Fractionation Design and Operations Reference

Tray Hydraulics

Module from the Fractionation Design and Operations Reference curriculum.

TRAY HYDRAULICS & DESIGN · Tray Capacity Limits

Limit	What Happens	Where It Occurs	Detection	Cause	Fix	Impact on Separation
Jet flooding	Excessive entrainment carries liquid to tray above	Rectifying section (high vapor)	Sharp ΔP increase, poor separation	Vapor velocity too high	Reduce throughput or increase column diameter	Severe — remixing of liquid
Downcomer flooding	Liquid backs up in downcomer to tray above	Stripping section (high liquid)	Level buildup, ΔP increase	Liquid rate exceeds downcomer capacity	Increase downcomer area, reduce liquid load	Severe — tray hydraulic failure
Weeping	Liquid falls through tray perforations instead of downcomer	Entire column at low rates	Loss of tray efficiency	Vapor velocity too low to support liquid	Increase vapor rate or use valves/bubble caps	Moderate — reduced efficiency
Dumping	Complete loss of liquid seal, all liquid falls through	Entire column at very low rates	Dramatic loss of separation	Severely reduced vapor rate	Cannot operate — increase to minimum rate	Complete — no separation
Downcomer backup	Froth in downcomer exceeds tray spacing	Column at high rates	Increasing ΔP, level instability	Insufficient downcomer clearance or area	Increase clearance, anti-jump baffles	Severe if floods
Tray Design Parameters
Parameter	Typical Range	Impact	Design Guideline
Tray spacing	18" (fouling) to 36" (high capacity)	Sets column height, flooding limit	24" standard, 30" for high fouling
Downcomer area	10-15% of column area	Sets liquid handling capacity	12% standard, 15% for high liquid
Downcomer clearance	1.0-1.5" under downcomer	Liquid seal, sets downcomer backup	1.0" standard, increase for fouling
Downcomer residence time	3-5 seconds minimum	Foam disengagement in downcomer	5 sec for foaming systems
Weir height	1-4" (liquid seal depth)	Efficiency, pressure drop	2" standard valve tray
Weir loading	<13 GPM/inch of weir	Overflow capacity	Design for maximum rates
Hole area (sieve)	5-14% of active area	Vapor distribution, weeping limit	8-12% typical
Valve weight (valve tray)	Light, medium, heavy	Turndown, ΔP, capacity	Medium standard; light for max capacity
Number of passes	1, 2, 3, or 4	Handles high liquid rates	2-pass above ~10 ft diameter

Source: Fractionation_Design_Ops_Reference_v1.xlsx · Sheet: Tray Hydraulics

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