ML20137Y704

AEOD/T515, RHR Svc Water Booster Pump Air Binding at Brunswick Unit 1, Technical Review Rept.Licensee Will Change RHR Svc Water Vent Line During Next Plant Shutdown

ML20137Y704
Person / Time
Site:	Brunswick
Issue date:	12/05/1985
From:	Salah S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20137Y618	List: ML20137Y613 ML20137Y704
References
TASK-AE, TASK-T515 AEOD-T515, NUDOCS 8603120362
Download: ML20137Y704 (6)

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AE00 TECHNICAL REVIEW REPORT

• UNIT: Brunswick Steam Electric TR REPORT NO.: AEOD/T515 Plant Unit 1 DATE: December 5, 1985 DOCKET N0.: 50-325 EVALUATOR / CONTACT: S. Salah NSSS/AE: General Electric /UE&C

SUBJECT:

RESIDUAL HEAT REMOVAL SERVICE WATER B0OSTER PUMP AIR BINDING AT BRUNSWICK UNIT 1

SUMMARY

During full power operation, an ' attempt was made to initiate suppression pool cooling at Brunswick Unit 1. When the A and C residual heat removal service.

water (RHRSW) booster pumps were started they ran for five seconds and tripped.

The pumps tripped due to air ingestion from the A loop suction header piping.

When the two booster pumps were started a trapped air pocket initiated a spurious low suction header pressure trip. At the time of this inci. dent, the redundant RHRSW loop (i.e., loop B) was out of service for maintenance.

Inoperability of both RHRSW loops rendered reactor shutdown cooling and sup-pression pool cooling inoperable. Following the trip of the A and C RHRSW booster pumps, the licensee flushed the trapped air out of the A loop suction header piping using the upstream service water (SW) pumps. The tripped RHRSW booster pumps were restarted within 15 minutes. The air had been trapped in the suction piping due to inadeouate venting arrangements for the line. Difficulty in venting the header was traced to the orientation of the suction header piping vent line. The licensee corrected the venting problem by rerouting of the vent line. Since RHRSW heat exchangers are normally supplied by the SW punps (which were unaffected by the event) and the RHRSW booster pumps only ensure a proper heat exchanger primary-to-secondary side differential pressure for heat exchanger leakage control, the actual and potential safety consequences of this event are considered minor.

DISCUSSION On January 19, 1984, during full power operation, the operators at Brunswick Unit I attempted to initiate suppression pool cooling using the "A" loop of the residual heat removal (RHR) system. However, both of the RHRSW booster pumps serving the A loop (i.e., "A" and "C" RHRSW booster pumps) ran only for approxi-mately five seconds before tripping on low suction pressure. At the time, the redundant (B) loop was out of service for maintenance and, therefore, was not available. As a result, the shutdown cooling and suppression pool cooling modes of RHR were inoperable.

The RHRSW system booster pumps are installed to raise RHR heat exchanger secon-dary side fluid-pressure during the shutdown cooling and suppression pool cooling modes of operation. With this arrangement, any leakage across the heat 8603120362 951205 PDR ADOCM 05000325 S PM This report supports ongoing AE00 and NRC activities and does not necessarily represent the position or requirement of the responsible NRC program office.

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exchanger will be inleakage (i.e., to the reactor coolant system side). Inleak-age minimizes the possible radiological releases to the environment if a heat exchanger tube failure were to occur following an accident. At Brunswick, cooling water is supplied to the RHR heat exchangers (HXs)'with the service water (SW) pumps. Water supplied to the RHR HXs with the SW pumps is delivered in sufficient quantity to meet all heat removal requirements. However, without the booster pumps, the positive differential pressure across the RHR HX between the service water and reactor water required for radioactive fluid outleakge control is lost. Figure 1 shows the RHRSW system arrangement for the SW and the RHRSW booster pumps.

An investigation by the licensee found that the A loop RHRSW booster pumps had tripped on a low suction pressure signal caused by an air pocket trapped in the suction header piping. The entrapped air caused pump cavitation and a low suction pressure condition which was sensed by suction line pressure switch PS 1175.

Air was retained in the suction header piping line because the suction line could not be completely vented with the existing suction header vent line orientation.

The trapped air was subsequently flushed from the suction line by establishing flow through the affected A loop using the conventional SW system pumps. The air was swept out of the line in approximately five minutes. The RHRSW booster pumps were restarted and returned to service within approximately 15 minutes.

For the near term, to preclude similar events, the RHRSW loops on each unit were flushed on a weekly basis to prevent air pockets from occurring. For the long term, a plant modification for Units 1 and 2 was developed and implemented by the licensee to correct the vent line problem. The plant modification con-sists of rotating the tap for SW suction line vent valve SW-V145 from its present 135 position (as measured from a vertical reference) to 0*. This will provide effective air venting of the A and C booster pump suction line. The original and new orientations of the vent valve tap are shown in Figure 2. This vent line orientation change has been completed for Unit 1. The vent line for Unit 2 will be changed at the next plant shutdown.

The licensee also cleaned the rust, sludge, and debris from several blind flanged weldolets and weldalets to which pressure switch PS 1175 and vent valve SW-V145 are connected to reduce buildup. It was expected that this would also increase the reliability of the RHR5W booster pumps by decreasing the potential for pump trips due to low pressure signals.

Finally, an LER search was rnade to detemine if any other plants reported similar problems. Although approxinately 200 RHRSW pumps failures were found, the search results indicated that the RHRSW booster pump air binding problem was unique to the Brunswick plants.

CONCLUSIONS The RHRSW booster pumps A and C tripped at Brunswick Unit 1. The cause of the trip was air ingestion fron the A loop suction header piping which caused the suction line pressure sensor to sense low pressure. With the original vent line tap location on the RHRSW suction header piping, all the air in the suction header piping could not be vented.

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As a result of this event, the licensee has relocated the vent line tap connected to the suction line vent valve from the original 135* to 0 for Un'it 1. This will provide effective venting capability for the A and C booster pump suction line. The licensee will also change the orientation of RHRSW vent line for Unit 2 during the next plant shutdown. This will eliminate the problem of RHRSW booster. pump air binding which could disable both the shutdown cooling and suppression pool cooling modes of the affected RHRSW loop.

The Brunswick SW pumps supply cooling water to the RHR heat exchangers in sufficient quantity to meet all heat removal requirements even without the use of the RHRSW booster pumps. Therefore, RHRSW booster pump failure due to air binding is of relatively minor safety sionificance.

i REFERENCES

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1. Licensee Event Report 84-01-00, Brunswick Steam Electric Plant Unit 1, Docket No. 50-325, March 9, 1984,

2. Licensee Event Report 84-01-01, Brunswick Steam Electric Plant Unit 1, Docket No. 50-325, March 12,1984.

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