United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-45: Potential Common-Mode Failure Mechanism for Large Vertical Pumps

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                 June 17, 1994


NRC INFORMATION NOTICE 94-45:  POTENTIAL COMMON-MODE FAILURE MECHANISM FOR 
                               LARGE VERTICAL PUMPS


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential common-mode failure mechanism for
large vertical pumps.  It is expected that recipients will review the
information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems.  However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.

Description of Circumstances

On March 3, 1994, while performing vibration tests on standby service water
pump "B," Grand Gulf personnel noted that vibration levels at all monitored
points were higher than the reference values.  Additional testing exhibited a
rapidly degrading trend in pump vibration performance.  The pump was declared
inoperable and disassembled for inspection.

The pump is a two-stage vertical line shaft pump with the pump impellers
coupled to the pump motor via a segmented vertical line shaft, approximately
18 meters [60 feet] long (see Figure 1).  Six couplings (see Figure 2) are
used to connect the shaft segments.  The pumps are located in large cooling
towers that contain water which is chemically treated to reduce biofouling. 
The pumps are run weekly for 24 hours to permit addition and circulation of
the chemicals.

Disassembly showed extensive corrosion of the carbon steel bolts and
lockwashers used in the pump shaft coupling assemblies.  Some lockwashers were
missing and were assumed to have corroded away completely.  The corrosion
allowed each retainer to move away from its contact surface on the sleeve. 
This allowed the gaps between the pump shaft sections to increase.  This
process increased the effective shaft length which reduced the clearance
between the pump impellers and the bottom of the bowls.




9406130197.                                                            IN 94-45
                                                            June 17, 1994
                                                            Page 2 of 3


Inspection of pump "B" revealed that the impellers had milled into the pump
bowls, extensively damaging the impellers and the bowls.  The licensee
estimated that more than 2.54 cm [1 inch] of material had been worn away from
the impellers and bowls.  Subsequent disassembly and inspection of the "A"
standby service water pump also showed corrosion of the coupling fasteners,
but there was only minor damage to the impellers and bowls.  

The standby service water pumps are tested in accordance with the licensee's
inservice testing program.  Neither pump indicated any degradation in
developed head, flow, or vibration during quarterly inservice testing
performed previously.  Vibration measurements are taken near the motor bearing
housing because the pump bearings are submerged.  Vibration measurements taken
at this location may not identify this type of failure mechanism until
significant damage has occurred.  In addition, the motor did not indicate any
increase in running current during testing.

Discussion

This style of coupling is used by several manufacturers of vertical line shaft
pumps.  The event at Grand Gulf indicates that current testing of vertical
line shaft pump hydraulic and mechanical performance may not identify, before
damage occurs, interference between the pump impellers and bowls caused by a
change in shaft length.  Current manufacturer's recommendations do not require
"as-found" measurements of pump lift to be taken and evaluated whenever the
pump is uncoupled from the motor.  The licensee of the Grand Gulf Station has
now added this inspection to its inservice testing program.  In addition,
after the pumps were rebuilt subsequent to this event, measurements of 
as-found total float between the impellers and the bowls were taken by the
licensee.  The measurements were taken by lifting the shaft until the upper
part of the impeller contacted the bowl, then measuring the change when the
impeller was lowered and allowed to rest on the bottom of the bowl.  This
measurement was taken after the pump was installed, and is to be used to
evaluate measurements taken during the service life of the pump.

Dissimilar materials used in the coupling assembly may have contributed to
this event.  The retainer bolts and lockwashers were made of carbon steel; the
pump shafts and remaining coupling components were made of stainless steel.

.                                                            IN 94-45
                                                            June 17, 1994
                                                            Page 3 of 3


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                    /S/'D BY BKGRIMES


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Rudolph H. Bernhard, RII
                     (601) 437-4620

                     Joseph Colaccino, NRR
                     (301) 504-2753
      
Attachments: (see file IN94045.WP1 for figures)
1.  Figure 1, Grand Gulf Standby Service Water Pump 
2.  Figure 2, Standby Service Water Pump Coupling

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