Subject: Two hard faces disadvantages 17-10

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:
  • chlorine
  • fluorine
  • bromine
  • astintine
  • iodine
• 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 a hard face. 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.

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