How to Select Gaskets for a PTFE Exchanger in a Cyclic Vacuum-Pressure Service?

A heat exchanger gasket is tortured by a procedure that alternates between pulling a strong vacuum and pushing several bar of pressure. The seal is continuously drawn in and blown out . It acts like a diaphragm . A simple soft elastomer gasket will get pressed out of shape or will not return to shape in short order. The PTFE heat exchanger is dealing with hostile chemicals in such cyclic settings. Hence, gasket selection PTFE exchanger cyclic vacuum pressure service is a vital dependability decision. The improper gasket causes leaks, loss of vacuum integrity and unscheduled down time.

Failure Mechanisms Under Cyclic Vacuum-Pressure Loading
It is necessary to know how gaskets behave under alternate pressure regimes before choosing a solution.

in vacuum
If the process side is evacuated to a hard vacuum (usually less than 10 mbar absolute), then the ambient pressure on the outside of the flange drives the gasket inward. A soft, uncompressed gasket can be pulled into the equipment bore or the space between flanges. This inward displacement can lead to:

Loss of compressive stress on sealing faces

Permanent deformation, or cold flow, of the gasket material.

Vacuum leakage path over the gasket ID

On the Spot
For a pressurised process (e.g. 3–6 bar gauge), the internal pressure will be working to force the gasket outwards. Where the gasket is not physically restricted, the gasket can extend into the flange space between the bolt holes. The extrusion gives:

Thinning of the gasket at the OD.

Loss of seal contact .

Sudden blowout or slow leakage

Combined Cyclic Effect
The alternating vacuum and pressure cause the gasket to slide back and forth with each cycle. This repeated flexing works the gasket material, speeding up fatigue, creep and relaxation. A gasket that depends on elastic recovery of a soft material will eventually take a permanent set and lose sealability.

PTFE Exchanger Suitable Gasket Types
For a PTFE heat exchanger, chemical compatibility with the process fluid is of crucial importance. In its own right, PTFE is a criterion for global chemical resistance. Thus, any gasket material that contacts the process must be PTFE or another perfluoropolymer or totally enclosed.

PTFE Filler Spiral Wound Gaskets
Spiral coiled gaskets are a very common and robust alternative. They are formed from a V-shaped metallic winding (usually 316L stainless steel) with a softer filler material in between. For PTFE exchanger service, the filler is flexible graphite or, more often, virgin PTFE tape. The metal winding gives spring-back and structural strength and the PTFE filler provides chemical inertness.

Benefits:

Great cyclic pressure resistance

Raised face, tongue and groove flanges available in common sizes

Good resistance to vacuum (assuming that the winding is squeezed enough)

Drawbacks:

The outer and, if any, interior guide rings can be subjected to the procedure and require careful selection of materials

PTFE filler is soft and might cold flow if pushed too much

Stainless Steel Corrugated Core Teflon Envelope Gasket
For the harshest cyclic vacuum-pressure service on a PTFE exchanger, the better option is a PTFE envelope gasket with a corrugated stainless steel inner core. This type of gasket is specifically designed to combine the chemical inertness of PTFE with the mechanical spring-back of metal.

Construction: A thin, corrugated (or perforated) stainless steel core, commonly 316L, is completely enclosed in a seamless or welded PTFE envelope. The corrugations constitute a number of leaf springs, assuring uniform elastic recovery along the sealing face. The PTFE envelope, normally 0.3–0.5 mm thick, is in contact with the flange faces and the process fluid, thus providing minimal corrosion or contamination.

Why it works under cyclic vacuum-pressure The steel core provides the mechanical 'spring-back' and structural strength to resist both vacuum and pressure. In vacuum the corrugation spring force pulls the PTFE envelope outwards, preserving contact with the flange faces and preventing inward suction. The solid steel core is resistant to extrusion under pressure and equally distributes the load of the bolt. The gasket is like a bellows and always returns to the original thickness after each pressure excursion .

Installation Requirement This type of gasket is required to be put on a tongue and groove or male/female flange design [4]. These flange styles ensure positive containment of the gasket by limiting the radial clearance in which the gasket could be extruded. The raised faces are not sufficient for flat-faced flanges as the soft PTFE envelope can still be forced sideways under significant cyclic stress. A well-designed tongue-and-groove flange will encircle the gasket entirely, making a blowout almost difficult.

Considerations for Flange Design and Surface Finish
The flange itself is essential to the performance of the gasket under cyclic vacuum-pressure.

Flange type: As indicated, tongue-and-groove or male-female flanges are strongly favored. This design physically restricts the gasket so that vacuum draw inward and extrusion outward are impossible. If the current flanges are of the raised face type, refit can be achieved with a gasket with an integral inner and outer metal ring (e.g. spiral wound with inner and outer rings) giving some containment, although full tongue-and-groove is better.

Flange surface finish The PTFE envelope requires a smooth surface finish for proper sealing, typically 32–64 µin Ra (0.8–1.6 µm Ra). But the coating must also have adequate "grip" so that the PTFE envelope does not slip out under pressure. Often, a serrated finish (concentric, spiral, etc.) with a controlled roughness (for example, 125–250 µin Ra for the serrated pattern) is specified. PTFE pours into the serrations to provide a mechanical interlock that inhibits lateral movement.

Installation and Torque Control
The right installation is as vital as the choice of the gasket. PTFE envelope gaskets:

Bolt torque must be regulated and applied in a star pattern.

Over-torquing over-compresses the PTFE envelope, creating cold flow and diminishing the spring action of the core

If you don't torque enough the compressive stress will be too low to hold the seal in place under vacuum.

It is advised to re-torque after the first thermal cycle, as PTFE relaxes under the initial load.

Cyclic Vacuum-Pressure Service Gasket Options Comparison
Gasket Type Cycling ResilienceChemical Resistance Vacuum Suitability Recommended Flange
Soft elastomer Bad Restricted Bad Any PTFE envelope (no core)Poor (cold flow) Good Bad Any Spiral wound (PTFE filler)Good Good Good Flanged face or T&G PTFE envelope + corrugated 316L core Excellent Excellent Excellent Tongue and groove
Conclusion: The Seal, the most dynamic element
The key to a leak free service life is a durable, mechanically robust gasket built for the particular cyclic duty. A PTFE-encased, metal-cored gasket in a constrained tongue-and-groove flange is the best standard for a PTFE exchanger of harsh chemicals under alternating vacuum and pressure. In a static assembly the most dynamic part is the seal and must be selected with the same care as the tubes. If flange design, surface finish and installation torque are all done correctly, a gasket that survives the first hundred cycles without failure will probably go on for thousands. When in doubt the corrugated stainless steel core within a PTFE envelope provides the spring-back needed for vacuum-pressure cycling.