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Information Notice No. 91-74: Changes In Pressurizer Safety Valve Setpoints Before Installation
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 November 25, 1991 NRC INFORMATION NOTICE 91-74: CHANGES IN PRESSURIZER SAFETY VALVE SETPOINTS BEFORE INSTALLATION Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to excessive changes observed in the setpoints of pressurizer safety valves (PSVs) after setting and acceptance testing but before installation. 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 are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances The Duke Power Company is investigating excessive changes in Dresser PSV setpoints that it has frequently identified during post-operational tests. The investigation has revealed that the setpoint changes may have been present before installation. Duke Power began the investigation when testing of as-found PSVs removed from the Catawba and Oconee Nuclear Stations in refueling outages early in 1991 revealed setpoints significantly outside the technical specification (TS) tolerance of �1 percent. Each of five valves tested failed to meet the TS limits, and the average deviation was over 5 percent (four failed high and one low). While performing subsequent investigative tests on two valves that had been acceptably set and tested to the TS criteria, but not installed, Duke found unacceptable setpoints. One valve setpoint was high by 2.2 percent and the other was high by 5.6 percent, as compared to the �1 percent TS limit. In its investigation, Duke initially examined its use of a pre-installation "jack-and-lap" procedure following setpoint testing as a possible contributor to the setpoint changes. This procedure involves partial disassembly of a PSV, maintaining its spring in compression, and polishing the seat surfaces to remove minor irregularities that may cause seat leakage. Licensee personnel questioned the adequacy of the controls previously applied to the jack-and-lap process. Procedural controls were added and an acceptably set and tested PSV was processed using the revised jack-and-lap procedure. Calculations indicated that any setpoint change caused by the new jack-and-lap procedure should 9111180310 . IN 91-74 November 25, 1991 Page 2 of 5 not exceed 0.5 percent. However, retesting following performance of the new procedure revealed a setpoint that was 2.2 percent high, which is greater than a 1 percent change from the previous setting. This is the 2.2 percent high setpoint previously referred to in the above paragraph. The setpoint increase beyond the 1 percent TS limit found in this test suggests that the jack-and-lap procedure has not been wholly responsible for the unreliable setpoints at the Duke plants. However, the fact that the setpoint deviated less than was recently experienced indicates that the previous jack-and-lap procedure may have contributed significantly to the problem. The valve referred to in the second paragraph above which tested 5.6 percent high in the pre-installation test had successfully completed the sequence normally used to prepare the valve as a replacement to be installed during the next refueling outage. It was obtained from storage at the Catawba Nuclear Station. Therefore, the licensee had not controlled the jack-and-lap procedure as closely for this valve as for the one which tested 2.2 percent high. Duke has identified a number of possible causes of the excessive changes in PSV setpoints and considers the following two the most probable: Valve leakage during setpoint testing that is eliminated by the subsequently performed jack-and-lap procedure: If a valve is leaking during setpoint testing, its huddle chamber may become partially pressurized, increasing the valve seat area and reducing the pressure required to cause lift. The pressure required to produce lift will be increased above the original setting if personnel adjust and test the setpoint with this leak and later repair the leak (through the jack-and-lap process) without verifying the setpoint again, as was the licensee's practice. Inadequate control of the jack-and-lap process The jack-and-lap process involves disassembling the valve partially and polishing the seating surfaces. This may introduce errors that can be limited with close controls but that may be inadequately quantified if the setpoint is not reverified afterward. Duke considered other possible causes of setpoint change including the following: � failure to recognize adverse trends in the three setpoint pops used in the verification test, � performance of ring adjustments without reverifying the setpoint, � temperature effects (as described in NRC Information Notice 89-90, "Pressurizer Safety Valve Lift Setpoint Shift," and Supplements 1 and 2), . IN 91-74 November 25, 1991 Page 3 of 5 � spring performance variables (e.g., corrosion or inadequate lubrication of spring/spring washer sliding surfaces), � seat adhesion, � handling or transportation shock, and � various test process parameters (e.g., steam quality, pressurization rate, valve installation variables, equipment calibration errors, and personnel errors). Duke reported that, based on its investigation, the refurbishment and test sequence it plans to specify for its PSVs each refueling outage is as follows: � Mount the valve on a test stand and strap on electric heaters and thermocouples. Two thermocouples to be placed 120� apart at each of the following locations: valve inlet flange, lower bonnet, and upper bonnet. � Supply the valve with saturated steam at 90 percent of the set pressure for heat-up. � Allow the steam to heat the valve's internal components while the heaters and thermocouples are used to maintain the proper valve temperature. � Leak test the valve with steam by placing a mirrored surface probe at the valve discharge and observing its surface for condensation. Use the following acceptance criterion: "No leakage shall be allowed." � If leaking is found, repair the valve by the jack-and-lap process. � Determine the initial actuation setpoint by raising the steam pressure at a ramp of 100 to 200 pounds per square inch per second until the valve stem lifts. � If the setpoint is not within the specified �1 percent limits, notify the licensee and adjust the valve setpoint and retest the valve until it is within the specified limits. � Require three consecutive successful tests within the specified setpoint limits for acceptance. � Leak test the valve with steam at 93 percent of the nameplate rating by passing a mirrored surface probe around the disc-seat interface and then visually inspecting the mirror for condensation. Use the following acceptance criterion: "No leakage shall be allowed." � If leaking is found, repair the valve by the jack-and-lap process. Following repair, install the valve on the steam header and verify the setpoint. . IN 91-74 November 25, 1991 Page 4 of 5 � If leakage is not found following the setpoint verification, package the valve and transport it to the station for storage until the next refueling outage. Subsequent to the investigative tests mentioned above, Duke retested a PSV for Catawba that had previously been refurbished and set in accordance with the above sequence and then placed in storage. The setpoint was determined during five actuations, and each lift was within 1 percent of the original setpoint. This indicated that the more strictly controlled refurbishment and testing may help reduce the magnitude of setpoint changes. Unfortunately, the valve failed the test because of minor leakage. Discussion The industry has continued to find that the setpoints of PSVs and main steam safety and safety relief valves can be unreliable. Recently, the NRC staff reviewed data from the Nuclear Plant Reliability Data System and found that, historically, as-found PSVs in U.S. plants have failed over 40 percent of the setpoint tests performed. The NRC staff believes that the number may actually be higher because of unreported failures. In performing the investigation described herein, the Duke Power Company identified possible causes of setpoint change that may have not been previously given adequate recognition. This investigation indicates the need to ensure that setpoint testing is performed after all operations that may cause changes. In this example, jack-and-lap procedures used to correct leakage before installation may have caused the setpoints to change. Many licensees prefer not to test the setpoint after performing procedures to correct leakage (such as the jack-and-lap) because the setpoint test itself appears to often lead to subsequent leakage, resulting in a repeating cycle that may be difficult to end. Leakage itself cannot be tolerated because even small pre-installation leaks may lead to steam cutting and increasing leakage during operation, which could cause the setpoint to change during operation. The licensees may need to maintain closer controls on certain maintenance, setting, testing, and other operations performed before the valve is installed. The Duke Power Company investigated setpoint changes on PSVs manufactured by Dresser Industries, which is understood to have supplied similar valves to approximately a third of the nuclear utilities in the United States. However, similar problems may exist with PSVs supplied by other manufacturers due to similarities in design, setpoint testing, maintenance practices, leakage correction practices, and other factors. Related Generic Communications Previous information notices issued on related topics include IN 86-56, "Reliability of Main Steam Safety Valves"; IN 86-92, "Pressurizer Safety Valve Reliability"; IN 88-68, "Setpoint Testing of Pressurizer Safety Valves with Filled Loop Seals Using Hydraulic Assist Devices"; and IN 89-90 and Supplements 1 and 2, "Pressurizer Safety Valve Lift Setpoint Shift." . IN 91-74 November 25, 1991 Page 5 of 5 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 (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Edward H. Girard, RII Charles G. Hammer, NRR (404) 331-4186 (301) 492-0791 Francis Jape, RII Mary S. Wegner, AEOD (404) 331-4182 (301) 492-7818 Attachment: List of Recently Issued NRC Information Notices .
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