ML20084U183

AO 50-265/75-9:on 750314,MSIV Local Leak Rate Testing Revealed Excessive Leakage Through Valve 2-203-2A.Caused by Slight Warpage.Valve Machined to Eliminate Warpage

ML20084U183
Person / Time
Site:	Quad Cities
Issue date:	03/20/1975
From:	COMMONWEALTH EDISON CO.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084U184	List: ML20084U183 ML20084U186
References
AO-50-265-75-9, NUDOCS 8306270123
Download: ML20084U183 (2)

Text

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.s O. O REPORT NUMBER: A0-50-265/75-9 REPORT DATE: March 20, 1975 OCCURRENCE DATE: March 14, 1975 FACILITY: Quad-Cities Nuclear Power Station Cordova, IL 61242 IDENTIFICATION OF OCCURRENCE:

Main steam isolation valve (MSIV) local leak rate testing revealed excessive leakage through MSIV 2-203-2A.

CONDITIONS PRIOR TO OCCURRENCE:

Unit 2 was in cold shutdown for reactor refueling. .

DESCP.iPTION OF OCCURRENCE:

On March 14, 1975, local leak rate testing was being performed on the Unit Two Main Steam isolation valves on line "A". Prior to the test, the MSIV's were placed in the closed position from the control room.

The leakage measurements were obtained by pressurizing the volume enclosed by the inboard and outboard MSIV's through a pressure test connection to a pres-sure of 25 psig. this' yielded a combined two valve (inboard and outboar,d) leakage of 259.0 scfh.

Because the inboard MSIV 2-203-1A was repaired on February 2, 1975, the out-board MSIV-2-203-2A was suspected of leaking excessively. This fact was veri-fled by raising the water level upstream of MSIV 2-203-1A, then pressurizing the volume between the inboard and outboard MSIVs. W o rk request no. 879 was immediately initiated to repair the outboard MSlV 2-203-2A.

Valve-1A showed an acceptable leakage value, but 2A exceeded the Technical Specification limitation of 11.5 scfh for any one MSIV.

DESIGNATION OF APPARENT CAUSE OF OCCURRENCE:

Upon disassembly of valve A0 2-203-2A, washing of the valve seat was observed.

Uneven heating produces a small warpage which eventually leads to a slight

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-erosion'of the stellite material. It -is this slight amount of expected warpage which resulted in an inability to drain the volume enclosed by the MSIVs of.the "A" steamline. -

ANALYSIS OF OCCURRENCE:

In the event of a main steam line break outside the primary containment, the total leakage possible through the "A" steam line would have been 129.5 scfh.

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The excessive leakage rates did not in any way render the MSlVs inoperable, nor i was the ability of the valves to perform their design function affected. Upon

receipt of a Group i isolation signal, these valves would have shut in the 3 required time, and performed the isolation function.

i The Main Steam isolation Valves utilize straight line flow to provide a good i flow pattern and upstream side of the valve. The balancing feature of the valve makes it possible to take advantage of the upstream pressure to aid in holding the valve closed and to have the advantage of requiring a small actu-3 ator cylinder to open the valve. This is accomplished by allowing the full j upstream line pressure to bleed into the chamber above the plug through the j balancing part to exert a force on the plug internals in a direction to hold

, it against the seat, j This design, combined with the fact that the MSIVs are open during normal operation, minimizes the safety implications of this occurrence.

CORRECTIVE ACTION:

j Valve A0 2-203-2A was disassembled and machined to climinate the slight warpage which was present. Following reassembly of the valve, the local leak rate i test revealed a leakage of 0.0 scfh. .

FAILURE DATA

! Main Steam isolation Valve A0 2-203-2A had not been individually leak rate j tested since the pre-operational tests in 1971. The combined leakage of the

in and 2A valves of the 'W" steam line was found to be 1.4 scfh at that time.

l Unit two MSIVs 2-203-1B and 2B were leak rate tested on April 27, 1974, during an Investigation of missing parts on electromatic relief valve 2-203-3E. The

!, measured leakage was 52.4 scfh, and repairs were initiated. In that case, the

inboard valve had two deep gashes on the pilot stem which were.the main cause i of the leakage. The pilot stems were machined and the seats lapped on both

! valves. A subsequent leak rate test was performed and the measured leakage l ~ was 3.25 scfh. .

1 l The Unit One MSIVs were leak rate tested on April 1, 1974 with valves A0 1-203-l 2C, ID and 2D indicating leak rates of 216.0 scfh, 34.7 scfh, respectively. Upon

disassembly of these valves, minor surface defects were noted on the main valve pilot stems. The pilot stem surfaces on valves ID and 20 showed significantly less defects than did the 2C valve.

i A Crane Valve Company representative was present during the valve repairs. The

. cause of the leakage was attributed to these surface defects. It is postulated

, that hard valve closure during Group i i solation conditions may have caused the main pilot stem surfaces to become defective.

4 I The main' valve seating surfaces were closely examined and no excessive corrosion or. surface marks were observed. Therefore, i t was determined that the major i

cause of the exce'ssive leakages on the three main steam isolation valves was the presence of' surface defects on the valve pilot stems. Subsequent leak rate Etests resulted in leakages of 8.64 scfh, 5 76 scfh and 0.0 scfh respectively for i valves A0 1-203-2C,DID and 2D.

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