1 Design Cases and Relief Scenarios

Sizing is driven by credible flare scenarios and expected liquid content. Typical inputs include:

• Maximum relief/vent gas flow (normal, upset, emergency, combined cases)
• Gas composition (H₂S/CO₂/hydrocarbons/water content) and temperature profile
• Condensation potential due to JT cooling, mixing, and ambient heat loss
• Expected liquid rate and slug volume (credible worst-case basis)

5.2 Separation Performance Targets

• Target droplet cut size (e.g., removal above a specified micron size)
• Allowable outlet liquid carryover aligned with flare tip/vendor requirements
• Mist vs. slug duty definition to select internals and holdup capacity appropriately

5.3 Pressure Drop and Backpressure Control

Flare systems are sensitive to backpressure. Nozzle sizing, internal layout, and mist eliminators should be selected to limit pressure drop—particularly under peak relief conditions.

5.4 Liquid Holdup and Drain Philosophy

• Define required holdup time/volume for continuous condensate and intermittent slugs.
• Confirm whether drainage is manual, automated, or controlled via level control.
• Ensure safe routing of sour liquids to a closed system (closed drain/sour water).

5.5 Mechanical Design Requirements

• Design pressure and temperature, including transient/upset conditions
• Corrosion allowance and erosion protection (inlet zone is often critical)
• Nozzle reinforcement and support loads (flare headers can impose significant piping loads)
• Maintainability: manways, demister access, lifting lugs, and inspection provisions

6. Materials, Corrosion, and Sour Service Considerations

Acid gas environments can be corrosive due to the combination of H₂S, CO₂, and water. Material selection should align with the client’s corrosion philosophy and project specifications, with particular attention to wet sour conditions.

• Carbon steel may be acceptable with proper corrosion allowance and controlled operating envelope.
• Stainless steel / duplex / nickel alloys may be required for severe corrosion, chloride exposure, or aggressive wet sour service.
• Weld quality and fabrication control are essential to long-term integrity in sour environments.

7. Instrumentation and Safeguards

7.1 Typical Instrumentation

• Level transmitter (LT) and (where permitted) local level indication
• High (LAH) and high-high level (LAHH) alarms to prevent downstream liquid carryover
• Pressure and temperature monitoring as required by the project
• DP monitoring across demister (optional) to detect fouling and performance degradation

7.2 Safeguards and Tie-ins

• High-high level actions may include drainage initiation, operator alarm, or upstream isolation depending on site philosophy.
• Consider H₂S gas detection around the KO drum area and suitable ventilation requirements.
• Drain routing should be to closed drain/sour water with proper isolation, flushing, and maintenance provisions.

8. Operation and Maintenance Recommendations

• Maintain effective drainage: avoid sustained high levels to reduce carryover risk.
• Monitor internals performance: rising DP across demister may indicate fouling; plan inspection intervals accordingly.
• Inspect high-wear zones: inlet and impingement areas are prone to erosion under high-velocity conditions.
• Corrosion monitoring: thickness checks, coupons/probes, and periodic inspections for wet sour service.
• Upset handling procedures: define actions for large slugs, abnormal condensation, and emergency drainage events.

9. RFQ / Datasheet Checklist

To enable accurate quotation and fit-for-purpose engineering, buyers typically provide:

• Design/operating pressure and temperature (including upset and relief conditions)
• Gas flow rates (normal, maximum, emergency, combined scenarios)
• Gas composition (H₂S, CO₂, hydrocarbons, water content) and dew point data
• Expected liquid composition and liquid rate/slug volume (credible worst case)
• Required separation target (droplet cut size or carryover limit) and internals preference
• Materials requirements, corrosion allowance, and sour service requirements
• Applicable design code (e.g., ASME Section VIII) and inspection/testing requirements
• Instrumentation scope (LT, LAH/LAHH, DP across demister, etc.)
• Drain routing philosophy (closed drain/sour water) and tie-in conditions
• Layout constraints (horizontal/vertical preference, footprint, maintenance access)

10. Conclusion

An Acid Gas Flare Intermediate KO Drum is a critical safety and reliability component in flare systems handling sour, condensable gases. By removing entrained liquids and managing slugs in a controlled vessel, it protects flare headers and flare tips, reduces operational upsets, and improves plant safety and compliance.

Fit-for-purpose performance depends on correct sizing for credible relief scenarios, disciplined pressure-drop control, and materials and safeguards aligned with sour service requirements.