ML20237L300
| ML20237L300 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Cook, Zion, 05000000, Trojan |
| Issue date: | 08/24/1987 |
| From: | NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20237L250 | List: |
| References | |
| TASK-AE, TASK-E709 AEOD-E709, NUDOCS 8708280041 | |
| Download: ML20237L300 (10) | |
Text
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AE00 ENGINEERING EVALUATION REPORT
- UNII:
Millstone 3 EE Report No.-
AE0D/E 709 DOCKET N0:
50-423 DATE: Auoust 24, 1987 LICENSEE:
Northeast Nuclear Energy Company EVALUATION / CONTACT:
C. HSV NSSS/AE:
Westinghouse /S&W SUBJECT-AUXILIARY FEEDWATER PUMP TRIPS CAUSED BY LOW SUCTION PRESSURE EVENT DATE: JANUARY 29, 1987 (LER 87-004-00)
SUMMARY
Licensee Event Report 87-004, dated January 29, 1987, for Millstone 3 describes an event in which each of the Unit's two motor-driven auxiliary feedwater (MDAFW) pumps A&B tripped immediately after being started during quarterly surveillance testings conducted on January 27 and 29,1987, respectively. The unit was at 100% power in Mode 1 during the testing.
Pump A tripped three times while pump B tripped once. All trips occurred several seconds after pump starts. Both pumps were declared inoperable.
The trips were determined to be caused by suction pressure oscillations that resulted in spurious low suction pressure trip signals. This spurious low pressure trip is a common mode failure for the two MDAFW pumps. The suction pressure oscillation was induced from hydraulic hammering in the pump discharge line.
The hydraulic hammering took place with the system alignment established for the tests, in which the pumps are isolated from the steam generators and the flow bypassed through the recirculating line back to the deminalized water storage tank.
Either the closed motor-operated valve or the system check valve in the pump discharge line could be the source of the hydraulic hammering.
A subsequent licensee's investigation revealed that significant pressure oscillations at pump suction could also occur during other operations.
Three additional events found in this review had similar spurious low-suction pressure trips that resulted in a partial loss of the AFW system.
The additional events occurred at Trojan, D.C. Cook 1 and Zion 2.
The low pressure condition causing the AFW pump trips in these three events were also generated by pressure oscillation or fluctuation in the suction lines during pump startup transient or in the middle of operation.
The pump fluctuation at Trojan and Zion 2 was the result of excessive suction flow, while that at D.C. Cook I was induced by turbine speed oscillation due to a faulty governor.
The pressure oscillation or fluctuation created a momentary low pressure in spite of sufficient available suction head. The low pressure was sensed by the pump protection system and actuated trip of the pumps.
The flow path and pump alignment combinations, which contributed to the occurrence of pressure oscillation, were not the normal operational configuration for the AFW system at these plants and appear not to have been fully specified or considered in design and operation of the pump low suction pressure protection systems.
Low suction pressure trips are generally provided for protection of the pumps against loss of suction head and cavitation. The events show that low suction pressure trips constitute a common mode failure that can potentially render
- This document supports ongoing AE0D and NRC activities and does not represent the position or requirements of the responsible NRC program office.
B708280041 070824 PDR ORG NEXD PDR
2 the AFW system inoperable and the plant would lose its redundancy in core heat removal capability. Although the trips may be removable in a relative'y short period and the pump can be manually restarted, the potential exists that system rc ponse time may increase beyond the time limit required by the technical specifications, and the reliability and capability of the ArW system would be compromised. Without some sort of low suction pressure trip, either automatic or manual, pump protection may not be adequate.
INTRODUCTION Licensee event report (LER)87-004 from Millstone 3 describes an event in which, with the unit at 100% power in Mode 1, each of the unit's two motor-driven auxiliary feedwater (MDAFW) pumps A&B tripped immediately after being started ouring quarterly surveillance testings conducted on January 27 and 29,1987.
Pump A tripped three times while pump B tripped once.
Each trip occurred several seconds after pump start.
Both pumps were declared inoperable. The trips were determined to be caused by suction pressure oscillations that resulted in spurious low suction pressure trip signals. This spurious low pressure trip was a connon mode failure of the two MDAFW pumps.
These surveillance testings were tests of initiation from the engineered safety features (ESF) actuation relays with the pumps isolated from the steam generators and recirculating minimum flow to the deminalized water storage tank (DWST).
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The actuation of the low suction pressure trip was attributed to suction pressure oscillations caused by hydraulic hammering in the pump discharge piping. The hydraulic hammering took place with the system valve alignment established for the quarterly ESF actuation relay testing in which the pump discharge flow control valve (FCU) remains open and the downstream motor-operated valve (MOV) is closed.
Either the MOV or system check valves may be the source of the hamme ring. The licensee's subsequent evaluations indicated that the pressure oscillations occurred several seconds following the pump startup. Although they dampened significantly, they were initially quite severe. The pressure dips of continuous oscillations were apparently sufficient to actuate the pressure switch and cause pump trip after the time delay which was set at 5 seconds.
Although the conditions which caused the pump motor trip existed only during the quarterly surveillance test, the licensee's investigations revealed l
significant pressure oscillations at pump suction during other operation as well. These pressure oscillations around 21 psig varied in magnitude between 10 and 25 psi (peak-to-peak) and were observed with or without flow to the steam generators. With the 10 psia trip actuation point, it appears that due to these oscillations, the MDAFW pumps could have tripped on low suction pres-sure before the design basis volume of water was pumped from the DWST to the steam generators.
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The licensee determined that the suction pressure trip was not part of the l
safety analytis and was an optional feature provided for equipment protection.
i The low suction pressure trip was removed from both MDAFW pumps to preclude
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i the possibility of spurious low suction pressure trips. Both pumps were I
classified as operable upon subsequent satisfactory completion of surveillance tests.
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DISCUSSION Since the low suction pressure protection feature is connonly used for pump protection, a review of operating experience for events involvine similar AFW purp trips at other nuclear plants was initiated.
In this review, three addi-tional events involving spurious low suction pressure that resulted in trip of AFW pumps were found in a search of the Sequence Coding and Searching System (SCSS) LER data base file. The low pressure conditions causing the AFW pump trips in these three events were also generated by pressure oscillation or fluctuation during pump startup transient or operation. Although the pressure oscillations dampened rapidly, initially the pressure dropped below the trip set point and actuated pressure trip switch. These three events occurred at Trojan, D. C. Cook 1 and Zion 2.
The low suction pressure trips at Trojan and Zion 2 occurred during pump startup transient, and that at D.C. Cook I occurred during operation.
The event at Tojan occurred on July 20, 1985, and was reported in LER 85-09.
The AFW pumps received a signal to automatically start after the plant tripped from 100% power because of a main turbine trip that was caused by the loss of the unit auxiliary transformer.
Both AFW pumps (one diesel-driven and one turbine-driven) started, but the diesel AFW pump tripped on low suction pressure during the starting sequence. The operator blocked the diesel AFW pump low suction pressure trip and restarted the pump successfully.
However, several minutes later, the turbine AFW pump tripped also on low suction pressure in -
spite of sufficient available suction head from the condensate storage tank (CST). The operator throttled down the AFW pump discharge valves and restarted i
the turbine AFW pump satisfactorily.
The low suction pressure trips were found to be caused by excessive combined flow from the two AFW pumps that draw from a single heater from the CST. The flow control valves were opened further than required, causing a turbine speed overshoot.
It was during this period that the suction pressure trips actuated.
The excessive opening of the control valves were attributed to improper post-modification adjustment and testing after retrofit of environmentally qualified controllers for the AFW system during a previous refueling outage. When the flow control valves were adjusted following the modification, only one AFW pump was run at a tine. Consequently, when both pumps were started following the i
reactor trip, the combined flow was excessive, thus creating a momentary low pressure at the pump suction.
After the change was made to the speed controllers of the AFW pumps, additional testing was performed with CST level as low as 60%. The test indicated that the pump suction pressure was still subatmospheric for a brief period during the diesel AFW pump overshoot. Since the low pressure condition only existed momentarily (the duration of low pressure condition was determined to have varied between 13 and 18 seconds), the time delays for bypass of the low suction pressure trips were adjusted from the original 10 seconds to 25 seconds and an I
administrative control was implemented to maintain water level in CST 60% or higher to prevent occurrence of spurious low-suction pressure trips. The turbine AFW pump time delay was unchanged at 7.5 seconds. The manufacturer of j
the pumps has indicated that the pumps can run with no suction source for 30 seconds without damage.
. j The event reported in LER 85-058 at D.C. Cook 1 occurred on October 27, 1985.
While the unit was in hot standby, the west motor-driven AFW pump tripped on low suction pressure while the turbine-driven AFW pump was inoperable for testing.
This violated the plant Technical Specification requirement that at j
least three AFW pumps be operable.
Investigation revealed that an unusual l
combination of factors was involved.
During the testing of the turbine-driven AFW pump, a faulty governor caused turbine Speed to oscillate at approXimately 2.2 cycles per second. This resulted in pressure oscillations of the suction side of the MDAFW pump which was in service.
Foreign material in the suction pressure gauge protectors on the MDAFW pump acted as a check valve that permitted the pressure sensors to sense only the low pressures of the oscilla-tions by preventing the higher pulses from reaching the sensors. That caused the sensors to ratchet progressively lower, resulting in a 2 out of 3 low suction pressure coincidence which tripped the pump.
The foreign material was removed to restore the pressure sensors, resulting in an operable MDAFW pump.
Following an engineering justification the automatic low suction pressure trips were disabled on the AFW system and replaced with equivalent safety j
function using an alarm and operator action.
The suction pressure gauge protectors and associated instrument piping were cleaned. Also, the governor and its valve linkage, cam and stem were all replaced. The pump was then successfully tested and returned to service.
i In order to have a reliable AFW system, the licensee's corrective action was to remove the low suction pressure trip and replace it with an alarm / operator action combination. The corrective action was justified based or, the following two bases.
1.
The condensate tank was capable of withstanding both operating and design b6 sis earthquakes.
i
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Sufficient time will be available for an operator to take action since there was a 14-minute interval between the moment when the water level in the tank was at the low suction pressure alarm position and the moment when the water level dropped to the center line of the suction pipe for the AFW pumps.
The ever.t at Zion 2 was described in LER 81-033. On December 11, 1981, upon a steam c,enerator low-low level signal following a reactor trip, the unit's two motor-driven AFW pumps started automatically but immediately tripped on low suction pressure. The pumps were promptly manually started from the control room. The turbine driven pump was out of service for maintenance at the time of this event.
The licensee's subsequent investigation indicated that the simultaneous start of the two MDAFW pumps caused their sensed suction pressure to drop below cheir low suction pressure trip setpoints, thus tripping the pumps. A split 'iischarge i
header arrangement had been used when the turbine-driven AFW pump was out of service, i.e., one of the MDAFW pumps was aligned to the discharge header of the TDAFW pump. This provided two separate feedwater paths to steam generators.
Due to this alignment, the transient effect upon pump start caused a momentary pressure reduction in the suction line near the pump and activated the pressure switch. Thus, the sensed suction pressure experienced a momentary drop even though sufficient suction head was available.
i l
J
i To eliminate the problem the licensee installed a time delay relay (about 2.5 seconds) in the control circuitry of the two MDAFW pumps. The relay momentarily bypasses the low suction pressure trip during pump start. This will provide sufficient time for suction flow to stabilize before actuation of the suction pressure trip without degrading the pump's protection from cavitation.
Following the modification, the pumps were tested satisfactorily. The licensee also installed the same modification on the Unit 1 MDAFW pumps.
These events illustrate a common mode failure of the AFW pumps that could result in a total or partial loss of the AFW system. The cited pressure oscillations in the suction line, which created momentary low pressure even j
though there were sufficient available suction heads and led to the pump trips, were apparently caused by hydraulic hammering, excessive suction flow, or turbine speed oscillation. The hydraulic hamering occurred in the pump discharge line at Miil stone 3 when the system was placed in the valve align-ment used for the quarterly surveillance testing, either the stop valve or check valve in the discharge line may be the source of the hammering. Exces-sive suction flow at Trojan were found to be caused by excessive combined discharge from the two AFW pumps that draw water from a single suction header.
The simultaneous start of the two motor-driven pumps with separate discharge headers at Zion 2 also caused excessive combined flow in the suction lines.
The turbine speed oscillation which occurred at D.C. Cook 1 was associated with a faulty governor.
It was this turbine speed oscillation that generated pres-sure fluctuation in the suction side of the motor-driven pump which shares the common suction het. der with the turbine-driven pump. The flow paths and pump alignment combinations described, which appeared to cause the pressure oscillation, were not the normal operational configuration for the system and had not been fully specified or considered in the operational condition for the design of the pump protection system.
In all cases, the pressure oscillations dampened quickly. Thus, the low pressure conditions at the pump suction only existed for a brief period (a few seconds).
The actuation of low suction pressure trips could be avoided by providing sufficient time delay in the low pressure trip circuitry to bypass the pressure fluctuation.
In the events reviewed, the operators were able to manually restart the tripped pump, once the cause of the trip was identified and corrected. The pump protective feature performed its intended function and in no case was any pump damaged.
Inade> Jate available suction head can cause cavitation and lead to vibration and damage to the pump. Other conditions, such as air or vapor binding of the suction line, or inadvertent closure of the suction valve can also cause cavitation and subsequent damage. Air binding of the suction line has occurred to the MDAFW pump at Ginna (Ref.1).
Because of venting problems, air accumulation in the suction line resulted in the pump running without water, l
Similarly, vapor binding in the suction line can cause a loss of suction to the pump. Both AFW pumps at Arkansas 2 (Ref. 2) have experienced cavitation as a result of water flashing to steam in the common suction header. The cause of this flashing was attributed to abnormally high temperature blowdown deminalizer effluent entering the AFW system. An event of suction valve closure occurred at Davis-Besse (Ref. 3) on May 11,1987. The suction valve to the MDAFW pump was closed while the pump was in operation. About eight minutes later the pump seized. The low suction pressure trip was not provided for this pump. Thus, without the low suction pressure protective feature, the pumps are potentially
7 susceptible to irreparable damage during sustained low suction pressure condi-tions. Thus without some means of low suction pressure protection, either automatic or manual, pump protection may be inadequate, and restart, under such circumstances, may not be possible.
The AFW system of PWR plants is required to provide feeawater to the steam generator to remove decay heat following a loss of main feedwater or following a loss of ac power. The low suction pressure trip constitutes a common mode f ailure that can potentially render all or some trains of the AFW system inoperable and the plant would lose its redundancy in heat removal capability.
The trip may be removable in a relatively short period and the pump manually i
restarted. However, there is potential that system response time may signifi-cantly increase and rated flow can not be achieved within the time limit recuired by the technical specifications.
In this case the reliability and i
capability of the AFW system would be compromised.
FINDINGS AND CONCLUSIONS Base on the preceding discussion and related follow-up activities conducted for the study, the following findings and conclusions are provided.
1.
Low-suction pressure trips have occurred to the AFW pumps of different system configurations at four plants, even though sufficient suction head was available. *ihese four plants were Millstone 3, Trojan, D.C. Cook 1 and Zion 2.
The spurious low pressure trips were attributed to pressure oscillations or fluctuations in the suction lines, which created a momen-i tary pressure drop. The momentary low pressure was sensed and actuated the pressure trips of the pumps, i
2.
The pressure oscillation or fluctuation was a result of hydraulic hammering, excessive suction flow or turbine speed oscillation. The hydraulic hammer-ing and excessive suction flow occurred during pump startup transient.
Either the stop valve or the check valve in the discharge line could be the source of hammering at Millstone 3.
Excessive suction flow was caused by excessive combined discharge from the two AFW pumps that drew water from a single suction header. The combined discharge at Trojan becoming excessive was due to overshooting of the flow control valves during pump startup.
The simultaneous start of the two motor-driven pumps with separate discharge headers at Zion 2 was the cause of excessive combined discharge. Turbine speed oscillation occurred at D.C. Cook 1.
A faulty governor caused the turbine speed oscillation that generated a pressure fluctuation in the suction line of the motor-driven pump sharing the common suction header with the turbine-driven pump.
3.
The flow path and pump alignment combinations, which appeared to contribute to the occurrence of pressure oscillation upon pump operation, were not the normal operational configuration for the system and had not been fully specified or considered in the operational conditions for the design of the pump low pressure protection system.
4.
Although the pressure oscillation or fluctuation dampened very rapidly, they were initially quite severe and were of sufficient magnitude that the pump suction pressure dropped below the trip setpoints for a brief period and actuated the low pressure trips.
5.
The low pressure condition only existed momentarily.
To prevent spurious low pressure pump trips from occurring, the licensee of Zion has installed a time delay relay in the control circuitry of the MDAFW pumps.
The relay momentarily bypasses the low suction pressure trip during pump start. The existing time relays for the AFW pumps at Trojan were also adjusted to provide sufficient time for suction flow to stabilize before actuation of the low pressure trips, without deg-ading the pump protection from cavitation.
6.
The corrective actions taken by the other two plants where the trip problem occurred are different.
The licensee of Millstone 3 removed the low suction pressure trip from both MDAFW pumps since it was not part of
{
safety analysis and was an operational feature provided for equipment i
protection.
"e low suction pressure trip was replaced with an alarm /
j operator act s combination at D.C. Cook 1.
The alarm will actuate when the water level in the CST drops to the point where there is still 14 minutes to reach the center line of the suction pipe for the AFW pumps.
The time is sufficient for the operator to take appropriate actions.
7.
Low suction pressure trip function is provided to protect centrifugal pumps from cavitation and subsequent damage.
It trips the pump within a desirable time upon a loss of adequate pressure to provide the necessary i
net positive suction head (NPSH) at the suction of the pump.
Without low l
pressure trip or using alarm / operator action combination, as in the cases l
of Millstone 3 and D.C. Cook 1, the automatic pump protection may not be j
adequate. There are conditions other than inadequate available suction head that can cause cavitation and damage the pump, such as air or vapor binding of tna suction line, or inadvertent closure of the suction valve.
An the alarm / operator action combination based on available suction head, such as water level in CST, by itself would not provide protection against vapor binding or suction valve closure.
REFERENCES I
1.
S. M. Stroller Corp., " Nuclear Power Experience," Vol. PWR 2, VI.
Turb. Cycle System.
E. Cond. & FW, P.13, " Air in suction - AFW pump Inope rable. "
2.
Licensee Event Report (LER)80-018, Docket Number 50-368, Arkansas 2, April 7,1980.
3.
Headquarters Daily Report dated May 11, 1987, Docket Number 50-346, Davis-Besse.
SSINS No.:
IN 87-XX UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C.
20555 July
, 1987 INFORMATION NOTICE NO. 87-XX: Auxiliary Feedwater Pump Trips Caused by Spurious low Suction Pressure Addresses:
All nuclear power reactor facilities holding a construction permit (CP) or an operating license (0L).
Purpose:
l This information notice is provided to alert recipients of potential problems of low suction pressure trips of auxili6ry feedwater (AFW) pumps which contribute to systems unavailability.
It is expected that the recipients will review the information for applicability to that facilities and consides 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:
Described herein are four events during which the AFW pumps tripped as a result of low suction pressure despite sufficient available suction heads from the source. The low pressure trips were attributed to pressure oscillation or j
fluctuations in the suction lines, which created momentary pressure drop.
The i
momentary low pressure was sensed and actuated the pressure trips of the pumps.
These four events occurred at Millstone 3, Trojan, D.C. Cook 1 and Zion 2.
Millstone 3 Event Each of the units two motor-driven (MD) AFW pumps A&B tripped after being started during quarterly surveillance testing conducted on January 27 and 29, 1987, respectively. The unit was at 100% power during the testing. All trips occurred several seconds after pump starts.
Both pumps were declared inoperable.
The trips were determined to be caused by suction pressure oscillations that resulted in spurious low suction pressure trip signals. The suction pressure oscillation was induced from hydraulic hammering in the pump discharge line.
The hydraulic hammering took place with the system valve alignment established for the tests in which the pumps were isolated from the steam generators and the flow bypassed through the recirculating line back to the demineralized water storage tank.
Either the closed motor-operated valve or the check valve in the pump discharge line could be the source of the hydraulic hammering. A
subsequent licensee investigation revealed that significant pressure oscilla-tions at pump suction could occur during other operations as well. The low suction pressure trip was removed from both MDAFW pumps to preclude the recurrence of spurious low pressure trips.
Trojan Event Following a reactor trip from full power on July 20, 1985, both AFW pumps (one diesel-driven and the other turbine-driven) automatically started, but the diesel-driven AFW pump tripped on low suction pressure during the starting sequence. The operator blocked the diesel-driven AFW pump low suction pressure trip and restarted the pump successfully. However, several minutes later, the turbine-driven AFW pump also tripped on low suction pressure despite sufficient available suction head from the condensate storage tank. The operator throttled down the AFW pump discharge valves and restarted the turbine-driven pump satisfactorily.
The low suction pressure trips were found to be caused by excessive combined suction flow from the two AFW pumps that draw froin a single suction header.
The excessive suction flow which resulted from an overshooting of the flow control valve, created a momentary pressure drop. Since the low pressure condition only existed momentarily, the time delays for bypass of the low suction pressure trips were adjusted to provide sufficient time for suction flow to stabilize.
D.C. Cook 1 Event On October 27, 1985, while the unit 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 while the TDAFW pump became inoperable during testing. This violated the technical specification requirement that at least three AFW pumps be operable. A faulty governor caused turbine speed to oscillate and resulted in pressure fluctuations in the suction side of the MDAFW pump which shares the common suction header with the TDAFW pump.
The licensee's corrective action was to remove the low suction pressure trip feature and replace it with an alarm / operator action combination. The alarm will actuate when the water level in the CST drops to the point where there still remains 14 minutes to reach the center line of the suction pipe fer the AFW pumps. The time is deemed sufficient for the operator to take appropriate actions.
Zion 2 Event On Deceinber 11, 1981, following a reactor trip and subsequent low-low level in a steam generator, the unit's two MDAFW pumps started automatically, but immediately tripped an low suction pressure. The pumps were promptly manually started from the control room and became operable. The turbine driven pump was out of service for maintenance at the time of this event.
The licensee's subsequent investigation indicated that the simultaneon start of the two MDAFW pumps with separate discharge headers caused their ski d scetion pressure to drop below the trip setpoints and tripped the pumps. The separate discharge arrangement provided separate feedwater paths to steam
generators.
Due to this alignment, the transient effect upon pump start caused a momentary pressure reduction in the suction line near the pump and activated the pressure switch. Thus, the sensed suction pressure experienced a mcmentary drop even though sufficient suction head was available. To eliminate the problem, the licensee installed a time delay relay in the control circuitry of the two MDAFW pumps to bypass the momentary low pressure condition during pump start.
These events illustrate a common-mode failure of the AFW pumps that could result in a total or partial loss of the AFW system. A variety of corrective actions, ranging from extending the time delay associated with the low suction pressure trip function to removal of the trip function, has been taken by the affected licensees.
It should be noted that the low suction pressure trip function is provided to protect 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 Regional Administrator oft he appropriate NRC regional office or this office.
l Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Peactor Regulation i
Technical
Contact:
Chuck Hsu, AE0D (301) 492-4443 I
)