SUBJECT : Selecting the correct elastomer (O- ring) for your mechanical seal application 4-9

This paper is to be used with the O-ring selection guide you'll find in the chart and data section of this web site. The guide is an attempt to select the fewest number of elastomers that will give you satisfactory sealing of most of the chemicals we find in the process industry. As you can see from the selection, most of the chemicals can be handled by either fluorocarbon (Viton/ Fluorel) or Ethylene Propylene.

The following paragraphs describe the codes used in the chart.

• V - Fluorocarbon. The compound specified is the specific one that has some water immersion capability. Dupont E60 Viton ®, 3M Fluorel 2174, Parker 747-75 and Parker V884-85 are typical examples .
• E - Ethylene propylene
• C - Perfluoroelastomers. Chemraz (a registered trademark of Greene, Tweed & Co.) or Kalrez ® are typical examples. These are very expensive compounds.
• N - Neoprene
• B - Buna N
• Bu- Butyl
• U - Unknown, or unreliable test data. Immersion testing or plant experience is your best bet. If no elastomer proves to be acceptable a non-elastomer seal may be your only answer.
  

Keep in mind that this o-ring selection chart is only a guide to help you in selecting the correct elastomer for your mechanical seal application. It was created from published information, various industry guide lines and many years of practical experience by field sales and engineering people. Most mechanical seals use at least one dynamic elastomer, so even small amounts of swelling or chemical attack is almost always un-acceptable. When using this chart please keep the following in mind:

• Chemical attack will usually double with a 10°C (18°F) increase in temperature. If the elastomer is located close to the seal face it will see the additional heat that is being generated by rubbing friction. Elastomers are poor conductors of heat. Cooling one side of the O-ring does not always allow the coolant to conduct to the hot side.
• If the chemical name is followed by (*), the chemical is called an oxidizer. Oxidizers spontaneously emit oxygen at either room temperature or under slight heating. The oxygen can then combine with the carbon in mechanical seal faces or the carbon black used to color O-rings, causing chemical attack. The largest group of oxidizing materials is comprised of peroxides. Hydrogen peroxide and benzoyl peroxide are typical. Permanganates, chlorates and some nitrates are also strong oxidizing agents. These materials additionally constitute a dangerous fire hazard so two seals may be required.
• Chemical companies routinely develope new compounds. Check with your seal suplier to learn if any new elastomers have recently come on the market

The degree of carbon and elastomer attack is determined by the chemical concentration and temperature. The higher the concentration and the higher the temperature, the more likely the attack.

Plant experience is your best protection, but if you have no experience in handling these chemicals it would be wise to immersion test both the black o-ring and carbon face prior to installing a mechanical seal. You could duplicate the temperature by placing the test vessel in an oven, or on a hot plate when practical.

• The product you're sealing is often a mixture of several chemicals and/ or may have a trade name. This chart normally shows only individual chemicals so you may have to rely upon plant experience, or immersion test to determine compatibility. Most plants have prior experience in handling their chemicals, so look for elastomers in other mechanical seals, valves, gages, filters, strainers, hoses, lined pipe, etc.
• In most cases Chemraz or Kalrez ® will handle the job if there is no plant experience, or if immersion testing is not practical. It is always worth a try.
• Remember that each of these elastomers has an upper and lower temperature limit. Although the elastomer may be chemically compatible with the sealing fluid it could still fail if the temperature limit is exceeded.
• Excessive temperature is usually indicated by a change in weight, shape or appearance of the o-ring. Compression set is often the first indication of high heat followed by a shrinking and hardening of the elastomer. If the stuffing box temperature is too high it's necessary to cool down the seal area. Using an installed pump jacket is the obvious solution. Keep in mind that quenching or the use of two seals with a cool barrier fluid between them, cools only one side of the o-ring. If cooling is not possible you'll have to use a non-elastomer seal.

Elastomer	Temperature range F.	Temperature range C.
Flurocarbon (Viton ®)	-15 to 400	-25 to 205
Ethyle propylene	-70 to 300	-55 to 150
Chemraz	-20 to 450	-30 to 230
Kalrez ®	0 to 500	-20 to 260
Neoprene	-45 to 300	-45 to 150
Buna N	-65 to 225	-55 to 105
Buna S	-75 to 250	-60 to 120

• Solvents, cleaners and steam are often used to flush lines and systems. Be sure the elastomer you choose is chemically and temperature compatible with these solvents, cleaners and steam.
• Some processes will not allow any thing colored "black" in the system. White colored o-rings are available for many compounds.
• Ethylene propylene rubber (EPR) is a very common elastomer mentioned in this chart. Be aware that EPR. is easily attacked by any petroleum product, so be careful with the type of lubricant you use to lubricate this elastomer. For all practical purposes silicone grease is probably your safest lubricant but to be sure check for compatibility. There is a high temperature version of this compound available (500°F or 260°C), but it cannot be used if air or oxygen is present on one side of the O-ring. In other words, the application is limited to the dynamic elastomer on the inboard side of a double seal application.
• Many of the chemicals listed are dangerous. Be sure to use an A.P.I. gland or better still, two mechanical seals in these applications.
• Nuclear, food products, and pharmaceutical often specify specific grades of elastomers and require cure date information for certain products. If you are working in any of these areas check for a list of approved materials.
• The term water does not describe a single product. For instance:
  • De-ionized and demineralized water has had various ions and minerals removed and as a result is constantly trying to replace them as the water moves through the pipes and around other hardware. The result is that sometimes the water can attack stainless steel and some seal face materials including carbon. You may have to do some immersion testing to be sure if your choices are satisfactory.
  • Water treatment varies with each application. These treatment chemicals and additives can attack some elastomers
  • Condensate often contains dissolved amines that could attack the elastomer.
  • Water hardness varies with geographic locations.
  • Waste water is liable to be any thing.

Ethylene Propylene Rubber (EPR) is the first choice in most water and water based applications, but the variance noted above can cause premature O-ring failure. If you have any doubt about your water, conduct an o-ring immersion test prior to installing the mechanical seal.

The four step procedure for selecting the correct elastomer:

• Look up the chemical in the O-ring compatibility guide in the CHARTS AND GRAPHS section of my home page. If your product is not on the list, or is a combination of several chemical on the list, go to step "2".
• Look around the plant for present or past experience. Look for elastomers in valves, other seals, gages, filters, strainers, etc. If you have no experience with elastomers in this fluid, go to step "3".
• Test is the next step. If possible, start with two elastomers of the same compound and immerse only one of them in the fluid and leave it there for one to two weeks. You can then compare that o-ring to the one that was not immersed. If the elastomer is not compatible with the fluid it will change weight, shape, or appearance. If the elastomer does not pass this test go to step "4".
• Chemraz or Kalrez ® is usually the end of the line. If neither is satisfactory you'll have to use a non-elastomer (metal bellows) seal. If a reliable flush is available the elastomer may be compatible with the flush, but remember that if you lose the flushing fluid, the product will attack the elastomer.

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