United States Nuclear Regulatory Commission - Protecting People and the Environment

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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                                                    IN 87-53
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                                                            Page 2 of 3


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.  

                                                    IN 87-53
                                                            October 20, 1987
                                                            Page 3 of 3


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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