SUBJECT: Shaft displacement cheat sheet
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
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
- An orifice has been installed in the discharge piping to limit
- Two pumps are being operated in a parallel mode with different
- Two pumps are being operated in series with different width
- The suction tank level is increasing or decreasing
- 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 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
- A globe valve has been substituted for a gate valve in the
- An oversized impeller was installed to satisfy a system
- The piping was damaged when a truck ran over it.
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
- A discharge bypass line is recirculating to the pump suction,
heatring the incoming fluid.
- 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
- 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..
CAUSES OF SHAFT DISPLACEMENT
- Pipe strain caused by either mechanical or thermal
- 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.
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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