SUBJECT: A sensible preventative
maintenance program for mechanical seals, cheat sheet
Eliminate as much shaft
deflection as possible:
- Use a C or D frame adapter to eliminate misalignment
- Use the pump centerline design to lessen pipe strain problems
and avoid wear ring damage.
- Specify a low L3/D4 pump shaft Below 60,
2.0 in the metric system.
- Make impeller adjustments as necessary to stop vibration
- Stabilize the shaft whenever possible. You can install a non
sparking bushing into the bottom of the stuffing box.
- Move the seal closer to the bearings.
- Be sure the face of the stuffing box is perpendicular to the
- Check to be sure the shaft is not bent.
- Try to operate as close to the best efficiency point as
possible. Sometimes it is just a matter of trimming an
- Check for excessive pipe strain.
Eliminate as much vibration as
- Dynamically balance the rotating components.
- Stop cavitation.
- Provide mechanical seal vibration damping to avoid "slip
Keep the stuffing
box temperature within the seal limits.
- Vent vertical pumps to prevent the trapping of air at the seal
- Use only hydraulically balanced seals that generate lower
- Use low friction seal face materials such as carbon against a
- Install the seal at the correct operating length.
- Use the heating or cooling jacket on the pump with a bushing
in the bottom of the box.
- Quenching is another option to provide heating or
- A dual seal with a barrier or buffer fluid can regulate the
seal face temperature.
- Be sure to set the correct seal face installation dimension
after you have made the initial impeller adjustment and
compensated for thermal expansion.
Keep the stuffing box
pressure within the seal limits.
- Discharge recirculation will raise the pressure.
- Suction recirculation will lower the pressure.
- Stage the pressure between dual seals as a last
Monitor any stuffing box environmental controls to keep them
functioning, especially when the pump is stopped. These controls
- Flushing. Be sure the pressure is at least one atmosphere
higher than the stuffing box.
- Quenching. Be sure the steam or water is not being directed
into the bearing case. Pipe the drain to a suitable location. The
vent should go to a flare or some other suitable location.
- Suction recirculation. Circulate from the seal faces not the
center of the stuffing box. Be sure to "lock in" the break down
bushing or it will move into the seal.
- Discharge recirculation. Do not aim the flow at the lapped
- Jacketed stuffing box. Use only condensate or steam to prevent
calcium build up.
- Dual seals with a barrier or buffer fluid and convection tank.
A pumping ring between the seals is always a good idea.
Make sure your
seals have anti-clogging features built into them:
- Multiple springs positioned out of the fluid.
- Be sure the elastomer (o-ring) moves to a clean location as
the seal faces wear.
- Use a Teflon or similar coating where possible to prevent
elastomer hang up and to keep solids away from the moving
- Keep the fluid solids at the seal outside diameter.
There are other desirable
- Use only hydraulically balanced seals.
- Do not isolate seal faces with a gasket that does not transmit
- Self centering is desirable.
- Use only known seal materials and never use stainless steel
springs or bellows. Use hastelloy C
- Specify stationary seal designs if possible (The springs do
- Use self aligning seal designs.
- Specify single seals that can pass fugitive emission
- Look for built in pumping rings when you specify dual
- Look for designs with built in environmental controls.
- There should be a vent in the face of cartridge seals to vent
the stuffing box in vertical applications.
Use cartridge seals to ease
- Be sure the cartridge sleeve is sealed at the inboard end or
solids will penetrate between the sleeve and the shaft making
removal very difficult.
- Stationary versions require some type of a self aligning
feature to prevent constant movement.
- Use hardened set screws to avoid slippage. The seal probably
came with soft corrosion resistant set screws. You will have to
Use back up seals to prevent an
unexpected shut down.
- Tandem is the best configuration.
- Be sure to specify two way balance to prevent the seal from
blowing open in a pressure reversal.
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