ML18081A617

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Technical Evaluation of Steam Generator Water Hammer
ML18081A617
Person / Time
Site: Salem PSEG icon.png
Issue date: 01/31/1979
From:
EG&G, INC.
To:
Shared Package
ML18081A615 List:
References
NUDOCS 7911280228
Download: ML18081A617 (15)
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Text

TECHNICAL EVALUATION SALEM UNIT NO. 1 JANUARY 1979 EG&G IDAHO, INC.

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CONTENTS I.

INTRODUCTION 1

II.

FEEDWATER SYSTEM.......................

2

1.

DESCRIPTION ***................... *,. 2

  • 2.

GENERAL OPERATION.............

.III.

MEANS TO REDUCE. THE POTENTIAL FOR WATER H.;MMER.

'l.

DESCRIPTION. * **.... * * *. *

. 2.

EFFECTIVENESS DURING TRANSIENTS AND CONDITIONS CONDUCIVE TO WATER H~~MER.

2.1 ReaCtor Trip.........

2.2 Loss of Main Feedwater Pumps..

2.3. Loss* of Off-Site Power 2.4 Operator Error.....

2.5 Steam Line Break....

2.6 Loss-of-Coolant Accident 4

5 5

7 7

8 8

9 9

10 IV.

CON CL US IONS AND RECOMMENDATIONS *.. * * * * *.. * * * *.

12 V.

REFERENCES **.*..*

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I.

INTRODUCTION An evaluation was performed for the Salem Unit No. 1 feedwater system. This evaluation was concerned with the effectiveness of the means to redµce the potential for water hamner during normal and hypothesized operating conditions. The potential of water harrrner due to steam-water slugging was considered.

A known steam generator water harrmer event has not occurred to date at Salem Unit No. l due to two-phase fluid instab~lities.

However, procedural and hardware modifications to further re.jucethe p.otential for water hamner at this faci1ity were implement~d. These modifications were selected based on a review of previous water hanmer e~ents at other facilities and measures considered effective in reducing the potential for feedwater water halffiler*.

There have been no water hanrner tests conducted to date at Sal em Unit No. 1 because the licensee concluded that the benefits that would be realized do not warrant subjecting the system to the conditions required in such testing.

The potential for water harrrner in PWR feedwater sy.stems is avoided if the system.is maintained full of water during normal and transient conditions. Therefore, this evaluation was based on the effectiveness of the means to maintain the feedwater systa~ full of water.

The information for this review was obtained from

1) discussions }ith the licensee, 2) its submittals of Jurie 21,*1975(l and January 10, 1978[2], 3) the Salem Fi,nal Faci1 ity Description and Safety Analysis Report[3], and 4) *"An Evaluation of PWR Steam Generator Water Hamner", NUREG-0291[ 41.

A description of the fegdwater system at Salem Unit No. 1 and its general operation are presented in Section II.

The means to reduce the potential for water hCiJ"l?Tier are presented in Sec~ion III including a general discussion of their effectiveness during transients and conditions conducive to water haiirner.

Finally, conclusions and recorrrnendations are presented in Section IV concerning the adequacy of the means to reduce the potential for water hamner at this facility.*

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II.

FEEDWATER SYSTEM

l. DESCRIPTION The feedwater system for Salem Unit No. 1 was designed to provide an adequate supply of feedwater to the secondary side of the four steam generators duri.ng all operationar conditions. The two main feedwater pumps are one-half capacity, high speed barrel-type pumps.

Each pump, rated at a flow rate of 18 1 600 gpm at a total developed head (TDH) of 884 psi, supplies main feedwater to the steam

  • ge.nerators. The pumps are each driven, during normal two pt.<<np operation, by a variable speed steam turbine supplied with steam from the outlet of the reheater moisture separators. Steam is supplied from the main steam header during low power and startup conditions.

The pumps are supplied with feedwater by the heater drain pumps and the condensate pumps via a coliil1on discharge header of the low pressur.e heater banks.

Feedwater from the main feedwater pu~ps is supplied to a main he~d~r via the h~g~ pressure heaters. The main header splits into four 14 in. feedwater lines to supply a feedring inside each steam gi;nerator. Feedwater i.s discharged downward through inverted "J" shaped tubes on top of the feedrings.

The auxiliary feedwater system provides feedwater to the steam generators for primary heat removal during low power operation and reactor startup and shut down.

Aux i 1 i ary f eedwater can be supp 1 i ed by

  • two redundant and diverse systems employing electric motor driven and steam turbine driven auxiliary feedwater pumps.

Lines from the auxiliary feedwater pumps carry water to the main feedwater lines at a point in each main 1ine just outside of the containment building.

The two motor driven pumps are each rated for 440 gpm at 1300 psi TDH and each supp 1 i es two steam generators. ihese pumps operate *...-i th

~ormal off-site power or, if off-site power is lost, power is supplied by the ~~ergency diesel generators. ihe turbine driven pump, rated t*:::::

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for 880 gpm at 1550 psi TOH to supply all four steam generators, is driven by steam suppl.ied from the outlet headers of two steam generators. The main water supply source for both auxiliary systems is the 220,000 gallon auxiliary feedwater storage tank.

Backup water sources for the auxiliary feedwater systems are the two demineralized water storage tanks (500,000 gallon capacity each), the two fire protection and domestic water storage tanks (350,000 gallon capacit.)I" each)/ and the station service water system.

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2.

GENERAL OPERATION During normal power operation of the reactor, the main feedwater system supplies fee.dwater to the steam generators for heat removal from the primary system. The feedwater flow is regulated to each steam generator by individual regulating valves in the main feedwater lines. The positions of the valves are automatically controlled ba~d on the steam generator level, secondary steam flow, and feedwater flow." At low power levels and during startup and hot standby conditions, feedwater is manually regulated to maintain adequate water

  • levels in the steam generators. During the*se conditions, lcw flow bypass lines. and feedwater bypass control valves provide more responsive and accurate feedwater flow adjustments by the operator.

After the loss of main feedwater flow to one or more steam generators, automatic initiation of auxiliary feedwater flow *..,.il1 res*u 1 t upon recei p_t of one or more aux Hi ary f eedwater pump startup signals. The motor driven auxiliary feedwater pumps auto~atically start on: 1) coincidence of two out of th.ree steam generator low-low water* 1 eve i s i gna 1S from any one steam generator, 2) the tripping of

-the main feedwater pumps, 3) a safety injection signal or 4) loss of offsite*power. The tur.bine driven auxiliary feedwater pump automatically starts on: 1) coincidence of two out of three steam generator low-low water level signals from any two steam generators or

2) loss of offsite power.

The motor driven and turbine driven pumps can also be started manually (local or remote).

Original plant design specifications allow for a maximum delay of one minute from.loss of main feedwater flow to delivery of auxiliary

Manual flow control is continued to bring the water leve*ls above the feedrings and to maintain adequa~e levels in the steam generators.

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III.

MEANS TO REDUCE THE POTENTIAL FOR WATER HAMMER

1.

DESCRIPTION The following means are employed at Salem Unit No. 1 to reduce the potential for water harrmer in the feedwater system:

1.

11 J 11 shaped discharge tubes were installed on top of the steam generator feedrings and the bottom discharge holes were plugged.

2.

"Loop seals" wel'.'e installed *in the feedwater piping to reduce the effective horizontal length adjacent to the steam generators.

3.
4.

Administrative control limits the feedwater flow to about 150 gpm pe~ steam generator when the water level in any steam generator is below the feedring.

ihe 11 J 11 shaped discharge tubes were installed on top of the feedrings to provide for top discharge of water rather than bottom dis.charge. During periods of feedring uncovery, this arrangement increases the time for complete drainage of the feedring and associated horizontal feedwater piping from less than 1 minute to' about 30 minutes.

Also, the maximum a.uxiliary feedwater flow (about 440 gpm per steam generator) was not sufficient to maintain the feedrings and feedwater piping full of water when the feedrings had bottom discharge holes. The feedrings equipped with "J" shaped discilarge tubes, however, permit feedwater flow rates as low as about 10 gpm per steam generator to keep the feedrings and feedwater piping

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fu.11 of water until feedring recovery occurs. Substantial drainage of the feedring ~nd pipipg doel not occur for about 5 minutes which allows time for 1.) automatic actuation of the auxiliary feed*,o1ater system after the loss of main feedwater flow during normal power operation or 2) the operator to reestablish steam generator water level during startup and low power operating conditions during which the water level drops below the feedrings in one or more.steam tit generators. Maintaining the feedrings* and feedwater piping full of water* while the feedrings are uncovered eliminates the potential for water hamTier.

"Loop seals" were installed in the main feedwater p1p1ng adjacent to each of the steam generators.

Each seal consists of two 45° elbows arranged so the bottom of the feedwater nozzle is six inches or more above the top of the feedwater piping upstream of the elbows.

  • This piping modification reduces the effective horizontal piping run

.to *about four feet. adjacent to the steam generators that could drain and become steam-filled during periods of feedring uncove~y. By reducing this length of piping, the potential for water harimer is also re.du.Ced~

Th~ prompt automatic startup of the 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.

In conjunction with the "J" shaped discharge tubes, auxiliary feedwater flow from either the motor driven pumps or the turpine driven pump is more than sufficient to maintain the feedwater

  • system fu 11 of water.

Tests conducted at Indian Point Unit No. 2 have confirmed that

educed feedwater flow to steam generators 'liith uncovered and draining feedrings reduces the potential for water harrrner.

Evidence of water h~-mier was observed in steam generators with uncovered and drained feedrings when feedwater was delivered at greater than 200 gpm per steam generator under hot standby test conditions.

The water har.:ner was hypothesized to be the result of turbulent, wavy flow patterns in

the feedwater piping creating a water slug that "sealed" the piping and isolated a pocket. of steam. Subsequent co11apse of the condensing steam pocket caused the slug to accelerate, resulting in the water hanmer.

Based on the tests, an administrative feedwater flow limitation of about 150 gpm (includes a 50 gpm uncertainty factor) was adopted~t Salem Unit No. 1. This flow limitation is implemented any time the steani generator water levels drop below the feedrings. A two minute delay time is included in the procedures to a11ow time-for the

  • operator to determine that feedring uncovery has occurred a~d to manua~ly regulate the fee~water flow accordingly.
2.

EFFECTIVENESS DURING TRANSIENTS ANO CONDITIONS CONDUCIVE TO WATER HAMMER 2.1* Reactor Trip.

A reactor trip with the plant in nor:ma1 power operation would resu"it in a turbine'trip and cause the water 1eve1 in all steam g_enerators to co11apse to a level below the feedrings. Within 60 seconds of the resulting steam generator low-low water level signals, the motor driven and turbine driven auxiliary feedwater pumps would automatically startup and supply auxiliary feedwater to the steam generators. If the initiating event for tne reactor trip did not close the main feedwater regulating valves (as would, for example, a s~fety injection signal), the valves would close upon receipt of low

  • primary coolant average temperature signals or steam generator high-high level signals. Auxiliary feedwater flow is normally under manual control shortly after an event that causes feedring uncovery.

The flow is regulated to refill and maintain the steam generator levels above the feedrings.

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The potential for water hamner occurring in the feedring or feedwater piping after a reactor trip is very low because* the main and auxi1 i ary feedwat_er keeps the feedrings and feedwater piping ful 1 of water until feedring recovery occurs. 2.2 Loss of Main Feedwater Pumps Any event that results in malfunction or isolation of the main feedwater pumps will result in automatic startup of the turbine driven and motor driven auxiliary feedwater pumps upon receipt of th,: steam

  • generator low-low water level signals. Either auxiliary feedwater system can provide more than sufficient flow to the feedrir.gs and associated horizontal feedwater piping of the steam generators since the 11 J 11 tubes reduce the drainage rate to about 10 gpm per steam generator during feedring uncovery periods *

. The loss of mairi feedwater flow and the likely uncovery of the ~eedrings would n6t result in substantial feedring and feedwater piping drainage since the auxiliary feedwater pumps would start up prcmptly to supply*feedwater to the steam generators. Therefore, the .potential for water harrmer is significantly reduced. - 2.3 Loss of Off-Site Power The interruption of the off-site power supply would result in a reactor trip and automatic startup of the emergency diesel generators. Automatic initiation of the motor 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 necessary electrical power to both systems.

As was the case for the loss of the main feedwater pumps, auxiliary feedwater ~aintains the feedrings and feedwater* piping full of water until feedring recovery occurs and again the potential fa~ water ha!Tl'iler is.very low. 2.4 Ooerator Error The potential for water harrrner iri the feedwater system increases greatly if uncovered feedrings are allowed to drain substantially after an event causes the steam generator water level* to drcp below the feedr5ngs. Admission of feedwater into the drained feedrings and feedwater piping could then result in water slugging and s~bsequent water harrrner. The uncovery of one or more feedrings is most likely when the plant is operating at low power or is shut down since feedwater is being regulated manually, rather than automatically. The use of ~he main feedwater bypass piping and associated regulating valves helps reduce ~he chance of feedring uncovery during low power situations since the feedwater flow rate is more easily and finely regulated by the operator. Should feedring uncovery occur, the "J" shaped discharge tubes ~eep the feedrings fro~ draining substantially for about 5 minutes. This time delay would allow sufficient time for the operator to become

  • aware of. feedring uncovery and to establish sufficient auxiliary feedwater flow to maintain the feedings and feedwatar piping full of water. Shouid feedring uncovery occur and the feedrings and hor.jzontal piping were to drain, feedwater flow would be limited to the administrative 1 imit of 150 gpm until recovery to ensure a low potential of water slugging.

2.5 Steam Line Break The potential for water harrmer events resulting from or con~urrent with the rupture of a steam line inside containment was considered. The sequence of events following such a failure was .9

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evaluated to determine if the break would result *in the 1) blowdown of one or more additional steam generators and/or 2) inab11ity to supply auxiliary feedwater to the unaffected steam generators. The rupture of a steam l}ne would automatically result in a safety injection signal (SIS) and subsequent isolation of all feedwater lines. The loss of main feedwater flow to the steam generators would result in the automatic startup of the motor driven and turbine driven auxiliary feedwater pumps. Feedwater would then be supplied for subsequent refill of the steam generators-and re:overy of

  • the feedrings.

The potential for water hal'!iller is low after a ste~~ line break since prompt delivery of auxiliary feedwater in conjunction with the "J"-tubes ~aintain full feedrings and feedwater piping in the unaffec~ed steam gen~rators until feedring recovery.* The turbine ¢riven auxiliary feedwater pump would receive adequate steam for driving power even if one of the two interconnecte_d steam.lines for the pump turbine. w~re supplied by the blQwndown steam generator. Check valves in each supply li~e would prevent "crossover" blowdown through the supply lines from one steam generator to the associated blowndoWn steam generator. Thus the means for avoiding water harrmer would be fully effective under the conditions of a steam line break. 2.6 Loss~of-Coolant Accident The potential for steam generator water harrmer during a postulated loss-of-coolant accident (LOCA) was examined because 1) a ruptura of the feedwater piping could increase the consequences of a LOCA and 2) the plant protective actions during a LOCA could result in conditions which are conducive to water hamner if the feedwater system is rrot kept full of water. r:: i.

~ A LOCA would result in a SIS, a reactor trip, and subsequent isolation of the feed~ater system. The startup of the motor driven and turbine dri~en auxiliary feedwater pumps would result and feedwater would be supplied ta the steam generators with 60 seconds of the reactor trip. Refill of the stea~ generators and recovery of the feedrings would occur in a manner typical of a reactor trip or the lass of off-site power. ~ The conditions conducive to water harrmer in the feedring and feedwater piping resulting from a LOCA would be very similar to those from a reactor trip. Therefore, the means to reduce the potential for water harrrner would be fully effective during a LOCA.

IY. CONCLUSIONS AND RECOMMENOAiIONS The assessment cif the capability of existing means to reduce the potential for water hal'ililer during various hypothesized transients and conditions was discussed in Section III. This assessment has shown that under conditions which are most conducive to water harrrner in the feedwater system (specifically, uncovered and draining feedrings an~ feedwater piping subjected to admission of cold auxiliary feedwater), the means to* r.educe the potential for water harrrner at Salem Unit No-. 1 are adequate* to maintain sufficiently full feedrings and feed,'fater . piping until feedring recovery occurs. Therefore, since keeping the feedrings and feedwater piping full of water eliminates ths potential for slugging, we find that the means to reduce the potential for stea~ generator water hamner at this facility are adequate and we recorrrnend acceptance by the NRC staff. , ?

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~ V. REFERENCES

1.

Letter from.R. L. Mittl to W. A. Giambusso, Subject - "Steam Generator Feed Line Water Hamner", June 27, 1975.

2.

Letter from F. P. Librizzi to G. Lear, Subject - 11Stearn Generator Feedwater Water Hamner 11, January 10, 1978.

3.

Final Facility Description and Safety Ana1ysis Report, Salem Unit Nos. 1 and 2, Public Service Electric and Gas Company, COE Docket No. 50-272.

4.

J, A. Block, et al, An Evaluation of PWR Steam Generator Water Hamner, Creare, Inc., NUREG-0291, December 1976.

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