Information Notice No. 87-53: Auxiliary Feedwater Pump Trips Resulting from Low Suction Pressure
SSINS No.: 6835
IN 87-53
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 20, 1987
Information Notice No. 87-53: AUXILIARY FEEDWATER PUMP TRIPS
RESULTING FROM LOW SUCTION PRESSURE
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems resulting from low suction pressure trips of auxiliary feedwater
(AFW) pumps which may contribute to system unavailability. 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:
Described herein are four events during which the AFW pumps tripped on low
suction pressure trip signals as a result of brief suction pressure oscilla-
tions or fluctuations which occurred despite the availability of sufficient
steady-state net positive suction head (NPSH). These events occurred at
Millstone 3, D.C. Cook 1, Trojan, and Zion 2.
Millstone 3 Event
On January 27, 1987, the "A" motor-driven auxiliary feedwater (MDAFW) pump
tripped seconds after being started on three different occasions during quar-
terly surveillance testing at Millstone Unit 3 while the reactor was operating
at 100% power. On January 29, 1987, the "B" MDAFW pump tripped seconds after
it was started during similar testing. Subsequently, both pumps were declared
inoperable. The licensee determined that the trips were caused by brief
suction pressure oscillations that resulted in low suction pressure trip
signals. The suction pressure oscillations were induced by hydraulic
hammering in the pump discharge line. The hydraulic hammering occurred with
the system valve aligned for testing: the pumps isolated from the steam
generators and the flow bypassed through the recirculating line back to the
demineralized water storage tank. The licensee speculated that the source of
the hydraulic hammering was either the closed motor-operated valve or the
check valve in
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October 20, 1987
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the pump discharge line. A subsequent investigation by the licensee revealed
that significant pump suction pressure oscillations could also occur during
other pump modes of operation. The licensee has removed the suction pressure
trips from the MDAFW pumps to preclude similar low pressure pump trips.
D.C. Cook 1 Event
On October 27, 1985, while the reactor was in hot standby, with the West MDAFW
pump in service and the TDAFW pump being tested, the West MDAFW pump tripped
on low suction pressure and the TDAFW pump was declared inoperable following
testing. This placed the plant in a technical specification action statement.
A faulty governor caused turbine speed oscillations that resulted in pressure
fluctuations on the suction side of the MDAFW pump. The MDAFW and the TDAFW
pumps share a common suction header.
As a corrective action, the licensee removed the low suction pressure trip
feature and replaced it with an alarm/operator action combination. The alarm
actuates when the water level in the condensate storage tank drops to the
point where 14 minutes remain before the level would reach the center line of
the suction pipe for the AFW pumps. This time is deemed sufficient for the
operator to take appropriate actions to prevent pump damage.
Trojan Event
Following a reactor trip from full power on July 20, 1985, the diesel-driven
AFW and the turbine-driven AFW (TDAFW) pumps automatically started; however,
the diesel-driven AFW pump tripped on low suction pressure during the starting
sequence. The operator blocked the diesel-driven AFW low suction pressure
trip and restarted the pump successfully. Several minutes later, the TDAFW
pump also tripped on low suction pressure while it was aligned to the
condensate storage tank. The operator throttled down the AFW pump discharge
valves and restarted the turbine-driven pump without further incident.
The low suction pressure trips occurred because excessive combined suction
flow was being drawn by the two AFW pumps from a single suction header. The
excessive suction flow resulted from an overshooting of the flow control
valve, which caused a turbine speed overshoot, and consequently created a
momentary pressure drop. Because the low pressure condition existed only
momentarily, the licensee adjusted the time delays for bypass of the low
suction pressure trips to provide sufficient time for the suction flow to
stabilize.
Zion 2 Event
On December 11, 1981, the unit's two MDAFW pumps started automatically follow-
ing a reactor trip and a subsequent low-low level in one of the steam genera-
tors; however, the pumps immediately tripped on low suction pressure. The
pumps were promptly manually restarted from the control room without further
complications. The turbine-driven pump was out of service for maintenance at
the time of the event.
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October 20, 1987
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The licensee's subsequent investigation indicated that the simultaneous start
of the two MDAFW pumps caused the suction pressure to briefly drop below the
trip setpoints and consequently tripped the pumps. A split discharge header
arrangement had been used when the TDAFW pump was out of service, i.e., one of
the MDAFW pumps was aligned to the discharge header of the TDAFW pump. This
arrangement provided separate feedwater paths to the steam generators.
Because of this alignment, pressure was momentarily reduced in the pump
suction line during pump start; this pressure reduction was sensed by the pump
trip sensors. To eliminate the problem, the licensee installed a time-delay
relay in the control circuitry of the two MDAFW pumps to bypass the low
suction pressure trips during the momentary low pressure condition during pump
start.
Discussion:
These events illustrate a common-mode failure of the auxiliary feedwater pumps
that could result in a total or partial loss of the AFW system. The pump
trips were caused by brief low pressure oscillations that occurred during pump
start despite sufficient steady-state net positive suction head (NPSH).
Although the four events described herein address only the AFW system, similar
pressure transients can occur in other systems where the pump arrangement is
susceptible to sudden suction pressure drops. Several corrective actions have
been taken by the affected licensees: extending the time delay associated
with the low suction pressure trip function, removing the trip function, or
replacing the automatic trip function with an alarm/operator action
combination. It should be noted that the low suction pressure trip function
is provided to protect the centrifugal pump from cavitation. The removal of
this trip function, either automatic or manual, may leave the pump without
adequate protection.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below, the Regional Administrator of the appropriate regional
office, or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Chuck Hsu, AEOD
(301) 492-4443
Attachment: List of Recently Issued NRC Information Notices
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