Collector Tray Leak Testing: How to Find Bypass Before Tower Startup

A liquid collector tray may appear mechanically complete while still leaking through panel joints, perimeter seals, welds, downpipe connections or sump seams. These leaks allow liquid to bypass the intended outlet and fall into the tower section below.

In a collector-and-redistributor arrangement, uncontrolled leakage can overload one packing region and leave another region short of liquid. In a product draw or pumparound service, leakage also changes the measured draw-off quantity.

A pre-commissioning leak test can identify these defects before packing or other internals make the collector inaccessible.

What the Test Is Intended to Prove

The purpose is not necessarily to demonstrate pressure-vessel-level tightness. Most collector trays operate with a shallow liquid head rather than high pressure.

The test should confirm that collected liquid follows the intended route, joints do not create significant bypass and the deck can hold the specified operating or test level long enough for inspection.

Acceptance criteria must be defined by the collector’s process duty. A total collector generally requires tighter control than a partial collector designed to allow a specified quantity of liquid to continue downward.

Identify Every Potential Leakage Path

Common leakage locations include:

Panel-to-panel joints

Gasketed seal plates

Field welds

Perimeter seals against the shell

Sump corners and bottom seams

Downpipe flange connections

Vapor-riser base welds

Temporary installation holes

Instrument connections

Emergency-overflow details

Inspectors should have an approved drawing showing which openings are intentional. Otherwise, a designed drain or overflow may be incorrectly repaired as a leak.

Testing Should Occur at the Correct Assembly Stage

Testing too early may miss leakage introduced during final tightening, downpipe installation or work on adjacent internals. Testing too late may make the underside impossible to inspect.

The most useful stage is normally after the collector, sump, seals and liquid outlets are fully assembled but before lower packing or distributors block access.

If later work requires personnel to stand on the collector or remove panels, a final inspection should confirm that the tested condition has not changed.

Water Is Useful but Not Always Representative

Water is convenient and makes leakage easy to observe. However, its density, viscosity, surface tension and wetting behavior may differ from the process liquid.

A joint that shows slight water seepage may behave differently with a viscous hydrocarbon. Conversely, a process liquid with lower surface tension may pass through a gap that appears tight during a water test.

Water may also be unacceptable in systems requiring strict dryness or where retained water can cause corrosion or contamination. The test medium must be approved for the service.

Test Head Must Match the Purpose

An excessively high water level can load the collector beyond its intended operating condition and produce leakage that would not occur in service. A very shallow level may fail to cover upper joints or sump seams.

The test procedure should define fill elevation, maximum allowable head, hold time and drainage route. Structural capacity must be confirmed before filling.

The test level should be referenced to a permanent feature such as the sump bottom or riser top rather than an uncertain chalk mark.

Visual Inspection Should Cover Both Sides

Observation from above can reveal falling liquid level but may not show where the liquid is escaping. Access below the deck allows inspectors to identify individual joints or welds.

Lighting, safe access and communication between the fill team and inspectors should be planned. Small leaks can travel along beams before dripping, so the first visible drop may not be directly beneath the defect.

Dry surfaces below the collector make detection easier. Existing rainwater, cleaning water or condensation should be removed before the test.

Leakage Quantity Can Be More Useful Than “No Drips”

Expecting absolute zero leakage from every segmented collector may be unrealistic unless the unit is seal-welded. The engineering team should define an allowable leakage rate based on downstream sensitivity.

Where measurement is required, leakage can be collected by zone or the water-level decline can be monitored after accounting for intentional outlets and evaporation.

The acceptance criterion should prevent arbitrary field decisions such as tightening every bolt until the panel distorts.

Repair Must Not Damage Expansion Provisions

A leaking sliding joint should not automatically be welded solid. Doing so may stop the cold test leak but create thermal buckling during operation.

Repairs should preserve the intended fixed and sliding support arrangement. Gasket compression, overlap and fastener condition should be checked before adding welds.

Any field welding must consider material grade, heat distortion, passivation requirements and fire-control procedures.

Drain and Dry the Collector After Testing

The test is not complete when the leakage inspection ends. Water must be removed from sumps, troughs, downpipes and hidden pockets.

Temporary test plugs and blind plates must be listed and removed. Remaining plugs can block the actual process outlet during startup.

Where dryness is critical, the procedure should include air blowing, wiping or another approved drying method followed by a final foreign-material inspection.

Required Test Record

A useful record includes:

Collector identification and drawing revision

Test medium

Fill level and calculated liquid load

Start and finish times

Observed leakage locations

Measured leakage where required

Repairs performed

Retest result

Drainage and drying confirmation

Temporary-plug removal

Photographs referenced to panel numbers