This guide is intended for process engineers reviewing the hydraulic calculations for tray columns, typically submitted by internals vendors during basic or detailed engineering. It provides key parameters, recommended values, and standard design limits for conventional cross-flow trays with floating valves. These guidelines help ensure a robust design for both new (grassroots) and revamped columns.
| Design Parameter | Recommended Values | Allowable Range / Limits | Remarks |
|---|---|---|---|
| Column Diameter | Sized to avoid flooding and meet capacity within pressure drop limits. | 1 ft to 45 ft (300 mm to 13800 mm) | The diameter must provide sufficient cross-sectional area to avoid both downcomer (liquid) and jet (vapor) flooding. |
| Tray Spacing | Economical: 18 in to 24 in (450 to 600 mm) General: 24 in (600 mm) |
12 in to 36 in (300 to 900 mm) | A key parameter for tray capacity. An optimum value minimizes column height without constraining liquid handling or maintenance access. |
| Bottom Tray Spacing | 750 mm | Normal Tray Spacing + 150 mm | The distance from the bottom tray floor to the seal pan floor should be ~150 mm greater than normal spacing to account for potential high liquid load or vaporization. |
| Downcomer Clearance (Downcomer Sealing) |
Typically 0.5 in (13 mm) less than the outlet weir height. | Minimum: 0.25 in (6 mm) | For high liquid rates, "zero seal" (no clearance) or "negative seal" (clearance > weir height) can be considered, but only after consulting the tray vendor. |
| Seal Pan Downcomer Clearance | Minimum: 2 in (50 mm) | To prevent issues from solids accumulation, the clearance between the seal pan floor and the bottom downcomer should be larger than normal. | |
| Downcomer Width a) Inlet d/c width b) Outlet d/c width |
a) Minimum: 8 in (200 mm) b) Minimum: 6 in (150 mm) |
For single-pass trays, the minimum side-downcomer width should be at least 10% of the column diameter. | |
| Outlet Weir Height | General Service: 2 in (50 mm) High Pressure: 3 in (75 mm) at 24 in spacing Vacuum Service: 1 in (25 mm) |
Max: 4 in (100 mm) Based on regime: β’ Froth: 50 to 80 mm β’ Spray: 20 to 25 mm |
An optimum weir height maximizes tray efficiency without causing excessive liquid backup in the downcomer. |
| Number of Flow Passes | 1 to 2 | 1 to 8 | If a column is limited by downcomer flooding, increasing the number of flow passes (e.g., from single-pass to two-pass) is a common solution. |
| Design Parameter | Recommended Values | Allowable Range / Limits | Remarks |
|---|---|---|---|
| Jet Flooding (%) | New Columns: 80% Max Revamp Columns: 85% to 90% Max Vacuum: 75-77% Max |
This is flooding caused by excessive vapor flow entraining liquid droplets up to the tray above. Tray capacity at jet flood is a function of tray spacing, hole area, hole size, and bubbling area. | |
| Ultimate Capacity (System Flooding) | In most systems, jet flooding occurs before this limit. | This is the absolute highest vapor load a tray can handle, where liquid is broken into fine droplets and cannot fall back. Exceptions include high-pressure columns (e.g., Demethanizers) or systems with very low surface tension (< 5 dyne/cm). | |
| Entrainment | Ideally, nil. | < 10% of the tray's liquid flow rate | Entrainment is liquid carried by vapor to the tray above. Designs should not exceed 10% entrainment, as this reduces efficiency. |
| Design Parameter | Recommended Values | Allowable Range / Limits | Remarks |
|---|---|---|---|
| Downcomer Flooding (%) (due to clear liquid backup) |
Max 40% for most services. If (ΟL - ΟV) β€ 480 kg/mΒ³, limit to 33%. |
This determines how close the tray is to flooding from excessive liquid height in the downcomer. It's the clear liquid backup divided by (tray spacing + weir height). | |
| Downcomer Choke Flooding (%) | Should ideally equal the Jet Flood rating for a "balanced" tray design. | 85% Max (Grassroots & Revamps) | Occurs when the downcomer inlet area is too small, and the froth on the tray cannot enter the downcomer effectively. A balanced design has equal "area" for vapor and liquid flow. |
| Downcomer Velocity (Clear Liquid) | Set by downcomer area. | Low Foaming: 0.12 to 0.21 m/s Medium Foaming: 0.09 to 0.18 m/s High Foaming: 0.06 to 0.09 m/s |
This velocity is limited to prevent choking and to ensure adequate vapor-liquid separation (de-entrainment) in the downcomer. |
| Velocity Under Downcomer | ~1.0 ft/s (0.3 m/s) | Max: 1.5 to 1.64 ft/s (0.45 to 0.5 m/s) | Calculates the head loss under the downcomer. If this velocity is too high, the downcomer clearance must be increased. |
| Head Loss Under Downcomer | Max: 1.5 in (38 mm) Min: 0.1 in (2.5 mm) |
This is a function of the downcomer clearance (seal) and the shape of the downcomer's edge. | |
| Weir Loading | 1 to 10 gpm/in (9 to 90 mΒ³/h-m) | Max: 120 mΒ³/h-m Min: 4.5 mΒ³/h-m |
This is the liquid flow rate per unit of effective weir length. If loading is too high (e.g., > 90 mΒ³/h-m), increase the number of tray passes. If too low, "picket-fence" weirs may be needed. |
| Percentage Weeping | Max: 20% to 25% (at turndown flow) | This is the percentage of liquid that leaks *through* the valves instead of flowing over the weir. Weepage up to 20% can result in a ~10% loss of tray efficiency. |
| Design Parameter | Recommended / Allowable Range | Remarks |
|---|---|---|
| Dry Tray Pressure Drop (ΞP) | Foaming Systems: 57 mm Max Non-Foaming: 114 mm Max General: 25 to 100 mm (1 to 4 in) Fixed Valves: > 13 mm |
This is the pressure drop from vapor flowing through the valves, *before* accounting for the liquid on the tray. |
| Total Tray Pressure Drop (ΞP) | Typically 8 mm Hg per tray (at 80% jet flooding) | This is the sum of the dry tray ΞP and the wet tray (liquid) ΞP. |
| Tray Turndown (Flexibility) | Sieve Trays: 2:1 to 3:1 Fixed Valve Trays: ~3:1 Moving Valve Trays: 3:1 to 4:1 |
This is the range of loadings (from max to min) over which the tray achieves acceptable efficiency and performance. |
| Fouling Service | Severe Fouling: Use Shed Decks / Baffle Trays Moderate Fouling: Use Fixed Valves (e.g., ProValve) or large-hole sieve trays. |
Moving valve trays are generally not recommended for fouling services. |
A "System Factor" is a de-rating factor applied to a tray's hydraulic capacity (both jet flood and downcomer flood) to account for system properties that reduce performance. This includes foaming and high vapor density effects.
These factors are best defined from existing operational experience for a specific chemical system and should be agreed upon with the client or licensor.
| System / Service | Suggested System Factor |
|---|---|
| 1. Non-Foaming (General System) e.g., most common clean distillation systems | 1.0 |
| 2. Low Foaming e.g., Vacuum Crude, Sulfolane Stripper, Atmospheric Crude | 0.85 - 1.0 |
| 3. Moderate Foaming e.g., Amine/Glycol Regenerators, Oil Absorbers, Demethanizer | 0.85 |
| 4. High Foaming e.g., Amine/Glycol Absorbers, Reclaimers | 0.7 - 0.75 |
| 5. Severe Foaming e.g., MEK Units, Sour Water Strippers, Caustic Wash Tower | 0.6 - 0.65 |
| 6. Foam Stable Systems e.g., Caustic Regenerators, Alcohol Synthesis Absorber | 0.3 - 0.35 |