Information Notice No. 86-14: PWR Auxiliary Feedwater Pump Turbine Control Problems
SSINS No: 6835
IN 86-14
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
March 10, 1986
Information Notice No. NO 86-14: PWR AUXILIARY FEEDWATER PUMP TURBINE
CONTROL PROBLEMS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This information is provided to alert recipients of potential problems of
overspeed trips of turbine-driven auxiliary feedwater (AFW) pumps which
contribute to systems unavailability Similar overspeed trips are employed
on turbine-driven high pressure injection pumps and reactor core isolation
cooling pumps
It is expected that recipients will review the information in this notice
for applicability to their facilities and consider actions, if appropriate,
to preclude similar problems occurring at their facilities However,
suggestions contained in this information notice do not constitute NRC
requirements; therefore, no specific action or written response is required
Description of Circumstances:
There have been four relatively recent events during which the
turbine-driven AFW pumps tripped as a result of overspeed to a "lockout"
condition that required manual reset at the turbine to return the component
to an operable status In two events, the cause was the presence of oil
pressure in the turbine governor before receipt of an initiating signal
These events occurred at Turkey Point and Crystal River In the other two
events, the overspeed trip was attributed to condensate in the feedwater
turbine steam lines These events occurred at Davis-Besse and Palo Verde
The events cited are examples of system malfunctions where individual
components responded as designed However, the system was unable to perform
its intended function because overall system dynamic problems were not fully
considered
At Turkey Point all three AFW turbines responded properly when called on
after a reactor trip When later required to restart, two of the AFW
turbines tripped to lockout on overspeed and the third turbine tripped (from
a different device) on overspeed
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IN 86-14
March 10, 1986
Page 2 of 4
At Crystal River, the AFW turbine tripped on overspeed during performance of
a routine operability test
At Davis-Besse one of two turbine-driven main feedwater pumps tripped while
the unit was operating at 90 percent power Steam to the second main
feedwater pump turbine was lost when the main steam isolation valves
spuriously closed After the resultant reactor trip, the turbine-driven AFW
pumps started on demand, but tripped on overspeed Various other problems
developed, which are discussed in Information Notice No. 85-50, IE Bulletin
85-03, and in greater detail in NUREG-1154
At Palo Verde, the AFW turbine tripped on overspeed during a startup test
In each of these instances an operator was required to go to the turbines
and manually reset the equipment for restart These events were similar in
consequences, but had different underlying causes
Discussion:
Historically, PWRs have been required to have redundant AFW systems to
remove residual reactor decay heat after scram Normally, at least one of
the systems has been powered by other than an electric motor, to address the
contingency of loss of all ac power Nearly all current PWR licensees have a
small steam turbine drive, most of which are Terry Corporation single-stage
noncondensing turbines with Woodward governors Woodward governors used with
Terry turbines do not have an internal overspeed trip device The turbine
has a mechanical overspeed trip that will trip the "trip-and-throttle" valve
at 150 percent of nominal speed Terry Corporation recommends visual
post-trip examination of the equipment, following mechanical overspeed trip
and has the trip linkage arranged to require local manual reset at the
turbine Many turbines are equipped with additional speed sensing devices
that can be used to trip the turbine at some lower overspeed, eg, 125
percent
Woodward mechanical-hydraulic governors sense the speed of the turbine
through gearing that rotates the mechanical fly weights and provides power
to supply hydraulic oil to the governor The governor acceleration control
feature is set to function properly on a start signal when the turbine
starts from a dead stop with no initial oil pressure in the governor control
system
Control oil pressure does not decay immediately when a governor is shut
down Depending on internal clearances, it may take as long as 30 minutes to
fully decay Some facilities utilize power-operated auxiliary devices to
provide for remote control of governor speed setting and to "dump" residual
oil from the hydraulic system However, the most commonly used device seems
to be a speed setting knob that must be adjusted manually at the governor
This requires that the governor speed be turned down to a minimum to dump
the excess oil and then reset to the desired speed This action introduces
two possibilities for human error: the operator may inadvertently not fully
dump the hydraulic system or may not correctly reset the speed control
IN 86-14
March 10, 1986
Page 3 of 4
Details of Recent Events:
At Turkey Point all three pumps started properly on the first demand During
recovery from the event, the pumps were no longer needed and were shut down
The turbine governors were reset per procedure, but as discussed above,
perhaps not properly reset During the continuing course of the plant
transient, the AFW systems received another auto-start signal Two of the
turbines experienced overspeed and tripped to lockout The third
turbine-driven pump had experienced problems with a feedwater discharge flow
control valve, and the steam supply valves had been placed in an unusual
configuration to facilitate troubleshooting on the water flow control valve
The turbine started automatically, but somewhat more slowly than normal It
experienced overspeed, as did the others, but in this case the acceleration
rate was slow enough that the electric overspeed device activated and
tripped the "trip-and-throttle" valve Immediately on closure the trip
signal cleared and a reset signal was generated The turbine governor had
not been reset, and as the valve opened, the turbine restarted, went to
overspeed, was intercepted and shut down by the electric device, and
continued this cycling until manually secured The initial corrective action
was to clarify the governor reset procedure and to retrain the operators
Since then the licensee has engaged in a major system design review and
upgrade
At Crystal River, the single turbine drive had apparently been properly shut
down, but during the interval between test operations, the steam supply
valve had developed a slight leak, enough so that the turbine was "idling"
at about 160 rpm, a speed sufficient to activate the governor hydraulic
system When the start signal was initiated, the turbine accelerated rapidly
and tripped to lockout on overspeed The licensee corrective action was to
overhaul the steam supply valve so that it no longer leaked
At Davis-Besse, the licensee reports that the turbine overspeed events were
caused by the presence of undrained condensation in the steam supply lines
The particular lines being used at the time of the overspeed events were
used only following certain accident sequences and were not normally
pressurized and heated These lines have long horizontal runs in which large
quantities of condensate could collect When the AFW start signal caused the
steam supply valves to open, much of the steam initially introduced was
condensed by the cold pipe and then swept into the turbines This water,
containing significantly less energy than an equivalent mass of steam,
caused the turbines to accelerate slowly and the governors to open the
control valves farther than normal When the condensate cleared the turbines
and was replaced by steam, the governors could not react rapidly enough to
prevent the turbines from overspeeding and locking out The auxiliary
feedwater system had not been tested in this configuration before the
overspeed events The initial corrective action during the transient was to
control the AFW turbines by local manual control Currently the system is
undergoing extensive design review and revision
At Palo Verde, the licensee reports that, like Davis-Besse, the overspeed
trip also occurred because of condensation in the steam supply line The
circumstances differ, however, in detail As at Davis-Besse, the branch
connection from the main steam supply lines was fairly long, but was
intended to be
IN 86-14
March 10, 1986
Page 4 of 4
pressurized to the turbine stop valve whenever steam was available Traps
and drain connections were provided to ensure that any condensation was
continuously removed However, these devices had not been verified to be
functional and properly adjusted, and in fact, significant quantities of
condensate were injected into the AFW turbine on startup Corrective action
was to verify that installed condensate removal devices functioned according
to design, and that procedures required periodic verification of
operability
All of the above-described events resulted in unavailability of the AFW
system requiring operator action to restore system operability The
underlying causes include inappropriate system design, poor operating
procedures, and inadequate maintenance
No specific action or written response is required by this information
notice If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or the Technical
Contact listed in this notice
Edward L Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: James B Henderson, IE
(301) 492-9654
Attachment: List of Recently Issued IE Information Notices
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