API 521 Flare Knockout Drum Sizing Calculator

Size horizontal flare knockout (KO) drums per API 521: calculate critical droplet settling velocity (300 - 600 µm), allowable vapor velocity, vessel diameter, and emergency liquid holdup.

Vapor Flow & Gas Properties

Peak Vapor Flow (lb/hr):

Vapor Density (lb/ft³):

Vapor Viscosity (cP):

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

Liquid Dropout & Emergency Holdup

Liquid Dropout Rate (GPM):

Liquid Density (lb/ft³): Condensate ~ 50 lb/ft³

Emergency Holdup Time (minutes): API 521 recommended: 20 to 30 minutes

Horizontal KO Drum Vessel Sizing

Recommended Drum Diameter x Length

-- ft x -- ft

-- m ID x -- m T/T length

Droplet Settling Velocity

-- ft/s

Terminal fall speed

Allowable Vapor Velocity

-- ft/s

Max non-entraining limit

Liquid Holdup Storage: -- Gallons (-- bbl)

Slenderness Ratio (L/D): -- : 1 (API target: 3.0 to 4.0)

Vapor Flow Space Fraction: --% of vessel cross-section

Actual Horizontal Gas Velocity: -- ft/s

Recommended Tools & Equipment

Tested hardware and components for high reliability

100% Free Tool Zero Sign-Up

Frequently Asked Questions

Why is a knockout drum essential upstream of an industrial flare?

When relief valves pop, liquid droplets and heavy condensates are carried along with high-velocity gas. If liquid enters the flare tip, it creates dense black smoke and can shower burning liquid droplets ("rain of fire") over plant units, initiating catastrophic ground fires.

What is the critical droplet size specified by API 521?

API 521 specifies sizing knockout drums to drop out liquid droplets between 300 and 600 microns in diameter. Droplets in this size range settle under gravity during the gas horizontal transit time before reaching the downstream outlet nozzle.

What determines the emergency liquid holdup capacity?

Per API 521, flare knockout drums must provide between 20 and 30 minutes of liquid storage under the maximum liquid relief rate. This provides operators sufficient time to safely troubleshoot, isolate the releasing unit, or start emergency pump-out.