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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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.
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