Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 6, 1990
Information Notice No. 90-45: OVERSPEED OF THE TURBINE-DRIVEN AUXILIARY
FEEDWATER PUMPS AND OVERPRESSURIZATION OF
THE ASSOCIATED PIPING SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to potential
problems resulting from the combined failures of the governor and the
overspeed trip mechanism (OTM) of turbine-driven auxiliary feedwater (AFW)
pumps that can result in turbine overspeed and overpressurization of the AFW
system. Because similar turbine-driven pumps are also used in the high
pressure core injection (HPCI) and reactor core isolation cooling (RCIC)
systems, they are also subject to the same failure mechanism. 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 do not
constitute NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances:
Two events involving overpressurization of the AFW system as a result of AFW
turbine overspeed are discussed in the following paragraphs. The turbine
overspeeds were caused by the combined failures of the turbine governor and
the OTM. The OTM did not function to close the steam trip valve when the
governor had failed to respond properly to turbine speed increases.
Rancho Seco Nuclear Generating Station
On January 31, 1989, the AFW turbine oversped and overpressurized both
trains of the AFW system. Both trains of the AFW system were declared
inoperable, and the plant was shutdown. The turbine overspeed occurred
while the licensee was testing a newly modified governor. The turbine speed
reached 6020 rpm in a few seconds after start and remained at that level for
three minutes. The governor failed to limit the turbine at the rated speed
of 3600 rpm and the turbine OTM also failed to trip the turbine at its
setting of 4500 rpm. An operator stopped the turbine by closing the steam
inlet valve. The AFW system pressure corresponding to the overspeed was
calculated to be 3850 psig. This pressure
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IN 90-45
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exceeded the AFW system design pressure of 1325 psig. Although only the
turbine-driven train was operating, the overpressurization also affected the
motor-driven train because of the system configuration which tied the two
trains together. The licensee's maintenance test program provided for no
specific valve lineup requirement, which could have limited the amount of
piping that was subject to overpressurization in the event of a turbine
overspeed.
The licensee determined that inadequate preventative maintenance and
inadequate periodic testing of the OTM were the root causes of the OTM
failure to function properly. As a result, the licensee included monthly
testing of the OTM and trip valve in the licensee corrective actions to
prevent recurrence. Maintenance test procedures for the AFW pump were also
modified. The discharge valve of the turbine-driven pump would be closed
before the licensee conducted a turbine surveillance test. In the case of
turbine overspeed, this closed valve would limit the quantity of piping that
is subject to overpressurization. In addition, the turbine overspeed
surveillance test would be performed with the turbine decoupled from the AFW
pump shaft to avoid any overpressurization of the AFW system in the event of
a turbine overspeed.
San Onofre Nuclear Generating Station Unit 2
On July 3, 1988, the AFW turbine oversped while personnel were performing
low-voltage testing of the steam inlet valve to the AFW turbine. Based on
the test procedures, when the licensee removed power from the inlet valve,
power was also removed from the governor valve. The governor valve failed
open and resulted in the turbine reaching overspeed conditions. The OTM did
not function properly to trip the turbine. An attempt to close the inlet
valve from the control room failed because the power had been removed. The
operator in the control room then closed the steam inlet isolation valve,
terminating the event. Control room indications showed that the
turbine-driven pump reached 5000 rpm (the overspeed trip setpoint is 3500
rpm). The licensee later calculated that the AFW system had been
pressurized to 2420 psig, which exceeded the system design pressure.
The licensee determined that the failure of the OTM to trip the turbine was
due to tappet ball damage. In addition to the replacement of the tappet
ball assembly, the licensee's corrective actions included performing annual
visual inspections of the tappet assembly and tests of the OTM every
refueling outage.
The isolation valves in the discharge interconnection line to the two motor-
driven trains were closed during the test. Therefore, the
overpressurization was limited to the discharge pipe section from the
turbine-driven pump up to the isolation valves and the motor-driven trains
were not affected. However, had the overspeed occurred during normal
operation of the AFW system, the overpressurization would have extended to
all of the AFW system piping because the isolation valve would have been in
its normally open position.
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IN 90-45
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Discussion of Safety Significance:
Failure of both the governor valve and OTM, as occurred in these events,
could result in severe damage to the turbine and pump due to overspeed.
Without prompt operator action to control the speed of this equipment, the
internal rotating elements could fail, possibly resulting in debris
missiles.
These events also reveal the vulnerability of the AFW system to
overpressurization caused by turbine-driven pump overspeed. The
vulnerability is inherent to the piping configuration and normal valve
lineup which allows for cross-connection between trains on the discharge
side of the pump. Although isolation valves are installed in the cross
connections, they are kept open during normal system operation to allow the
trains to share the pumps. With this system configuration,
overpressurization of the turbine-driven train immediately affects the other
trains and causes the entire AFW system to be vulnerable to
overpressurization. With the AFW system isolated from the steam generators,
there is no pressure relief device with sufficient capacity to limit
overpressure. Also, the pump recirculation lines do not have sufficient
capacity to discharge enough pump flow to limit the overspeed pressure. The
Woodward governor, the OTM, and the Terry turbine that experienced the
overpressure events described herein are used in the AFW systems of nearly
all pressurized water reactors.
Similar Woodward governors, OTMs, and Terry turbines are used in the HPCI
and RCIC systems of boiling water reactors. Therefore, these systems are
also vulnerable to overpressurization from the simultaneous failure of the
governor and OTM.
These events also highlight the importance of adequate maintenance and
testing of this equipment to assure the reliability of these safety systems.
Related Generic Communications:
The turbine overspeeds described herein were caused by the combined failures
of the governor and the OTM. Although the OTM is installed to provide
protection for the pump and turbine, in the event of governor failure, both
the governor and OTM may fail simultaneously due to various problems
affecting their operation and reliability. Failure of the governor and OTM
have been previously described in the following information notices:
1. NRC Information Notice 88-67, "PWR Auxiliary Feedwater Pump Turbine
Overspeed Trip Failure."
2. NRC Information Notice 88-09, "Reduced Reliability of Steam-Driven
Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type
Governors."
3. Information Notice No. 86-14, "PWR Auxiliary Feedwater Pump Turbine
Control Problems."
4. Information Notice No. 86-14, Supplement 1, "Overspeed Trips of AFW,
HPCI, and RCIC Turbines."
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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 NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Chuck Hsu, AEOD
(301) 492-4443
John B. MacKinnon, AEOD
(301) 492-9097
Charles G. Hammer, NRR
(301) 492-0791
Attachment: List of Recently Issued NRC Information Notices
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