SUBJECT: Selecting the correct hard face seal material 5-2

A good mechanical seal should run leak free until the carbon/graphite seal face wears away. This is the same way we decide if we're getting good life with our automobile tires. The tires should not go flat, or the sidewalls "blow-out". The tire tread should wear at a rate that is consistent with our driving habits.

An inspection of your used seals will show that 85% or more of mechanical seals fail long before the faces wear out. The seal starts to leak and an inspection shows that there's plenty of wearable face visible. Some of these failures are caused by the wrong choice of seal face materials, so we have to be knowledgeable about those materials that are available to us. The ideal hard face material would incorporate the following features:

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 chose 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're purchasing, or you will never be able to trouble shoot a seal failure caused by a wrong material selection.

Here is some information about the common hard face materials we use in the seal business:

Reaction bonded silicon carbide

There are many manufacturers of reaction bonded silicon carbide. They include:

COMPANY
DESIGNATION

BNFL

REFEL

COORS

SC-2

NORTON

DD-630

PURE CARBON

PS 9242

Shunk and Hoechst of West Germany are also manufacturers of reaction bonded silicon carbide .

* Results vary with temperature and concentration. These chemical can leach out the silicon leaving a weakened structure that can act like a grinding wheel against the softer carbon face.

Self sintered silicon carbide (sometimes called direct sintered or pressure less sintered)

COMPANY
DESIGNATION

Carborundum

SA-80

Kyocera

SC-201

ESK

EKasicD (the standard)

ESK

Tribo 2000 (Controlled porosity)

ESK

Tribo 2000-1 (Controlled porosity + graphite)

Siliconized graphite

Tungsten Carbide

Here are some additional thoughts about hard seal faces:

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 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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