Type B PTFE Envelope Gasket

Type B PTFE envelope gasket is machined from PTFE block into a square cross-section, fully encapsulating the filler core with no slit. Better sealing integrity than Type A. Lower cost than solid PTFE.

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What Is Type B PTFE Envelope Gasket?

A Type B PTFE envelope gasket consists of a PTFE outer jacket machined from solid PTFE block or sheet, fully encapsulating a soft filler core. Unlike Type A (which has a slit cut from the outer edge), Type B is machined with a square cross-section and has no slit — the filler core is completely sealed within the PTFE envelope.

Structure:

PTFE Jacket: Machined from PTFE block or sheet into a square cross-section, fully encapsulating the filler core with no exposed slit

Filler Core: Non-metallic material such as non-asbestos rubber sheet, compressed fiber, or graphite composite sheet

Type B Construction: Full envelope with machined square cross-section, no slit, no exposed filler, better sealing integrity

Key characteristics:

No slit: The filler core is completely sealed within the PTFE jacket, no exposed weak point

Better sealing integrity: The machined construction provides superior sealing performance compared to Type A

Inner diameter matching: The inner diameter can be machined to match the flange bore, preventing flow turbulence and reducing erosion

Comparison & Selection Guide

Type A vs Type B vs Type C vs Solid PTFE

Quick Selection Guide

Primary Applications

Technical Specifications

Type B PTFE Envelope Gasket Properties

Available Filler Options

Case Study 

At a large petrochemical plant in Shanghai, the outlet pipeline valve of an ethylene cracker gas compressor (Class 300) operated under H₂S-containing service conditions with exposure to corrosive media, elevated temperatures, pressure fluctuations, and frequent start-stop cycles. The original spiral wound gaskets gradually lost sealing performance during long-term operation due to corrosion of the winding materials and filler components, creating potential leakage risks and increasing maintenance requirements.

To address these challenges, the plant replaced the existing gaskets with Type B PTFE-encapsulated gaskets. Featuring a fully PTFE-jacketed flexible filler core, this design provides excellent resistance to H₂S, acids, alkalis, and other aggressive chemicals while preventing direct contact between the process medium and the internal filler material. The compressible core also delivers improved recovery performance, allowing the gasket to compensate for flange irregularities and pressure fluctuations during operation.

After installation, the Type B PTFE-encapsulated gaskets achieved stable sealing performance in the ethylene cracker gas compressor outlet valve application. Throughout the 4-year turnaround cycle, the system maintained zero leakage, demonstrating the long-term reliability of PTFE-encapsulated gasket technology in demanding petrochemical sealing applications while reducing maintenance frequency and improving operational safety.

FAQ

Q: What is a Type B PTFE envelope gasket, and when should I use it?

A: A Type B PTFE envelope gasket is a machined PTFE jacket with a square cross-section, fully encapsulating a soft filler core with no slit. The full envelope provides superior sealing integrity compared to Type A (slit type), no exposed filler core, no potential weak point. The inner diameter can be machined to match the flange bore, preventing flow turbulence.

Use Type B when: You need better sealing reliability than Type A for medium-to-high pressure corrosive service (up to 5.0 MPa) but don't want to pay for solid PTFE.

When NOT to use Type B: Low-pressure, cost-sensitive applications where Type A is sufficient. For high-pressure or large diameter service, consider Type C. For ultimate chemical purity and no permeation risk, consider solid PTFE.

Q: Type B vs. solid PTFE gasket, which one should I choose?

A: Type B (envelope) offers chemical resistance from the PTFE jacket with compressibility from the filler core, at a lower cost than solid PTFE.

However, the filler core can be attacked if media permeates through the PTFE jacket over time. Solid PTFE gasket offers maximum chemical resistance and purity, no filler core, no permeation risk.

But it costs significantly more and is less resilient (no filler core to provide spring-back).

Quick rule:

Medium pressure, cost-conscious, corrosive service → Type B.

High pressure, ultrapure, critical service, no permeation risk acceptable → Solid PTFE.

Q: What are the limitations and what are the most common mistakes?

A: Temperature: Up to +200°C.

Pressure: Up to 5.0 MPa (50 bar).

Chemical: pH 0-14 universal resistance, NOT compatible with molten alkali metals or elemental fluorine at high temperatures. PTFE can be permeated by some chemicals (nitric acid, chlorine, ethylene oxide) over time.

Common mistakes:

(1) Using Type B in applications where media permeates PTFE: some chemicals can permeate through PTFE over time and attack the filler core, replace gaskets regularly or use solid PTFE gaskets. 

(2) Over-tightening: excessive compression can damage the PTFE jacket, use controlled torque. 

(3) Wrong size selection: A and B types are recommended for sizes up to DN500, for larger diameters, Type C is preferred.