PTFE Envelope Gaskets
Envelope gaskets feature virgin PTFE enclosures that offer protection to insert rings, which are made of a highly compressible, liquid-resistant material. These asbestos-free gaskets include a corrugated metal ring made of stainless steel, for long life in most services (a special benefit in corrosive atmospheres). Alternative materials of construction are available, such as monel and hastelloy. Additional features like grounding tabs for antistatic use or retaining clips to assist in retaining gaskets on manways and handholes. Different design options are also available, including:
· Single Envelope Gaskets: Designed for both small and larger nozzles, the virgin PTFE slit envelope is constructed of formed tape with a smooth, butt-welded junction (especially important for high-vacuum service). This envelope is used to protect the compressible inserts from the process environment. There are a lot of different envelope gaskets available, so it is important to make sure you are purchasing one that is appropriate to use with glass-lined equipment.
· Double Envelope Gaskets: This is similar to the single-envelope, except that a second envelope is used to “enclose” the entire gasket. The inner envelope protects the process side while the outer envelope protects from any atmospheric contaminants. A virgin PTFE slit type or butt-welded type envelopes are used to fully encapsulate the gasket from both the process side and the atmospheric side of the joint. Again, it is critical to ensure you are using a design suitable for glass-lined equipment.
· U-cut Gaskets: A multi-purpose U-channel gasket is an exceptionally high quality gasket for normal service applications on all glass-lined and alloy process equipment where PTFE is compatible with the product and process operating conditions. Instead of a butt-welded envelope, this design incorporates an envelope that is machined from a solid piece of PTFE, eliminating the need for any welds in the fabrication. The U-cut design combines a unique convex geometry to allow the gasket to conform more uniformly to the compressive forces when the nozzle bolts are torqued, making it highly attractive for food and pharmaceutical applications where product build-up is not acceptable and CIP is required.
For even more information, download a copy of our Gasket Portfolio, that contains specs and part numbers for our most popular envelope gasket designs.
Expanded PTFE is created by rapidly stretching PTFE under the right conditions to create a very strong, microporous material. The resulting material, known ePTFE, exhibits the array of properties found in virgin PTFE, such as strength, chemical inertness, and high thermal resistance.
ePTFE is much softer and more flexible than regular PTFE, giving it the ability to be manufactured into tape that can be fitted and installed onto a flange with an end product similar to how a traditional gasket would function. The added benefits of ePTFE tape are ease of compression, minimal creep and cold flow, and the ability for on-site customization (which eliminates the long lead times and complicated logistics that are brought about by offsite shipping, handling and installation).
Gasket and Torque Requirements
A gasket is only as good as its compression, so proper bolt torquing is essential to provide consistent sealing. If your gasket is under-compressed it will be prone to leaks (and for users of glass-lined equipment that can often equate to leakage of a highly corrosive product!). Under compression also creates risk of entrapment areas between the gasket and flange, which are not only dangerous, but they make cleaning difficult. On the flip side, over compression produces high stresses on the gasket material, clamps, and flanges which can lead to damage or premature failure.
By controlling the torque applied to the gasket clamps, you can achieve correct compression during initial installation. The correct torque setting depends on the gasket material. Generally after the final torque is reached a re-torque should be done after the first heat cycle or 24 hrs. For more detailed procedures, instructions, and recommendations on bolting torques for our AFII gaskets, consult Section V of our Glass-Lined Equipment Maintenance Manual.
PTFE’s unique properties and unmatched versatility make it a popular choice of gasket construction in many industrial applications, particularly in chemical processes that involve aggressive media and require high-purity conditions. Unfilled PTFE, filled PTFE, and ePTFE tape are all viable choices and offer distinctive advantages based on different process conditions. Factors such as flange size, operating pressure and temperature will help to determine which type will perform the best for you.