ML20086E732

AO 74-03:on 740204,LPCI Differential Pressure Indicating Switch 2-137B Failed to Indicate Properly Upon Isolation from Normal Sys Pressure W/Bypass Valve Open.Caused by Improper Sensor over-range Stop Setting.Sensors Calibr

ML20086E732
Person / Time
Site:	Vermont Yankee
Issue date:	02/11/1974
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086E739	List: ML20086E732 ML20086E747
References
AO-74-03, AO-74-3, VYV-3159, NUDOCS 8312070503
Download: ML20086E732 (2)

Text

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. VERMONT YANKEE NUCLEAR POWER CORPORATION SEVENTY SEVEN OROVE STRCCT RuTLAxo, VERMONT OG701 VYV-3159 P.O. BOX 157 VERNON. VERMONr 05354 February 11, 1974 D  %

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Director Directorate of Licensing C-OfC f[  %

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United States Atomic Energy Cocnission 2 Washington, D.C. 20545

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REFERENCE:

Operating License DPR-28 $}{4 Docket No. 50-271 Abnormal Occurrence No. A0-74-03 N

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l Gentlemen. i As defined by Technical Specifications for the Vermont Yankee Nuclear Power Station, Section 6.7.B.1, we are reporting the following Abnormal Occurrence as A0-74-03.

On February 4, 1974, at 1300, during conduct of a monthly functional test of the Low Pressure Coolant Injection System, Differential Pressure Indicating Switch 2-137B failed to indicate properly when it was isolated frou normal system pressure with its bypass valve open. Under this condition, the sensor should have indicated zero PSID, however, it continued to indicate approximately 8 PSID. It was determined that this condition was in violation of Technical Specifications, Section 1.0.A.4 in that it could have prevented the performance of the intended safety function of an engineered safety feature system. Functionally, this sensor operates as a pressure permissive in the Low Pressure Coolant Injection System Loop Selection Logic and has a redundant sensor which was tested and found to be operating properly thereby maintaining proper system availability. The reactor was operating at approximately 97% power at the time of this incident.

The sensor was immediately replaced, functionally tested and verified to be operable.

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Directorate of Licensing February 11, 1974 Page 2 The cause of the failure was due to improper setting of the sensor over-range stop which is installed to afford over-pressure.

protection for the indicating mechanism. The sensor design is such that it can be safely subjected to pressures in excess of the maximum visual indication. This is possible due to the installation of over-r4nge valves in the rupture-proof bellows unit assembly.

The bellows unit moves in proportion to pressure difference and transmits a rotary motion to the sensor torque tu'ae assembly. As pressure increases, the torque tube tends to " wind up" and imparts clockwise motion to the indicating portion of the sensor through pivoting linkage. As the indicator exceeds the full scale reading, it is allowed to overtravel by an amount predetermined by the setting of the over-range stop. If torque tube motion, and associated ,

indicator travel, is greater than the over-range stop poaition, the pivoting linkage can become captured as it is forced over the stop, thus preventing its accurately indicating a low or zero differential pressure.

As a result of this incident, all differential pressure sensors monitoring this parameter were calibrated and checked to verify that a similar condition was not present. This investigation indicated that the over-range stops were set properly.

The Plant Operations Review Committee meeting to review the incident, agreed that the corrective measures taken were appropriate and further action was not required.

Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION s

B.W. Riley

. Plant Superintendent WFC/kbd

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