As shown in Figure 1, Newman envelope gaskets consist of an outer envelope of relatively stiff Teflon, and an inner rubber-like elastomeric core. The outer envelope provides the chemical resistance of Teflon while the inner cores provides a cushion to seal more evenly and securely when the gasket “flats” come in contact with the mating hardware (ferrules) and the gasket is clamped in place (Figure 2). The alignment beads are only there to keep the gasket centered in the ferrules, not to seal.
Figure 3 shows the forces exerted on an envelope gasket when it is clamped between ferrules as shown in Figure 2 and then a vacuum is applied inside the piping. The clamping forces (or pressure) exerted by the ferrules and clamp are predominately vertical (in this view), on the “flats” of the gasket, squeezing the gasket and making it thinner. The elastomeric (rubber) core is designed to take this pressure, but the Teflon layer in between also sees the same pressure. To the surprise of some, Teflon is actually a plastic. Plastics deform or change shape under pressure (“plastic deformation” or “cold-flow”). Since the alignment beads are captured in the ferrules, the only way the plastic can move is outward from the beads. Outside the bead, the Outer Diameter may get larger, but this does not affect performance. But on the inside of the bead, the Teflon begins to move inside of the ID of the pipe and may close off the flow through the pipe.
Figure 3 also shows the effects of applying a vacuum to the pipe. Vacuum applies a suction to the ID surface of the gasket. The pull of the vacuum is not just on the flats, but on the entire ID section of the gasket exposed to the process, accelerating the effect. The end-result is a sometimes, dramatic extrusion of Teflon into to process stream.
Figure 3 - Envelope Gasket Cross-Section
Other factors that increase the rate of the cold flow of the Teflon Envelope are show below:
Factors that Increase the Rate of Plastic Deformation of Teflon Clamp Gaskets:
Higher Temperatures
Higher Vacuums
Higher Torque on the Clamp
Vibration
Thermal Cycling
The results of extrusion can be seen in the pictures below in Figure 4 and 5:
Suggested Alternatives
There are other options that could be tried for vacuum service. The closest to envelope gaskets that Newman manufactures that might be adequate for your conditions are our Newman ORCATM Gaskets:
Newman ORCA™ Gaskets
As shown in Figure 6, instead of having the detached sleeve of PTFE (Teflon, center illustration) of an envelope gasket, Orca Encapsulated Gaskets have an adhered layer of PTFE (right illustration). The layer is thinner and cannot slide during use. This means that there will be less extrusion of the PTFE layer into the process stream. The customer still has the chemical resistance and purity of Teflon, but with less cold flow.
Newman Solid Elastomeric Gaskets:
Vacuum applications do not typically use plastic gaskets (like Teflon or Teflon composites), but use solid elastomeric gaskets. This is because plastic containing gaskets usually need retightening to maintain a vacuum and tend to deform. Solid elastomeric gaskets are chemically bonded throughout the gasket and are elastic, meaning that they maintain their shape and maintain a better seal during the normal cycles of processing.
Figure 6 – Different Types of PTFE Gaskets
Elastomeric gaskets are available in a variety of materials with different hardness, chemical resistance and temperature ranges. We can help you select the proper material. For example, for the conditions mentioned: 21 Deg C , 4% Sodium Hydroxide and 0.3% Acetic Acid Solution (CIP) and SIP at 121 Deg C and Vacuum of 0.85 barG, I would suggest starting by trying our 2107 EPDM or our new 2107LE (low extractable) EPDM. These are solid elastomeric gaskets with good chemical resistance and vacuum sealing.
[1] The information contained herein is offered in good faith and is believed to be accurate at the time of writing. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer’s tests to ensure that our products are safe, effective, and fully satisfactory for the intended end use. NEWMAN SANITARY GASKET CO. AND NEWMAN DIAPHRAGMS, LLC. (NSG) SPECIFICALLY DISCLAIM ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. NSG DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. TeflonTM is a trademark of Chemours Company.