IE Circular No. 78-14, HPCI Turbine Reversing Chamber Hold down Bolting
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 IE Circular No. 78-14 Date: July 17, 1978 Page 1 of 2 HPCI TURBINE REVERSING CHAMBER HOLD DOWN BOLTING Recent inspections by the Philadelphia Electric Company of HPCI turbines at Peach Bottom 2/3 facilities disclosed broken cap screws and missing clamping plates which secure the turbine reversing chambers. The cap screws are used to fasten clamping plates and reversing chamber rails to the support ring. Additional detailed information is available in Section 6 of the HPCI Terry Turbine instruction manual. Missing parts were recovered from components in the turbine steam exhaust piping and unrecovered parts are believed to be in the torus. Subsequent examination of the cap screws indicated the failures resulted from fatigue. The design of the cap screws includes a 1/4 inch deep concentric counterbore from the threaded end. Failures generally occurred at the base of the counterbore with a few failures occurring at the base of the screw head. The screw length of 1-1/4 inches allows about 5/16 inches of thread engagement in the support ring. After review, the vendor (Terry Steam Turbine Company) recommended modification of the bolting design to prevent recurrence. An interim solution was to replace the 3/8-16 X 1-1/4" long ASTM A574 socket head cap screws with similar 1-1/2" long cap screws. This required deeper drilled and tapped holes in the support ring to prevent the longer screws from bottoming. Later, the General Electric Company advised of a further modification from Terry Turbine consisting of bolting the reversing chamber directly to the support ring, lockwiring the cap screws and eliminating the clamping plate. In addition, the replacement cap screws were changed to 3/8-16 X l" long ASTM A193 Gr. B6 with a Rockwell Hardness of Rc28 or less. Information supplied by the Terry Steam Turbine Company indicates that similar turbines were supplied for HPCI systems at the following facilities: Browns Ferry Unit Nos. 1, 2, and 3; Brunswick Unit Nos. 1, and 2; Cooper; Vermont Yankee; Fermi Unit No. 2; J. A. FitzPatrick; Hatch Unit Nos. 1 and 2; Hope Creek Unit Nos. 1 and 2; Limerick Unit Nos. 1 and 2; Pilgrim Unit No. 1; Shoreham; Susquehanna Unit Nos. 1 and 2. In addition, HPCI Terry Turbines were also supplied for Monticello and Duane Arnold; . IE Circular No. 78-14 Date: July 17, 1978 Page 2 of 2 however, these applications involved smaller machines which incorporated a single turbine wheel and reversing chambers which are supported with out clamping plates, and therefore are not subject to similar failure of cap screws. All holders of BWR operating licenses or construction permits for facilities with a similar HPCI Terry Turbine should be aware of the potential for failure of the turbine reversing chamber hold down bolting. It is recommended that inspection of the clamping plate cap screws be included in the next inspection of these turbines. If the present bolting design is inadequate, it is further recommended that the nuclear steam supplier and vendor be contacted for assistance in determining the appropriate modification. No written response to this Circular is required. If additional information is needed regarding this matter, contact the Director of the appropriate NRC Regional Office.
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