Subject : The non metallic seal 12.9

A portion of the chemical industry uses non-metallic pumps. You'll find them manufactured from Teflon®, graphite, Carbamate, and a variety of other materials.

Common sense dictates that if the application engineer selected a non metallic pump, he must of had a good reason for doing so, and it would not be logical for you to equip it with a mechanical seal containing metal components exposed to the product, unless you had specific knowledge that doing so would make some kind of sense.

Most non-metallic seals are installed outside the stuffing box and fall into two categories :

Both these designs have several features in common :

Because these seals are designed with no metal parts in the sealing fluid, they are occasionally used in exotic metal pumps to save the cost of purchasing an exotic metal seal.

The following illustration describes the Teflon® version of a non metallic seal :

  • A glass filled Teflon® face is commonly fused into the Teflon® bellows. This means that you are somewhat limited in your choice of face materials.
  • This version slips on the shaft easier than the O-ring type.
  • Teflon's® cold flow properties also works against you when you clamp it to the shaft. Unlike an O-ring, Teflon® will relax under pressure.
  • Teflon's chemical compatibility makes this version more universal. You do not have to pick the correct material for the O-ring.

The next illustration describes the O-ring version :

  • Although the dynamic O-ring is touching the shaft, it will not frett the shaft because the sliding or flexing takes place on the O-ring outside diameter.
  • The rotating carbon face should be banded as a safety precaution because carbon is weak in tension
  • In this design the metal holder can be drilled and tapped to provide a better holding force if the seal is used in "exotic metal" applications

 

® DuPont Dow elastomer

 

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