What is a Flare KOD | Working, Functions, Sizing, and Design of a Knock Out Drum

Function of a Flare Knockout Drum in Relief Systems

The primary function of a Flare Knockout Drum is to serve as a safety buffer between the process plant’s relief headers and the flare stack. In any industrial facility, relief valves (PSVs) discharge a mixture of gases, vapors, and entrained liquids during overpressure events. If these liquids reach the flare tip, they can cause mechanical damage, extinguish the pilot, or result in “burning rain.” By drastically reducing the velocity of the incoming relief stream, the KOD allows gravity to pull liquid droplets out of the gas phase, ensuring only dry or “lean” gas reaches the combustion point.

Beyond simple separation, these vessels act as a critical storage point. During a massive plant-wide emergency, a Flare Knockout Drum must provide sufficient residence time to hold the cumulative liquid volume from multiple sources. This prevents the liquid level from rising too quickly and being re-entrained into the flare line. According to the American Petroleum Institute (API), maintaining the integrity of this separation is the single most important factor in preventing environmental non-compliance and hazardous ground-level fires.

Critical Design Configurations for Flare Knockout Drum Sizing

When initiating Flare Knockout Drum Sizing, engineers must first select the vessel orientation. The choice between horizontal and vertical configurations is rarely arbitrary; it is driven by the liquid-to-gas ratio and the available footprint within the plant’s battery limits.

• Horizontal Knockout Drums: These are the industry standard for large-scale refineries. The horizontal orientation provides a significantly larger surface area for liquid-vapor disengagement and can handle massive liquid “slugs” much more effectively than vertical units. They are generally preferred when the liquid load is high or when low-pressure drop is a critical design constraint.
• Vertical Knockout Drums: Often utilized in gas processing plants or upstream facilities where liquid loads are relatively low. The vertical configuration occupies less plot space and is easier to equip with high-efficiency mist eliminators. However, they are prone to “slugging” issues if the relief event contains high liquid fractions, as the upward gas velocity can easily re-entrain falling droplets.

API 521 Guidelines for Flare Knockout Drum Sizing and Design

The gold standard for Flare Knockout Drum Sizing is the API 521 standard (Pressure-relieving and Depressuring Systems). The sizing methodology is built upon the principle of terminal settling velocity. The goal is to ensure that the time it takes for a liquid droplet to fall from the top of the vessel to the liquid surface is less than the time it takes for the gas to travel from the inlet to the outlet.

To achieve this, engineers calculate the maximum allowable vapor velocity (V_c) using the Souders-Brown equation. This equation incorporates a “K-factor” (typically ranging from 0.15 to 0.35 for KODs) which accounts for the physical properties of the fluid and the vessel’s internal geometry. In 2026, modern computational fluid dynamics (CFD) are often used to validate these Flare Knockout Drum Sizing calculations, especially for non-standard relief scenarios involving cryogenic or high-viscosity fluids.

Step-by-Step Selection of Droplet Size and Vapor Velocity

The core of Flare Knockout Drum Sizing involves calculating the terminal settling velocity of a liquid droplet. Under API 521, the vessel diameter must be sufficient to ensure the horizontal vapor velocity remains below the terminal velocity of a 300 to 600-micron droplet. This calculation relies on the drag coefficient (C_d) and the Reynolds number of the particle. If the vapor velocity exceeds this limit, droplets are carried overhead into the flare tip, leading to incomplete combustion and hazardous environmental releases.

Liquid Holdup and Residence Time Requirements

Beyond vapor separation, Flare Knockout Drum Sizing must account for emergency liquid storage. High-integrity systems require the vessel to hold the liquid volume from the single largest contingency for 20 to 30 minutes. This “holdup time” provides operators with a critical window to stabilize the plant or initiate an emergency shutdown (ESD) before the drum reaches its High-High Level (LAHH) trip point.

Strategic Locating of the Knockout Drum for Optimal Flow

The physical location of the KOD is as critical as its dimensions. It must be situated at the low point of the flare header to allow for natural slope and gravity drainage of condensed liquids. Furthermore, it should be placed as close to the flare stack as possible—typically within 60 to 100 meters—to minimize the risk of further condensation occurring in the line after the gas has already been processed by the Flare Knockout Drum.

Technical Differences: Flare Knockout Drum Sizing vs Standard Separators

While a standard process separator is designed for continuous operation at steady-state conditions, Flare Knockout Drum Sizing focuses on transient, high-velocity “shock” loads. Standard separators prioritize high-efficiency separation (removing droplets down to 10 microns), whereas a KOD is a safety device designed to handle massive volumes and prevent catastrophic “burning rain” during a total plant power failure or fire.

Preventing the Burning Rain Phenomenon in Flare Knockout Drums

The term “burning rain” refers to unvaporized liquid hydrocarbons exiting the flare tip and igniting in mid-air. This occurs when Flare Knockout Drum Sizing fails to account for peak vapor velocities during a “fire case” relief scenario. To prevent this, engineers must ensure the vessel length-to-diameter (L/D) ratio is optimized, typically between 3:1 and 4:1 for horizontal drums, to maximize the distance available for droplet dropout.

Maintenance and Inspection Requirements for Industrial KODs

Regular maintenance of a KOD is governed by API 510 (Pressure Vessel Inspection Code). Critical focus areas include checking for internal corrosion caused by stagnant acidic condensates and ensuring that the high-level alarms/trips are calibrated and functioning. Because the KOD is an “end-of-line” safety asset, its failure during an emergency can lead to immediate loss of primary containment.

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