ML19259D226
| ML19259D226 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 09/25/1979 |
| From: | ENERGY ENGINEERING GROUP |
| To: | |
| Shared Package | |
| ML19259D208 | List: |
| References | |
| NUDOCS 7910170400 | |
| Download: ML19259D226 (13) | |
Text
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STEAM GENERATOR WATER HAMMER TECHNICAL EVALUATION TROJAN POWER STATION September 1979 EG&G Idaho, Inc.
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CONTENTS I.
INTRODUCTION.........................
1 II.
FEE 0 WATER SYSTEM.......................
2 1.
DESCRIPTION.......................
2 3
2.
GENERAL OPERATION....................
Ill.
ME/NS TO REDUCE THE POTENTIAL FOR WATER HAMMER........
5 1.
DESCRIPTION.......................
5 2.
EFFECTIVENESS DURING TRANSIENTS AND CONDITIONS CONDUCIVE TO WATER HAMMER................ 7 7
2.1 Reactor Trip....................
2.2 Loss of Main Feedwater Flow..
8 8
2.4 Operator Error...................
9 2.5 Steam Line Break..................
9 2.6 Loss-of-Coolant Accident.............
10 IV.
CONCLUSIONS AND RECOMMENDATIONS..............
12 V.
REFER ENCES.........................
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I.
INTRODUCTION An evaluation was performed for the Trojan Power Station (TPS)
The purpose of this evaluation was to assess the feedwater system.
effectiveness of the existing means to reduce the potential for water hammer in the feedwater systems (steam generator water hamer) during The steam-water slugging normal and hypothetical operating conditions.
in the feedwater systems (specifically, the steam generator feedrings A known and adjacent feedwater piping) was considered in this review.
steam generator water hamer has not occurred at the TPS.
The potential for steam generator water hammer is avoided if the Hence, this evaluation feedwater system is maintained full of water.
was based on the effectiveness of the means utilized at the TPS to maintain the feedwater system full of water during normal and hypothetical operating conditions.
The information for this evaluation was obtained from:,
- 1) discussions with the licensee, 2) licensee submittals of August 6, 2
3 and October 22, 197S, October 21, 1975, July 12, 1976,
1 1976, 3) the " Trojan Nuclear Plant Final Safety Analysis Report" 4
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- 4) "An Evaluation of PWR Steam Generator Water Hamer", NUREG-0291, 7
and 5) Westinghouse Technical Bulletin, NSD-T3-75-7.
A description of the feedwater system at the TPS and its general The means to reduce the operation is presented in Section II.
potential for steam generator water hamer are presented in Section III, including a discussion of their effectiveness during Finally, conclusions operating conditions conducive to water hammer.
and recomendations are presented in Section IV concerning the adequacy of the existing means to reduce the potential for steam generator water hamer at this f acility.
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II.
FEEDWATER SYSTEM 1.
DESCRIPTION The feedwater system for the TPS was designed to provide an adequate supply of feedwater to the secondary side of the four steam generators during all operational conditions. Feedwater is supplied to the main feedwater pumps by the heater drain pumps and by the condensate pumps via the low pressure heaters.
Feedwater from the main feedwater pumos is supplied to a main header via the high pressure heaters. The main header splits into four 14-inch feedwater lines to supply a 10-inch feedring inside each steam generator.
Feedwater is discharged downward through inverted "J" snaped tubes uniformly distributed on top of each feedring.
The two main feedwater pumps are each rated for a flow rate o'f 19,800 gpm at 1330 feet total developed head (TDH).
The pumps are each driven by a 13,000 bhp variable speed turbine.
The turbine drivers are supplied with steam, during normal two pump operation, from the outlet of the reheater moisture separators.
Steam from the main steam header is used during startup and low power operation.
TM auxiliary feedwater system provides feedwater to the steam generators for residual heat removal during reactor startup and shutdown, low power operation, and in the event of loss of main feedwater flow. Auxiliary feedwater can be supplied by two redundant systems employing a turbine driven auxiliary feedwater pump in one system and a diesel driven auxiliary feedwater pump in the other system.
Auxiliary feedwater can be supplied to each of the four steam generators via four 3-inch lines. A single line connects to the main feedwater line of each steam generator just outside the containment building.
Each three inch line can be supplied by either or both auxiliary feedwater pumps. All valves between the pumps and the steam generators are normally open and fail as is.
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The turbine driven and diesel driven auxiliary feedwater pumps are each rated to deliver 880 gpm to the steam generators at 3375 feet TDH.
The normal water supply source for the auxilir.ey feedwater system is the condensate storage tank with a backup supply available from the service water system.
The diesel driven auxiliary feedwater pump is supplied with diesel fuel by a 500 gallen (10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> of operation) day tank with automatic transfer of fuel available from the emergency diesel fuel oil transfer system. The turbine driven auxiliary feedwater pump operates with steam taken from all four main steam neaders. The electrical requirements for each system are normally supplied by independent offsite power sources via the startup transformers with backup power provided by separate diesel generators.
DC power is used to operate the speed governors for both pumps and the throttle valve in the turbine driven pump steam supply line.
2.
GENERAL OPERATION During normal power operation of the reactor, the main feedwater system supplies feedwater to the secondary side of the steam generators for heat removal from the reactor coolant system.
The feedwater flow is regulated by individual regulating valves in the main feeawater lines to each steam generator.
The positions of the valves are automatically controlled based upon steam generator level, steam flow, and feedwater flow.
During plant shutdown, startup, and for feedwater requirements up to about 4% of full reactor power, feedwater is normally supplied by the diesel driven auxiliary feedwater pump.
Feedwater flow is manuall/
regulated to maintain adequate water levels in the steam generato) 2 As power is increased and sufficient high pressure steam is available, a main feedwater pump is started and the diesel driven auxiliary feedwater pump is shut down.
For feedwater requirements of 3
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about 4% to about 20% of full power, feedwater is manually controlled and supplied via low flow bypass lines which bypass the main feedwater regulating valve in each main feedwater line. The bypass regulating valve in each bypass line allows more accurate and responsive feedwater flow control than would be possible with the larger main regulating valves during low power (and low feedwater flow) operation.
Above feedwater requirements of about 20% of full reactor power, feedwater control is shifted to the main regulating valves. As power is increased, the second main feedwater pump is started and feedwater flow is placed under automatic control.
Automatic initiation of auxiliary feedwater flow will result upon receipt of one or more auxiliary feedwater pump startup signals.
The diesel driven and turbine driven pumps both start on:
- 1) the coincidence of two out of three steam generator low-low water levels from any steam generator, 2) the tripping of both main feedwater pumps,
- 3) a safety injection signal (SIS), or 4) a turbine trip coincident with the loss of both offsite power sources ~.
Plant design saecifications allow for a maximum delay of one minute from receipt of any auxiliary feedwater pump startup signals to delivery of auxiliary feedwater to the steam generators.
Administrative guidelines do not currently exist for limitation of auxiliary feedwater flow during periods of feedring uncovery.
At the operator's discretion, auxiliary feedwater is manually controlled subsequent to actuation to maintain proper water levels in all steam generators.
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III. MEANS TO REDUCE THE POTENTIAL FOR WATER HAMMER 1.
DESCRIPTION The following are means currently employed at the TPS to reduce the potential for steam generator water hammer:
1.
"J" shaped discharge tubes are installed on all steam generator feedrings and all bottom discharge holes are plugged.
In addition, the present design allows for prompt automatic initiation of auxiliary feedwater flow upon the loss of main feedwater flow and/or steam generator feedring uncovery.
2.
A downward turning elbow on each steam generator nozzle eliminates the effective horizontal length of feedwater piping at the entrance to the steam generators.
The "J" shaped discharge tubes were installed on top of the feedrings to provide for too discharge of water rather than bottom discharge.
During periods of feedring uncovery, this arrangement increases the time for complete drainage of the feedrings and associated horizontal feedwater piping from less than one minute to about 30 minutes. Also, the maximum auxiliary feedwater flow (about 440 gpm cer steam generator) was not sufficient to maintain the feedrings and feedwater piping full of water when the feedrings nad bottom discharge holes.
The feedrings equipped with "J" shaped discharge tubes, however, permit feedwater flow rates as Icw as about 10 gpm oer steam generator to keep the feedrings and feedwater piping full of water until feedring recovery occurs.
Substantial drainage of the feedrings and piping via the feedring fitting clearance does not occur for about five minutes which allows time for 1) automatic actuation of the auxiliary feedwater system after the loss of main feedwater flow or 2) the operator to reestablish steam generator water level during startuo and low power operating conditions during which 3
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the water level drops below the feedrings in one or more steam generators.
The potential for water hammer is avoided if the feedrings and feedwater piping are kept full of water.
The prompt automatic startup of one or both auxiliary feedwater pumps after the loss of main feedwater flow provides feedwater flow to keep the feedrings and feedwater piping full of water until feedring recovery.
Because the "J" shaped discharge tubes reduce the leakage from the feedring, the auxiliary feedwater flow from either the diesel driven pump or the turbine driven pump is more than sufficient to keep the feedwater system full of water.
The main feedwater piping was modified to minimize the horizontal piping sections at the entrance to the steam generators. A downward turning elbow was attached to the feedwater nozzle of each steam generator which effectively eliminates any horizontal piping at or above the elevations of the steam generator feedwater nozzles.
This arrangement eliminates the sections of piping adjacent to the steam generators that could drain and become steam-filled during periods of feedring uncovery. The potential for conditions conducive to steam generator water hamer are therefore avoided in these piping sections.
Tests were conducted on September 28, 1975 at the TPS to determine the effect of auxiliary feedwater flow rates on the inducement of steam generator water ha= er.
The eight tests simulated various normal and hyDothetical operating conditions. Auxiliary feedwater was admitted into a steam generator at flow rates ranging from 120 gpm to 440 gpm.
The steam generator pressure was varied from 400 psig to 1100 psig and the feedring drain times (feedring full at time zero) were varied from 1 minute to 120 minutes.
Water hammer was not observed in any of the tests at the TPS.
The tests show that the TPS feedwater system may therefore not be susceptible to steam generator water hammer regardless of auxiliary feedwater flow, reedring drain time, or steam generator pressure.
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2.
EFFECTIVENESS DURING TRANSIENTS AND CONDITIONS CONDUCIVE TO WATER HAMfiER The normal and hypothetical transients and conditions conducive to steam generator water hammer are discussed in this section. With the exception of subsection 2.4 entitled " Operator Error", each subsection describes a transient resulting from a single initiating event or f ailure with the unit in normal power operation.
Potential component or system f ailures as a direct result of a hypothetical steam generator water hammer are accounted for in the analysis.
A single criterion was the basis for evaluating the effectiveness of the means to adequately reduce the potential for steam generator water hammer.
The criterion is to maintain the feedwater system full of water during the time from the initiating event resulting in feedring uncovery to subsequent feedring recovery and stabilized steam generator water inventory.
2.1 Reactor Trio A reactor trip with the plant in normal power operation would result in a turbine trip and cause the water level in all steam generators to collapse to a level below the feedrings. Within 60 seconds of the resulting steam generator low-low water level signals, the diesel driven and turbine driven auxiliary feedwater ' umps would automatically start and supply auxiliary feedwater to the steam generators.
If the initiating event for the reactor trip did not close the main feedwater regulating valves, the valves would close upon receipt of:
- 1) low primary coolant aver.ge tem::erature signals,
- 2) steam generator high-high water leve'. signals, or 3) an SIS.
Auxiliary feedwater would then be manually controlled to restore the water levels in the steam generators and maintain the levels above the feedrings.
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The potential for water hammer occurring in the feedring or feedwater piping after a reactor trip is very low because the main and auxiliary feedwater keeps the feedrings and feedwater piping full of water.
2.2 Loss of Main Feedwater Flow The main feedwater supply could be interrupted due to the 1) loss of offsite power, 2) malfunction or tripping of the main feedwater pumos, 3) loss of suction to the main feedwater pumps, or 4) closure of the main feedwater regulating and/or isolation valves. A reactor trip would occur upon receipt of the resulting steam /feedsater flow mismatch signals and icw steam generator water level signals.
The reactor trip would cause the water levels in all steam generators to collapse to a level below the feedrings.
The diesel driven and turbine driven auxiliary feedwater pumps would start upon receipt of the subsequent low-low steam generator water level signals.
Auxiliary feedwater would then be used to refill the steam generators and recover the feedrings.
The loss of main feedwater flow and the likely uncovery of the feedrings would not result in substantial feedring and feedwater piping drainage since the auxiliary feedwater pumps would start promptly to sucoly feedwater to the steam generators.
Therefore, the potential for water hammer is significantly reduced.
2.3 Less of Offsite Power The complete interruption of off-site power would result in a reactor trip and automatic startup of the emergency diesel generators.
Automatic initiation of the diesel driven and turbine driven auxiliary feedwater systems would occur to supply feedwater to the steam generators. The redundant auxiliary feedwater systems are fully functional without off-site power since the diesel generators and DC batteries can supply all necessary electrical power to both systems.
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As was the case for the loss of main feedwater flow, auxiliary feedwater flow would maintain the feedrings and feedwater piping full of water until feedring recovery occurs and again the potential for water hammer would be very Icw.
2.4 Ooerator Error The potential for water hammer in the feedwater system increases if uncovered feedrings are allowed to drain substantially after an event causes the steam generator water levels to drop below the feedrings. Admission of feedwater into the drained feedrings and horizontal feedwater piping could then result in water slugging and subsequent water hammer.
The uncovery of one or ;oth feedrings is most likely when the plant is operating at low power or during startup since feedwater is being regulated manually, rather than automatically.
For this situation, an administrative limit of 150 gpm on auxiliary feedwater flow has been recommended in Reference 6 based on tests Howe'er, the tests conducted at performed at Indian Point Unit No. 2.
v the TPS (described in Section III.1) show that no water hammer occurred under these conditions.
Therefore such administrative controls are not necessary for the TPS.
2.5 Steam Line Break The potential for steam generator water hraner events resulting from or concurrent with the rupture of a steam line inside containment was considered.
The sequerce of events following such a f ailure was evaluated to determine if the break could result in the 1) blowdown of one or more additional steam generators and/or 2) inability to supply auxiliary feedwater to the unaffected steam generators.
The rupture of a steam line would automatically result in an SIS cauing a reactor trip, a turbine trip, and isolation of all main feedwater lines.
The loss of main feedwater flow to the steam generators would result in the automatic startup of the diesel driven and turbine driven auxiliary feedwater pumps upon receipt of low-low 9
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steam generator water level signals.
Isolation valves in the auxiliary feedwater lines to the affected steam generator would cut off auxiliary feedwater upon receipt of high flow (600 gpm) signals sensed in the lines. Auxiliary feedwater would continue to be supplied fur subsequent refill of the unaffected steam generators and recovery of the feedrings.
The blowdown of a steam generator would not deprive the turbine driven auxiliary feedwater pumo of driving steam.
A check valve in each steam supply line would prevent " crossover" blewdown through the supply lines from the unaffected steam generators to the associated blowndown steam generator.
The potential for water hammer is low after a steam line break since prompt delivery of auxiliary feedwater in m junction with the "J" shaced discharge tubes maintain full feedrings and feedwater piping in the unaffected steam generators.
2.6 Loss-of-Coolant Accident The potential for feedwater water hammer during a postulated loss-of-coolant accident (LOCA) in either unit was examined because
- 1) a water hammer could increase the consequences of a LOCA and 2) the plant protective actions during a LOCA could resul; in conditions which are conducive to water hammer if the feedwater system is not kept full of water.
A LOCA would result in an SIS, a reactor trip, a turbine trip, and subsequent isolation of the feedwater system.
The startup of the diesel driven and turbine driven auxiliary feedwater pumps would result and feedwater would be supplied to the steam generators within 60 seconds of the reactor trip.
Refill of the steam generators anc recovery of the feedrings would occur in a manner typical of a reactor trip or the loss of offsite power.
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The conditions conducive to water hat:ner in the feedrings and feedwater piping resulting from a LOCA would be very similar to those resulting from a reactor trip. Therefore, the means to reduce the potential for water hammer would be fully effective during a LOCA.
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IV.
CONCLUSIONS AND RECOMMENDATIONS The asset.sment of the capability of existing meant to reduce the potential for steam generator water hammer during normal and hypothetical operating conditions was discussed in Section III.
This assessment has shown that under conditions which are most conducive to water hammer in the feedwater systems (specifically, uncovered and draining feedrings and feedwater piping subjected to sdmission of cold auxiliary feedwater), the means available to reduce the potentisl for water hammer at the TPS are adequate to maintain sufficientif full feedrings and feedwater piping. Keecing the feedrings and feedwater piping full of water avo* the potential for water hammer.
Therefore, we conclude that the
...c..., to reduce the potential for steam generator water hammer at this facility are adequate and we recommend acceptance by the NRC staff.
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V.
REFERENCES 1.
J. L. Williams, Portland General Elec ric Company (PGEC), letter to W. R. Butler, NRC, Subject
" Transmittal of report entitled
' Report of Steam Generator Feedwater Hammer Design Considerations for Trojan Nuclear Plant of the PGEC'", August 6, 1975.
2.
J. L. Williams, PGEC, letter to W. R. Butler, NRC, Subject -
" Transmittals of report entitled ' Report on Testing of Auxiliary Feedwater Addition, Following J-Tube Modification to the Steam Generators of the Trojan Nuclear Plant'", October 21, 1975.
3.
A. J. Porter, PGEC, letter to A. Schwencer, NRC, Subject -
" Transmittal of report entitled ' Report on Testing of J-Tube Modification to the Steam Generators with Reactor Decay Heat at the Trojan Nuclear Plant'", July 12, 1976.
4 J. M. Cohner, Dynamic Resoonse of the Trojan Feedwater Pice Due to,
Feed eter Water Hammer for PGEC, Bechtel Power Corporation, Transnatted to NRC on October 22, 1976.
5.
Final Safety Analysis Report, Trojan Nuclear Plant, PGEC, NRC Docket No. 50-344.
6.
J. B. Block, et al, An Evaluation of PWR Stean: Generator Water Ham g, Creare, Inc. NUREG-0291 (December..,76).
7.
W. E. Bennett, Waterhammer in Steam Generator Feedwater Lines, Westinghouse Technical Bulletin, NSD-T3-75-7 (June 10,1975).
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