ML20039F453

Forwards Addl Info Re 811023 & 1119 Failure of Diesel Generator Cooling Pumps.Final Mods & Installation Schedule Will Be Provided Upon Completion of Engineering Study. Separate Gages Installed to Indicate Pressure Readings

ML20039F453
Person / Time
Site:	Dresden
Issue date:	12/17/1981
From:	Scott D COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
81-998, NUDOCS 8201130007
Download: ML20039F453 (2)

Text

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/ ) . Dresden Nucle:r Pow son g i ~ b"# / PRINCIPAL STAFF

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• ll/ 1 D/D PA0 At A/D no gp M' Q} N 6rt (N v &PI EU D mber 17, 1981 y P1ITT U DEP&Of File DJS Ltr. #81-998 Mr. James G. Keppler, Regional Administrator Directorate of Regulatory Operations - Region III U. S. Nuclear Regulatory Connission 799 Roosevelt Road Glen Ellyn, IL 60137

SUBJECT:

Additional Information Concerning the Failure of the Diesel Generator Cooling Water Pumps at Dresden Station on October 23, 1981, and November 19, 1981

REFERENCE:

(a) D. Scott letter to J. Keppler dated December 4, 1961

Dear Mr. Keppler:

On December 4, we provided information to your office concerning our investigation into the cause of the overheating of the Unit 3 and Unit 2/3 diesel generators at Dresden Station on October 23, 1981, and November 19, 1981. This information was provided in reference (a). We concluded that the overheating was the result of insufficient cooling water flow, which occurred because of flow restrictions caused by the failure of the cool-ing water pump discharge check valves. As a result of continuing dis-cussions with members of your staff, we are submitting the additional information below.

We are unable at this time to commit to a firm schedule for modify-ing the instrumentation in the diesel generator rooms to assist the oper-atora in determining flow through the diesel generator heat exchangers.

The engineering study discussed in reference (a) has just been initiated, and the detailed modifications that will result from that study have not yet been determined. When the final modifications are identified, it is our intention to proceed as expeditiously as possible in procurring equip-ment, scheduling the necessary outages, and installing the recommended instrumentation. The final modifications and their schedule for install-ation will be availabl=. for review by your resident inspectors upon com-pletion of the engineering study.

Also, as discussed with members of your staf f, we believe that the difference that we observed in " indicated" pressure at the diesel gener-ator cooling water pumps for the Unit 3 and the Unit 2/3 pumps (See page 3 of reference (a) ) was the result of significantly greater bearing lub-ricating water flow in the Unit 2/3 cooling water pump. At the time this test was conducted, we believe the bearing tolerances on the Unit 2/3 pump were greater than those of the Unit 3 pumps and allowed a signifi-cantly greater lubricating flow through the water lines. This increased flow led to a lower " indicated" pressure and contributed to the difference between the Unit 3 and Unit 2/3 pumps as reported to you in reference (a).

8201130007 811217 Il I[M J pon AnocK 05o00237 DEC S PDR 2319&f ~ f

We continue to believe that the operators had opened both the vent valve and the strainer blowdown valve in the lubricating water supply line, and that this resulted in the " apparent" low pump discharge pressure during the October and November events. We have now installed separate gages which indicate the pressure directly off the discharge volute of all three diesel cooling water pumps.

We believe this letter answers the questions raised by your staff regarding reference (a) and should permit them to allow us to return the operation of the diesel generator cooling water pumps to normal in accor-dance with the conditions of your Confirmatory Action Letter dated Novem-ber 25, 1981.

Very truly yours,

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D. . Scott Station Superintendent Dresden Nuclear Power Station DJS/DLF/jrh cc: C. Reed F. Palmer J. Abc]

D. Farrar J. Brunner T. Rausch File /NRC File / Numerical

Commonwealth Edison Dresden Nuclear Power Sttion R.R. #1 M rris, tilinois 60450 Telephone 815/942-2920 December 4, 1981 DJS Ltr. #81-951 Mr. James G. Keppler, Regional Administrator Directorate of Regulatory Operations - Region III U. S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, IL 60137

SUBJECT:

Results of an Investigation into the Unit 3 and Unit 2/3 Diesel Generators at Dresden Nuclear Power Station on October 23, 1981, and November 19, 1981

Dear Mr. Keppler:

While conducting surveillance tests on October 23, 1981, Dresden Station experienced trips of both the Unit 2/3 and Unit 3 Emergency Diesel Generators. The cause of the trips of the diesels is believed to have been insufficient cooling water flow to the diesel heat ex-changers, resulting in protective action trips of the diesels from high temperature. On November 19, 1981, a similar high temperature trip of the Unit 3 Emergency Diesel Generator occurred. The cause of this trip is also believed to have resulted from insufficient cooling water flow to the diesel heat exchanger.

As a result of concerns raised about the operability of the diesel generator cooling water pumps, we have taken action to preclude recurr-ence of the diesel generator trips and to ensure that the pumps remain operable. Some of these actions have been detailed in Confirmatory Action Letters from your office on November 20, 1981, November 25, 1981, and December 2, 1981. As described in your December 2, 1981, 1cteer, the Unit 2/3 and the Unit 3 diesel cooling water pumps are presently operating continually, and all three cooling water pumps are being vented or inspected twice during each shift.

Operability of the Diesel Generator Cooling Water Pumps ,

We have conducted an extensive review of the operability of the Unit 2/3 and Unit 3 Diesel Generator Cooling Water Pumps. Our investi- .

gation has included the following:

1. Extensive testing of the pumps utilizing DOS 6600-11 (the diesel generator cooling water pump operability surveillance) and DOS 6600-1 (the diesel generator operability surveillance),

as well as special pump tests with different suction and discharge piping arrangements.

2. Review of the cooling water system piping and design. ,N [

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3. Review of the dicaal gensrator cooling watzr pump dssign and discussions with the pump manufacturer.

4. Review of testimony of the operators involved in these events.

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5. Review of the diesel generator alarms received and printed on -

the computer alarm typer during the events.

l 6. Review of the diesel generator control and alarm system logic 1

and circuitry.

.The investigating team consisted of station personnel, personnel from our corporate Station Nuclear Engineering Department, and an out-side consultant. Based on our investigation, we conclude that the-Unit

2/3 and Unit 3 diesel generator high temperature trips resulted from re-stricted flow in the cooling water lines to the heat exchangers. The flow restriction resulted from failure of the discharge check valves in both the Unit 2/3 and Unic 3 cooling water pump discharge lines. In j both cases, the check valve disc had separated from the valve operating arm. This check valve disc causing the flow restriction is consistent with the tests performed and the observed behavior of the pumps. Both check valves have been replaced with new valves, and the similar valve for the Unit 2 diesel has also been replaced.- We are revising the diesel preventative maintenance procedures to require that these check j valves be inspected annually and replaced or repaired as required.

Further, we conclude that the diesel generator cooling water pumps were operable and did operate normally during the.0ctober and November

events. The pumps have been operable throughout the period of our in-ves tigation. None of our testing has shown any indication of pump ab-normal behavior.

Instrumentation The results of our investigation indicated that two significant problems exist in determMug pump operability and in troubleshooting potential cooling water pump malfunctions.

At the heat exchangers in the Diesel Generator rooms, personnel are hampered in determining that the heat exchangers are receiving proper flow because of inadequate instrumentation. Pump running lights are installed in the diesel generator room, which are operated by. contacts at the pump breaker, to provide one indication of proper pump operation. However, the operators must infer pump flow from readings of cooling water pressure at the inlet and outlet of the heat exchangers. These pressure readings are indicated on gages which have a range of 0-200 psig. However, because the pressure drop across the heat exchanger is less than two psi, and because of vibrations at the heat exchangers, the installed gages provide l an unreliable indication of cooling water flow. We have concluded that a more accurate indication of flow should be installed, and we have initiated an engineering study to design and provide that instru-mentation. Currently, we believe that pressure gages with a narrower range, combined with temperature indication across the heat exchanger, should resolve this deficiency.

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b

Wa have clzo daterminrd th:t tha prazent pump discharga gtga piping and arrangement can result in unreliable readingo, or readings not in-dicative of the actual pump discharge conditions. Because these are canned rotor pumps, there are no local indications other than pump discharge pressure that the pumps are actually operating. The cooling water pump discharge pressure gage is installed on the line supplying lubricating water to the pump motor bearings. (See Sketch on page 6).

a During testing of the Unit 2/3 diesel cooling water pump, the following was observed:

1. The discharge pressure gage read 30 psig with the vent valve closed, "Y" strainer blowdown valve closed and the remaining two valves full open.

2. Upon opening the "Y" strainer blewdown valve, the discharge pressure gage reading decreased to zero.

The above test was then repeated on the Unit 3 diesel generator cooling water pumps. The discharge pressure gage reading decreased 25 psig (50 psig to 25 psig) when the strainer blowdown valve was opened. Thus, opening the vent and blowdown valves dramatically lowers the " indicated" pump discharge pressure, and can mislead an operator about the actual (higher) pump discharge pressure. We be-lieve this phenomena resulted in the " apparent" low pump discharge pressuras reported by the operators during the October events. We

.have added an additional discharge pressure gage directly off the dis- '

charge volute of all the diesel generator cooling water pumps to pro-vide an accurate indication of pump discharge pressure. (See sketch)

Also reported during these events was a low pressure stream of water while venting the pump. Subsequent tests showed that when vent-ing the pump with both the vent valve and the strainer blowdown cpen that the stream of water is only slightly greater when a pump is running. The observation of low pump noise during these events was proved to be consistent with flow restriction by subsequent tests which indicated that pump is actually quieter with the discharge valve closed than with that valve open.

, The review of the November 19 event has indicated that the same condi-tions were present as those that had occurred on October 23 with one exception.

A reported air / water mixture from the pump vent is now believed, based on subsequent tests, to be an atomizing effect that can occur only by par-tially opening the vent valve (gate valve) . We believe the air / water mixture which the operator reported as being observed was caused by this penomena.

Other Items Investigated As a result of the reported pump low discharge pressure during the October events, we suspected that the suction piping to the diesel generator cooling water pumps might have been blocked by debris or other material, or that the pumps may have become airbound during maintenance operations on the circulating water pumps which required dewatering the 3B suction bay. Also considered was the possibility

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$ of a pump runout condition. As a racult, we did ths following:

1.- Pumps were vented daily - no signs of air.

2. Checked for air in the suction piping using ultrasonic and X-ray techniques - no indications of air.

3. Injected air into the suction of the Unit 3 cooling water pump - were unable to airbind the pump.

4. Inspected the cooling water side of the diesel generator heat exchangers for debris and foreign material - no evi-dance of foreign material that might have blocked _the cooling water pump suction lines.

5. Opened another suction line to provide redundant suction lines to the Unit 2/3 diesel generator cooling water pump and chained and locked all pump suction, discharge, and cross connect valves in their proper position. We are reviewing the procedures for dewatering and filling a circulating water pump bay to ensure air is not trapped in the lines or pumps as a result of these operations.

6. Based on the review by Station Nuclear Engineering Department and an outside consultant, it was determined that a' large flow and low discharge piping restriction condition would be necess-ary to cause pump runout - this condition was not observed dur-ing the events.

In addition, the following should be noted:

1. All three diesel generator cooling water pumps were_ operated successfully on October 5,1981. (Surveillance DOS 6600-11) _.

which was after dewatering suction bay 3B.

2. The diesel cooling water pumps have a suction head (positive pressure at the pump suction) rather than suction lif t and air inlaakage is unlikely.

3. Blockage of the suction would reduce the net positive suction head available to pump resulting in cavitation and noisy pump operation - this was not observed during the events.

Based on the above and our investigation, we conclude that air binding and/or suction piping blockage or pump runout were not contrib-uting factors to the October and November events.

We also evaluated the possibility of a locked rotor. In this case, the pumps would trip on thermal overload and manually resetting at the motor control center would be required. This was not experienced during these events.

All parts of the electrical system related to the operation of the cooling water pumps have been checked and/or tested with no abnormalties -

noted.

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Conclustons We believe that the cause of the diesel generator trips in October and November was flow blockage by the disc of the cooling water pump discharge check valves. As described above, the conflicting observations to a check valve failure have been erniained. The apparent spurious starts and stops are now believed to have been either operator manual starts and stops or as an expected result of resetting the diesel generator high temperature. Therefore, the check valve failure is consistent with all the observations made during the event. The discharge check valves on all three diesel generator cooling water pumps have now been replaced with new valvea. Our review furrher indicates that no other failures have occurred. We have, however, made some additional changes (Page 4, Item 5) to further ensure that the diesel generator cooling water pumps re tain operable, and we are providing additional instrumentation to assist the operators in troubleshooting any further problems.

Based on our investigation, we believe that the circumstances which account for the observed behavior during the October and November events with the Unit 2/3 and Unit 3 diesel generator cooling water pumps have been identified and explained, and that the probability of similar e-vents occurring has been significantly reduced. We feel that the con-ditions of your November 25, 1981, Confirmatory Action Letter have been satisfied. After concurrence by your office, we will return the oper-ation of the Unit 2/3 and Unit 3 diesel cooling pump to their normal standby condition, and we will resume normal surveillances of thair operability.

Very truly yours, O

D. 'J. Sco tt Station Superintendent Dresden Nuclear Power Station DJS/DLF/jrh cc: C. Reed F. Palmer J bel

. Farrar File /NRC File / Numerical

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4 Installed

\ Pressure Gage Vent X Strainer Discharge Check Valve X .

Strainer Bearing ,

X Blow Down Lube Valve v Water

\ Ne w Gage O

r 5 0 Stator N Cooling g N Jacket Suction O

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