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Information Notice No. 89-90, Supplement 2: Pressurizer Safety Valve Lift Setpoint Shift
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 5, 1991 NRC INFORMATION NOTICE 89-90, SUPPLEMENT 2: PRESSURIZER SAFETY VALVE LIFT SETPOINT SHIFT Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to Information Notice (IN) 89-90 to alert addressees to information about more recent pressurizer safety valve setpoint tests using steam, loop seal water, and nitrogen as test fluids. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice supplement do not constitute NRC requirements; therefore, no specific action or written response is required. Background The staff issued IN 89-90 to inform addressees about problems that could result from operating pressurizer safety valves in environments that are different from those used to establish the lift setpoints for the valves. The information notice focused on problems encountered when valves were operated with a loop seal containing water after setting the valve lift setpoints using steam under the seat. The staff issued IN 89-90, Supplement 1, to inform addressees about the possible effects of temperature variations on safety valve setpoints. Description of Circumstances In IN 89-90, the staff noted that the Westinghouse Electric Corporation tested the setpoints on several plant pressurizer safety valves (PSVs) and found that when the valve setpoints are set with steam as the test fluid, the setpoints could shift upward 4 to 8 percent if the PSVs are installed on water-filled loop seals. Westinghouse discovered the shift while attempting to set PSVs more correctly by using the same fluid against the valve seats as is found during normal operation (subcooled loop seal water). Westinghouse tested and adjusted the setpoint of several plant PSVs by mounting the valves on a test facility loop seal piping configuration, filling the loop seal piping with water, and increasing the inlet pressure to the observed lift setpoint of the PSVs. After Westinghouse performed these tests, three events occurred in which PSVs, which had been set by this method, actuated at normal plant operating pressure, about 200 psi below the required setpoint. These three events 9108290122 . IN 89-90, Supplement 2 September 5, 1991 Page 2 of 3 occurred at the Virgil C. Summer Nuclear Station in May and August of 1989 and at the Surry Power Station, Unit 2, in October 1989. As discussed in IN 89-90, a loss of the loop seal water appeared to cause the valve setpoints to shift downward. In late 1990, the Westinghouse Owners Group had Westinghouse perform additional diagnostic tests on several PSV models to address concerns regarding the three plant events and findings from the preliminary test results that indicated a 4 to 8 percent difference between the resultant setpoints for the steam and for the subcooled loop seal water. The results of these diagnostic tests showed that by making very precise measurements of valve spindle motion (i.e. disk lift), Westinghouse found a much smaller corresponding shift in setpoint between the steam and the loop seal liquid inlet conditions. These tests indicated that what had earlier appeared in the above three events as downward shifts in setpoint caused by a loss of loop seal water probably occurred because the actual PSV setpoints were too near the system operating pressure. By not measuring the disk lift very precisely at the loop seal test facility, Westinghouse had apparently adjusted the PSV setpoints too low. Westinghouse also found that the time needed to purge the loop seal was longer than had been previously measured in the Electric Power Research Institute (EPRI) test program for safety valves in 1981. Discussion In performing the recent diagnostic tests of PSV setpoints with saturated steam and subcooled loop seal water, Westinghouse did not observe the large shift in setpoint that it had observed earlier and that the staff had reported in IN 89-90. When the disk lift was measured very precisely, testing with subcooled loop seal liquid provided setpoint results nearly the same as for steam, even with a large difference in temperature between the two fluids. Therefore, these test results indicate that valves set with steam should begin to open at approximately the same pressure when installed with steam or subcooled loop seal water against the seats. However, in performing the most recent tests, Westinghouse also found that more time was needed to purge the loop seal than had been earlier determined from the EPRI test program data. Safety valves will not relieve the system steam pressure until the loop seal has purged at which time the valves will fully open. Therefore, the longer time needed to purge the loop seal could cause the maximum pressure in the primary system to increase. If the system design margin is small, a rapid pressurization rate could cause the pressure to exceed the allowable limit. Responding to concerns regarding setpoint testing with water-filled loop seals, Westinghouse also submitted inquiries regarding the issue of setpoint testing of PSVs to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code committees that establish setpoint testing standards. The ASME Section III and Operation and Maintenance Code committees addressed this issue by clarifying the testing requirements for the Code setpoint to specify that safety valves for saturated steam service, which are installed on water-filled loop seals, should be tested with steam. In addition, the Section III Code committee stated that the effects of the water-filled loop seals should be considered in evaluating the maximum system pressure. . IN 89-90, Supplement 2 September 5, 1991 Page 3 of 3 In conducting the recent safety valve tests, Westinghouse also conducted tests using nitrogen and compared the setpoints with those obtained for saturated steam. Westinghouse found that the setpoint varies more between nitrogen and steam than it does between loop seal water and steam. Westinghouse also found that the setpoints for nitrogen and steam may need to be correlated for specific valves individually. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Charles G. Hammer, NRR (301) 492-0791 Mary S. Wegner, AEOD (301) 492-7818 Frank C. Cherny, RES (301) 492-3945 Attachment: List of Recently Issued NRC Information Notices .
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