PREVENTATIVE MAINTENANCE, SEALS P035
Here are some thoughts about a sensible preventive maintenance program for mechanical seals.
- Stabilize the shaft whenever possible. You can install a non-sparking bushing into the bottom or end of the stuffing box.
- Eliminate as much shaft deflection as possible:
- Use a C or D frame adapter to eliminate misalignment problems.
- Use the pump centerline design to lessen pipe strain problems and avoid wear ring damage.
- Move the seal closer to the bearings.
- Be sure the face of the stuffing box is perpendicular to the shaft.
- Provide mechanical seal vibration damping to avoid slip stick problems.
- Insure the mechanical seal was installed with the correct face load.
- Check that the mechanical seal environmental controls are hooked up properly and functioning.
Keep the stuffing box temperature within the seal limits.
- Vent vertical pumps to prevent the trapping of air at the seal faces.
- Use only hydraulically balanced seals that generate lower heat.
- Use low friction seal face materials such as carbon against a hard face.
- 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 cooling.
- 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 if you install a close fitting bushing in the end of the stuffing box.
- Suction recirculation will lower the pressure.
- Stage the pressure between dual seals as a last alternative.
Monitor any stuffing box environmental controls to keep them functioning, especially when the pump is stopped. These controls include:
- 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 logical location.
- Suction recirculation. Be sure to circulate from the seal faces and not the center of the stuffing box. Also “lock in” the break down bushing or it will slide into the seal.
- Discharge recirculation. Do not aim the flow at the lapped faces.
- 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-slurry features built into them:
- Multiple springs positioned out of the fluid.
- Be sure the elastomer 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 parts.
- Keep the fluid solids at the seal outside diameter.
There are other desirable seal features that will prevent some premature seal failures:
- Do not isolate seal faces with a gasket that does not transmit heat.
- Self-centering of the seal faces is desirable. Most cartridge seals have that feature.
- Use only known seal materials and never use stainless steel springs or bellows.
- Specify stationary seal designs if possible (The springs do not rotate).
- Use self-aligning seal designs.
- Specify single seals that can pass fugitive emission standards.
- Look for built in pumping rings when you specify dual seals.
- 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 installation
- 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 setscrews to avoid slippage. The seal probably came with soft corrosion resistant setscrews. You will have to change them.
- 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.