DAMAGE OF THE ELASTOMER ST010
Compression set. The elastomer has changed shape. It started off round, but now the O-ring is almost square.
- High heat is almost always the cause unless you are dealing with Kalrez® , Chemraz, or a similar Perfluroelastomer where a certain amount of compression set is normal because these materials are not true elastomers.
Shrinking, hardening or cracking.
- High heat is the main cause of this one. First you get the compression set and then comes the hardening, cracking etc.
- The shelf life was exceeded. This is a big problem with “buna N” that has a shelf life of only twelve months.
- Cryogenics will freeze just about any elastomer.
- Chemical attack normally causes swelling, but in rare cases can harden an elastomer.
- Oxidizing liquids can attack the carbon that is used to color most elastomers black.
Torn nibbled, or extruded.
- Sliding over a rough surface.
- The O-ring is being forced out of the O-ring groove by high pressure. The elastomer will then extrude into sliding components of the seal. The solution to this extrusion problem is to go to a back up ring.
- The liquid has penetrated the elastomer, vaporized inside and blowing out pieces. This can happen with ethylene oxide.
- Halogenated fluids can penetrate the Teflon® coating on an elastomer and cause the base material to swell up, splitting the Teflon® jacket or coating. The new perfluroelastomers have made dynamic, jacketed and coated O-rings obsolete.
Swelling, changing color, weight or size. This is almost always caused by chemical attack.
- Be careful of the lubricant used to install the elastomer. A petroleum product such as grease or oil can attack ethylene propylene rubber.
- Solvents or cleaners used in the system may not be compatible with the elastomer.
- Some compounds are sensitive to steam. Most grades of Viton® are a good example of this problem.
- The elastomer is not compatible with something in the fluid you are sealing.
Torn rubber bellows.
- The bellows did not vulcanize to the shaft because you used the wrong lubricant during installation. The proper lubricant would attack the bellows and cause it to swell up so that it would stick to the shaft.
- If the shaft is too smooth the rubber bellows will not stick. The shaft finish should be no better than 40 rms.
- The shelf life of the rubber material was exceeded.
- The seal faces stuck together and the shaft spun inside the bellows.
- The pump discharge recirculation line was aimed at the rubber bellows. Solids entrained in the high velocity liquid are abrading the bellows material.