SUBJECT: Preventing premature seal failures.11-5

What is the best way to prevent a premature seal failure? Find out what is causing the failure and then take the necessary steps to prevent it. If you understand this simple logic you will be pleased to learn that mechanical seals fail for only two reasons:

Please look at the following diagram. It describes a typical original equipment seal (O.E.M.) with five potential leak paths. If the carbon face were pressed into a metal holder there would be an additional leak path between the carbon and the holder.

ORIGINAL EQUIPMENT SEAL

 

1. Teflon ® dynamic seal

2. Seal between the faces

3. Stationary face seal

4. Sleeve to shaft seal

As you look at this seal remember that there is a continual axial and radial movement of the shaft due to run out, whip, wobble, vibration, operating off the best efficiency point, etc. Regardless of this movement the lapped seal faces must remain in contact.

The seal is normally set screwed to the shaft or sleeve causing the metal case to follow any and all shaft movement. The springs continuously flex because they are pushing the Teflon® wedge to the shaft as well as putting an axial load on the seal faces. This means that there is a constant relative movement between the Teflon wedge and the shaft sleeve. We see evidence of this rubbing when we observe the frett mark on the sleeve caused by the removal of the sleeve's protective oxide layer. This passivated oxide layer forms on all corrosion resistant metal parts and can easily be rubbed or polished off exposing the base active metal.

Most fluids contain a certain amount of suspended solids. If the lapped faces open, these solids will penetrate between the seal faces, imbed or stick into the softer carbon face, and cause a scoring or damaging to the hard face. This is the most common seal failure that you will experience.

In addition to suspended solids there are other problems with fluids that can cause the premature opening of the lapped faces. The product we are trying to seal could:

There are certain operating conditions that can cause the lapped faces to open:

The condition of the sleeve can cause the seal faces to open:

Maybe something went wrong during the installation process that is causing the seal faces to open:

The seal design is causing the face opening problems:

The rotating shaft is not centered in the stuffing box. A rotating part of the seal is contacting a stationary part of the stuffing box causing the seal faces to open:

The typical metal bellows seal has five leak paths also:

® E.I.Dupont

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