United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-11: Failure of Condensate Piping Because of Erosion/Corrosion at a Flow-Straightening Device

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               February 24, 1995

                               EROSION/CORROSION AT A FLOW-STRAIGHTENING


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to possible piping failures caused by flow
disturbances that are not accounted for in erosion/corrosion programs.  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

Description of Circumstances

On November 29, 1994, the Sequoyah Unit 1 reactor tripped from 100-percent
power.  Approximately 3 hours after the plant trip, Tennessee Valley Authority
(TVA, the licensee) observed water pouring from a 16 inch nominal size
diameter condensate line between the 1B4 and 1B3 feedwater heaters.  A
licensee investigation found a 180 degree circumferential crack in the reduced
section of a nominal 14 inch pipe.  This pipe section was part of a
Westinghouse flow-metering device that had been installed during the first
refueling cycle to test turbine performance.

The metering device consisted of three flanged sections of pipe: the first
section reduced the pipe diameter from 16 inch to 14 inch; the last section
expanded the diameter back to 16 inch; and the middle section contained a
flow-straightening device, a nozzle, and flow taps.  The flow straightener
device consisted of three 0.95 cm [0.375 inch] thick, circular plates with
drilled flow holes.  The plates were spaced about 0.3 meter [1 foot] apart and
held together by four 1.27 cm [0.5 inch] rods.  The first circular plate fit
the pipe flange face and held the fixture in place.  The other two plates fit
the machined, inside surface of the 14 inch diameter pipe section.

9502210050.                                                            IN 95-11
                                                            February 24, 1995
                                                            Page 2 of 3


TVA found that the pipe failure occurred at the interface of the edge of the
middle plate and the inner surface of the pipe wall.  The failure resulted
from bypass flow around the edge of the plate, which caused very localized
erosion along a narrow band, approximately 1.27 cm [0.5 in] wide and        
360 degrees around the pipe wall.  A 7.6 cm [3 inch] wide, 0.32 cm       
[0.125 inch] deep machined surface, 360 degrees, on the outer surface of the
pipe in the same area of the internal erosion may have contributed to the pipe
failure.  This surface had been machined to serve as a reference surface and
the inner surface was machined to ensure a snug fit of the flow straightener
inside the pipe.  At the failure area, erosion had further thinned the pipe
wall to approximately 0.127 cm [0.05 inch].

The condensate line containing the flow-metering device was in the
erosion/corrosion program and modeled with CHECMATE, but it was modeled as a
straight 16 inch pipe section without any diameter or thickness change. 
CHECMATE is a program used by a majority of licensees that predicts
erosion/corrosion rates in piping components, ranks the components in order of
damage potential, and calculates the time remaining before reaching a user
defined acceptable wall thickness.  The licensee personnel responsible for
operations and engineering were aware that the flow-metering device was
installed; however, ambiguities in drawings prompted the personnel responsible
for the erosion/corrosion program to assume that these sections had been
removed.  The pipe configuration had not been visually inspected and it had
been modelled as a straight section.

After the pipe failure, the CHECMATE model, including the condensate line with
the flow-metering devices, was re-analyzed.  The CHECMATE program did not
include a model for the flow straightener; the closest model for this device
was a straight pipe section.  The CHECMATE model would have indicated a high
rank for erosion downstream of the nozzle, which would have been modelled as
an orifice.  Therefore, knowledge that the metering device was installed still
may not have prompted an inspection of the area of piping that failed (the
area of the flow straightener).  Even if the area had been inspected, the band
of erosion was so localized that it could have been missed since only grid
intersections are inspected.

The licensee determined that the parallel condensate lines still had the
temporary metering devices installed and replaced those sections with straight
16 inch sections of pipe.  The licensee also determined that the heater drain
system had two of these temporary metering sections but decided to leave the
lines in service because the flow straighteners had been removed in an outage
and present thickness measurements indicated no unacceptable erosion.

The root cause of the failure was the bypass flow around the middle plate of
the flow straightener.  This bypass flow was not anticipated and the NRC staff
is not aware of any previous industry experience that would have demonstrated
a need to have the CHECMATE program indicate a high rank for flow
straighteners.  This example is an indication of how flow disturbances not
accounted for by modelling tools can affect the reliability of licensee
erosion/corrosion programs..                                                            IN 95-11
                                                            February 24, 1995
                                                            Page 3 of 3

Related Generic Communications

In NRC Bulletin 87-01, "Thinning of Pipe Walls in Nuclear Power Plants,"   
July 9, 1987, the staff requested licensees and applicants to inform NRC about
their programs for monitoring the wall thickness of carbon steel piping.

By NRC Generic Letter (GL) 89-08, "Erosion/Corrosion-Induced Pipe Wall
Thinning," May 2, 1989, the staff requested licensees and applicants to
implement long term erosion/corrosion monitoring programs.

The NRC also issued several information notices on erosion and corrosion.

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 Project Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  B. R. Crowley, RII
                     (404) 331-5579

                     N. Economos, RII
                     (404) 331-5580

                     K. I. Parczewski, NRR
                     (301) 415-2705

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