Information Notice No. 92-07: Rapid Flow-Induced Erosion/Corrosion of Feedwater Piping

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                               January 9, 1992

                               OF FEEDWATER PIPING


All holders of operating licenses or construction permits for pressurized 
water reactors. 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to problems discovered at the Catawba Nuclear 
Station, Unit 2, involving the rapid flow-induced erosion/corrosion of 
feedwater piping.  The problem discovered at Catawba affects the piping 
supplying the upper feedwater nozzles and may apply to all Westinghouse 
units employing steam generator models D-4, D-5, and E, in which a portion 
of main feedwater is diverted to the upper feedwater nozzles when the unit 
is operating.  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 November 4, 1991, while performing ultrasonic testing on a section of 
4-inch feedwater system piping on Catawba's Unit 2 A steam generator, the 
licensee, Duke Power Company, detected several locations that were at or 
near minimum acceptable wall thickness.  The licensee detected similar 
results on the other three steam generators.  The licensee was performing 
this testing as part of its prescribed piping erosion/corrosion control 

The licensee removed sections of the A106B carbon steel piping for analysis 
or replacement.  This analysis revealed that the piping had undergone 
significant, symmetric, flow-induced, single phase erosion/corrosion.  In 
some areas, the 4-inch schedule 80 pipe (nominal thickness 0.337 inch) had 
been thinned to 0.185 inch in only four operating cycles.  This implies an 
erosion/corrosion rate of approximately 0.040 inch per cycle.  The licensee 
identified approximately 90 feet of affected pipe for replacement. 

It appears that the thinning is directly attributable to the design of the 
steam generators (Model D5) on Catawba Unit 2, which requires a split-flow 
feedwater delivery system.  In this design, typically 10-20 percent of the 
total feedwater flow is delivered to the upper feedwater nozzles to prevent 


                                                            IN 92-07
                                                            January 9, 1992
                                                            Page 2 of 2

flow-induced vibration damage to the steam generator feedwater preheater 
section when full feedwater flow is delivered through the main nozzles.

The original Catawba feedwater system design did not employ split-flow 
delivery at power.  As a result, the piping was not sized to accommodate the 
operating flow rates, resulting in flow velocities of 30-35 feet per second 
in sections of 4-inch piping.  This high flow rate has led to rapid 
erosion/corrosion. In the case of Catawba, Unit 2, if the subject piping in 
the feedwater preheater bypass line had ruptured, the break would not have 
been isolable and would have resulted in the steam generator fluid being 
released outside containment. 

This problem applies to Westinghouse units employing steam generator models 
D-4, D-5, and E if the flow reaches a high velocity when a portion of normal 
feedwater is diverted to the upper feedwater nozzles.  In numerous generic 
communications, the staff has previously stated that high velocity flows may 
cause rapid flow-induced erosion/corrosion in carbon steel piping. 

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.

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contacts:  William Orders, RII
                     (803) 831-2963

                     John Zeiler, RII
                     (803) 831-2963

                     Krysztof I. Parczewski, NRR 
                     (301) 504-2705

Attachment:  List of Recently Issued NRC Information Notices

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