How PTFE Gasket Cold Flow Causes Leakage in Tower Internals

PTFE gaskets are widely used in corrosive tower services because they resist many acids, solvents, and aggressive chemicals. They may seal segmented liquid collectors, distributor flanges, internal pipe joints, inspection covers, and removable panels.

Chemical compatibility, however, is only part of gasket performance.

PTFE deforms continuously under compressive load. This time-dependent deformation—commonly called creep or cold flow—can reduce gasket thickness and bolt preload even when the temperature is well below the polymer’s melting point.

A joint that passes its initial leak test may therefore begin leaking after startup.

What Cold Flow Means

When a PTFE gasket is compressed, part of its deformation is elastic and part is time-dependent. The material gradually moves away from highly loaded areas.

As the gasket becomes thinner, the joint stack shortens. If the bolts cannot maintain their stretch, clamping force falls.

Cold flow becomes more severe with:

High gasket stress.

Elevated temperature.

Wide unsupported gasket width.

Large flange gaps.

Thermal cycling.

Soft or unfilled PTFE.

Uneven flange surfaces.

Excessive bolt spacing.

Pressure acting on an exposed gasket edge.

Chemical swelling or permeation in certain services.

The nuts do not need to rotate for preload to disappear.

Why Internal Joints Are Vulnerable

Tower-internal flanges are often thinner and less rigid than external piping flanges. Segmented collectors and distributors may use formed sheet, narrow angles, or lightweight plastic components.

When bolts are tightened, flexible flanges can bend. Contact pressure becomes concentrated near the bolts while gasket areas between bolts remain lightly compressed.

PTFE flows away from the high-pressure zones. The initially uneven stress becomes even less uniform over time.

Internal joints may also experience:

Liquid head.

Local vapor differential pressure.

Vibration.

Thermal expansion.

Assembly misalignment.

Limited access for controlled tightening.

Inability to retorque during operation.

A small leak may bypass calibrated distributor holes and seriously alter hydraulic performance without escaping outside the vessel.

Failure Consequences

PTFE gasket cold flow can cause:

Leakage through collector seams.

Distributor bypass flow.

Loss of liquid seals.

Vapor bypass.

Corrosion of unprotected joint surfaces.

Gasket extrusion into the flow path.

Loose bolts and joint movement.

Contamination by torn gasket fragments.

Progressive enlargement of leakage gaps.

If liquid bypasses the intended outlets, one region of packing may receive excessive flow while another becomes under-irrigated.

A leaking collector deck may allow liquid to fall directly into the bed below instead of being withdrawn or redistributed.

Gasket Extrusion

Cold flow becomes more dangerous when the gasket has an unconfined edge exposed to pressure. The PTFE can extrude into a flange gap.

Extrusion reduces the remaining gasket cross-section and may create a rapid leakage path. Thin sheet flanges that separate locally under load increase this risk.

Raised faces, tongue-and-groove arrangements, gasket retainers, or controlled recesses can limit lateral movement. The geometry must still permit assembly through the manway and field replacement.

Expanded PTFE and filled PTFE products may offer different creep behavior, but they are not interchangeable without reviewing chemical compatibility, compressibility, recovery, and sealing stress.

Bolt Load and Flange Stiffness

Applying very high torque is not a reliable solution. Excessive load can distort thin flanges, crush plastic components, or force the gasket to extrude faster.

The joint design should balance:

Minimum seating stress.

Maximum gasket stress.

Bolt spacing.

Flange rotation.

Operating pressure.

Thermal movement.

Material strength.

Long-term relaxation.

Large washers or backing bars can distribute bolt load on thin internal flanges. In polymer assemblies, compression limiters may prevent local crushing.

Spring washers or disc-spring systems can help maintain load as the gasket relaxes, provided their material and operating range are suitable.

Surface and Alignment Requirements

PTFE can conform to moderate surface irregularity, but it should not be used to compensate for major flange mismatch.

Misaligned segments produce uneven gasket compression. Sharp edges can cut the gasket during assembly. Deep scratches crossing the sealing direction can become leak paths.

Before assembly, inspectors should verify:

Flange flatness.

Parallelism.

Surface condition.

Bolt-hole alignment.

Correct gasket dimensions.

Absence of folds and handling damage.

Proper gasket centering.

Clean, dry contact surfaces.

Joint parts should not be forced into alignment by tightening the bolts.

Tightening Procedure

Bolts should be tightened in a balanced sequence using several passes. The procedure should define final torque or another controlled measure of gasket compression.

Important controls include:

Correct fastener lubrication assumption.

Uniform initial snugging.

Cross-pattern tightening.

Incremental torque steps.

Final rotational pass where appropriate.

Waiting period for initial gasket relaxation.

Approved retorque procedure.

Witness marking.

Final flange-gap measurement.

Documentation of inaccessible joints.

Retightening rules must be specific. Repeatedly applying full torque can over-compress the gasket or damage thin flanges.

Temperature Effects

PTFE creep increases with temperature. A joint assembled at ambient temperature may lose preload rapidly during the first heat-up.

Differential thermal expansion between metal flanges, bolts, polymer components, and the gasket further changes joint stress.

The design should evaluate normal operation, steaming, hot washing, upset temperature, and shutdown cooling. A gasket selected only from a chemical-resistance chart may not provide adequate mechanical stability.

Inspection and Acceptance

A hydrostatic or water leak test demonstrates initial tightness but does not prove long-term performance. Where practical, the joint should be tested after an agreed relaxation period or thermal cycle.

During shutdown, look for:

Gasket protrusion.

Uneven flange gaps.

Liquid staining.

Loose fasteners.

Torn gasket edges.

Polished surfaces caused by movement.

Corrosion near leak paths.

Local flange distortion.

A gasket that has undergone significant compression set should normally be replaced rather than reused.

Procurement Requirements

The gasket specification should identify PTFE type, filler or expansion structure, thickness, dimensions, maximum temperature, chemical service, required seating stress, and reuse restrictions.

The supplier should also define flange-finish requirements, bolt-loading procedure, retorque guidance, storage conditions, and traceability.