ML20070R928

Interim Deficiency Rept Re Limitorque Limit Switch Rotor Failures Due to Tension Stress Buildup.Initially Reported on 820731.Corrective Action Plan Not Finalized.Next Rept Will Be Submitted by 830427

ML20070R928
Person / Time
Site:	Fermi
Issue date:	12/28/1982
From:	Wells D DETROIT EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
EF2-61525, NUDOCS 8302040090
Download: ML20070R928 (2)

Text

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, Donald A. '# ells ungen Cumy Ass e ca

31 M 237 W4 7 2000 Secc,nd Avenue 31 3- 0 December 28, 1982 EF2-61525 0 -3 4-)

Mr. James G. Keppler, Regional Administrator Pegion III U.S. Nuclear Regulatory Ccanission 799 Roosevelt Road Glen Ellyn, Illinois 60137

Subject:

Interim Report of 10CFR50.55(e) Item on Limitorque Limit Switch Rotor Failures (#76)

Dear Mr. Keppler:

This interim report on the Lhnitorque limit switch rotor failures has been prepared to provide the current status of this problem.

This itan was originally reported to Mr. F.14xnax of NRC - Washington D.C.

Duty Officer, by Detroit Edison's Mr. D. Ferencz, Acting Supervisor - Con-struction Quality Assurance, on July 31, 1982.

This item involved cracking and breaking of plastic rotors in Limitorque limit switch assemblier. Three (3) OA Level I valve operators, and several other non OA Level I valve operators, had been found to date. The failures are occurring at the point of attachment, where they are secured by a steal rolled pin.

The type of failure of this plastic ccxnponent are all similar and appear to be the result of a tension stress buildup at the point where the rotor is fastened to the rotor drive gear assembly. The source of this tension stress can be attributed to two possible causes: Mishandling of the part by untrained personnel, or improper installation of the split pin which holds the limit switch rotor to the mating rotor drive shaft.

In the limit switch assembly, the rotor which is failing is the part which makes and breaks the contacts for the limit switches which in turn, stop the operator when the valve is in its open or closed position depending on the direction of the valve stemg travel. The novement of the limit switch rotor is limited to a simple 90 rotation about its axis. External forces which are applied during normal operation are limited to the equally oppos-ing and thus cancelling force applied by the limit switch contacts, the tor-sional force inputted into the rotor of the split pin connection equal to the drag of the limit switch contacts on the rotor and, the cantilevered weight of the rotor minus the friction force of the contacts acting on the rotor at the split pin connection. All these forces are negligable and could not induce sufficient stress buildup to result in the type of failure being observed.

8302040090 821228 JAN31$83 PDR ADOCK 05000341 PDR

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, Mr. James G. Keppler, Regional Administrator Decmber 28, 1982 Page Two EF2-61525 Inspection of the connection of the limit switch rotor to the rotor drive shaft does indicate that a potential for a severe local stress buildup in the rotor does exist d' the split pin is inproperly installed. The plastic rotors as received by Limitorque have holes molded in than for the split pin. Limitorque procedures for assembly of the rotor to the rotor drive shaft state that the rotor must be reamed out with a #40 drill prior to installing the split pin. This step is necessary for two reasons. First, it aligns the axis of the hole to assure the hole is straight and true, and second, it assures that the holn is the proper diameter to received the split pin. Either a misaligned hole or an undersized hole can result in the cmpressed split pin imparting a concentrated stress into the rotor at the point of the observed failure. Thus, it is very important both during ini-tial assmbly and during rotor replacement that the procedure for reaming the rotor hola be followed.

The tension type of failure being experienced can also be the result of an external force being applied to the assembled rotor. The rotor is attached to its driver shaft such that it becmes a simple cantilever anchored by the split pin. I.ny external force applied to the shaft will result in a bending moment in the region of the split pin. This bending mment will produce a localized stress in the rotor. Given a large enough external force through careless handling, inexperience or simple accident and the rotor could fail.

A correct 1T action plan has not been finalized at this time. This informa-tion will be included in a future report.

Another report on this itan, either interim or final, is scheduled to be sent on or before April 27, 1983. If you have questions concerning this mat-ter, please contact Mr. G.M. Trahey, Assistant Director - Project Quality Assurance.

p Very truly [yours,&[ &

DAW /DF/WRW/pn cc: Mr. Richard DeYoung, Director Office of Inspection and Enforcement U.S. Nuclear Regulatory Commission Washington, D.C. 20555 l Mr. Bruce Little, Senior Resident Inspector i U.S. Nuclear Regulatory Comnission

! 6450 North Dixie Highway l Newport, Michigan 48166 l

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