SUBJECT: A sensible preventative maintenance program for mechanical seals, cheat sheet 9-6

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.
• Specify a low L³/D⁴ pump shaft Below 60, 2.0 in the metric system.
• Make impeller adjustments as necessary to stop vibration problems.
• 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 shaft.
• 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 impeller.
• Check for excessive pipe strain.

Eliminate as much vibration as possible:

• Dynamically balance the rotating components.
• Stop cavitation.
• Provide mechanical seal vibration damping to avoid "slip stick" problems.

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.
• 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 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 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-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 parts.
• Keep the fluid solids at the seal outside diameter.

There are other desirable seal features:

• Use only hydraulically balanced seals.
• Do not isolate seal faces with a gasket that does not transmit heat.
• 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 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 set screws to avoid slippage. The seal probably came with soft corrosion resistant set screws. 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.

For information about my CD with over 600 Seal & Pump Subjects explained, click here  

 Link to the Mc Nally home page