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
- 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
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
- 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.
Temperature range F.
Temperature range C.
Flurocarbon (Viton ®)
-15 to 400
-25 to 205
-70 to 300
-55 to 150
-20 to 450
-30 to 230
0 to 500
-20 to 260
-45 to 300
-45 to 150
-65 to 225
-55 to 105
-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
- Some processes will not allow any thing colored "black" in the
system. White colored o-rings are available for many
- 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
- 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
- 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
four step procedure for selecting the correct elastomer:
- Look up the chemical in the O-ring
compatibility guide in the CHARTS
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
- 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
- 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.
® Registered trademark of DuPont Dow Elastomers
For information about my CD
with over 600 Seal & Pump Subjects