Subject: The ten most important features you can specify in a
mechanical seal design.
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A mechanical seal should operate leak free until the wearable face
(normally carbon) is worn smooth. Ninety percent of mechanical seals
leak prematurely. Surprisingly the seals fail for only two
- One of the seal components becomes inoperable from either
corrosion or physical
- The lapped seal faces open.
If these are the only reasons why seals fail prematurely, and they
are, then any sensible mechanical seal design would address these two
problems. Here are the ten most important features you can specify in
any mechanical seal design
1. Deal only with known seal materials. Be sure these materials
are chemically compatible with what you are sealing and any cleaners
or solvents that might be flushed through the lines.
- Metal parts including the
seal barrel, cartridge sleeve, springs, set screws, etc.
- Choose a low friction face combination to reduce unwanted heat
at the lapped faces. Carbon
vs.the hard face silicone
carbide is probably your best choice
- Select rubber parts that
include O-rings, and gaskets.
2. Choose seal designs with built in slurry
or anti-clogging features:
- Keep the seal springs out of the fluid. This will make the
springs less likely to clog up with solid materials.
- Springs are subject to chloride
stress corrosion problems.
- Be sure any dynamic elastomer moves to a clean surface. Do not
let the elastomer move into any potential solids that could
restrict its' movement and open the seal faces.
- Choose designs that keep the sealing fluid at the outside
diameter of the seal face. Centrifugal force will then work with
you to throw solids away from the lapped seal faces.
3. Avoid spring loaded elastomers or rubber parts
- Spring-loaded elastomers cannot flex or roll. They have to
slide as they move axially and frequently "hang up" and open the
- Spring loaded elastomers are a major cause of shaft
4. Specify seal designs that are hydraulically
- Hydraulically balanced seal faces generate less heat than the
unbalanced version used by original equipment manufacturers
5. Specify stationary seal
designs where the seal springs do not rotate with the shaft
- Stationary seal designs are not as sensitive as rotating seal
designs to pump misalignment and pipe strain problems.
6. Cartridge designs simplify the installation process and make
the important impeller adjustment of semi-open impeller pumps
7. Be sure the lapped seal faces say flat. If they are not flat to
within three helium light bands the lapped seal faces will probably
- Avoid "shrunk in" carbon faces. Monolithic or "pressed in
carbon" designs are better.
8. Use the correct environmental
control to stop the product you are sealing from changing state
and opening the lapped seal faces.
9. Use an API type gland if you have the option
- The disaster bushing will protect the seal and pump if you
experience a catastrophic bearing failure.
- The API type gland can be used for quenching if it is
10. Stay with non-fretting designs
- Most metal bellows seals
fall into this non-fretting category
- Stationary seals seldom frett shafts and sleeves
- Flexible O-ring designs are better than Teflon wedges, V-rings
- Rubber bellows designs
will not frett a sleeve unless the rubber bellows does not stick
to the sleeve.
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