Process Equipment

Tray Hydraulics Fundamentals

Design and analyze distillation column trays using Fair's flooding correlation, GPSA methods, and FRI guidelines for weeping, entrainment, and pressure drop.

Launch Calculator Flooding Limits

Lower Limit

Weeping

Liquid drains through holes when vapor velocity is too low.

Upper Limit

Flooding

Excessive entrainment or downcomer backup at high vapor rates.

Design Target

70-85% Flood

Optimal balance of capacity, efficiency, and turndown margin.

Contents

Use this guide to:

  • Size distillation tower trays
  • Diagnose flooding/weeping problems
  • Estimate tray efficiency
  • Calculate pressure drop per tray

1. Tray Types & Terminology

Tray columns (plate columns) achieve vapor-liquid contact through horizontal trays stacked inside a vertical shell. Vapor rises through openings in each tray, contacts liquid flowing across the tray, and continues upward. Liquid flows over an outlet weir, down a downcomer, to the tray below.

Cutaway cross-section of distillation tray column showing three horizontal trays with 24-inch spacing in 6 ft ID column, vapor flow arrows rising through perforations and bubbling through liquid creating froth zones, liquid flow arrows moving horizontally across trays and down through alternating downcomers, labeled components including tray deck, outlet weir, inlet weir, seal pan, and active bubbling area with froth
Distillation tray column cross-section showing countercurrent vapor-liquid contact on trayed internals.

Common Tray Types

Tray Type Description Turndown Best For
Sieve Perforated plate with round holes (⅜"-¾" dia) 2-3:1 Clean service, lowest cost
Valve Movable caps over holes, self-adjusting 5-8:1 Variable loads, general service
Bubble Cap Fixed caps with slots for vapor passage 10+:1 Severe turndown, no weeping

Key Tray Terminology

Plan view of single distillation tray showing 6 ft diameter circular tray with inlet and outlet downcomer areas as crescent shapes at 12% of tower area each, central active bubbling area at 76% with triangular pitch hole pattern, outlet weir at 4.5 ft length and 2-inch height, liquid flow path arrow from inlet downcomer across active area to outlet weir, dimensions labeled for tower diameter, weir length, downcomer width, and hole area percentage
Plan view of distillation tray showing active area, downcomers, weir, and liquid flow path.
  • Active Area (Aa): Bubbling zone where vapor-liquid contact occurs; typically 70-85% of tower cross-section
  • Hole Area (Ah): Total open area of perforations; typically 8-14% of active area for sieve trays
  • Downcomer Area (Ad): Vertical channel for liquid flow to tray below; typically 10-15% of tower area per side
  • Weir Height (hw): Height of outlet dam; typically 2-3 inches; sets minimum liquid level
  • Tray Spacing: Vertical distance between trays; typically 18-24 inches for standard service
  • Net Area (An): Tower area minus one downcomer; vapor flow area above tray
Design Consideration: Active area determines capacity. Higher active area (smaller downcomers) increases vapor capacity but may limit liquid handling. Balance based on L/V ratio.

2. Flooding Correlations

Flooding occurs when vapor rate is high enough to prevent proper liquid drainage, causing liquid accumulation and loss of separation. Two flooding mechanisms exist: jet (entrainment) flooding from excessive vapor velocity, and downcomer flooding from liquid backup.

Fair's Flooding Correlation

The most widely used method for predicting jet flooding is Fair's correlation (1961), adopted in GPSA Section 19. It uses the Souders-Brown capacity factor:

Souders-Brown Flood Velocity: Vflood = CSB × √[(ρL - ρV) / ρV] Where: Vflood = Maximum vapor velocity through active area (ft/s) CSB = Capacity factor at flooding (ft/s) ρL = Liquid density (lb/ft³) ρV = Vapor density (lb/ft³) Fair's Capacity Factor: CSB = K₁ × (σ/20)^0.2 Where: K₁ = Capacity constant from Fair's chart (function of FLV and tray spacing) σ = Surface tension (dyne/cm), baseline = 20 dyne/cm

Flow Parameter (FLV)

The flow parameter relates liquid-to-vapor loading and determines K₁:

Flow Parameter: FLV = (L/V) × √(ρV/ρL) Where: L = Liquid mass flow rate (lb/hr) V = Vapor mass flow rate (lb/hr)
Fair's flooding correlation chart (GPSA Fig. 19-25) showing log-log plot of capacity factor K1 in ft/s (0.05-0.5) versus flow parameter FLV (0.01-3.0) with four curves for tray spacings of 36 inch (dark blue), 24 inch (blue), 18 inch (green), and 12 inch (orange), horizontal dashed line at typical design point K1=0.20, vertical dashed line at low L/V ratio FLV=0.1, note showing surface tension correction formula
Fair's K₁ flooding correlation for tray capacity design (GPSA Fig. 19-25).

K₁ Values (24-inch Tray Spacing)

FLV K₁ (ft/s) FLV K₁ (ft/s)
0.010.380.200.21
0.020.340.500.17
0.050.281.000.13
0.100.242.000.10

Tray spacing correction: K₁ scales with (Ts/24)^0.5 where Ts is tray spacing in inches.

Design Operating Point

Percent of Flood: % Flood = (Vactual / Vflood) × 100 Design Recommendation: • Normal operation: 70-85% of flood • Maximum design: 80% of flood (10-15% margin for upsets) • Below 50%: May have efficiency issues • Above 90%: Risk of flooding with minor upsets
Troubleshooting: Flooding symptoms include sharp pressure drop increase, loss of separation, and liquid carryover to overhead. Gamma scanning can diagnose location without shutdown.

3. Pressure Drop Calculations

Tray pressure drop consists of three components: dry tray drop (vapor through holes), liquid head on tray, and residual drop (surface tension effects).

Total Tray Pressure Drop: ht = hd + hL + hr (inches of liquid) Where: hd = Dry tray pressure drop hL = Clear liquid height on tray hr = Residual pressure drop

Dry Tray Pressure Drop

Sieve Tray Dry Drop: hd = 0.186 × (ρV/ρL) × (Vh/C₀)² Where: Vh = Vapor velocity through holes (ft/s) C₀ = Orifice coefficient (typically 0.72-0.78) Result in inches of liquid Orifice Coefficient C₀: • Depends on plate thickness / hole diameter ratio • For t/dh = 0.5-1.0: C₀ ≈ 0.73-0.76 • Higher hole area ratio → higher C₀

Liquid Head Components

Clear Liquid Height: hL = β × (hw + how) Where: β = Aeration factor (froth density ratio, typically 0.3-0.7) hw = Weir height (inches) how = Height of liquid over weir (inches) Francis Weir Formula: how = 0.48 × Fw × (QL/Lw)^(2/3) Where: QL = Liquid flow rate (gpm) Lw = Weir length (inches) Fw = Weir constriction factor (≈1.0 for straight weirs)

Bennett Aeration Factor

The aeration factor accounts for vapor holdup in the froth on the tray:

Bennett Correlation: β = exp(-12.55 × Ks^0.91) Where: Ks = Va × √[ρV/(ρL - ρV)] Va = Vapor velocity through active area (ft/s) Typical values: • Low vapor rate (F-factor < 0.5): β ≈ 0.7-0.8 • Normal operation: β ≈ 0.4-0.6 • High vapor rate (F-factor > 2): β ≈ 0.3-0.4
Side view cross-section of distillation tray hydraulic profile showing dimension labels for weir height hw=2 inches, height over weir how=1 inch, clear liquid height hL=2.5 inches, froth height hf=5 inches with aeration factor beta=0.5, downcomer backup hdc=8 inches, tray spacing Ts=24 inches, pressure drop components hd dry tray plus hL liquid head plus hr residual equals ht total, vapor bubbles rising through perforations, clear liquid in downcomer
Tray hydraulic profile showing liquid levels, froth height, and pressure drop components.

Typical Pressure Drop Values

Tray Type ΔP Range Notes
Sieve tray 2-4 in H₂O Lowest ΔP; simple design
Valve tray 2-5 in H₂O Valve weight adds to dry drop
Bubble cap 4-8 in H₂O Highest ΔP due to slot resistance
Vacuum Service: Minimize pressure drop for vacuum distillation. Use low weir height (1-1.5"), high hole area (12-15%), and consider structured packing instead.

4. Weeping & Turndown

Weeping occurs when vapor velocity is too low to support the liquid on the tray. Liquid drains ("weeps") through the holes, bypassing vapor-liquid contact and reducing efficiency.

Weeping Criterion

Minimum Hole Velocity (Lockett): Vh,min = K × √[(hL/12) × (ρL - ρV)/ρV] Where: K = Weeping constant (0.10-0.13 depending on hole area ratio) hL = Clear liquid height (inches) Result in ft/s Weeping Factor K: • Hole area < 8% of active: K ≈ 0.10 • Hole area 8-12%: K ≈ 0.11 • Hole area 12-15%: K ≈ 0.12 • Hole area > 15%: K ≈ 0.13 Design Margin: Actual hole velocity should be at least 20% above minimum to ensure adequate turndown capability.

Turndown Ratio

The operable range between weeping and flooding defines turndown capability:

Turndown Ratio: Turndown = Vflood / Vweep Typical values: • Sieve tray: 2-3:1 (limited by weeping) • Valve tray: 5-8:1 (valves close at low rates) • Bubble cap: 10-20:1 (vapor always bubbles through liquid)
Tray operating envelope diagram for 24-inch spacing 6 ft diameter column showing vapor rate (0-25 ft3/s) versus liquid rate (0-500 gpm) with boundary lines for jet entrainment flooding (red solid top), downcomer flooding (red dashed right), weeping limit (blue solid bottom), dumping limit (blue dashed left), green shaded operating region, darker green 70-85% flood design target zone, design point marked at 250 gpm and 15 ft3/s with annotations for entrainment increases, efficiency drops, and liquid backup
Tray operating envelope showing operable region bounded by flooding and weeping limits.

Downcomer Flooding

Liquid backup in the downcomer must not exceed available height:

Downcomer Backup: hdc = ht + hw + how + hda Where: ht = Total tray pressure drop (inches liquid) hw = Weir height (inches) how = Height over weir (inches) hda = Downcomer apron loss (typically 0.5-1.0 inches) Flooding Criterion: hdc < 0.5 × (Tray spacing - 3") Design limit is typically 50% of available downcomer height.
Valve Tray Advantage: At low vapor rates, valve caps close to maintain hole velocity, preventing weeping. This provides 5-8:1 turndown vs 2-3:1 for sieve trays.

5. Tray Efficiency

Tray efficiency quantifies how closely actual separation approaches theoretical equilibrium. Higher efficiency means fewer trays required for a given separation.

Murphree Vapor Efficiency

Murphree Efficiency (EMV): EMV = (yout - yin) / (y*out - yin) Where: yout = Actual vapor composition leaving tray yin = Vapor composition entering tray (from below) y*out = Equilibrium vapor composition with liquid leaving tray Typical values: 50-90% for hydrocarbon distillation

O'Connell Correlation

Quick estimate of overall column efficiency:

O'Connell Correlation: EO = 0.492 × (α × μL)^(-0.245) Where: α = Relative volatility (key components) μL = Liquid viscosity (cP) Example: α = 2.5, μL = 0.3 cP EO = 0.492 × (0.75)^(-0.245) = 53% Actual trays = Theoretical stages / EO

Typical Efficiency by Service

Service EMV Notes
Hydrocarbon distillation 60-90% Low viscosity, good mass transfer
Deethanizer/depropanizer 50-70% High pressure effects
Amine absorber 20-40% Reaction kinetics limited
Glycol contactor 15-30% High viscosity glycol
Vacuum tower 30-50% High viscosity, low pressure

Entrainment Effect on Efficiency

Liquid carryover to the tray above reduces apparent efficiency:

Entrainment Correlation (Hunt): ψ ≈ 0.02 × (%Flood/70)^4 × (24/Ts)^0.5 Where: ψ = Fractional entrainment (liquid entrained / total liquid) Ts = Tray spacing (inches) Effect on Efficiency: Eactual ≈ EMV × (1 - ψ/2) Design Limit: Keep ψ < 0.10 (10%) for good efficiency At 80% flood with 24" spacing: ψ ≈ 4-6%
Design Margin: Add 10-15% extra trays above calculated requirements to account for efficiency uncertainty, fouling degradation, and future capacity increases.

Factors Affecting Efficiency

Factor Effect Remedy
Low vapor rate Poor contacting, weeping Increase reflux or use valve trays
High vapor rate Entrainment reduces efficiency Reduce throughput or increase spacing
Fouling Plugged holes, reduced active area Regular cleaning, larger holes
Damaged trays Liquid bypassing, maldistribution Inspection and repair

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