The ten most important features you can specify in a mechanical seal design. S024_1
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.
- Mount the seal as close to the pump bearings as possible.
- Address the problem of cartridge mounted stationary seal designs
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.
- Control the temperature in the stuffing box
- Control the pressure in the stuffing box
- Replace the fluid in the stuffing box with a fluid more friendly to the seal.
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.