Sealing Mechanism

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Rubber-edged Composite gasket solutions are well-suited for more demanding applications with:

• Out-of-flat flanges
  
• Non-uniform loading
  
• Rough or non-machined flange-surface finishes
  
• High shear stresses
  
• Wide bolt spans

Bolt-Load Retention Stability

There are three basic types of static elastomeric seals. Fixed-compression seals, such as molded rubber, seal by generating surface pressure, but the majority of the bolt load is carried through metal stops. Full-compression seals such as composite gaskets carry the entire bolt load. Adhesion seals such as liquid sealants have no compression load, but seal by surface adhesion.

Rubber-edged composite gaskets are designed to carry the full compressive load of tensioned bolts. When the bolts are tightened, they develop a measured amount of load. The amount of load that is retained over time is called bolt-load retention (BLR). There are two important aspects of BLR: the amount of retained load, and how stable the load is over time. The amount of retained load is influenced by the gasket carrier properties, as well as joint design parameters such as initial load, bolt stiffness, temperature exposure, etc. The stability of BLR is influenced primarily by the material composition, especially at high temperatures.

The Select-a-Seal composite fiber-based carrier is specifically engineered for bolt-load retention (BLR) as well as durability at relatively high temperatures. The carrier’s high density and low organic content deliver long-term, stable BLR in a wide range of joint designs and at temperatures up to 175°C. The graph above shows BLR at 175°C for 2000 hours at several different bolt stiffnesses (expressed in million lbf per inch). As the chart demonstrates, the specific amount of BLR is a function of joint design parameters, but the flat lines show BLR stability over time at very high temperatures.

By comparison, many fixed-compression seals, such as O-rings, retain only 10 to 20% of their original surface sealing stress, and only fairly expensive rubber compounds can be exposed to 175°C for 2000 hours without experiencing thermal degradation.

Adhesion Sealing Mechanism

While REC gaskets retain a high percentage of their compression stress, they also have another unique sealing mechanism. In addition to being a full-compression seal, REC gaskets retain their ability to seal over time by forming a strong adhesive bond with the flange surfaces. The initial compression stress on the edge during bolt-up initiates the adhesion mechanism, and exposure to heat causes the adhesive force to continue increasing over time.

The following graphs show how quickly the adhesive bond of an REC gasket is formed. The chart at top-left shows a non-adhesive compression seal being first tensioned, then exposed to heat (150°C for 3 hours), and then loosened. After loosening, the air leak rate returns to its original, high value. By contrast, the REC gasket shown in the top-right graph retains perfect sealability after the bolt tension is removed.

To evaluate long-term adhesive strength, REC edge polymers were tested on both clean and oily surfaces of fixtures that can be pulled apart in a tensile testing machine to get an exact measure of the true sealing force. RTV Silicone was used as a baseline, since it is the most widely used adhesive seal. As shown in the bottom two charts, REC edge polymers develop a much stronger bond with the flange surfaces than RTV Silicone. In addition, the bond strength of REC edge polymers is almost unaffected by oil on the flange surfaces, whereas RTV silicone is unable to develop any bond whatsoever with an oily surface.