Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 19, 1994
NRC INFORMATION NOTICE 94-66:OVERSPEED OF TURBINE-DRIVEN PUMPS CAUSED BY
GOVERNOR VALVE STEM BINDING
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 recent problems regarding binding of governor
valves for turbine-driven pumps that have resulted in overspeed trips. 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
River Bend
On September 8, 1994, the River Bend Station experienced an automatic reactor
trip on a reactor water level high (Level 8) trip signal. In the course of
the recovery, the operators initiated the reactor core isolation cooling
(RCIC) system, but the RCIC pump turbine tripped on overspeed. The operators
then initiated the high pressure core spray (HPCS) system to provide
condensate makeup to the reactor vessel. The reactor vessel level was
maintained above the emergency core cooling system (ECCS) actuation setpoints,
and no ECCS initiations occurred.
The RCIC turbine tripped on overspeed due to the governor valve failing in the
open position. Attempts to manually stroke the valve were unsuccessful. The
valve bonnet was disassembled. The valve stem was found to be stuck and had
to be forcibly removed. Close inspection indicated that the apparent failure
mechanism was galvanic corrosion between the valve stem and spacers in the
packing assembly. The licensee reviewed the maintenance history of the valve
and found that the stem and spacers had been recently replaced during the
refueling outage that ended in July 1994. The valve, which is on a quarterly
test schedule, had not been tested since the outage. The licensee has
contacted the valve manufacturer, Dresser-Rand Steam Turbines, for information
on the material specifications for the valve stem and spacers. In addition,
the licensee has submitted the spacers for chemical analysis and is presently
awaiting the results.
9409160230.IN 94-66
September 19, 1994
Page 2 of 4
North Anna
On July 29, 1994, the North Anna Power Station Unit 2 turbine-driven auxiliary
feedwater pump (AFW) pump tripped on overspeed during an initial startup
following maintenance. During valve cycling by hand, the licensee noted that
the valve appeared to stick and the stem would only move approximately
one-half inch. After several more attempts, the valve was freed and traveled
an additional one-half inch. The valve was disassembled, and the licensee
noted pitting and corrosion on the valve stem. The licensee replaced the stem
and packing assembly. The post-maintenance test was successfully completed.
Surry
On January 11, 1992, the Surry Power Station Unit 1 turbine-driven AFW pump
tripped on overspeed during shutdown of the pump; it was noted that the valve
stem was sticking. On December 2, 1993, the Surry Power Station Unit 1
turbine-driven AFW pump tripped on overspeed after being started for a monthly
surveillance test. The pump was started a second time and again tripped on
overspeed. The governor valve linkage was disconnected from the turbine
governor and the governor valve stem was checked for freedom of movement. The
valve stem was found stuck in the open position. In this condition, the
turbine governor was unable to reposition the governor valve and the turbine
tripped at the overspeed trip setpoint. The governor valve bonnet, stem and
packing were replaced and the turbine-driven AFW pump operated satisfactorily.
On June 15, 1994, the Unit 1 turbine-driven AFW pump again tripped on
overspeed after being started for a monthly surveillance test. The pump was
started a second time and operated satisfactorily. Corrective maintenance was
not performed between the first and second starts. The governor valve linkage
was disconnected from the turbine governor and the governor valve stem was
checked for freedom of movement. The valve stem was difficult to move and it
was concluded that governor valve stem binding caused the turbine-driven AFW
pump to trip on overspeed during the initial start. The governor valve
bonnet, stem and packing, which were previously replaced in December 1993,
were again replaced and the turbine-driven AFW pump operated satisfactorily.
To evaluate whether a similar condition existed in Unit 2, the Unit 2
turbine-driven AFW pump governor valve linkage was disconnected from the
turbine governor and the governor valve stem was checked for freedom of
movement on June 25, 1994. It also had been checked on January 17, 1994. In
both instances governor valve stem binding was noted. The Unit 2
turbine-driven AFW pump had satisfactorily passed its monthly surveillance in
this condition. The governor valve stem and packing were replaced each time
stem binding was identified.
Based on metallurgical analysis, the licensee determined that the corrosion
products are formed due to galvanic corrosion, crevice corrosion, and pitting
corrosion. Dresser-Rand stated that the packing binder material has
approximately 1.29% sulfur and traces of other chemicals. The licensee and
the vendor are currently investigating to determine what effect the sulfur and
other chemicals have in accelerating the corrosion process.
.IN 94-66
September 19, 1994
Page 3 of 4
Arkansas Nuclear One
On May 9, 1991, an event occurred at Arkansas Nuclear One where a
turbine-driven emergency feedwater pump tripped on overspeed. The cause of
the overspeed trip was determined to be corrosion of the governor valve stem.
The buildup of corrosion products on the stem caused binding which prevented
the valve from closing as the turbine accelerated on startup. In this
instance, the valve stem had been in service for one month prior to the
failure. Discussion
Turbine driven pumps used in both RCIC and the auxiliary feedwater systems are
classified as engineered safety features which are designed for removing decay
heat and for providing a redundant means of supplying water to either the
reactor vessel or the steam generators.
The turbine-driven pumps at River Bend, North Anna, Surry, and Arkansas
Nuclear One were manufactured by Terry Steam Turbine Company. Dresser-Rand
Steam Turbines is the current vendor for the equipment. The governor valve
for the turbines is designed such that the valve stem travels through a
packing assembly made up of carbon spacers and stainless steel washers with
very tight clearances. The governor valve stem material is 410 stainless
steel that is nitrided for hardness. The packing space is composed of carbon
discs and stainless steel washers. A gland packing leakoff is located in the
outer portion of the packing/stem assembly. In one case, the licensee has
determined that there was corrosion and a buildup of mineral deposits on the
governor valve stem in the area adjacent to the packing leakoff. The deposits
were extremely hard and adherent to the stem. The corrosion and deposits
interfered with the movement of the stem through the packing causing the stem
to bind. The clearance between a new stem and packing assembly is
approximately 0.005 cm [0.002 inches]; therefore, only a small buildup of
deposits on the stem will result in interference between the stem and packing.
One licensee reported that the ambient temperature of the governor valve is
significantly warmer, which may affect the corrosion rate. (The steam supply
valve is in close proximity to the governor valve.)
In a letter dated March 24, 1993, to Surry Nuclear Power Station from
Dresser-Rand Steam Turbines, nuclear plant experience with corrosion-related
valve stem binding was discussed. The letter stated that eight plants had
reported governor valve stem binding problems. Dresser-Rand stated that
Terry model ZS and GS turbines using two and one half-inch veeport and
three-inch venturi governor valves with 410 stainless steel stems and carbon
packing rings were subject to this phenomena. The letter also stated that
the environment of the governor valve was critical and that a dry environment
for stem and packing assembly was desired when the equipment was not
operating. When unable to assure a dry environment the letter recommended
using a chrome plated stem. However, in conversation with the staff, the
Dresser-Rand Steam Turbines representative indicated that the new stems may
also be subject to binding. .IN 94-66 September 19, 1994
Page 4 of 4
The root cause for the formation of deposits and corrosion on the governor
valve stems is under review by several licensees, Dresser-Rand Steam Turbines,
and the EPRI Terry Turbine Users Group. Until a root cause is determined,
some licensees have elected to perform more frequent operational checks or
inspections of the stem and packing. At Surry Power Station, a maintenance
procedure to stroke the governor valve stem is performed weekly at Unit 2 and
every few days at Unit 1. In addition, the speed of the turbine and the
initial movement of the governor valve stem are correlated during monthly
surveillance testing. Trending has identified that turbine speed at initial
governor valve stem movement increases as the deposits on the stem are formed.
Also, the governor valve packing drain line at Surry is periodically removed
and the area of the stem inside the governor valve bonnet is inspected for
corrosion and deposit buildup with mirror/flashlight and mini-camera.
Related Generic Communications
Turbine-driven pump overspeed trip events attributed to various causes are
described in the following information notices:
NRC Information Notice 93-51, Repetitive Overspeed Tripping of Turbine-Driven
Auxiliary Feedwater Pumps.
NRC Information Notice 90-45, Overspeed of the Turbine-Driven Auxiliary
Feedwater Pumps and Overpressurizaton of the Associated Piping Systems.
NRC Information Notice 88-67, PWR Auxiliary Feedwater Pump Turbine
Overspeed Trip Failure.
Information Notice No. 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 project manager.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: M. Branch, RII G. Hornseth, NRR
(804) 357-2101 (301) 504-2756
S. Tingen, RII F. Grubelich, NRR
(804) 357-2101 (301) 504-2784
Attachment:
List of Recently Issued NRC Information Notices
