Selecting the correct hard face seal material 5-2
A good mechanical seal should run leak free until the carbon/graphite seal face wears away. This is the same way we decide if we’re getting good life with our automobile tires. The tires should not go flat, or the sidewalls “blow-out”. The tire tread should wear at a rate that is consistent with our driving habits.
An inspection of your used seals will show that 85% or more of mechanical seals fail long before the faces wear out. The seal starts to leak and an inspection shows that there’s plenty of wearable face visible. Some of these failures are caused by the wrong choice of seal face materials, so we have to be knowledgeable about those materials that are available to us. The ideal hard face material would incorporate the following features:
- Excellent corrosion resistance.
- It would be self lubricating.
- High strength in compression, shear and tension.
- High modulus of elasticity to prevent face distortion.
- Good heat conductivity.
- Good wearing characteristics (hardness).
- Low friction.
- High temperature capability.
- Temperature cycling capability.
- Easy insertion into a metal holder
- Low coefficient of friction.
- The ability to be molded in thin cross sections.
- And of course, low cost
Needless to say all of these characteristics are not available in the same face material. The idea is to get as many of them as you can in a properly chosen face combination.
With just a few exceptions, seal companies purchase hard face materials from outside vendors. Be sure the face component you chose is identified by material, type and grade so that you can check out the “physicals”. Some companies change the generic name of the material to confuse you. Make sure you know exactly what you’re purchasing, or you will never be able to trouble shoot a seal failure caused by a wrong material selection.
Here is some information about the common hard face materials we use in the seal business:
Reaction bonded silicon carbide
- Reaction bonded silicon carbide is produced by adding molten silicon to a mixture of silicon carbide and carbon. A reaction between the silicon and carbon bonds the structure while the excess silicon metal fills the majority of the pits left in the resultant material. There is almost no shrinkage during the process.
- The silicon content is about 8% to 15%. High pH chemicals such as caustic can attack this grade of silicon carbide .
- As of this writing, carbon/ graphite vs. reaction bonded silicon carbide has been demonstrated to have the best wear characteristics of all the possible face combinations.
- Reaction bonded silicon carbide is difficult to insert into a metal holder so it is usually supplied in a solid rather than a composite configuration.
There are many manufacturers of reaction bonded silicon carbide. They include:
|PURE CARBON||PS 9242|
Shunk and Hoechst of West Germany are also manufacturers of reaction bonded silicon carbide .
- Reaction bonded silicon carbide has proven to be more chip resistant than the sintered version
- Avoid the following chemicals when using reaction bonded silicon carbide :
- Sodium Hydroxide
- Potassium Hydroxide
- Nitric Acid *
- Green Sulfate Liquor *
- Calcium Hydroxide *
- Hydrofluoric Acid
- Caustics and strong acids
- Most high pH chemicals
* Results vary with temperature and concentration. These chemical can leach out the silicon leaving a weakened structure that can act like a grinding wheel against the softer carbon face.
Self sintered silicon carbide (sometimes called direct sintered or pressure less sintered)
- This material begins as a mixture of silicon carbide grains and a sintering aid which is pressed and subsequently sintered as its name implies. Unlike Reaction bonded SiC, there is no free silicon present These direct sintered materials have no metal phase and are therefore more resistant to chemical attack.
- There are two grain shapes available to the manufacturer. Alpha (Hexagonal Structure), and Beta (Cubic Structure). There does not appear to be any difference in the chemical resistance, wear or friction of these two grain shapes.
- These self sintered materials will not be attacked by most process chemicals.
- Here are a few of the bigger manufacturers:
|ESK||EKasicD (the standard)|
|ESK||Tribo 2000 (Controlled porosity)|
|ESK||Tribo 2000-1 (Controlled porosity + graphite)|
- Sintered silicon carbide is impossible to shrink into a metal holder.
- Self sintered silicon carbide carries a slight price premium compared to the reaction bonded version.
- Although the preferred seal face material, it is sometimes too brittle for some designs.
- The manufacturing process uses a permeable form of carbon graphite that is reaction sintered in silicon at elevated temperature. This forms an outer layer of silicon carbide on the graphite base.
- A resin impregnate is added to increase the density.
- Cobalt and nickel are the common binders. Each is susceptible to selective chemical attack of this metallic binder that will leave a skeletal surface structure of tungsten carbide particles.
- Galvanic corrosion can take place between a passivated stainless steel shaft, or seal face holder and the active nickel in the nickel base tungsten carbide seal face. This can be a real problem in caustic and other high pH fluids. The temperature at the seal face is higher than the temperature of the sealing fluid so the attack takes place quicker.
- The metallic binders in tungsten carbide are also subject to galvanic attack near copper, brass or bronze.
- Tungsten carbide is easy to insert into a metal holder so it is the most common material used in metal bellows and other hard face&endash;metal composite designs.
Here are some additional thoughts about hard seal faces:
- Many sales people promote two hard faces as the ideal face combination for slurry and similar services. Keep in mind that solids cannot penetrate between seal faces unless they open. Seal faces are lapped to a flatness of less than one micron (three helium light bands), and as long as they stay in contact solids are filtered out. Here are some of the main disadvantages of using two hard faces in a seal application:
- Higher cost compared to using carbon as a seal face.
- If either face is “out of flat” it is almost impossible for the faces to lap them selves back together again.
- Carbon graphite provides an additional lubricating film if you are sealing a poor or non lubricating fluid. It should be noted that many fluids fall into that category. It takes a film thickness of at least one micron at operating temperature and face load to be classified as a lubricating fluid.
- Carbon graphite can easily be inserted into a metal holder.
- In the event the equipment is run dry, carbon/ graphite is self lubricating.
- Use two hard faces in the following applications. or any place carbon is not acceptable:
- If you are sealing hot oil or almost any hot hydrocarbon. Most oils coke between the seal faces and can pull out pieces of carbon , causing fugitive emissions problems.
- If the product tends to stick the faces together.
- Some DI water applications can attack any form of carbon.
- Halogens can attack all forms of carbon. These chemicals include:
- If the product you are sealing is an oxidizer that will attack all forms of carbon, including black O-rings.
- If you are pumping a slurry and you cannot keep the two lapped faces together by flushing, suction recirculation, a large diameter stuffing box or some other method usually employed to seal a large percentage of solids.
- If nothing black is allowed in the system because of a possible color contamination of the product you are pumping.
- Plated or coated faces can “heat check” and crack due to the differential expansion of the coating and the base material.
- PV factors as a design tool are unreliable because carbon is sensitive to “Pressure” but not to “Velocity”.
- Water can cause cracking problems with both 85% and 99.5% grade ceramic. The cause is not fully understood, but hydrogen embrittlement is suspected as the culprit. Cracks have been observed after seven to eight temperature cycles.
Unfilled carbon should be your first choice for a material to run against the above mentioned hard faces. Use an unfilled carbon in all applications except an oxidizing agent, halogen, cryogenic fluid, or if color contamination is a potential problem. See another paper in this site for details about how carbon/graphite seal faces are manufactured.