Flare Knockout Drum Droplet Sizing Calculator (API 521)

Determine critical liquid droplet settling velocity ($u_t$), vapor dropout diameter, drum length-to-diameter ratio, and emergency liquid holdup volume.

Relief Vapor & Liquid Inflow

Vapor Mass Flow Rate W_v (kg/hr):

Operating Pressure P (bara):

Vapor Molecular Weight MW:

Operating Temp T (°C):

Liquid Density ρ_L (kg/m³):

Target Droplet Cut Diameter (µm): API 521 standard: 300 to 600 µm

Emergency Liquid Inflow Rate (m³/hr):

Liquid Holdup Time (minutes):

Drum Dimensions & Settling Velocity

Terminal Settling Vel (u_t):

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Recommended Drum Diameter (D):

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Recommended Drum Length (L):

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Aspect Ratio (L / D):

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Required Liquid Holdup Volume:

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Horizontal Gas Velocity (v_gas):

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API 521 Flare Knockout Drum Principles

A flare knockout drum separates entrained liquid hydrocarbon droplets from emergency relief vapors before they reach the flare stack. If liquid enters the flare tip, burning droplets rain down upon plant personnel and facilities as flaming "fireballs" or "burning rain", creating catastrophic secondary plant fires.

Critical Cut Droplet Diameter ($300 - 600\text{ µm}$)

API Standard 521 specifies that flare KO drums must capture all liquid droplets larger than $300$ to $600\text{ µm}$. The drum length and horizontal vapor velocity are sized so that the time required for a droplet to settle gravitationally across the vertical vapor space is shorter than the gas transit residence time from the inlet nozzle to the outlet nozzle.

Flare Knockout Drum Droplet Sizing Calculator (API 521)

Relief Vapor & Liquid Inflow

Drum Dimensions & Settling Velocity

API 521 Flare Knockout Drum Principles

Critical Cut Droplet Diameter ($300 - 600\text{ µm}$)

Frequently Asked Questions