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:
Use two hard faces in the following applications. or any place carbon is not acceptable:
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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