ASUBJECT: The problems associated with original equipment mechanical seals 4-4

The next time you purchase a pump, mixer or some other type of rotating equipment and request that it be supplied with mechanical seals, you're going to be very disappointed in the performance of those seals unless you specify exactly which brand, model and materials you want.

If you fail to do this you'll be supplied with a hydraulically unbalanced mechanical seal, manufactured from unnamed grades of materials and you will find that the seal will leak long before the sacrificial seal faces are worn out. In other words, the seal will have plenty of wearable carbon face left when it starts to leak.

In addition to supplying a poor performing seal the manufacturer will attempt to put the seal into a piece of equipment that was designed for soft packing. The result will be less than idealr seal and bearing performance and premature failure of both of them.

What's wrong with these O.E.M. seals? The following paragraphs will describe a few of the problems. The seal that came in your pump was probably supplied with:

• An unknown grade of carbon/ graphite with binders and fillers that can be attacked by the product or cleaners used to flush the lines. In most cases the seal will be too porous for higher temperature applications
• An unknown grade of elastomer that has a temperature limit, chemical compatibility problems, possible shelf life, and sensitivity to steam cleaning, or some lubricants that might be put on the seal during the installation process.
• Stainless steel springs that are subject to chloride stress corrosion failure.
• Multiple springs located in the sealing medium that can easily clog when solids are present.
• A spring loaded dynamic elastomer or Teflon® piece that is not free to flex and roll on the shaft. These wedge and v-ring designs will damage the expensive shaft (this is called fretting) so the manufacturer will be forced to supply a sleeve that will weaken the shaft.
• Without a method of compensating for axial growth or impeller adjustment. This is a major problem with seals that position against a shoulder on the shaft.
• Seal designs that are sensitive to the diameter, tolerance and surface finish of the shaft or sleeve.
• A discharge recirculation line that will fill the limited space stuffing box with solids and abrasives that can interfere with the free movement of the seal, or even worse, a filter in this line that will clog up and cause overheating in the stuffing box.
• Rubber bellows designs that are very sensitive to the installation lubricant, shelf life and heat. These designs will experience massive failure as the bellows ruptures (and it will)
• Piloted glands, with drilled bolt holes that require a large inventory of glands and prevent you from using the same gland on most pumps of the same shaft size
• Single spring seal designs that are wound in one direction and therefor sensitive to the direction of shaft rotation.

Please look at the following diagram.

This diagram describes a rotating "back to back" dual seal with a whole series of problems that include:

• Spring loaded elastomers that cause fretting damage in two places on the shaft or sleeve.
• Designs in which the spring load on the seal faces increases and decreases as the shaft moves axially.
• The inner rotating face moves into the solids as the carbon wears, causing "face hang up" and premature failure, along with product dilution.
• Faces that are kept closed by barrier fluid pressure and will open if there is a surge of pressure in the system or the barrier fluid pressure is lost.
• Dirt and solid particles are centrifuged into the lapped faces of the inner seal.
• If the outside seal fails or wears out (and it is supposed to) the inner seal will blow open when the barrier fluid pressure is lost, making the design unsafe with dangerous or expensive products.
• Seals that are unable to compensate for shaft radial movement because they are located too far away from the support bearings and the hard face is too narrow causing the carbon or softer face to "run off" during radial movement of the shaft.
• No provision for environmental controls that are necessary in many applications.
• No way of venting air from the seal faces when the seal is mounted in vertical applications.

What can you do about these problems? The answer is obvious; do not use original equipment seals. Do you remember how you handled the packing that came in pumps? You threw that stuff away and used the type that worked in your applications. Tell your supplier that you want balanced O-ring seals made from proper materials and the material grades must be identified because you chose not to run your facility on "mystery materials".

To insure longer mechanical seal life do the following:

• Specify hyfraulically balanced o-ring seals.
• Require that all seal materials be identified by compound and grade.
• Use split or cartridge seals for ease of assembly.
• Connect a suction recirculation line between the front of the stuffing box and the suction side of the pump or any other low pressure point in the system.
  • CAUTION : This will lower stuffing box pressure, so do not do this if you are pumping close to the vapor point of the liquid. In this case you would connect the line between the stuffing box and the discharge side of the pump to raise stuffing box pressure.

® E.I.Dupont

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