Special elastomers, Mechanical seals
The super rubber compounds 10-6
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 (moving) 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.
- 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 paper is an attempt to put these “super compounds” into a proper perspective. There are several of these compounds that you should know about.
- 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 also 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 selection 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 super compounds for the solution. At other times you will tempted to standardize on a super compound to avoid the selection process altogether. The attached chart will help you to avoid a mistake in both of these instances.
The Special Elastomers chart link at the bottom of this page 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.
- 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.
You can look to see if your chemical is listed in the charts. Just click on the appropriate box:
®DuPont Dow elastomer
- On February 13, 2018