Information Notice No. 90-45: Overspeed of the Turbine-Driven Auxiliary Feedwater Pumps and Overpressurization of the Associated Piping Systems
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 July 6, 1990 Information Notice No. 90-45: OVERSPEED OF THE TURBINE-DRIVEN AUXILIARY FEEDWATER PUMPS AND OVERPRESSURIZATION OF THE ASSOCIATED PIPING SYSTEMS Addressees: All holders of operating licenses or construction permits for nuclear power reactors. Purpose: This information notice is intended to alert addressees to potential problems resulting from the combined failures of the governor and the overspeed trip mechanism (OTM) of turbine-driven auxiliary feedwater (AFW) pumps that can result in turbine overspeed and overpressurization of the AFW system. Because similar turbine-driven pumps are also used in the high pressure core injection (HPCI) and reactor core isolation cooling (RCIC) systems, they are also subject to the same failure mechanism. 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 do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: Two events involving overpressurization of the AFW system as a result of AFW turbine overspeed are discussed in the following paragraphs. The turbine overspeeds were caused by the combined failures of the turbine governor and the OTM. The OTM did not function to close the steam trip valve when the governor had failed to respond properly to turbine speed increases. Rancho Seco Nuclear Generating Station On January 31, 1989, the AFW turbine oversped and overpressurized both trains of the AFW system. Both trains of the AFW system were declared inoperable, and the plant was shutdown. The turbine overspeed occurred while the licensee was testing a newly modified governor. The turbine speed reached 6020 rpm in a few seconds after start and remained at that level for three minutes. The governor failed to limit the turbine at the rated speed of 3600 rpm and the turbine OTM also failed to trip the turbine at its setting of 4500 rpm. An operator stopped the turbine by closing the steam inlet valve. The AFW system pressure corresponding to the overspeed was calculated to be 3850 psig. This pressure 9006290323 . IN 90-45 July 6, 1990 Page 2 of 4 exceeded the AFW system design pressure of 1325 psig. Although only the turbine-driven train was operating, the overpressurization also affected the motor-driven train because of the system configuration which tied the two trains together. The licensee's maintenance test program provided for no specific valve lineup requirement, which could have limited the amount of piping that was subject to overpressurization in the event of a turbine overspeed. The licensee determined that inadequate preventative maintenance and inadequate periodic testing of the OTM were the root causes of the OTM failure to function properly. As a result, the licensee included monthly testing of the OTM and trip valve in the licensee corrective actions to prevent recurrence. Maintenance test procedures for the AFW pump were also modified. The discharge valve of the turbine-driven pump would be closed before the licensee conducted a turbine surveillance test. In the case of turbine overspeed, this closed valve would limit the quantity of piping that is subject to overpressurization. In addition, the turbine overspeed surveillance test would be performed with the turbine decoupled from the AFW pump shaft to avoid any overpressurization of the AFW system in the event of a turbine overspeed. San Onofre Nuclear Generating Station Unit 2 On July 3, 1988, the AFW turbine oversped while personnel were performing low-voltage testing of the steam inlet valve to the AFW turbine. Based on the test procedures, when the licensee removed power from the inlet valve, power was also removed from the governor valve. The governor valve failed open and resulted in the turbine reaching overspeed conditions. The OTM did not function properly to trip the turbine. An attempt to close the inlet valve from the control room failed because the power had been removed. The operator in the control room then closed the steam inlet isolation valve, terminating the event. Control room indications showed that the turbine-driven pump reached 5000 rpm (the overspeed trip setpoint is 3500 rpm). The licensee later calculated that the AFW system had been pressurized to 2420 psig, which exceeded the system design pressure. The licensee determined that the failure of the OTM to trip the turbine was due to tappet ball damage. In addition to the replacement of the tappet ball assembly, the licensee's corrective actions included performing annual visual inspections of the tappet assembly and tests of the OTM every refueling outage. The isolation valves in the discharge interconnection line to the two motor- driven trains were closed during the test. Therefore, the overpressurization was limited to the discharge pipe section from the turbine-driven pump up to the isolation valves and the motor-driven trains were not affected. However, had the overspeed occurred during normal operation of the AFW system, the overpressurization would have extended to all of the AFW system piping because the isolation valve would have been in its normally open position. . IN 90-45 July 6, 1990 Page 3 of 4 Discussion of Safety Significance: Failure of both the governor valve and OTM, as occurred in these events, could result in severe damage to the turbine and pump due to overspeed. Without prompt operator action to control the speed of this equipment, the internal rotating elements could fail, possibly resulting in debris missiles. These events also reveal the vulnerability of the AFW system to overpressurization caused by turbine-driven pump overspeed. The vulnerability is inherent to the piping configuration and normal valve lineup which allows for cross-connection between trains on the discharge side of the pump. Although isolation valves are installed in the cross connections, they are kept open during normal system operation to allow the trains to share the pumps. With this system configuration, overpressurization of the turbine-driven train immediately affects the other trains and causes the entire AFW system to be vulnerable to overpressurization. With the AFW system isolated from the steam generators, there is no pressure relief device with sufficient capacity to limit overpressure. Also, the pump recirculation lines do not have sufficient capacity to discharge enough pump flow to limit the overspeed pressure. The Woodward governor, the OTM, and the Terry turbine that experienced the overpressure events described herein are used in the AFW systems of nearly all pressurized water reactors. Similar Woodward governors, OTMs, and Terry turbines are used in the HPCI and RCIC systems of boiling water reactors. Therefore, these systems are also vulnerable to overpressurization from the simultaneous failure of the governor and OTM. These events also highlight the importance of adequate maintenance and testing of this equipment to assure the reliability of these safety systems. Related Generic Communications: The turbine overspeeds described herein were caused by the combined failures of the governor and the OTM. Although the OTM is installed to provide protection for the pump and turbine, in the event of governor failure, both the governor and OTM may fail simultaneously due to various problems affecting their operation and reliability. Failure of the governor and OTM have been previously described in the following information notices: 1. NRC Information Notice 88-67, "PWR Auxiliary Feedwater Pump Turbine Overspeed Trip Failure." 2. NRC Information Notice 88-09, "Reduced Reliability of Steam-Driven Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type Governors." 3. Information Notice No. 86-14, "PWR Auxiliary Feedwater Pump Turbine Control Problems." 4. Information Notice No. 86-14, Supplement 1, "Overspeed Trips of AFW, HPCI, and RCIC Turbines." . IN 90-45 July 6, 1990 Page 4 of 4 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 NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Chuck Hsu, AEOD (301) 492-4443 John B. MacKinnon, AEOD (301) 492-9097 Charles G. Hammer, NRR (301) 492-0791 Attachment: List of Recently Issued NRC Information Notices .ENDEND
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Page Last Reviewed/Updated Tuesday, March 09, 2021