ML20210E214
| ML20210E214 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 03/24/1986 |
| From: | Tucker H DUKE POWER CO. |
| To: | Harold Denton, Youngblood B Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8603270279 | |
| Download: ML20210E214 (7) | |
Text
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DUKE POWER GOMPAhT P.O. HOx 33180 CHAMLOTTE, N.C. 28242 HALU. TUCKER (704) ora-4mn lutELEAR,30BCCTBDM March 24, 1986 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C.
20555 Attention:
Mr. B. J. Youngblood, Project Director PWR Project Directorate No. 4 Re: Catawba Nuclear Station Docket Nos.40-413 and 50-414
Dear Sir:
By letter dated March 6,1986, NRC staff requested additional information regarding elimination of arbitrary intermediate pipe breaks for the pressurizer surge lines and main feedwater system at Catawba.
Accordingly, please find attached the Duke response to this request.
It there are any questions please advise through normal licensing channels.
Very truly yours, Yd //
t
/
w Hal B. Tucker ROS:slb Attachment xc:
Dr. J. Nelson Grace, Regional Administrator U. S. Nuclear Regulatory Commission l
Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Dr. K. Jabbour Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission l
Washington, D. C.
20555 NRC Resident Inspector gDk Catawba Nuclear Station
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8603270279 860324
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PDR ADOCK 05000413 P
SYSTEM DESIGN AND OPERATING PROCEDURES FOR MINIMIZING WATER HAMMER IN THE CATAWBA FEEDWATER AND AUXILIARY FEEDWATER SYSTEMS General The Catawba Auxiliary Feedwater System (CA) and Feedwater System (CF) design and operating procedures include provisions to prevent water hammer in these systems. Westinghouse has provided design and operating recommendations concerning water hammer prevention in these systems.
The Catawba design and operating procedures are in compliance with these Westinghouse recommendations.
Anti-water hammer design and operating considerations are described below.
Feedwater System A separate Feedwater System (CF) serves each of th'e two Catawba units.
The Unit 1 and Unit 2 CF Systems are similar but not identical.
The major differ-ences result from the use of different model steam generators in the two units.
Differences between the two units' CF Systems will be stated throughout this description as appropriate.
Design features and operational procedures are provided in the design of the Feedwater System to preclude the possibility of water hammer.
Studies and tests by the steam generator manufacturer have indiciated that pressure tran-sients may be induced in the preheater section of the steam generators under certain conditions of operation.
Pressure pulses can be produced in the preheater or feedwater piping by the formation of one or more steam bubbles which subsequently come into contact with cold (relative to the temperature of the steam in the bubble) water.
Very rapid condensation of the steam bubble may cause a pressure pulse.
The Feedwater Bypass System is implemented to allow the operator to avert possible pressure transient conditions in the preheater and feedwater piping.
The Feedwater Bypass System provides a connec-tion between the auxiliary feedwater nozzle and the main feedwater line as shown on the attached diagrams.
To minimize the potential for occurrence of pressure transients in the steam generator preheater and in the feedwater piping connecting to the steam genera-tor, it is necessary to prevent the introduction of cold water to the steam generator through the main feedwater nozzle at any time when significant void may be present.
During startup, shutdown, hot standby, and low load operation, the feedwater is delivered via the bypass line and through the steam generator auxiliary feedwater nozzle.
After proper conditions have been established, feedwater flow is transferred from the bypass line to main feedwater line.
One of the conditions which must be assured prior to transfer consists of establishing the minimum required feedwater temperature throughout the Feed-water System to ensure that cold feedwater will not enter the preheater.
This requires purging the cold feedwater from the main feedwater line between the feedwater isolation valve (FIV) and the main feedwater nozzle.
This is accom-plished by reverse purging a low controlled rate of flow from the steam genera-tor main feedwater nozzle through.the feedwater purge valve (FPV) and to the condenser.
MN30134D/1
Replacing cold water in the feedline with hot water whose temperature is above the required value is accomplished by reverse purge to ensure that all cold water is removed and no cold pockets allowed to remain.. Temperature measure -
ments are provided in the feedline at the bottom of the feedwater piping seal loop immediately upstream of the main feedwater nozzle, immediately upstream of the feedwater isolation valve (FIV), and immediately downstream of the feed-water isolation valve.
These temperature measurements are provided to ensure that all water in the feedwater line _is above the required temperature before the feedwater isolation valve is open.
A small flowrate of hot water from the steam generator, bypassed around the feedwater check valve, is employed to purge the cold water from the feedwater line between the main feedwater nozzle and the feedwater isolation valve.
The arrangment shown in Figure 1 & 2 utilizes a small reverse purge line containing the feedwater purge valve (FPV).
The reverse purge line routes the purge flow to the tempering flow line.
The tempering flow line is used during purge to route purge flow to the condenser.
The maximum permissible reverse flushing flowrate is 80,000 lbm/hr per steam generator.
This maximum flow-is specified to prevent voids which could be present in the preheater from being swept into the piping dur'ng the 'puring operation.
Reverse purge flowrate is controlled by the feedwater reverse purge orifice (FRP0).
This orifice is sized to pass 80,000 lbm/hr total, or 20,000 lbm/hr per steam generator.
Reverse purge must be maintained for a time greater than or equal to 30 minutes, and the feedwater temperature to the main nozzle must be greater than or equal to 250 F before opening the feedwater isolation valve (FIV).
A connection is provided between the auxiliary feedwater nozzle of each steam generator and the main feedwater equalization header following the feedwater heaters.
A small tempering flow is provided to the auxiliary nozzle at all times when other feedwater flow is not diverted to the auxiliary nozzle, except when a feedwater isolation signal is activated.
This flow cools the inner surfaces of the auxiliary nozzle and. adjacent connection piping and maintains the water temperatures in the piping connected to the nozzle at approximately feedwater temperature, which should cause the thermal stresses induced in the nozzle and connecting pipe to be reduced when main feedwater flow is trans-ferred to the au>iliary nozzle.
Under normal operating conditions, this tempering flow ensures that the piping connecting to the auxiliary feedwater nozzle is kept full of water preventing backleakage into this piping from the steam generator.
Steam generator tube wear problems have been experienced by other nuclear stations with steam generators identical to Catawba Unit 2 steam generators.
These tube wear problems are caused by tube vibrations within the preheater section of the steam generators.
These vibrations are caused by high localized velocities in the preheater.
In order to minimize ~the potential for tube vibration, modifications have been made to the original design to reduce feedwater flowrate into the preheater.
Additionally, selected tubes within the preheater have been expanded to give a tighter fit with the tube sheet and reduce vibration.
On Unit 2, a split feedwater flow arrangement is used to limit flowrate into the main feedwater nozzle at high loads.
This split feedwater delivery arrange-ment is shown on Figure 2.
Feedwater flowrate into the main feedwater nozzles is limited at high loads by diverting a portion of the flow to the auxiliary feedwater nozzle through the feedwater preheater bypass valves (FPBV) and the i
MN30134D/2
feedwater. bypass piping.
A flow restricting orifice is installed _ in the main feedline downstream of the feedwater bypass piping takeoff to provide the necessary pressure differential to attain the desired bypass line flowrate.
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The main feedwater flow restricting orifice (MFFR) is sized to give a. bypass
-flow of 13% of rated flow at 100% load.
Waterhammer is avoided during initial phases of startup by filling and venting the Condensate and Feedwater Systems at a controlled low flowrate.
All air or steam voids are removed prior to initiating full flow through the system.
Voids in the piping upstream of the feedwater control valves are removed by.
slowly filling the system using the hotwell pumps with flow controlled by a flow restricting orifice.
Piping' downstream of the feedwater control valves including the preheater bypass piping and tempering flow piping is purged of voids by slowly filling the piping using the feedwater control bypass valves and either a manual valve or flow restricting orifice to limit the fill rate.
Auxiliary Feedwater System The Auxiliary Feedwater System provides feedwater to the steam generator auxiliary nozzle via the same piping utilized by the Feedwater Bypass System.
Piping to the steam ~ generator auxiliary. nozzle is routed to minimize steam voids by utilizing a 90 elbow at the nozzle. connected directly.to a vertical pipe run.
Reverse flow of steam or hot water from the steam generators into
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piping connecting to the auxiliary feedwater nozzles can result.in void forma-tion within the preheater bypass piping and the possibility of water hammer.
4 This condition should not occur during normal operating conditions since some form of feedwater flow is directed to the auxiliary feedwater nozzles at all times the unit is at load.
Additionally, the steam generator programmed water level is above the auxiliary feedwater nozzles during 100% power operation of Unit 1 and at all loads for Unit 2.
If back leakage does occur, it would normally be water, not steam. However, to guard against any abnormal operating conditions which might result in void formation, instrumentation is installed to detect potential voids and alarm the control room.
A temperature element is located immediately upstream of each steam generator auxiliary feedwater nozzle in the associated feedwater preheater bypass piping.
This instrumentation monitors temperature in this piping and compares it to the-corresponding loop main steam temperature.
A computer alarm is given or, decreasing temperature differential at' anytime the differential setpoint is reached and main steam temperature is greater than 212 F.
If this alarm is actuated, the piping should be slowly flushed with feedwater or auxiliary feedwater, or the unit may be brought to a cold _ shutdown condition feeding only those steam generators which have.not experienced reverse flow..To flush this piping with feedwater, tempering flow throttling valve CF105 should be used to achieve a low flush flowrate by cracking the valve'open.
This method can only_
be used if feedwater flow has been lost to all steam generator auxiliary feedwater nozzles. -To flush this piping with auxiliary feedwater, the appro-priate CA pump should be started against closed control: valves, and the appro-priate control valve should be cracked open to slowly flush the line.
Backleakage of hot feedwater into low pressure portions of a normally non-operating CA System can result in steam void formation and disabling of the system.
Check valves and isolation valves separate the portion of the system MN30134D/3 4
normally under low pressure from th'e high pressure and high temperature feed-water bypass or tempering' flow during normal operation.
Temperature elements have been installed upstream of these check valves to monitor for backleakage.
The operator aid computer indicates these temperatures and provides a high temperature alarm and additional alarms on a given incremental increase.
If temperature at any of.the temperature monitoring points reaches the high setpoint which will actuate a high temperature alarm, the associated CA pump should be run to flush the.line.
The appropriate CA pump should be started against closed control valves, and the control valve in the hot line should be cracked open to slowly flush the line.
Depending on the severity of the backleakage, it may be advisable to leave the pump running until the check valve can be repaired.
If temperature at any of the temperature monitoring points reaches the high high setpoint which will actuate the incremental temperature increasing alarm, a steam void may be present and special precautions should be taken to prevent water hammer in the affected line and assure the other CA trains remain avail-able. Under these conditions, the extent of the possible steam void or hot water ' slug should be determined by making temperature nieasurements at appropri-ate points along the pipe.
If high temperatures are limited to points down-stream of the associated control valve, the appropriate CA' pump should be started against closed control valves, and the control valve in the hot line should be cracked open to slowly flush the line.
If hign temperatures extend upstream of. the associated control valve, the possible void or associated CA pump should be isolated.
The leaking check valve should be isolated and repaired as soon as possible.
If repairs cannot be achieved in the time frame allowed in the Catawba Technical Specificiations, then Technical Specification required actions should be followed.
There are two check valves in each flow path by which backleakage could occur into the Auxiliary Feedwater System.
Both check valves in a given flow path would have to leak for a steam void to form.
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MN30134D/4
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UNIT I FEEDtIATER BYPASS ARRANGEMENT
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to condenser l
i from auxillary E
7 FRPO Steam from main
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Generator
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'l' t
M feedwater equalization "I.l la ry feedwa ter FDTV FBTO CF105 header AFFE nozzle j
T g
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FPV FPBV feedwa ter H
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from main g
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hd A
1 1
feedwater equalizction e
j
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n 1
FCBV i
I l.Ist of Abbreviations
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j AFFE auxillary feedwater flow element FBTO feedwater. bypass tempering orifice F8TV feedwater bypass temper,Ing valve:
FCBV feedwater. control bypass valve FCV feedwater control valve Fly feedwater*lsolatIon valve FPDV feedwater preheater bypass valve j
FPV feedwater purge valve l
FRPO feedwater reverse purge orifice HFFE main feedwater flow element j
Figure I
UNIT 2 FEEDWATER BYPASS ARRANGEMENT to condenser from Auxiliary E
=
E Feedwater Steam FRP0 ~
Generator
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feedwater FBTV FET'O from main i
' Aux.
ATFE feedwater equalization CF105 header lozzle N
A FPBV P
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FPV main H
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from main FI r}f FR IIEE CV feedwater equalization header
,-CB List of Abbreviations AFFE auxiliary feedwater flow element FBT0 feedwater bypass tempering orifice FBTV feedwater bypass tempering valve FCBV feedwater control bypass valve FCV feedwater control valve FIV feedwater isolation valve FPBV feedwater preheater byp. ass valve FPV feedwater purge. valve FRP0 feedwater reverse purge or,1fice MFFE main feedwater flow element FIGURE 2 MFFR main feedwater flow restricting orifice
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