How to Size a Flare Knockout Drum in 15 Minutes

v2 revision (2026-04-11): Corrected unit conversion errors in Step 6 (droplet diameter and viscosity conversions). Steps 1–5 unchanged.

Purpose and Scope

A flare knockout (KO) drum is a horizontal or vertical pressure vessel installed upstream of a flare system to separate entrained liquid droplets from the relieving vapor before it reaches the flare stack. Its core function is to prevent liquid slugs and high liquid rates from reaching the flare tip, which would cause:

Smoking and incomplete combustion at the flare tip
Liquid carryover that damages downstream equipment
Flare radiation and noise issues
Safety hazards from liquid mist in the flare stack

This guide applies to sizing KO drums for: - New relief scenarios (expansion project, increased capacity, new equipment relief load) - Relief load increases (uprating of existing equipment) - Safety case reviews or HAZOP sessions requiring a quick sanity check on flare system adequacy - Preliminary engineering and design reviews

This is a 15-minute method — fast enough for a design review room, but not a substitute for detailed simulation when foaming, high viscosity, or complex phase behavior is involved.

When This 15-Minute Method Applies

Assumptions: - Horizontal or vertical cylindrical vessel with flat ends - Single-phase gas with fine suspended liquid droplets (mist) - Gravity-driven separation (no demister pads or coalescers initially) - Design basis per API 521, 6th edition (2014) - Relieving fluid is hydrocarbon gas or gas mixture with water or hydrocarbon liquid condensate - Liquid hold-up is modest (typically 1–10% by mass of total relief flow) - No foaming liquids, no significant emulsion formation - Relieving conditions are above the pour point of the liquid

This method does NOT apply to: - Two-phase relief streams with foam formation (crude oil tanks, evaporators) - Highly viscous liquids (heavy oil, synthetic fluids with VI > 100) - Slug flow or intermittent discharge - H₂S or caustic flares (separate design standards) - Cryogenic service or sub-zero relieving temperatures - Liquid loading > 20% by mass (go to rigorous design)

For those cases, engage a flare vendor or run rigorous CFD/separator simulation.

Step-by-Step Sizing Procedure

Step 1: Establish Design Basis

Define your relieving scenario:

Maximum credible relief flow (W) — total mass flow rate in lb/hr
Source: relief valve sizing, vessel emergency depressuring, or process upset scenario
Example: 20,000 lb/hr total relief
Fluid composition at relieving conditions
Vapor phase: identify the gas (methane, ethane, N₂, etc., or mixture)
Liquid phase: identify condensate (water, naphtha, methane, etc.)
Liquid mass fraction: f_L = (mass of condensate) / (total mass relief flow)
Example: 20,000 lb/hr total; 5% condensate → 1,000 lb/hr liquid, 19,000 lb/hr vapor
Relieving conditions (P, T)
Pressure (psig)
Temperature (°F)
Example: 50 psig, 100°F
Fluid properties at relieving conditions (from tables or thermodynamics software):
Vapor density: ρ_V (lb/ft³)
Liquid density: ρ_L (lb/ft³)
Liquid viscosity: μ (cP) — needed for droplet settling check
Example: for methane at 50 psig, 100°F: ρ_V ≈ 2.8 lb/ft³; ρ_L ≈ 30 lb/ft³ (if condensate is light hydrocarbon)

Step 2: Calculate Maximum Allowable Vapor Velocity

Use the Souders-Brown equation, which balances gravity settling of droplets against upward vapor flow:

$$u_{max} = K \times \sqrt{\frac{\rho_L - \rho_V}{\rho_V}}$$

Where: - u_max = maximum allowable superficial vapor velocity (ft/s) - K = Souders-Brown constant (ft/s) — depends on vessel orientation and service type (see Quick Reference Table below) - ρ_L = liquid density (lb/ft³) - ρ_V = vapor density (lb/ft³)

For this guide, use: - Vertical drum: K = 0.25 ft/s (from API 521, Table 5, mist separation, vertical) - Horizontal drum: K = 0.35 ft/s (typical for gravity separation, horizontal)

These K values are conservative; they ensure most > 300-micron droplets will settle within the drum residence time.

Worked example (vertical drum):

$$u_{max} = 0.25 \times \sqrt{\frac{30 - 2.8}{2.8}} = 0.25 \times \sqrt{9.71} = 0.25 \times 3.12 = 0.78 \text{ ft/s}$$

Step 3: Calculate Required Cross-Sectional Area

The vapor volumetric flow rate is:

$$Q_V = \frac{W \times (1 - f_L) \times R_V \times T}{P}$$

Or more simply, use the vapor density directly:

$$Q_V = \frac{W \times (1 - f_L)}{\rho_V}$$

Where: - W = total mass relief flow (lb/hr) - f_L = liquid mass fraction (dimensionless) - ρ_V = vapor density (lb/ft³) - Q_V = vapor volumetric flow rate (ft³/hr)

Then calculate the required cross-sectional area:

$$A_{vapor} = \frac{Q_V}{u_{max} \times 3600}$$

Where: - A_vapor = cross-sectional area (ft²) — this is the flow area (π D²/4 for round, or L × W for rectangular sections) - The 3600 converts ft³/hr to ft³/s

Worked example:

$$Q_V = \frac{20,000 \text{ lb/hr} \times (1 - 0.05)}{2.8 \text{ lb/ft³}} = \frac{19,000}{2.8} = 6,786 \text{ ft³/hr}$$

$$A_{vapor} = \frac{6,786}{0.78 \times 3600} = \frac{6,786}{2,808} = 2.42 \text{ ft²}$$

Step 4: Calculate Drum Diameter (or L:D Ratio)

For a vertical drum:

Assume a cylindrical vessel. The cross-sectional area equals the circular area:

$$A_{vapor} = \frac{\pi D^2}{4}$$

$$D = \sqrt{\frac{4 \times A_{vapor}}{\pi}} = \sqrt{\frac{4 \times 2.42}{3.14159}} = \sqrt{3.08} = 1.76 \text{ ft}$$

Round up to the next standard size. Common drum diameters are 24”, 30”, 36”, 42”, 48”, etc. In this case, use 24 inches (2.0 ft).

For a horizontal drum:

Typical L:D ratio for KO drums is L/D = 3 to 5. Use L/D = 4 as default.

If we used the same area requirement (2.42 ft²): - For a horizontal vessel, the flow area in the vapor space is approximately the full cross-sectional area. - Using A = πD²/4 and L = 4D: - D ≈ 1.76 ft (same as vertical) - L = 4 × 1.76 = 7.04 ft

Standard horizontal: 24” ID × 7 ft long (or 24” × 8 ft).

Step 5: Liquid Surge Volume Check

The KO drum must accommodate a 10-minute surge of maximum condensate production during the relief event. This prevents liquid backup into relief lines or liquid overflow at the drum outlet.

Liquid surge volume required:

$$V_{liquid} = \frac{W \times f_L \times 10 \text{ min}}{60 \text{ min/hr} \times \rho_L}$$

$$V_{liquid} = \frac{20,000 \times 0.05 \times 10}{60 \times 30} = \frac{10,000}{1,800} = 5.56 \text{ ft³}$$

Usable liquid volume in a vertical drum (24” ID = 2.0 ft diameter):

Assume liquid occupies 40% of the drum height (standard operating rule to avoid carryover):

$$V_{drum} = 0.4 \times \frac{\pi D^2}{4} \times L$$

For a vertical drum 8 ft tall (typical for vapor space of 60%, liquid space 40%):

$$V_{drum} = 0.4 \times \frac{3.14159 \times 2.0^2}{4} \times 8 = 0.4 \times 3.14 \times 8 = 10.05 \text{ ft³}$$

This is > 5.56 ft³, so the drum is adequate. If not, increase length or diameter.

Step 6: Liquid Droplet Settling Check

Confirm that liquid droplets settle to the bottom within the drum residence time. For a 300-micron droplet (typical mist):

Terminal velocity (Stokes’ law, sphere):

$$v_{terminal} = \frac{d_p^2 \times (\rho_L - \rho_V) \times g}{18 \times \mu}$$

Where: - d_p = droplet diameter (ft); 300 microns = 9.84 × 10⁻⁴ ft (Convert: 300 × 10⁻⁶ m × 3.2808 ft/m = 9.84 × 10⁻⁴ ft) - g = 32.174 ft/s² - μ = liquid viscosity (lb/(ft·s)); convert cP: 1 cP = 6.72 × 10⁻⁴ lb/(ft·s) (Note: 1 cP = 2.419 lb/(ft·hr). Divide by 3600 s/hr: 2.419/3600 = 6.72 × 10⁻⁴ lb/(ft·s)) - ρ_L, ρ_V = densities (lb/ft³)

Vapor residence time in drum:

$$t_{residence} = \frac{V_{vapor}}{Q_V} = \frac{V_{drum} \times (1 - 0.4)}{Q_V \text{ [ft³/s]}}$$

For the vertical drum example: - V_vapor ≈ 15 ft³ (60% of 8 ft × π × 1² ≈ 24 ft³) - Q_V = 6,786 ft³/hr = 1.885 ft³/s

$$t_{residence} = \frac{15}{1.885} = 8 \text{ seconds}$$

Terminal velocity of 300-micron droplet (assume light hydrocarbon, μ ≈ 0.3 cP = 2.016 × 10⁻⁴ lb/(ft·s)):

$$v_{terminal} = \frac{(9.84 \times 10^{-4})^2 \times (30 - 2.8) \times 32.174}{18 \times 2.016 \times 10^{-4}}$$

$$= \frac{9.68 \times 10^{-7} \times 873.1}{3.63 \times 10^{-3}} = \frac{8.45 \times 10^{-4}}{3.63 \times 10^{-3}} \approx 0.233 \text{ ft/s}$$

Distance to fall (drum height available): 4.8 ft (vapor space = 60% of 8 ft)

$$t_{fall} = \frac{4.8}{0.233} = 20.6 \text{ seconds}$$

Since t_residence (8 s) < t_fall (20.6 s), droplets will not fully settle in a single pass. This is acceptable because the Souders-Brown equation already accounts for this: the 0.25 ft/s K value is set so that most droplets > 300 microns settle; some finer mist carries through. Use a demister pad or mesh coalescer if finer separation is required.

Revision note (v2): v1 contained two unit conversion errors in this step: droplet diameter was stated as 3 × 10⁻⁴ ft (= 91 microns, not 300) and viscosity conversion used 1 cP = 2.419 × 10⁻⁵ lb/(ft·s) (off by a factor of 27.8). The corrected terminal velocity is 0.233 ft/s vs. v1’s 0.2 ft/s. The qualitative conclusion (mist carryover expected; demister may be needed) is unchanged.

Quick Reference Table

Souders-Brown K Values (API 521, Table 5)

Service	Vertical Drum	Horizontal Drum	Notes
Mist separation (gas + entrained droplets)	0.25	0.35	Typical KO drum application
Free liquid surface (disengaging space)	0.15	0.20	Higher ρ_L/ρ_V ratios, foaming fluids
Mesh demister pad	0.35–0.45	0.45–0.55	Additional coalescence device

Typical Densities (at 50 psig, 100°F)

Fluid	ρ (lb/ft³)
Methane (vapor)	2.8
Ethane (vapor)	4.5
Light naphtha (liquid)	42–45
Water (liquid)	62.4
Crude oil (liquid)	48–58

What the 15-Minute Method Misses

This method is a screening tool, not a final design. It omits:

Foaming liquids — crude oil, soap solutions, surfactant-laden water. These require demister pads or mesh devices; Souders-Brown alone is unsafe. Go to API 521 Section 7 or vendor simulation.
High-viscosity liquids — μ > 10 cP. Terminal velocity becomes very slow; you may need much larger drums or active coalescence. Use rigorous settling calculations or pilot tests.
Slug flow or intermittent discharge — some relief scenarios (blocked outlet) produce gas-liquid plugs. This method assumes continuous mist and fails for slugs. Requires transient simulation.
Very high liquid loading — > 20% mass fraction. The liquid space becomes dominant; normal KO sizing breaks down. Consider two-stage separation or dedicated liquid handling.
Phase transitions and solubility — if the relieving stream is near saturation or includes dissolved gases that flash, composition changes through the drum. Use thermodynamic equilibrium calculations (HYSYS, ProMax).
Flashing liquids — if the liquid pressure-drops significantly in the outlet nozzle, it may flash and generate secondary vapor. Requires sub-cooled liquid outlet design and outlet nozzle velocity checks.
H₂S and caustic flares — These have separate design standards (API RP 75, sulfidic corrosion concerns). Not covered here.
Sub-zero service — Freezing of condensate, ice formation, viscosity changes. Requires cryogenic vessel design and insulation.
Corrosion and material selection — This method assumes carbon steel. If corrosive condensate (acid, salt), specify stainless steel or coatings. Check API 522 and NACE standards.
Uncertainty and safety factor — This method uses nominal fluid properties. Add 20–30% safety margin to drum size or K-value reduction for production/operating variance.

Design Checklist

Before signing off on a KO drum design, verify:

[ ] Relief flow basis documented — valve sizing, depressuring rate, or process scenario with source reference (P&ID, relief calc sheet, safety case)
[ ] Fluid properties confirmed — vapor and liquid density, viscosity at relieving T and P from thermodynamic data or HYSYS; not assumed
[ ] Orientation decided — horizontal or vertical; impacts K value, installation footprint, maintenance access, and cost
[ ] Diameter and length calculated — Souders-Brown equation applied; dimensional check against standard ASME vessel sizes
[ ] Liquid volume adequate — 10-minute surge rule applied; usable volume verified against 40% fill level rule
[ ] Residence time acceptable — droplet settling or demister performance confirmed; if demister, K value and mesh selection verified
[ ] Outlet nozzle sized — liquid outlet ≥ 2” pipe, velocity < 2 ft/s; vapor outlet velocity < 10 ft/s (check if mist carryover is concern)
[ ] Inlet nozzle arranged — tangential or dip-pipe entry to reduce splash and promote liquid drop-out; inlet velocity < 10 ft/s
[ ] ASME code compliance checked — vessel designed to ASME Section VIII, stamped, relief valve set at correct pressure, hydrotest performed
[ ] Installation reviewed — location upstream of flare system confirmed, liquid drain routed to safe disposal (tank, sump, or recycle), vapor outlet vented to flare stack, thermal relief provided if isolated

References

API Standard 521, Pressure-Relieving and Depressuring Systems, 6th Edition. Washington, DC: American Petroleum Institute, 2014.
Table 5: Souders-Brown Constants
Section 7: Flares
Section 9: Liquid Separation
GPSA Engineering Data Book, 14th Edition. Tulsa, OK: Gas Processors Supply Association, 2015.
Section 7: Fluid Dynamics and Separator Design
Density and property correlations
Perry’s Chemical Engineers’ Handbook, 9th Edition. New York: McGraw-Hill, 2019.
Section 6: Fluid and Particle Dynamics
Stokes’ law and droplet settling
Souders-Brown equation derivation
API Recommended Practice 14C, Analysis, Design, Installation, and Testing of Safety Systems for Offshore Platforms — Sections on liquid knockout drums for emergency depressuring.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1. New York: American Society of Mechanical Engineers, current edition.
Vessel design and construction standards
Hydrostatic testing requirements

Last revised: April 2026 (v2 — Step 6 unit corrections)
For corrections, clarifications, or case studies, contact the engineering team.