United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-66, Supplement 1: Overspeed of Turbine-Driven Pumps Caused by Binding in Stems of Governor Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                 June 16, 1995


NRC INFORMATION NOTICE 94-66, SUPPLEMENT 1:  OVERSPEED OF TURBINE-DRIVEN PUMPS
                                             CAUSED BY BINDING IN STEMS OF
                                             GOVERNOR VALVES


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 (IN) supplement to alert addressees to a potential problem with some
licensee actions taken to prevent binding of the valve stems of turbine
governor valves and the resulting overspeed trips of the associated turbine-
driven 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

Licensees continue to experience failures of steam turbine-driven auxiliary
feedwater pumps and reactor core isolation cooling pumps to start upon actual
or test demand and continue operating.  A number of information notices have
been written on aspects of the problem.  The most recently issued information
notice, IN 94-66, "Overspeed of Turbine-Driven Pumps Caused by Governor Valve
Stem Binding," describes sticking of the governor valve stem in the control
system for the steam-driven turbine.  Since IN 94-66 was issued, additional
instances of overspeed turbine trips or stem binding have been reported at
Calvert Cliffs Unit 2, Beaver Valley Unit 2, South Texas Unit 2, and, most
recently, at Comanche Peak Unit 1.

Comanche Peak

On June 11, 1995, the Unit 1 turbine-driven auxiliary feedwater pump started
and immediately tripped on overspeed when it automatically started during a
plant transient.  Inspection of the governor valve stem revealed signs of
corrosion.  The root cause of the overspeed trip has not yet been determined,
but it appears that corrosion may have been a contributing factor.  The
liquid-nitrided 410 stainless steel (410 SS) stem had been in service for only
8 weeks.



9506150217.                                                            IN 94-66, Supp. 1
                                                            June 16, 1995
                                                            Page 2 of 4


Calvert Cliffs

The governor valve on one of the two turbine-driven auxiliary feedwater pumps
at Calvert Cliffs 2 first failed in November 1993.  The licensee could not
determine a root cause and decided to increase the test frequency.  No further
failures occurred until May 1994 when only small movement of the governor
valve stem was observed prior to binding during a surveillance test.  

Examination of the governor valve stem showed corrosion in the area where the
stem interfaces with the packing gland.  No excessive moisture was found. 
This stem was the original stem fabricated from a gas-nitrided type 410
stainless steel and had been in service for 17 years.  The stem was replaced
with one fabricated from liquid-nitrided 410 SS.

The governor valve failed again during a surveillance test in September 1994. 
Corrosion was found in the same area on the governor valve stem.  The corroded
stem was replaced with another liquid-nitrided 410 SS stem.  The licensee
began exercising the governor valve every four days following the failure in
September.  In December 1994, the exercise frequency was changed to once a
week.  In January 1995, the valve stem failed to move during weekly valve
stroke testing and required the use of a rubber mallet to free it. 
Examination showed corrosion in the same area seen in the previous instances
of binding.

Beaver Valley

On March 29, 1995, at the end of a refueling outage, the governor valve stem
for the turbine-driven auxiliary feedwater pump at Beaver Valley 2 was
replaced after only 18 months of service, when inspection found that it was
pitted due to corrosion.  The stem was fabricated from liquid-nitrided 410 SS. 
It was replaced with a similar liquid-nitrided 410 SS stem with the nitride
layer machined off.

After 25 days (April 23, 1995), the replacement valve stem became bound during
pull testing.  The plant had been in cold shutdown throughout the period. 
Upon disassembly of the valve, a buildup of corrosion products and significant
pitting of the stem was found.  The licensee decided to supplement the monthly
surveillance test with a regular pull test performed on a biweekly basis.

South Texas

The original gas-nitrided 410 stainless steel governor valve stem for the
turbine-driven auxiliary feedwater pump was replaced with a liquid-nitrided
410 SS stem during maintenance in April 1994 after several years of
satisfactory service with only minor corrosion evident.  During a surveillance
test in December 1994, the pump tripped on overspeed.  The licensee determined
that corrosion of the governor valve stem was the root cause.  The stem was
replaced with another liquid-nitrided 410 SS stem.

.                                                            IN 94-66, Supp. 1
                                                            June 16, 1995
                                                            Page 3 of 4 


In April 1995, the same pump tripped on overspeed during a test following
planned preventive maintenance unrelated to the governor valve.  The licensee
determined that corrosion of the governor valve stem, similar to the corrosion
observed in December 1994, was the root cause.

Discussion

The visible cause of valve stem binding is corrosion product buildup on the
valve stem.  The corrosion product hinders movement of the valve stem within
the surrounding packing assembly because of the small tolerances between the
stem and surrounding stainless steel washers.  However, the root cause has not
been definitively determined.  Corrosion may be initiated by a combination of
moisture, heat, trace impurities in stem packing, materials used for the valve
stem and washers, and mechanical factors.

At South Texas and Calvert Cliffs, a valve stem replacement was soon followed
by additional failures.  It appears that a change in valve stem material
processing (i.e. from gaseous to liquid nitriding) in conjunction with
conditions conducive to corrosion may lead to rapid failures.

Recently, a study performed for Calvert Cliffs reported that severe corrosion
was known to have occurred at nine plants, and that all nine had valve stems
made of 410 SS nitrided by using a liquid nitriding process.  This nitriding
process was first used in 1977, but product parts were not put into service
until the late 1980s and early 1990s.  Many of the original stems, which are
still installed, were nitrided using a gas process, which gives a deeper layer
of nitriding.

Independent studies of the effect of nitriding of stainless steel have shown
that 410 SS with liquid nitriding has better surface corrosion resistance than
410 SS with gas nitriding.  However, either type of nitriding is subject to
galvanic attack when coupled to 410 SS without nitriding.  If the layer of
nitriding is mechanically damaged, the underlying 410 SS may cause galvanic
corrosion of the nitrided layer.

As an interim solution, South Texas has replaced one stem with an Inconel 718
(Inconel) stem because of its superior corrosion resistance and has plans to
make a similar replacement on the other governor valve.  The licensee also
plans to monitor starting times and governor valve temperatures during testing
to get data on performance of the Inconel stem.  The Inconel stem will be
inspected at the end of a six-month period.

Calvert Cliffs replaced both Unit 2 governor valve stems for the turbine-
driven auxiliary feedwater pumps with Inconel stems.  The licensee plans to
periodically exercise the Unit 1 stems until the next refueling outage, when
installation of Inconel stems is planned.  Beaver Valley installed a used stem
from warehouse stock with the nitrided layer mechanically removed as an
interim measure and plans to install an Inconel stem when one becomes
available through the vendor, Dresser-Rand.  Comanche Peak replaced the
corroded stem with an Inconel stem. 

.                                                            IN 94-66, Supp. 1
                                                            June 16, 1995
                                                            Page 4 of 4


The vendor, Dresser-Rand, has been investigating a solution involving Inconel
as the stem material but has not completed qualification testing.  The NRC
continues to follow the efforts of the Terry Turbine Owner's Group, Dresser-
Rand, and licensees to resolve this issue.

Related Generic Communications

Turbine-driven pump overspeed trip events attributed to various causes are
described in the following NRC information notices:

.     IN 93-51, "Repetitive Overspeed Tripping of Turbine-Driven Auxiliary
      Feedwater Pumps"

.     IN 90-45, "Overspeed of the Turbine-Driven Auxiliary Feedwater Pumps and
      Overpressurization of the Associated Piping"

.     IN 88-67, "PWR Auxiliary Feedwater Pump Turbine Overspeed Trip Failure"

.     IN 86-14, Supplement 1, "Overspeed Trips of AFW, HPCI, and RCIC
      Turbines"

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:  James Davis, NRR
                     (301) 415-2713

                     David Skeen, NRR
                     (301) 415-1174

Attachment:  List of Recently Issued NRC Information Notices
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