The emergency service pump cannot be tested in a normal pumping mode.  Tests are done on a quarterly basis pumping through a small recirculation line that pumps to a tank.  The result is that the pump is running far off the pump curve.  There are 4 pumps at the site.

The pump has erratic vibration amplitudes.  Levels do not cycle up and down with any consistency.  The other pumps have “damping” braces at the floor level in the room where the motors are located.  This is at the top of the pump “can”.  The braces on this pump were removed because the attachment to the containment vessel wall were causing cracks in the concrete wall.  Engineering calculations showed that the braces were unnecessary so they were removed.

A couple years ago the the recirculation piping was rerouted and ever since the flows in the piping of this particular pump have been over 200 feet per second velocity.  While all the pumps have similar velocities this particular pump has the highest velocities.  This is compounded by the fact that the piping has numerous elbows and few pipe hangers.

A few quick calculations showed that the pump was operating near it's “shut-off” head.  This creates extremely turbulent flow as the fluid in the pump churns and the liquid in the pump discharge column collapses on itself within the pump casing.  The turbulent flow creates a broadband energy which tends to excite natural frequencies.  Finally, there is a tendency for vertical pumps with long fluid-lubricated bearings to “whirl” (that is orbit) within their bearings at a rate of 40-50% of running speed.

A review of data collected by the site personnel and provided before arrival indicated that all the excessive vibration energy was centered around 46% of RPM.  This coincides with the whirl frequency tendency of vertical pumps.

This is how the vibration varied over a period of 1.5 minutes.