1. System Overview
Flare disposal systems safely collect, transport, and combust relief discharges from pressure safety valves (PSVs), blowdown valves, and emergency depressuring systems. The system must handle transient high-flow events while maintaining acceptable backpressure on relief devices.
Flare header
Collect & transport
Routes relief gases from multiple sources to the flare stack or ground flare.
Knockout drum
Liquid separation
Removes entrained liquids before combustion to prevent liquid carryover to flare tip.
Liquid seal drum
Flashback prevention
Water seal prevents flame propagation back into the header system.
Flare stack/tip
Combustion
Elevated or ground-level combustion with proper radiation protection.
Key Design Criteria
- Backpressure limit: Total backpressure at PSV outlet must not exceed allowable (typically 10% of set pressure for conventional valves)
- Mach number: Velocity must remain below sonic to prevent choking and acoustic fatigue
- Liquid separation: KO drum must capture entrained liquids to protect flare tip and prevent liquid rainout
- Seal integrity: Liquid seal must prevent flashback under all operating conditions
Critical design consideration: Flare header sizing is governed by two competing constraints: (1) Mach number limit to prevent acoustic fatigue and sonic choking, and (2) pressure drop limit to maintain acceptable PSV backpressure. Both must be checked for the design relief scenario.
Typical Relief Scenarios
| Scenario | Duration | Flow Character | Design Consideration |
|---|---|---|---|
| Fire case | Extended (hours) | Steady, vapor | Simultaneous relief from fire-exposed vessels |
| Power failure | Minutes | Peak then decay | Loss of cooling/compression across plant |
| Blocked outlet | Minutes | Steady, single source | Usually single PSV, check header routing |
| Emergency depressuring | 15-20 min | High initial, decaying | Largest single relief load typically |
| Tube rupture | Minutes | Steady, two-phase | High liquid fraction possible |
2. Flare Header Design
Flare header sizing ensures adequate flow capacity while limiting velocity to prevent acoustic vibration, erosion, and excessive pressure drop.
Pressure Drop Calculation (Darcy-Weisbach)
Frictional Pressure Drop:
ΔP = f × (L/D) × (ρ × V²) / (2 × gc)
Where:
ΔP = Pressure drop (lbf/ft² or psi after /144)
f = Darcy friction factor (dimensionless)
L = Pipe length (ft)
D = Internal diameter (ft)
ρ = Gas density at flowing conditions (lb/ft³)
V = Gas velocity (ft/s)
gc = 32.174 lbm-ft/(lbf-s²)
Converting to psi:
ΔP_psi = f × (L/D) × (ρ × V²) / (2 × gc × 144)
Friction Factor (Churchill Equation)
Churchill Equation (all flow regimes):
For turbulent flow (Re > 4000):
1/√f = -2 × log₁₀(ε/3.7D + 2.51/(Re×√f))
Where:
ε = Pipe roughness (ft)
D = Internal diameter (ft)
Re = Reynolds number = ρVD/μ
Typical roughness values:
Commercial steel: ε = 0.0018 in = 0.00015 ft
New steel pipe: ε = 0.0006 in = 0.00005 ft
Corroded steel: ε = 0.01-0.02 in
Example (Re = 1,000,000, ε/D = 0.0001):
f ≈ 0.014 (Moody diagram)
Mach Number Calculation
Mach Number:
Ma = V / c
Where:
Ma = Mach number (dimensionless)
V = Gas velocity (ft/s)
c = Sonic velocity (ft/s)
Sonic Velocity:
c = √(γ × Z × R × T / M)
Where:
γ = Specific heat ratio (Cp/Cv), typically 1.25-1.35 for hydrocarbons
Z = Compressibility factor
R = Gas constant = 1545 ft-lbf/(lbmol-°R)
T = Absolute temperature (°R = °F + 459.67)
M = Molecular weight (lb/lbmol)
For natural gas (SG = 0.65, T = 100°F):
M = 0.65 × 28.97 = 18.8 lb/lbmol
c ≈ √(1.3 × 0.95 × 1545 × 559.67 × 32.174 / 18.8)
c ≈ 1,350 ft/s
API RP 521 Mach Limits:
• Recommended: Ma ≤ 0.5 (prevents acoustic fatigue)
• Maximum allowable: Ma ≤ 0.7 (prevents choking)
• Above 0.3: Acceleration ΔP becomes significant
Header Sizing Criteria
| Parameter | Limit | Basis |
|---|---|---|
| Mach number | ≤ 0.5 (recommended) / ≤ 0.7 (max) | API RP 521 acoustic fatigue prevention |
| Gas velocity | Typically 100-300 ft/s | Function of Mach and sonic velocity |
| Pressure drop | < 10% of PSV backpressure allowable | PSV capacity depends on backpressure |
| Erosion velocity | V × √ρ < 100 | API RP 14E erosional velocity |
Two-Phase Flow Considerations
Lockhart-Martinelli Two-Phase Multiplier:
When liquid is entrained in the gas stream, pressure drop increases:
ΔP_2phase = ΔP_gas × φ²
Where φ² is the two-phase multiplier:
For turbulent-turbulent flow:
φ² = 1 + 20/X_tt + 1/X_tt²
X_tt = [(1-x)/x]^0.9 × (ρG/ρL)^0.5 × (μL/μG)^0.1
Where:
x = Gas mass fraction (quality)
ρG, ρL = Gas and liquid densities
μG, μL = Gas and liquid viscosities
Typical values:
• x = 0.95 (5% liquid): φ² ≈ 2-3
• x = 0.90 (10% liquid): φ² ≈ 3-5
• x = 0.80 (20% liquid): φ² ≈ 5-10
Rule of thumb: For liquid fractions > 5%, use two-phase
pressure drop methods or increase header size by 20-50%.
Header sizing approach: Start with Mach = 0.5 to calculate required diameter. Verify pressure drop is acceptable. If pressure drop too high, increase diameter. If Mach < 0.3, smaller diameter may be economical but check erosion and backpressure.
3. Knockout Drum Sizing
Knockout drums (KO drums) separate entrained liquid droplets from the vapor stream before the flare tip. Proper sizing prevents liquid carryover (burning liquid rain) and ensures adequate liquid storage capacity.
Souders-Brown Vapor Velocity
Maximum Allowable Vapor Velocity:
V_max = K × √[(ρL - ρG) / ρG]
Where:
V_max = Maximum vapor velocity (ft/s)
K = Souders-Brown coefficient (ft/s)
ρL = Liquid density (lb/ft³)
ρG = Gas density (lb/ft³)
The K-value depends on:
• Droplet size distribution
• Vessel internals (mesh pad, vanes)
• Foaming tendency
• Operating pressure
API RP 521 K-Values (vertical drums, no internals):
| Service Type | K (ft/s) | Application |
|--------------|----------|-------------|
| Dry gas | 0.35 | Clean gas, no liquid expected |
| Wet gas | 0.25 | Some liquid, light hydrocarbon |
| Average | 0.17 | Typical oilfield gas with condensate |
| Dirty/foaming | 0.125 | High liquid, surfactants present |
| Glycol/amine | 0.10 | Foaming chemical services |
With mesh pad demister: K can increase 50-100%
With vane pack: K can increase 20-40%
Drum Diameter Sizing
Required Diameter from Vapor Velocity:
A_req = Q_actual / V_max
D = √(4 × A_req / π)
Where:
A_req = Required cross-sectional area (ft²)
Q_actual = Actual volumetric flow rate (ft³/s)
V_max = Maximum allowable velocity (ft/s)
D = Drum diameter (ft)
Actual Volumetric Flow:
Q_actual = W_gas / (ρG × 3600)
Where:
W_gas = Gas mass flow rate (lb/hr)
ρG = Gas density at operating conditions (lb/ft³)
Gas Density:
ρG = P × M / (Z × R × T)
Where:
P = Absolute pressure (psia)
M = Molecular weight
Z = Compressibility factor
R = 10.73 psia-ft³/(lbmol-°R)
T = Absolute temperature (°R)
Liquid Holdup and Height
Liquid Holdup Volume:
V_liquid = Q_liquid × t_retention
Where:
V_liquid = Liquid holdup volume (ft³)
Q_liquid = Liquid volumetric flow rate (ft³/min)
t_retention = Retention time (minutes)
Typical Retention Times:
| Application | Retention (min) |
|-------------|-----------------|
| Emergency KO drum | 10-20 |
| Normal service | 5-10 |
| High liquid rate | 15-30 |
| Intermittent service | 5 |
Drum Height (Vertical):
For vertical drums, use Height/Diameter (H/D) ratio:
H = D × (H/D ratio)
Typical H/D = 2.0 to 4.0
H/D < 2: May have poor vapor distribution
H/D > 4: Structural/cost considerations
Liquid Level Allocation (typical):
• High-High (HH): 70% of height
• High (H): 60% of height
• Normal: 50% of height
• Low (L): 30% of height
• Low-Low (LL): 20% of height
K-Value Selection Guide
| Service | K (ft/s) | Characteristics |
|---|---|---|
| Dry gas (no liquid) | 0.35 | No liquid expected; emergency vent only |
| Wet gas (some liquid) | 0.25 | Light liquid carryover; methane/ethane |
| Average service | 0.17 | Typical oil/gas; condensate + gas |
| Dirty/foaming | 0.125 | High liquid; surfactants; crude oil |
| Glycol/amine | 0.10 | Highly foaming; chemical contamination |
Design margin: Size KO drum so actual vapor velocity is 80-90% of allowable. This provides margin for flow variations and prevents liquid carryover during transient events. Never operate above 100% of V_max.
4. Design Examples
Example 1: Flare Header Pressure Drop
Given:
• Mass flow rate: 50,000 lb/hr
• Gas SG: 0.65 (MW = 18.8)
• Temperature: 100°F (559.67°R)
• Pressure: 50 psig (64.7 psia)
• Header diameter: 16" ID
• Header length: 500 ft
• Pipe roughness: 0.0018 in
Step 1: Calculate gas density
Z ≈ 0.95 (estimated from correlations)
ρ = (64.7 × 18.8) / (0.95 × 10.73 × 559.67)
ρ = 1216.4 / 5704.7 = 0.213 lb/ft³
Step 2: Calculate velocity
D = 16/12 = 1.333 ft
A = π × 1.333² / 4 = 1.396 ft²
Q = (50,000 / 0.213) / 3600 = 65.2 ft³/s
V = 65.2 / 1.396 = 46.7 ft/s
Step 3: Calculate Mach number
c = √(1.3 × 0.95 × 1545 × 559.67 × 32.174 / 18.8)
c = 1,350 ft/s
Ma = 46.7 / 1350 = 0.035 ✓ (well below 0.5)
Step 4: Calculate Reynolds number
μ ≈ 0.012 cp = 8.1×10⁻⁶ lb/(ft·s)
Re = (0.213 × 46.7 × 1.333) / 8.1×10⁻⁶
Re = 1,640,000 (highly turbulent)
Step 5: Calculate friction factor
ε/D = 0.00015 / 1.333 = 0.000113
f ≈ 0.014 (Colebrook)
Step 6: Calculate pressure drop
ΔP = 0.014 × (500/1.333) × (0.213 × 46.7²) / (2 × 32.174 × 144)
ΔP = 0.014 × 375 × 464.5 / 9266
ΔP = 0.26 psi ✓ (acceptable)
Example 2: Knockout Drum Sizing
Given:
• Gas mass flow: 45,000 lb/hr (90% of total)
• Liquid mass flow: 5,000 lb/hr (10% of total)
• Gas SG: 0.65, Liquid density: 50 lb/ft³
• Temperature: 100°F, Pressure: 50 psig
• Service: Average (K = 0.17)
• Retention time: 10 minutes
• Target H/D ratio: 3.0
Step 1: Calculate gas density
ρG = 0.213 lb/ft³ (from Example 1)
Step 2: Calculate allowable velocity
V_max = 0.17 × √[(50 - 0.213) / 0.213]
V_max = 0.17 × √234.0
V_max = 0.17 × 15.3 = 2.60 ft/s
Step 3: Calculate required area
Q_gas = (45,000 / 0.213) / 3600 = 58.7 ft³/s
A_req = 58.7 / 2.60 = 22.6 ft²
Step 4: Calculate drum diameter
D = √(4 × 22.6 / π) = 5.36 ft
Round up: D = 5.5 ft (66 inches)
Step 5: Verify actual velocity
A_actual = π × 5.5² / 4 = 23.76 ft²
V_actual = 58.7 / 23.76 = 2.47 ft/s
% of allowable = 2.47 / 2.60 × 100 = 95% ✓
Step 6: Calculate drum height
H = 5.5 × 3.0 = 16.5 ft
Step 7: Verify liquid holdup
Q_liquid = (5,000 / 50) / 60 = 1.67 ft³/min
V_holdup = 1.67 × 10 = 16.7 ft³
Liquid height (at 50% fill):
V_drum/2 = π × 5.5² × 16.5 / 8 = 196 ft³ > 16.7 ft³ ✓
Result:
Drum size: 5.5 ft diameter × 16.5 ft height (66" × 198")
5. Best Practices
Header Design
- Sloping: Install headers with 1:500 minimum slope toward KO drum to drain liquids
- Pockets: Avoid low points that trap liquid; install drip legs with level switches where unavoidable
- Lateral connections: Connect subheaders to main header at 45° in flow direction
- Velocity balance: Size laterals so velocity increase at junction doesn't exceed Mach limit
- Thermal expansion: Provide adequate flexibility for temperature excursions during relief events
Knockout Drum Design
- Inlet device: Use half-pipe, impingement plate, or tangential entry to reduce inlet velocity
- Vapor outlet: Locate in top head with vortex breaker; avoid direct path from inlet
- Liquid outlet: Provide vortex breaker; size for twice normal liquid rate
- Level control: Install HH/H/L/LL level switches; HH triggers automatic pump start
- Drainage: Connect to closed drain system; do not drain to atmosphere
Common Design Errors
| Error | Consequence | Prevention |
|---|---|---|
| Undersized header (Ma > 0.7) | Choking, acoustic vibration, high backpressure | Size for Ma ≤ 0.5 at maximum flow |
| Wrong K-value selection | Liquid carryover to flare tip | Use conservative K for uncertain service |
| Inadequate liquid holdup | Drum overflow, loss of seal | Size for 2× expected liquid rate |
| Header low points | Liquid accumulation, slugging, seal loss | Maintain continuous slope; add drip legs |
| Ignoring two-phase flow | Actual ΔP 3-10× calculated | Apply Lockhart-Martinelli for liquid > 5% |
Safety margin: Flare systems must work when everything else fails. Apply 20-30% margin on flow capacity and design for worst credible scenario. Never optimize flare systems for minimum cost at the expense of reliability.
Ready to use the calculator?
→ Launch Calculator