CHOOSING THE CORRECT ELASTOMER SA005
The O-ring selection chart is an attempt to select the fewest number of elastomers that will give you satisfactory sealing. As you can see from the selection, most of the chemicals can be handled by either fluorocarbon (Viton® and Fluorel are typical examples) 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® (a registered trademark of E.I. Dupont Dow) are typical examples.
- 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 (metal bellows) 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 guidelines 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 unacceptable. 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, so cooling one side of the O-ring does not always allow the lower temperature to conduct to the hot side.
- If the chemical name is followed by (*), it 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.
The chemical concentration and temperature determine the degree of carbon and elastomer attack. The higher the concentration and the higher the temperature, the more likely the attack.
Plant experience is your best protection in elastomer selection, but if you have no experience in handling these chemicals it would be wise to immersion test both the black O&endash;ring and carbon face prior to installing a mechanical seal. Sometimes you can duplicate the operating temperature by placing the test vessel in an oven or on a hot plate when practical.
- The product you are 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 will be necessary to cool down the seal area. Using an installed stuffing box heating or cooling jacket is the obvious solution. Keep in mind that quenching or the use of two seals with a cool barrier or buffer fluid between them cools only one side of the o-ring. If cooling is not possible you will have to use a metal bellows or some other type of non-elastomer seal.
- 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 “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 dual seal application.
- Many of the chemicals listed are dangerous. Be sure to use an API (American Petroleum Institute) 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 have had various ions and minerals removed and as a result they are constantly trying to replace the minerals as the water moves through the pipes and 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.
- Wastewater is liable to be any thing.
- The chloride concentration in salt water varies widely.
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 is:
- Look up the chemical in the O-ring selection chart . If your product is not on the list or is a combination of several chemical on the list, go to the next step.
- 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 the last step
- Chemraz or Kalrez® is usually the end of the line. Check the special elastomers chart. If neither of these materials is satisfactory you will have to use a non-elastomer seal such as a metal bellows design. 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.
When you are selecting an O-ring, or any other elastomer shape for your mechanical seal application remember that with the exception of solvents, most chemicals and chemical compounds can be successfully sealed with either ethylene propylene or a good grade of Viton® as the dynamic elastomer.
Most mechanical seal designs incorporate both dynamic and static elastomers.
- Dynamic O-rings are required to flex and roll with the shaft movement. This means that a very low shaft squeeze is important to prevent seal hang up or hysteresis. They must also be free to flex and roll to compensate for mechanical seal face wear.
- Static O-rings do not have to move. They are used as a gasket and are a lot more forgiving than dynamic O-rings because a small amount of swell can be tolerated that might even improve their sealing.
There are many elastomer shapes available to you; individual seal companies use wedges, V-rings, U-cups, Quad rings etc, but O-rings have a lot of advantages over these other elastomer shapes in mechanical seal design. As an example:
- They can seal both pressure and vacuum.
- They can flex 0.003 to 0.005 inches (0.08 to 0 0.13 mm) before they roll, and then they can roll up to half of their diameter, making it a lot easier for the seal faces to follow shaft run out and end play.
- O-rings reduce shaft fretting dramatically because of this ability to flex and roll.
- They are available in a variety of compounds.
- They are the first shape available when a new compound is introduced.
- Most of the O-ring compounds are available in a wide range of durometer or hardness. The average mechanical seal uses a durometer of 75 to 80 (as measured on the shore A scale), but harder durometers are available for high-pressure applications similar to those we find in pipe line sealing.
- The O-ring configuration is usually the first shape available when a new compound becomes available from the manufacturer.
- They are the most precision rubber part that you can purchase. O-rings are manufactured to a tolerance of ± 0.003 inches (0.08 mm)
- You can buy them anywhere. There are plenty of distributors.
- Unlike other shapes, most designers have settled on only a few O-ring cross sections, making spare parts and inventory a lot easier.
- Their cost is low compared to other shapes.
- Because they are self-energizing there is no need to spring-load them to the shaft or sleeve. This means that the seal spring or springs can be designed for face loading only.
- You cannot put them in backwards.
In recent years the elastomer industry has produced a variety of newer compounds that appear to be getting closer to the universal rubber that we are all seeking.
Unfortunately we are not there yet, so this article is an attempt to put these “super compounds” into a proper perspective. There are several of these compounds that you should know about.
KALREZ®, a Dupont product that is not a true elastomer so you will experience some compression set depending upon the compound you select. You have a few choices of compounds:
- Compound 4079, A “low compression set” compound (about 25% compression at 400°F) (205°C). Can be used to 600°F (316°C) Not recommended for hot water or steam applications, or in contact with certain hot aliphatic amines, ethylene oxide and propylene oxide.
- Compound 1050, Slightly harder than 4079. Can be used to 500°F (260°C) in non-oxidizing environments. Not recommended for pure water or steam at higher temperatures. This compound is scheduled to be phased out of production.
- Compound 2035, To 425°F (218°C) It is the compound recommended for Ethylene Oxide and Propylene Oxide service. It also exhibits low swell in organic and inorganic acids, esters, ketones, and aldehydes.
- Compound 1018, To 550°F (288°C). It has better hot water/ steam resistance than all other compounds except 3018. Not recommended for use in organic or inorganic acids at high temperature or for rapid temperature cycling applications.
- Compound 3018, To 600°F (315°C). It has the best hot water/steam resistance and the best high-pressure extrusion resistance. It is too hard for most mechanical seal applications at temperatures below 400°F (205°C).
The following compounds are exhibited on the special elastomers chart
CHEMRAZ is distributed by Greene, Tweed & Company, telephone (714) 875 3301. It is similar to KALREZ and can be used to 400°F (205°C). It is available in both black and white O-rings.
FLUORAZ – is another product distributed by Greene Tweed & Company, telephone (714) 875 3301. It can be used to 400°F (205°C). Field experience indicates that in operation it appears t o be very similar to AFLAS.
AFLAS is distributed through the 3M company, telephone (612) 733 5353. It can be used to 400°F (205°C)
To be classified as a true elastomer you should be able to compress the O-ring and have it return to 90% of its original shape in less than five seconds after the compression force is removed. It is this elasticity that gives the compound its memory and eliminates the need for spring loading the elastomer to the seal shaft or sleeve. If the compound does not return to 90% of its original shape in five seconds or less it is called a plastic and becomes less desirable as a dynamic seal in mechanical seal design. Many of these “super compounds” are plastics and present sealing problems in some seal configurations. You are going to have to depend upon your experience to select individual seal designs that work well with these materials.
Some distributors of these compounds recommend the use of mechanical seals with spring loaded dynamic O-rings. They do this to booster their sales of the compound. They forget to mention that when you spring load one of these compounds you will experience shaft fretting under the O-ring. This shaft fretting increases the probability of seal failure, and dictates the use of shaft sleeves that raise the L3/D4 rating of the shaft, contributing to excessive shaft deflection.
There are many charts available to help you pick the correct elastomer compound for your application. Unfortunately your fluid may not be shown on some of these charts and the temptation is to go to one of the special elastomers for the solution. At other times you will tempted to standardize on a special elastomers to avoid the selection process altogether. The next chart will help you to avoid a mistake in both of these instances.
The special elastomers chart is unique in that it shows you where these “super compounds” cannot be used. This does not imply that if the chemical is not listed, or if no notation is made, that the compound is suitable for your service.
It means nothing more than what it says; these are the chemicals that each manufacturer has designated as not suitable for a dynamic O-ring application.
- A = Aflas
- C = Chemraz
- C* = White colored Chemraz
- F = Fluoraz
- K = Kalrez®
- n = According to the manufacturer this compound is not suitable for either dynamic or static mechanical seal O-ring service. In some cases a compound was given an “n” rating when field experience proved that the published compatibility information was incorrect.
- c = Caution. May be suitable for static service, but probably not for a dynamic application. The higher the fluid operating temperature the less acceptable. You may want to check for experience in your plant or test the O-ring in your fluid to be sure.
If there is any question about the use of one of these compounds in a given service you can test the compound by immersing the O-ring in the fluid to be tested for about ten days to two weeks.
If the fluid is going to attack the compound, the O-ring it will change weight, shape, or appearance. If the application is going to be at a hot temperature, you might want to put the test container in an oven to duplicate the seal operating conditions.