Gaskets, degradation

In applying the definition of rehabihty, the concept of adequate performance must be estabhshed clearly. Products usually do not fail suddenly, but degrade over time. Gasket leaks on equipment, for example, may start as a slow weep and increase in volume over time. The point at which this undesirable occurrence is called a failure must be clear before rehabihty can be measured objectively. Changing the failure definition for a product changes its rehabihty level, although the product itself has not changed. 

Certain elastomeric materials used as gaskets, seals, and hoses can be degraded by aromatic fuel components. Elastomers such as natural rubber, neoprene, Buna-N, and ethylene-propylene will all swell in the presence of aromatic fuel components. 

D. Little, Inc., report for Contract 210-76-0130 to the National Institute for Occupational Safety and Health(11) and are shown in Fig. 1. The cell was constructed of Pyrex 1720 glass with stainless steel membrane holders and stainless steel top covers. Gaskets for the cell were constructed from Teflon (trademark DuPont) with the test material placed between the gaskets and with the bolts tightened to a torque of 5 2.5 pounds. This torque was chosen because values less than 2.5 in pounds did not seal the protective material firmly,and torques greater than 7.5 in pounds would put such pressure on the material that it would tear or degrade rapidly at the points of contact. 

Rapid seal and gasket material degradation in the presence of high pH (basic) environments such as inhibitors, carbonates (completion fluids). 

Higher alcohol content may cause the degradation of rubber gaskets and should be limited to under 1%. The acidity number reflects the presence of free fatty acids, which can contribute to accelerated engine aging. For the same reasons the water content should be kept quite low, at max. 300ppm. Finally, in order to ensure proper storage the oxidation stability of the fuel should be at least 5h at 110°C. 

Processing PCTFE is by the conventional methods (extrusion, injection, etc.) processing temperatures are high, and any degradation of PCTFE can cause severe corrosion and environmental problems. Applications include wire and cable insulation, electronic flexible printed circuits, packaging material (transparent film and sheet), pharmaceutical industry in particular strip and blister packs for tablets and capsules, etc. In the chemical industry, used as gaskets, 0-rings, valve seats, chemical tank liners, etc. 

There are a number of grades of fluorocarbon rubber (copolymers, terpolymers and tetrapolymers) and they are mainly used in applications where the temperatures would degrade ethylene-propylene rubber products. They are able to withstand prolonged use at temperatures up to 200 °C. Typical conditions are high temperature (<150 °C) gaskets under flow or static conditions, in contact with aqueous or fatty foods (including oils). 

Materials incompatibility is one of the most frequent causes of process incidents. Degradation of seals and gaskets that have become softened by solvent effects can lead to minor leaks or major loss of containment, and hence to fires, explosions, or more serious accidents. If seal or gasket leakage is identified in a process, then the plant engineer should consult with the manufacturer to confirm the material is suitable for the service. If necessary, all the seals or gaskets of that material should be replaced with something more resilient to the process conditions. 

Leaks from degraded equipment, including damaged seals, gaskets, and packings and corroded or eroded vessels and pipes ... 

Materials Compatibility Einally, any lubricant is required to be compatible with non-metallic components used in the engine, such as plastics, resins and elastomers. In particular, polymeric materials used in seals and plastics need to retain their integrity when in contact with the lubricant. ACEA and most OEMs have material compatibility tests to ensure that the lubricant will not cause undue degradation in key physical parameters of the polymer. These parameters include tensile strength, hardness, volume and crack formation. Any such loss of polymer integrity could be manifest as oil seal leaks or in more extreme cases as a blown gasket. Current engine test examples for American, European and selected OEM specifications are shown in Table 9.5. 

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