ML20042E961
| ML20042E961 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 05/31/1990 |
| From: | Caroline Hsu NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20042E956 | List: |
| References | |
| TASK-AE, TASK-E90-03, TASK-E90-3 AEOD-E90-03, AEOD-E90-3, NUDOCS 9005070026 | |
| Download: ML20042E961 (13) | |
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e AEOD ENGINEERING EVALUATION REPORT l
UNIT:
Rancho Seco EE REPORT NO.: AE0D/E90-03 DOCKET NO.: 50-312 DATE:
May, 1990 LICENSEE:
Sacramento Municipal Utility EVALUATOR / CONTACT:
C. Hsu District NSSS/AE:
B&W/Bechtel
SUBJECT:
OVERPRESSURIZATION OF AUXILIARY FEEDWATER SYSTEM EVENT DATE:
January-31,1989 (LER 89-001)
SUMMARY
Rancho Seco experienced an overspeed-excursion of the auxiliary feedwater-(AFW)
I turbine resulting in an overpressurization of both AFW trains on-January-31, t
1989. Both trains of-the AFW system were declared inoperable, and the plant.
was shutdown. The turbine overspeed occurred while testing a newly modified governor. The turbine speed. reached 6020 rpm and remained at this level for 3-minutes. The governor failed to limit the turbine at the rated speed of 3600 rpm and the turbine overspeed trip mechanism (OTM) also failed to trip the turbine at the trip point of 4500. rpm. 'The turbine was stopped by an operator 1
closing the steam inlet valve. The system-pressure _ corresponding-to the over-speed was calculated to be 3850 psig, which exceeded the. system design pressure of 1325 psig. Although only the turbine-driven train was in operation,.the overpressurization affected the other train as well, due=to the system configuration and the valve-lineup.. This event revealed the vulnerability of the AFW system to system overpressurization.
Inadequatecpreventive maintenance and periodic testing were' determined to be the root cause of the OTM failure to function properly.
Overpressurization of the AFW system as a result of turbine overs)eed also occurred at San Onofre 2 on July 3,1988. The. turbine; speed reacled 5000. rpm (vs 3500 rpm trip point), and produced a s also exceeded the system design pressure. ystem pressure of 2420 psig which The turbine overspeed was due to simultaneous failures of the governor and OTM. ;The overspeed occurred during steam inlet valve testing. Since the isolation valves in the cross-connection line between the AFW trains were closed during this test, the overpressurization l.
was limited to the. turbine-driven train. The other trains were not affected; The events show that the potential exists for both governor and 0TM to. fail simultaneously due to various problems affecting the operation and reliability of the governor and the OTM. Generally, all trains of the AFW system at the:
-operating plants are cross-connected at the discharge lines.of.the pumps and the isolation valves in the cross-connection are normally open during normal l
operation of the system to ensure sharing of pumps among the' trains. With this
. configuration, it appears that overpressurization of one. train could affect the other trains of the system. There is no overpressure protection, such as-relief valves downstream of the AFW pumps.
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The governor, OTM, and turbine that experienced the overspeed events were manufactured by the same companies that provide the equipment in the AFW system of nearly all o)erating PWRs, and in the RCIC system in many BWRs.
- Hence, these failures lave generic applicability.
Overpressurization of a system due to turbine overspeed could impair piping integrity and is a potential comon cause failure mode of the AFW system.
INTRODUCTION This evaluation was initiated by an event at Rancho Seco which occurred on
-January 31, 1989, involving the total inoperability of the AFW system due to piping overpressurization of both the AFW trains.
The. intent of this evaluation was to determine the generic extent of the overpressurization problem and whether additional licensee or NRC actions are necessary.
Rancho Seco experienced an overspeed excursion of the auxiliary feedwater (AFW) turbine resulting in an overpressurization of both AFW trains on January 31, 1989. Both AFW trains were declared inoperable, and the plant was shutdown.
The turbine overspeed occurred while maintenance technicians were conducting a post-maintenance installation test of the turbine governor of auxiliary feedwater pump P-318. The governor had just been refurbished by the governor-manufacturer.
During the test, maintenance personnel were locally monitoring the turbine.
The turbine was started by opening the' inlet valve from the control room.
Within 4 seconds, AFW pump P-318 discharge pressure exceeded 1500 psig (off-scale). The turbine casing relief valve lifted immediately. Ambi2nt noise i
level was deafening, making verbal communication extremely difficult. All i
attempts to adjust the governor control.-the manuel speed adjusting knob, or i
the control-oil-dumping solenoid, were unsuccessful. There were no responses by the governor. The mechanical overspeed mechanism trip (OTM) did not-function properly to close the trip valve and holate steam to be turbine.
The pump speed, as indicated by a hand held tachometer, was 6020 rpm.
j Approximately 2 minutes-after starting the turbine, the inboard pump packing i
began to smoke.
The maintenance personnel imediately exited the site, and radioed the control room to close the steam inlet valve to secure the turbine (Ref. 1).
The discharge pressure of the AFW pump during the event was calculated to be l
3850 psig, which exceeded the system design pressure of 1325 psig. This overpressure condition existed for about 3 minutes prior to closure of the steam inlet valve. Based on analysis, the AFW train components were l
subjected to pressures in excess of three times the system operating pressure.
The licensee imediately conducted an engineering evaluation and determined the l
overpressurization also affected the other train of the AF4 system. This-was L
due to the AFW system configuration and valve line-up.
Both AFW trains were i
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declared inoperable and the plant was' shutdown.
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Altnough the AFW pump was. operated on recirculation to the condenser during i
the post-maintenance testing, the extent of overpressurization included almost all the piping, valves, and some instrumentation of both trains.
Both trains of AFW are cross-connected to ensure water can be fed to either steam generator in case one of the AFW pumps fail. The valves in the cross-connection line are normally open, but can be closed from the control room to isolate.one train from the other.
The AFW pump shaft driven by the turbine also connects to a motor L
in tandem. This motor was successfully tested and deenergized prior to the overspeed event.
Following the event, disassembly of the motor and pump revealed that damage had occurred to the bearings and labyrinth oil seals.of the motor,-
and severe rubbing had occurred to the pump casing with minimal material removal.
Excessive rub and burn marks on the inboard shaft sleeve and signs of overheating in the packing of the pump were also found.-
The causes of the AFW turbine overspeed was the failure of both the governor and the mechanical overspeed trip mechanism (OTM). The failure of governor to control turbine speed was due to an internal modification at the vendor's l
facility which resulted in the governor being able to function only when rotating.in the clockwise direction.
Previously, the governor could function while rotating in either direction.
The internal modification by:the vendor caused-the governor to be unidirectional, in that oil-pressure would be built up with increasing speed only while turning in one direction. Because of' inadequate communication between the vendor and the licensee, the turbine was 4
thought to be rotating in the clockwise direction, when in fact, it rotates counter-clockwise..Thus, the governor as modified did not develop the required internal oil pressure and was, therefore, not able to control the turbine-speed.
The OTM's failure to close (trip))the trip valve when turbine speed reached the overspeed trip setpoint (4450 rpm was due to a number of mechanical problems with the linkage which resulted in the trip mechanism not being able to perform reliably. A weak trip. rod spring was the most significant' contributor to the OTM failure.
Its compression measured at 20 lbs. ys, a required 30 lbs. Also, i
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the latch hook loading spring was corroded and added resistance to unlatching of the latch hook. With a weak trip rod spring, insufficient force -is imparted to the latch hook to unlatch the valve and close the valve by spring action.
The binding of the trip rod-to-latch hook linkage due to physical distortion also contributed to the unreliable performance of the OTM. A subsequent evaluation indicated that the program for periodic surveillance and preventive maintenance, and testing of the linkage mechanism and associated valve, was inadequate. Although the turbine vendor's operation and maintenance manual requires checking the proper operation of OTM prior to performance of post maintenance testing of the governor, the OTM was not tested to verify its i
l operability before the post maintenance testing of the governor..
It appeared that there was no specific valve lineup requirement provided :in the '
Rancho Seco Terry Turbine maintenuce test program.= Figure 1 shows the affected portions of the AFW system and the valve-lineups that; existed at the time of the event. Following the event, the licensees modified their maintenance test procedures for the AFW pump. The discharge block valve of the turbine pump is to be closed prior to conducting'a turbine surveillance test.
In the case of turbine overspeed, if the discharge block valve had been closed, the. turbine overspeed would have only over-pressurized the turbine driven train up to the discharge block valve. The rest of'the AFW system downstream of the closed discharge block valve would be protected from overpressurization.
Moreover, the quarterly turbine overspeed surveillance test of the overspeed trip setpoint is to be performed'with the turbine decoupled from the AFW pump motor shaft (the turbine-driven pump also connects to a motor in tandem).
In this case, if the turbine fails to trip on overspeed it cannot overpressurize the AFW system since it is not connected to the AFW pump.
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s The root cause of the OTM failure to function properly at Rancho Seco was the lack of preventive maintenance and periodic testing of the CTM and its associated trip valve to ensure operability. The plant had previously exserienced problems with the AFW turbine governor and the OTM and had identified the need for testing of the components years earlier.
However, the OTM and trip valve were not incorporated in a periodic testing program prior to the governor replacement.
Furthermore, the inadequate control-of post-maintenance testing significantly.
contributed to.this AFW overpressurization event. The vendor manual for the governor requires that the overspeed devices be-tested prior to adjusting an installed governor on an operating compeaent. However, this requirement was not included in the test procedure.
It was assumed that the new governor would operate properly, and relied solely on the OTM to protect the AFW pump, turbine and motor in the event of a governor failure. An NRC inspection team investiga-tion subsequent to the event concluded (Ref. 2) that had proper periodic testing been developed and performed following turbine governor maintenance-work, the AFW overpressurization' event could possibly ~have been avoided.
Monthly testing of the OTM and trip valve was included in the licensee correc-tive actions to prevent a recurrence.
A search of the LER and other operatioral experience data bases identified a similar event involving overpressurizat wn of the AFW system as a result of turbine overspeed. This event occurred at San Onofre 2 on July 3,1988..This turbine overspeed was also due to the combined failures of the turbine governor and the OTM.
The overspeed occurred while personnel were performing testing of the steam inlet valve to the AFW turbine.
Based on the test-procedures, power
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was removed from the inlet valve which also led to power being removed from the governor valve. The governor valve failed open and resulted in the turbine reaching an overspeed condition. 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. Turbine speed
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reached 5000 rpm (the overspeed trip setpoint is 3500 rpm). The licensee's calculation indicated that the AFW system had been overpressurized.- The system pressure corresponding to the overspeed was 2420 psig, which exceeded system j
design pressure of 2367 psig.
I The failure of the OTM to trip the turbine was due to tappet ball. damage.
Upon disassembly of the mechanical trip device, visual ins)ection showed excessive wear on the polyurethane tappet ball. Approximately 1alf the polyurethane was 4
missing from the tappet ball so that the emergency governor weight could not contact the tappet ball with sufficient force-to actuate the trip mechanism.
In addition to the replacement of tappet ball assembly, the: licensee's corrective actions included annual visual inspection of the tappet assembly'and test of the mechanical overspeed device every refueling outage.
The isolation valves in the discharge interconnection lines to the other two motor-driven trains were closed during the test, so that the overpressurization O
was limited in the discharge pipe sections from the turbine driven pump up to those isolation valves. The motor-driven trains were not affected. However, had the overspeed occurred during demand operation of the AFW system, the over-pressurization would-have extended to the entire system piping since the two isolation valves vould have been open.
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Inadequate preventative maintenance and testing program for the OTM and trip valve was the cause of an event involving the AFW turbine' failure to trip at Diablo Canyon on February 12, 1989.
The trip valve of the AFW turbine failed
-to close during a manual actuation.
The pump was declared inoperable.
Subsequent maintenance. investigation discovered the problem was due to corrosion and hardened lubricant nn some parts of the-trip valve and the OTM, restricting the movement of these parts. The parts involved were the sliding nut in the valve yoke assembly which was corroded, and the valve trip lever which had corrosion and hardened lubricant preventing full ~ disengagement.of the hook on the trip lever. Although.the' vendor technical manual recomends
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testing the OTH. monthly and lubrication of the device weekly, the licensee's maintenance program does not include these vendor recommendations, j
i Since the Diablo Canyon monthly surveillance. test procedure for the AFW pump-K requires the pump discharge valve to be closed during the test, the piping c
downstream of the discharge valve cannot be affected if overpressurization wetc to occur due to turbine overspeed. Also, the procedures for turbine overspeed test during refueling outage requires the.AFW pump to be decoupled from the turbine. With these arrangements, the turbine overspeed would not overpressurize i
the AFW system. However, during ncrmal operation of the AFW system, turbine overspeed could overpressurize the entire AFW system as the discharge.. valve of the turbine-driven pump and the isolation valve in the discharge cross-connection are open.
DISCUSSION A typical AFW system usually consists of one turbine-driven pump and 'one or two matcr-driven pumps with interconnection between the trains. Most of the AFW turbines at operating PWR plants are manufactured by the Terry Corporation and are equipped with a Woodward governor. Terry turbine and Woodward governor' combination is also used in the HPCI 'and RCIC systems at most of the operating.
BWR plants. There are various problems affecting the reliability of the.
Woodward governor and Terry OTH. These problems have been discussed in a l
number of reports in the past. These reports include NRC IN-88-67, (Ref. 2),
88-09 (Ref. 4), 87-34 (Ref. 5) 86-14 (Ref. 6), 86-14 Supp(. 1-(Ref 7) AC Study C-602 (Ref. 8), NSAC/53 (Ref. 9) and GE/RICSIl. 037 Ref.10). The NRC information notices identified a number of comon mcde failures pertaining to the operation and control of governor and OTM. The AE0D case study listed the factors contributing to turbine overspeed. An industry study inoicated that governor and 0TM failures were-the largest contributor to equipment failure of the steam driven auxiliary feedwater system.. Similar findings also was reported in the NSAC/53 study of HPCI & RCIC reliability by EPRI. The study found that a large percr.ntage of the HPCI and RCIC failure problems involve turbine governors.
Lac 4 of preventive maintenance was cited as a significant contributing factor to the governor failure.
The GE/RCISIL 037 indicated that the new design of Terry Turbine OTMs for the HPCI/RCIC are still experiencing problems which may prevent its operation.
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-7, The Rancho Seco event revealed the vulnerability of the AFW system to overpres-sure caused by overspeed of a single turbine-driven pump in the system. The turbine overspeed was caused by the failure of the turbine governor and the 0TM to control turbine speed and trip the turbine, respectively. Although only the turbine-driven train was in operation, the event resulted in both trains of AFW system p_iping being pressurized beyond the design stress determined by:
analysis, thereby causing the system to be declared inoperable due to questionable piping integrity. Such an overspeed condition of a single turbine driven pump has not been considered in the original design of the system.
Similar to Rancho Seco Diablo-Canyon, and. San Onofre 2, the design of the AFW
- system at other operating PWR plants.also provide discharge side cross-connection with isolation valves between trains to provide sharing of pumps between the trains. This system arrangement is designed to redundancy to accommodate-a single active component failure provide adequate and/or a. loss of all offsite ac power without; impairing the ability of the system to remove decay heat following a loss ~of the main feedwater system. -To provide mitigation of this specific scenario, the isolation valves in cross-connections are normally kept open during system operation to ensure that feedwater can'be supplied' to the steam generators from any one of the AFW pumps. ' Because of -
this valve lineup,,it is apparent that overpressurization of turbine-driven train due to turbine overspeed could immediately. affect the other trains of the
-system. Since there is no overpressure protection downstream of the AFW pumps, 4
such as relief valves, overpressure in the system could impair piping integrity and has the potential for a common cause failure of the entire system. This suggests turbine overspeed could be a potential single cause failure for the AFW system that is not considered in the design base.
The failures of governor and OTM identified in these studies have not overpres-surized all-the trains of the AFW system.
However, the potential exists for'the combined failure of the governor and the OTH that would lead to an overpres-surization event.
The primary concern is that' the system is not designed for overpressurization-and has no overpressure protection such as relief valves in r
the line. Therefore, the overpressurization could impair. system piping integrity causing f ailure of the AFW system to perform its safety function.
l FINDINGS AND CONCLUSIONS Based on the preceding discussion and related follow-up activities conducted for the study, the following findings.and conclusions are provided:
A typical AFW system in PWRs usually consists of-one turbine-driven pump and two motor-driven pumps with interconnection-between the trains to ensure sharing of pumps. With this system configuration, overpressurization of a
turbine-driven train due to turbine overspeed could immediately affect the other trains and cause the system to be vulnerable to failure. The turbine-overspeed could be a potential single cause failure for the system which is not considered in the design bases.
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System overpressurization due to turbine overspeed has occurred in the AFW 4
systems at Rancho Seco and San Onofre 2. _ The overpressurization was caused by the combined failure of the turbine governor and the turbine OTM which led the turbine to reach an overspeed condition.
In the Rancho Seco event, the turbine overspeed occurred during a post maintenance test of the governor while j
the isolation valves in the cross-connection were.in the open position.
The overpressurization of the turbine-driven train overpressurized the other train.
The isolation valves at San Onofre 2 during the event were in the closed posi-tion and. therefore, the overpressurization was limited to the discharge line of the turbine-driven pump..'The event occurred during a low voltage test of the j
steam inlet valve. Had the event occurred during a normal operation of the system, all trains would have pressurized and the entire system could have been l
damaged.
The failures of the OTM and the trip valve to function properly were believed to be attributed to the inadequacy of preventive maintenance and periodic testing program.
The preventive maintenance test and surveillance test require-ments in the vendor marxal had not been fully incorporated and implemented by the licensees.
Inadequate preventive maintenance and surveillance testing programs were also the cause _of the AFW turbine failure to trip at Diablo Canyon.
The vendor manual recommends testing the OTM monthly and lubricating the device weekly. However, the licensee maintenance programs did not include all of the recommendations.-
In addition to the inadequacy of preventive maintenance and surveillance testing programs, there are other various problems affecting the reliability of the_ Woodard governor and Terry OTM which are installed in most of the AFW systems _at operating plants. Several industry studies have found that the governor and OTM failures were the dominant cause of AFW system failures.
Although the failures of the governor and the OTM identified in these studies had not occurred at the same time, the potential exist for the governor and the OTH to fail simultaneously. The events of overpressurization should not be considered random or plant specific failures but as a scenario leading to potential loss of AFW capability due to a single cause failure.
l SUGGESTIONS 9
Based on the findings and conclusions, the following suggestions are provided to reduce the incidents of turbine overspeed and consequent system overpressu-rization of the AFW system in PWRs (and RCIC and HPCI systems in BWRs).
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ForthePWRs(andBWRs), maintenance'programsshouldbeevaluatedtoensure L
that appropriate preventative maintenance and testing programs for the governor, the steam trip valve, and the overspeed trip mechanism are o
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implemented, to identify and prevent degradation of timc romponents, l'
Vendor's recommendations for maintenance and test should be included in the maintenance program.
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To limit overpressurization to a single train during testing, either a change in the piping arrangement or valve alignment should be considered to avoid a crossconnection between the AFW trains.
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REFERENCES 4
1.
Sacramento Municipal Utility District, " Root Cause Investigation 89-08, Auxiliary Feedwater Pump (P-318) Overspeed on January 31, 1989,"
', Rev. O, Docket No. 50-312, March 1989.
2.
Inspection Report 50-312/89-01, U.S. Nuclear Regulatory Commission, Region V. March 27, 1989.
3.
NRC Information Notice No.' 88-67, "PWR Auxiliary Feedwater Pump Turbine Overspeed Trip Failure," August 22, 1988.
4 NRC Information Notice No. 88-09, " Reduced Reliability of Steam Driven AFW Pumps Caused by Instability of Woodward PG-PL Type Governors,"
May 18, 1988.
5.
NRC Information Notice No. 87-34, " Single Failure in AFW System," July 26,
-1987.
1 6.
NRC Information Notice No. 86-14, "PWR AFW Pump Turbine Control Problems."
7.
NRC Information Notice 86-14 Supp.1, "Overspeed Trips of. AFW,.HPCI, and t
RCIC Turbine.
1
- 8. : NRC, AE0D Case Study No. C-602, " Operational Experience Involving Turbine Overspeed Trips," August'1986.
9.
Nuclear Safety Analysis Center,:" Reliability of BWR High Pressure Core Cooling, "NSAC-53, August 1982.
- 10. General Electric, RICSIL No. 037, "HPCI and RCIC Turbine Oyerspeed Trip -
Tappet Assembly-Binding," January 20, 1989.
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w SSINS NO.
IN 90-XX UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D. C. 20555 February,1990 NRC INFORMATION NOTICE NO. 90 55: OVERPRESSURIZATION OF AUXILIARY FEEDWATER SYSTEM Addresses:
All holders of operating licensees or construction permits for nuclear power
- reactors, purposes:
This information notice is being provided to alert addressees to potential problems resulting from combined failures of the governor and the overspeed trip mechanism (OTN) of the auxiliary feedwater (AFW) turbine that can cause a total loss of the AFW system.
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 conteined in this information notice do not constitute NRC requirements; therefore, no specific action or written response is requiu d.
Description of Circumstances:
1 Described herein are two events involving overpressurization of the AFW system as a result of the AFW turbine overspeed.
The turbine overspeeds were caused by the combined failures of the turbine governor and the OTM. The OTM did not function properly to close the steam trip valve when the governor failed to rdpond to the turbine overspeed. These events occurred at Rancho Seco and-San Onofre 2.
Rancho Seco Event:
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On January 31, 1989, tr.e AFW turbine oversped and overpressurized both trains
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of the AFW system. Both trains of the AFW system were declared inoperable, and the plant was shutdown. The turbine overspeed occurred while testing a newly modified governor. The turbine speed reached 6020 rpm in a few seconds 4
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af ter start, and remained at this level for 3 minutes. The governor failed to 4
limit the turbine at the rated speed of 3600 rpm and the turbine overspeed trip mechanism also failed to trip the turbine at the trip point of 4500 rpm.
The turbine was stopped by an operator closing the steam inlet valve. The system l
pressure corresponding to the overspeed wcs 3850 psig, which exceeded the l
system design pressure of 1325 psig. Although only the turbine-driven train was-in operation, the overpressurization affected the other train as well, due j
to the system configuration and the valve lineup'. There was no specific valve lineup requirement provided in the licensee's maintenance test program to avoid I
overpressurization of the AFW piping in the event of turbine overspeed.
inadequate preventive maintenance and periodic testing of the OTM were deter-i mined to be the root cause of the OTH failure to function properly. Monthly testing of the OTH and trip valve was included in the licensee corrective actions to prevent a recurrence. Maintenance test procedure for the AFW pump also was modified. The discharge block valve of the turbine-driven pump is to be closed prior to conducting a turbine surveillance test.
In the case of turbine overspeed, it would only overpressurire the turbine-driven train up to the discharge valve.
The quarterly turbine overspeed surveillance test is to be performed with the turbine decoupled from the AFW pump shaft to avoid overpressurization of the AFW system in the case of turbine overspeed.
San Onofre 2 Event:
On July 3,1988, while personnel were performing low-voltage testing of the r
l steam inlet valve to the AFW turbine, the AFW turbine oversped.
Based on the test procedures, power was removed from the inlet valve which also led to power being removed from the governor valve. The governor valve failed open and resulted in the turbine reaching an overspeed condition.
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 (overspeedtripsetpointis3500 rpm).
The subsequent licensee's calculation indicated that the AFW system had been overpressurized.
The pump discharge pressure corresponding to the overspeed was 2420 psig, which exceeded the system design pressure.
The failure of the OTH to trip the
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-3 turbine was due to tappet ball damage.
In addition to the replacement of tappet ball assembly, the licensee's corrective actions included annual visual inspection of the tappet assembly and test of the OTM every refueling outage.
The isolation valves in the discharge interconnection lines to the other two motor driven trains were closed during the test, so that the overpressurization was limited in the discharge pipe section from the turbine-driven pump up to i
the isolation valves. The motor-driven trains were not affected. However, had f
the overspeed occurred during normal operation of the AFW system, the over-pressurization would have extended to the entire system piping as the isolation
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valves would have been in an open position, j
Discussion:
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These events revealed the vulnerability of the AFW system to overpressure caused by overspeed of a single turbine-driven pump in the system. The vulnerability is inherent to the piping configuration and valve lineup, which allow for discharge side cross-connection with isolation valves between trains to provide sharing of pumps between the trains.
The isolation valves are normally kept open during system operation. With this system configuration, overpressurization of turbine-driven train due to turbine overspeed could immediately affect the other trains and cause the system to be vulnerable to failure. The turbine overspeed could be a potential single cause failure for the system.
The turbine overspeed were caused by combined failures of the governor and the l
OTM, Although the OTM is installed to provide protection for the AFW pump and turbine in the event of governor failure, the potential exists for both the governor and OTM to fail simultaneously due to various problems affecting their r
operation and reliability.
In the events described herein, the deficiency of preventive maintenance and periodic testing programs was the major contributor l
ic the failure of the OTMs.
The governor, the OTM, and the turbine that experienced the overpressure events are used in the AFW system of nearly all operating PWRs. They are also used in the RCIC system in operating BWRs.
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4 No specific action or written response is required by this information notice.
If you have any questions about this natter, please contact the technical contact listed below, the Regional Administrator of the eppropriate regional
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office, or this office.
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Chariva E. Rossi. Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation l
i Technical
Contact:
Chuck Hsu, AEOD (301) 492 4443 f
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