SUBJECT: What is really happening between the mechanical seal faces? 9-7

The answer is, "lots of things", and it's hard to predict exactly what is going on in any particular application. As you can guess there are many theories about face lubrication, but few rules. Here are some of the more popular theories:

Regardless of what is happening between the seal faces, the rules for operating mechanical seals successfully always remains the same. Keep the two lapped flat faces together and the seal will not leak.

Now, let's define leakage:

You will note that in a previous paragraph I said the seal faces had to be flat. The term flat is often confused with the term smooth. We want the seal faces flat, not smooth. Flatness can be measured with a straight edge, or by rubbing the part on a known flat that has been coated with machinist bluing or dye. Neither of these methods is satisfactory for checking the flatness of mechanical seal faces.

Flatness must be measured by an optical flat and a monochromatic light. This equipment generates helium light bands that measure distances of 0.0000116 inches (0,3 microns). If a seal face is lapped to less than three helium light bands (slightly less than one micron) and the faces kept in physical contact, the seal should be able to pass any fugitive emission test.

If you want to understand the relative size of one micron then realize that the smallest object that can be seen with the human eye is forty (40) microns. The average coffee filter mesh is some where between ten and fifteen microns, and we know that both water and coffee will pass through this small opening easily, but solids have a tough time getting through ten microns. This implies that the lapped seal faces make the best filter you own. In other words, dirt or solids will not penetrate between lapped seal faces unless they open for some reason. Testing has shown that visible seal leakage occurs at about five helium light bands or a space that measures slightly less than two microns.

Surface finish is a measure of smoothness or polish. In the United States we commonly measure surface finish with some sort of a comparative gauge that has been polished to various degrees of gloss. The readings are made using the units RMS (root mean square) which is a number representing the square root of the mean or middle distance between the peaks and the valleys in the surface of the seal face. The metric system commonly uses CLA or center line average which is the average distance between the peaks and valleys in the face. If a seal face is too smooth there is the possibility of galling or sticking of the seal faces.

People often ask if seal faces can run dry, and the answer is yes if you're using a good grade of carbon graphite without any fillers or binders that would be injured by the additional heat caused by the friction between the faces. We all know that electric motors have run carbon brushes for years without lubrication.

Friction between the lapped seal faces will cause the softer graphite to coat or deposit on the hard face, leaving the harder carbon behind, automatically creating a non-smooth carbon surface. It is important to understand that this will only happen if moisture is present. This is the reason that children will lick the tip of a graphite pencil to make the writing darker. Dry gases, hot air, and cryogenic fluids do not have this moisture present, so a special carbon/ graphite must be used in these applications. This special grade has organics imbedded into the carbon/graphite mixture to release the softer graphite.

Experienced seal people know that excessive carbon wear is seldom a problem with mechanical seals. Most leaking seals have plenty of carbon/ graphite remaining when the seal begins to leak. The more common problem with running seal faces dry is that the friction heat will harden or destroy the elastomer or O-ring that is often located in, or close to the seal face.

Since there is a direct correlation between seal face flatness and seal face leakage it is important to know that here are a number of circumstances that can cause the seal faces to go "out of flat". Here are some of them:

Damage is a separate subject, but we know that seal faces will leak if they become damaged. Watch out for:

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