SUBJECT: Shaft displacement cheat sheet 9-2

We know that seals fail for only two reasons:

• One of the seal components becomes damaged.
• The lapped faces open.

Common sense dictates that the more the shaft deflects from the center of the stuffing box, the more likely the lapped faces are to separate. Rotating seals (the spring loaded face rotates with the shaft) are very sensitive to this type of shaft displacement or any other form of misalignment between the stationary and rotating faces.

THE PUMP IS OPERATING OFF THE BEST EFFICIENCY POINT (BEP)

• The wrong size pump was selected because safety factors were added to the computations.
• A discharge valve is throttled to decrease the excessive capacity .
• An orifice has been installed in the discharge piping to limit flow.
• Two pumps are being operated in a parallel mode with different diameter impellers.
• Two pumps are being operated in series with different width impellers.
• The suction tank level is increasing or decreasing dramatically.
• The centrifugal pump is discharging into the bottom of the tank instead of the top. The head is changing.
• The system pressure is being maintained by a head tank. The centrifugal pump is acting like an accumulator because it starts when the head tank pressure falls and stops when the pressure tank pressure is reached.
• You are using a variable speed motor, trying to maintain a flat system curve.
• The impeller diameter has been changed.
• The specific speed of the impeller is too high or too low for the application.
• The piping system has been altered:
  • There have been piping additions and deletions since the pump was originally sized.
• Extra pumps have been installed into the system.
• The piping inside diameter is reduced because of product build up.
• A globe valve has been substituted for a gate valve in the system.
• An oversized impeller was installed to satisfy a system requirement.
• The piping was damaged when a truck ran over it.

THE PUMP IS CAVITATING

• Suction vaporization. The suction temperature is too high or the suction pressure is too low.
• Vane passing syndrome. There is not enough clearance between the tip of the impeller and the pump cut water.
• The suction specific speed number is in excess of 8500 (5000 in the metric system)
• Air ingestion. The fluid is vortexing at the pump suction or air is entering the system through packing, valves above the water line, flange gaskets, etc...
• Turbulence&emdash;there is an elbow too close to the suction.
• A discharge bypass line is recirculating to the pump suction, heatring the incoming fluid.

THE PUMP IS VIBRATING

• Dynamic unbalance of the rotating assembly caused by erosion, corrosion, or damage.
• Harmonic vibration. The shaft is vibrating in harmony with something close by.
• Slip stick. The seal faces are slipping and sticking due to poor lubrication.
• Water hammer.
• The pump is hitting a critical speed.
• Bent shaft
• Bad bearings
  • Poor lubrication.
  • Contamination of the lubricant.
  • Poor quality.
  • Bad installation.
  • Over lubrication.
• The radial bearing is being retained by a snap ring..

OTHER CAUSES OF SHAFT DISPLACEMENT

• Pipe strain caused by either mechanical or thermal expansion.
• Misalignment between the pump and driver.
• Pulley driven designs.
• Start up thrust.
• Water hammer
• High L³/D⁴ number
• Thermal growth, both axial and radial.
• Impeller adjustment.
• The pump pedestal is not five times the mass of the hardware sitting on it.

THE STUFFING BOX IS CAUSING THE MISALIGNMENT PROBLEM

• The face is not machined square to the shaft.
• The stuffing box is not concentric with the shaft.
• Some bolted on stuffing boxes can slip with vibration.

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