Original Problem:
Excessively High Radial Vibration
The pumps were originally installed with the original equipment manufacturer (OEM) standard stainless steel impeller
and inducer. Typical synchronous vibration levels for the pumps
were approximately 3-mils to 4-mils at an operating speed of
25,000-rpm. As could be expected, this was very detrimental to
the reliability of the seals.
The end-user questioned the OEM about the possibility
of a resonance problem with the high speed output shaft and
its first critical speed. Originally, the OEM had calculated that
the first critical was above the operating speed. However, reverification of the calculations showed that the operating speed
was nearly on top of the first critical, which was predicted at
24,500-cpm (see Figure 3).
Since the overhung weight of the impeller greatly affected
the critical speed, the impeller and inducer were changed to
titanium. This raised the predicted critical speed to approximately 31,500-cpm (see Figure 4). This lowered the measured
vibration in the field to approximately 1.0-mils to 1.5-mils and
greatly improved the reliability of the pumps.
Figure 3. Revised OEM unbalanced response calculations with
steel impeller/inducer.
Figure 4. OEM calculated unbalanced response with titanium
and steel impeller/inducer.
Current Problem
Prior to 2001, the service for the pumps was rarely in need
more than two to three months at a time. The pump might be
idle for several months and then restarted based on business
needs.
Since then, the business climate changed and the pump
service was required nearly 100 percent of the time. The pumps
were only shut down due to turnaround or other unit downtimes. During a nine-month period of time beginning in 2001,
there were seven failures of the three pumps. The total cost of
these failures was approximately $375,000. The most common
causes of failures were:
Gearbox Oil Leaks
There were constant problems with upper gearbox seal leaks.
The upper gearbox seal was originally a single lip seal. To reduce
the leakage, another lip seal was installed on top of the original,
but this did not reduce the leakage significantly. Even a slight
gearbox oil leak could be a problem because the gearbox case
was the lube oil reservoir, which gave it a small capacity.
High Vibration
Even though the vibration had been reduced substantially
with the change from stainless steel to titanium hardware,
synchronous vibration continued to be a problem. Vibration
levels would begin to exceed 2-mils, which at 25,000-cpm was
excessive.
After approximately six months of operation this vibration
would begin to damage the bearings and gears in the gearbox.
Since the vibration was all synchronous, it was suspected that
the increase in vibration was caused by the buildup on the
rotor, described below.
Failed Secondary Process Seal
The pump was equipped with a tandem, pusher style seal
Figure 5. Secondary seal stationary face, evidence of dry running/
cracked face.
