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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 April 30, 1992 NRC INFORMATION NOTICE 92-33: INCREASED INSTRUMENT RESPONSE TIME WHEN PRESSURE DAMPENING DEVICES ARE INSTALLED 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 increased response times for pressure sensing instruments that occur when pressure dampening devices are installed in the instrument sensing lines. 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 On September 25, 1991, the GPU Nuclear Corporation, licensee for the Oyster Creek Nuclear Generating Station, initiated a plant shutdown after determining that seven of eight isolation condenser line break pressure sensors did not meet the plant technical specification requirements for instrument response times. The licensee's trouble shooting determined that pressure dampening devices in the sensing lines for these differential pressure sensors had caused an increase in the response times. Furthermore, the licensee found that the time delay caused by these devices is significant at both low and high pressures. Discussion The pressure dampening devices (snubbers) which utilize sintered stainless steel elements are generally installed in instrument sensing lines to dampen pressure oscillations or to protect the instruments from particulate contamination. When the sensors failed to meet the required response times during surveillance tests, the licensee reviewed the pressure sensing system and found that the snubbers were causing unacceptable time delays. The licensee later removed the snubbers because they are not needed when using the upgraded Barton pressure sensors that were installed. 9204240146 . IN 92-33 April 30, 1992 Page 2 of 2 Snubber time delay will not be detected in response time testing conducted directly at the sensing instrument (see Figure 1.) The effect of a snubber will differ from sensor to sensor because of differences in the volumetric displacement of fluid within the pressure sensing mechanisms. System response time can also be degraded by the accumulation of foreign material in sensing line snubbers. The NRC's Office of Nuclear Regulatory Research is conducting a generic study on the performance of pressure instrumentation at nuclear power plants. The staff plans to publish the results in NUREG/CR 5851, "Long Term Performance and Aging Characteristics of Nuclear Plant Pressure Transmitters." The staff has completed a part of this effort, and the results of tests conducted on pressure sensor response time testing were published in an ISA (Instrument Society of America) transaction 91-720, "Response Time Testing of Pressure Transmitters in Nuclear Power Plants." This publication addresses various causes for delays in sensor response and documents that significant time delays can occur. 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: Thomas Koshy, NRR (301) 504-1176 Iqbal Ahmed, NRR (301) 504-3252 Attachments: 1. Figure 1. Test Configuration that Excludes the Effect of Snubbers 2. List of Recently Issued NRC Information Notices .
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