SUBJECT : Why do most Mechanical Seals fail 2-2

A mechanical seal can either wear out, or fail. To determine which one your seals are doing, look at the wearable face. In most instances this will be the face manufactured from some grade of carbon/ graphite.

Since the seal face is the only sacrificial part of the mechanical seal, a worn out seal is identified as one that has no carbon nose piece left at the time it started to leak. A failed seal is identified by the fact that it has substantial carbon remaining at the time it started to leak.

The above illustrations show the difference between a worn out and a new mechanical seal.

Most consumers experience seal failure rates in excess of 85%, and for the most part these seal failures are easily correctable. Seal failures fall into only two broad categories, either the seal faces opened, or one of the seal components was damaged by contact, heat or corrosion. Whenever we try to troubleshoot any mechanical seal it's wise to remember that only three things are visible to a troubleshooter:

Here are some reasons why a mechanical seal face would open:

The dynamic elastomer is not free to slide or move on the rotating shaft or sleeve.

Something is restricting the free movement of the seal.

The shaft is being displaced causing the seal to hit something as it rotates, or to cause the rotating face to run off of the stationary face.

The seal face is being distorted by either temperature or pressure.

The product is vaporizing between the seal faces causing the faces to blow apart.

An environmental control has failed. There are many types used with Mechanical Seals, here are a few of the common environmental controls:

Unbalanced seals can open their lapped faces in vacuum applications.

When a seal face opens it allows solids to penetrate between the lapped surfaces. The solids imbed themselves into the softer carbon/graphite face causing it to act like a grinding wheel. This grinding action will cause severe wear in the hard face. It should be noted that seal face opening accounts for the largest majority of mechanical seal failures.

The second major cause of seal failure is when one of the seal components is attacked by the sealing fluid or a chemical being used to clean or flush the lines. Chemical attack is easy to see:

FAHRENHEIT
COLOR OF THE METAL
CENTIGRADE
700 - 800
Straw Yellow
370 - 425
900 - 1000
Brown
480 - 540
1100 - 1200
Blue
600 -650
> 1200
Black
> 650

Here are a few things to consider when you suspect corrosion is the problem :

The corrosion rate of almost all chemicals doubles with each 18 degree Fahrenheit (10 C.) rise in temperature.

When you look for corrosion be sure to check out any cleaners or solvents that are used to flush out the system or clean the lines. Many grades of Viton® can be attacked by cleaning the lines with steam or caustic. It is important to identify all of the materials used in the seal components.

Some hard coatings have very little flexibility and will crack with a small differential temperature. Be careful of tungsten carbide with a cobalt binder; nickel binder would be a much better choice.

®DuPont Dow elastomer

For information about my CD with over 600 Seal & Pump Subjects explained, click here  

 Link to Mc Nally home page