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 :

• The Teflon® bellows version.
• The O-ring version.

Both these designs have several features in common :

• These rotating seals are clamped, not set screwed to the shaft. This is a necessary feature on glass shafts or any shaft where the set screws would cause damage.
• Because of the clamping arrangement these seals are limited in their pressure capability. Too high a stuffing box pressure can cause them to slip on the shaft.
• The fluid is at the inside diameter of the seal. This means that if there are solids present in the fluid (slurry) the solids will be thrown into the lapped faces. It also means that as the softer face wears, the rotating face will move towards the slurry with a possibility of "hanging up" on the solids and opening the lapped faces.
• They are hydraulically balanced for fluctuating pressure, but are limited by the clamping force on the shaft.
• They are both impossible to vent in a vertical application, so you want to be aware of potential heat problems; especially at the higher speeds.
• Both versions should have a shroud install over them to protect personnel in the area in the event of a seal failure.
• These seals can be used to "back up" an inside rotating seal. When installed in this manner the resultant dual seal is refereed to as "the face to face" version.

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.

• Because of the lack of set screws in the design, you should install a clamp of some type behind the seal to prevent it from sliding in higher pressure applications.
• Exotic metals are often used with dangerous products. You really should be using dual seals made from the exotic metal and not these seals, even though their price is attractive.

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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