Important informations about mechanical seals
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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 reasons:
- One of the seal components becomes inoperable from either corrosion or
physical damage.
- 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 lapped faces
- Spring loaded elastomers are a major cause of shaft damage
4. Specify seal designs that are hydraulically
balanced.
- 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 possible.
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 leak
- 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 necessary
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 or U-cups.
- Rubber
bellows designs will not frett a sleeve unless the rubber bellows does not
stick to the sleeve.
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