Sealing technology manual for the process industry

Twisting during installation – Large O-rings with a small cross-section are not very dimensionally stable and can be twisted when mounted. After re- moval, a circumferential track in the elastomer is visible in case of heavy strain and wear. If one puts the O-ring on a flat surface, a permanent defor- mation can be noticed as a twist. The use of an assembly grease suitable for the material and great care during the installation of O-rings with an unfa- vorable cross-section to diameter ratio is recommended. Grease swelling – EPDM swells in min- eral oil and mineral fat and becomes softer. The damage is evident in shell- like disruptions with relatively round edges. Even individual pieces can break off. It is necessary to check whether the correct assembly grease was used. Gen- erally, EPDM may only be lubricated with silicone grease. In addition, the product-specificinformation,suchasfat concentration, temperature etc., should be recorded in order to verify the possi- bility of using EPDM. Failure due to excessive friction – The elastomer “scrapes” by friction on the opposite surface, whereby particles are abraded. Depending on the type of sur- face, one can observe different damage patterns. To remedy the situation, the conditionoftheoppositesurfaceshould be thoroughly investigated and the product and purity parameters should be documented. Explosive decompression – Cracks in longitudinal directions can be found on the surface. The elastomer satu- rates itself with gas under pressure. When rapid depressurization occurs, the amount of gas cannot escape fast enough from the matrix of the elas- tomer and forms gas bubbles (similar to diver’s paralysis/aeroembolism). Gap extrusion – Provided that the de- sign is consistent, the factors which can push the O-ring into a gap and lead to a shearing-off include varying pressures, dynamic stress and media expansion. Depending on the individ- ual case, there are diverse manifesta- tions, as shown in the sample illustra- tions above. With dynamically stressed seals, one colloquially refers to “mouse nibbling” in the event of damage. Image above: Gap extrusion in static application Image below: Gap extrusion in dy- namic application Typical cases of O-ring damage APPENDIX PRODUCT PORTFOLIO MATERIALS RESISTANCE & PURITY ENGINEERING PLASTICS ELASTOMERIC MATERIALS TECHNICAL FUNDAMENTALS INTRODUCTION 11

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