SELECTING THE CORRECT HARD FACE MATERIAL SA004
The ideal hard face material would incorporate many features including :
- Excellent corrosion resistance.
- High strength in compression, shear and tension.
- High modulus of elasticity to prevent face distortion.
- Good heat conductivity.
- Good wearing characteristics (hardness).
- 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.
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 choose 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 are purchasing or you will never be able to trouble shoot a seal failure caused by a wrong material selection.
Takde a look at the chart labeled: “HARD FACE MATERIALS” This chart lists the physicals for some of the most common hard face materials used in the mechanical seal industry. Most of the information was supplied by the Pure Carbon Company of St. Mary’s, Pennsylvania.
Use these numbers only as a guide. Individual manufacturers use different testing methods and express the results in different metric and imperial units. I have also listed some of the hard face manufacturers so that you can contact them directly for test results, latest specifications, newer materials, etc.
There is some additional information you should know about the materials listed in the chart:
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. The following chart shows some of them.
- ESK, 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.
The above chemicals can leach the silicon out of the silicon carbide leaving a weakened, hard matrix 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 that 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 great difference in the chemical resistance, wear or friction of these two grain shapes.
- Most process chemicals will not attack these self sintered materials.
- In the following box you will find some of the bigger manufacturers of self sintered silicon carbide:
- 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 often is too brittle for some seal face 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 used to hold the tungsten particles together. 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 less difficult to insert into a metal holder so it is the most common material used in metal bellows and other hard face metal composite designs.
Here are some additional thoughts about hard seal faces:
- Many sales people promote two hard faces running against each other 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-graphite as a seal face.
- If either face is “out of flat” it is almost impossible for the faces to lap themselves 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:
- 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.
- If the product you are sealing is an oxidizer that will attack all forms of carbon, including black O-rings. Oxidizing chemicals are listed in another section of this manual.
- Halogens can attack all forms of carbon. These chemicals include:
- 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.
- Some deionized (DI) water applications can attack any form of carbon.
- Plated or coated faces can “heat check” and crack due to the differential expansion of the coating and the base material.
- PV (pressure x velocity) factors as a design tool are unreliable because carbon is sensitive to “P” but not to “V”.
- Water can cause cracking problems with both 85% and 99.5% 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 in those applications that require two hard faces and:
- Cryogenic and dry running applications require a special carbon with an embedded organic to release the graphite.
- Hot oil if the seal has to meet fugitive emission standards.