Information Notice 1986-14, Overspeed Trips of Afw, HPCI & RCIC Turbines

Overspeed Trips of Afw, HPCI & RCIC Turbines

ML031220596
Person / Time
Issue date:	08/26/1991
Revision:	0
From:	Rossi C E Office of Nuclear Reactor Regulation
To:
References
IN-86-014, Suppl 2, NUDOCS 9108200172
Download: ML031220596 (6)

UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555August 26, 1991NRC INFORMATION NOTICE NO. 86-14, SUPPLEMENT 2: OVERSPEED TRIPS OF AFW, HPCIAND RCIC TURBINES

Addressees

All holders of operating licenses or construction permits for nuclear powerreactors.

Purpose

This information notice supplement is intended to alert addressees to a recentlyidentified condition in which turbine-driven pumps may trip on overspeed becauseof the sluggish response of the turbine speed governor caused by an accumulationof dirt and grit in the governor's control oil system. Recent overspeed tripsof turbine-driven pumps have also prompted the staff to issue this supplementto reemphasize previously identified causes of overspeed trips. It is expectedthat recipients will review the information for applicability to their facilitiesand consider actions, as appropriate, to avoid similar problems. However,suggestions contained in this information notice supplement do not constituteNRC requirements; therefore, no specific action or written response isrequired.Background:On March 10, 1986, the NRC issued Information Notice (IN) 86-14, "PWR AuxiliaryFeedwater Pump Turbine Control Problems," to alert addressees to certainconditions that could cause turbine-driven pumps to trip on overspeed. InAugust 1986, the NRC Office for Analysis and Evaluation of Operational Data(AEOD) issued study AEOD/C602, "Operational Experience Involving TurbineOverspeed Trips." On December 17, 1986, the NRC issued IN 86-14, Supplement 1,"Overspeed Trips of AFW, HPCI and RCIC Turbines," which summarized the resultsof the AEOD study.Recent operating experience has shown that overspeed trips of turbine-drivenpumps continue to occur from the same basic causes identified in the AEODreport. A description of recent operating experience and a newly identifiedcondition follows.

Description of Circumstances

On November 13, November 29, and December 6, 1990, during three separateoperational tests of the turbine-driven auxiliary feedwater (AFW) pump atArkansas Nuclear-One (ANO), Unit 2, the turbine tripped on overspeed during9108200172 IN 86-14, Supplement 2August 26, 1991 initial acceleration. After each of the three overspeed trips, the licenseemanually reset and successfully started the turbine several times. After thethird overspeed trip, the licensee determined that fouling of components in thecontrol oil system had caused the governor's response to be too slow to controlthe turbine's initial acceleration. Subsequent turbine starts were successfulbecause the governor's components had been sufficiently exercised and loosenedto permit faster response.On June 18, 1990, during fast start surveillance testing at the LaSalle CountyNuclear Station, Unit 1, the reactor core isolation cooling (RCIC) turbinetripped on overspeed and the licensee declared the system inoperable. Thelicensee determined that contaminated oil had fouled the components of thegovernor, slowing the governor's response and causing the turbine to trip onoverspeed.In October 1990, the licensee at the Millstone Nuclear Power Station, Unit 3,declared the turbine-driven AFW pump inoperable when the turbine trippedrepeatedly on overspeed during testing. The licensee noted that, before eachtest start, the turbine was rolling because of steam leaking past the steamadmission valves. The licensee determined that the turbine rolling caused oilto be admitted into the governor's speed setting cylinder which resulted in theoverspeed trips.Discussion:ANO-2 has one AFW pump powered by a motor and one powered by a steam turbine.Upon initiation of a start signal to the turbine-driven pump, a bypass valvearound the normally closed isolation valve in the steam supply line to theturbine opens and the turbine accelerates to a minimum idle speed. Following apreset time delay, the isolation valve opens and the turbine governor valvepositions to allow the turbine to accelerate to rated speed. The governorvalve is positioned by an electronic governor type-R (EG-R) hydraulic actuatorin conjunction with a remote servo valve. The EG-R actuator converts theelectrical speed demand signal to a hydraulic signal which is then sent to theservo to adjust the governor valve's position. The hydraulic medium for thegovernor control system is filtered oil taken from the turbine lube oil system.After the December 6, 1990, overspeed trip, the licensee for ANO-2 brought afield representative of the Woodward Governor Company onsite to help determinethe cause of the recurring overspeed trips. Upon examination, the control oilwas found to be contaminated with dirt and grit. A thick gelatinous coating ofdirt and hardened oil was observed on some governor components including theEG-R actuator and remote servo. The three overspeed trips resulted fromcontaminated oil that slowed the response of the governor. To correct thiscondition, the licensee changed the turbine lube oil, replaced the filter,cleaned the remote servo and control tubing, and replaced the EG-R actuator.The licensee tuned the governor to ensure proper response and successfullytested the turbine. The licensee declared the pump operable and returned it toservice.The ANO preventive maintenance (PM) program provided for sampling the turbinelube oil each month and for changing the lube oil and filter every six month IN 86-14, Supplement 2August 26, 1991 Maintenance records showed that the licensee had changed the lube oil andfilter on September 2, 1990, approximately two months before the overspeed tripon November 13, 1990. However, the PM program did not provide for periodicinspections of the oil sump and other components of the governor control oilsystem. The vendor manual for the Terry Corporation turbine contained a notestating that oil used to fill the turbine lube oil system should be filteredthrough a 5-micron filter. The licensee had overlooked this note and had notperformed this step when filling the system. Since the inline filter in thelube oil system is a 25-micron filter, the lube oil system contained a largequantity of particles of approximately 5 to 25 microns. This condition and thelow flow rate of oil through the governor resulted in a heavy accumulation ofimpurities in the governor. Because the accumulation occurred over a period ofyears, the periodic oil sampling and changing of the oil and filter in theturbine lube oil system failed to control or detect the accumulation of parti-cles inside the governor. The licensee revised the PM program to includeperiodic cleaning or replacement of the EG-R actuator and its associated remoteservo valve. The licensee plans to clean the turbine lube oil system duringthe next refueling outage.On June 18, 1990, the licensee at LaSalle County Nuclear Station, Unit 1,identified a similar problem. During fast start testing, the RCIC turbinetripped on overspeed. The licensee's investigation included the removal andinspection of the EG-R actuator. The licensee found sediment inside theactuator and on the actuator's components. The licensee tested the oil forparticles between 5 and 250 microns and found that the amount of these parti-cles greatly exceeded allowable limits. To prevent the problem from recurring,the licensee revised the plant procedure to require the oil to be filteredbefore filling the turbine lube oil system. Also, the licensee will flush theoil system and disassemble, inspect, and clean the EG-R actuator during eachoutage.In October 1990, during testing of the turbine-driven AFW pump at MillstoneUnit 3, the turbine tripped repeatedly on overspeed. The licensee noted thatthe turbine shaft was rotating before each of the test starts caused by steamleaking past the steam admission valve. The turbine rolling caused oil to beadmitted into the governor's speed setting cylinder. The combination of theturbine's initial rolling and the position of the speed setting bushing wassufficient to cause the turbine to trip on overspeed during the turbine'sinitial acceleration. The licensee developed a maintenance program to elimi-nate the steam leaking past the admission valve and also to periodically checkif the turbine is rolling.In AEOD report C602, the staff identified several turbine overspedd eventsrelated to oil contamination. The events at ANO and LaSalle have revealed anadditional mechanism by which contaminated oil can cause turbine-driven pumpoverspeed trips. These events demonstrate that turbine governor control oilsystems are sensitive to the accumulation of impurities in the oil or onsurfaces exposed to the oil medium. To compensate for this sensitivity,licensees may wish to periodically examine and clean these critical componentsin addition to the traditional practice of periodically changing the lube oiland filter IN 86-14, Supplement 2August 26, 1991 In the AEOD report, the staff also noted that steam valve leakage and undrainedcondensate can cause overspeed trips. The staff listed three events of turbineoverspeed caused by steam valve leakage. Those events occurred at theSt. Lucie Plant, Unit 2, the Crystal River Plant, Unit 3 and theVirgil C. Summer Nuclear Station. The turbines at these three plants areequipped with Woodward PG-PL governors which are set to control turbineacceleration properly when the turbine starts from rest. These turbinestripped on overspeed because the turbines were rolling before being started whichincreased the oil pressure and caused oil to flow into the governors' speedsetting cylinder. The oil pressure in the cylinder prevented the governor fromresponding fast enough to close the governor valve and control the initialturbine acceleration. This overspeed problem is not limited to the PG-PL typegovernor. Other types of Woodward governors that use a ramp bushing to controlacceleration may also trip on overspeed. In addition, the increased oilpressure in the speed setting cylinder does not decrease immediately and mustbe released by locally exercising and resetting the speed setting knob. Thischaracteristic may cause the turbine-driven pump to be unavailable forimmediate starts or quick restarts.At Crystal River, the licensee installed a modified governor with an automaticbleed feature to relieve oil pressure in the speed setting cylinder. Thismodification should prevent the turbine from tripping on overspeed as a resultof the turbine rolling before the pump is started.In the AEOD report, the staff identified nine turbine overspeed trip eventsthat occurred as a result of undrained condensate in the turbine steam supplylines. Although steam lines are usually designed to separate and removecondensate, it is possible that during a cold start the condensate may not beseparated or removed fast enough to prevent it from reaching the turbine.Because this condensate contains significantly less energy than an equivalentmass of steam, the turbine's initial acceleration is slower than expected. Inresponse to the slower acceleration, the governor opens the governor valvefurther to allow more steam to enter. However, once the condensate clears, thegovernor cannot respond fast enough to prevent the turbine from tripping onoverspeed.The actual condition causing such an overspeed trip is often not determinedbecause subsequent restarts are usually successful as the steam line has beenheated and the condensate removed. To prevent similar trips, these plantsincreased the capacity of the condensate removal process or minimized thecondensate formation by keeping the steam supply line in a hot and pressurizedcondition.Previous Similar Occurrences:The staff reviewed LERs received since the middle of 1985 and identified twoturbine overspeed trips caused by undrained condensate. It is likely thatother turbine overspeed trips have occurred but were not reported. Theoverspeed trips caused by undrained condensate occurred at San Onofre NuclearGenerating Station, Unit 2, in August 1990, and at the Crystal River Plant,Unit 3, in November 1986. The turbine overspeed trip at San Onofre occurredduring testing. Initially, the licensee could not determine the cause of the IN 86-14, Supplement 2August 26, 1991 trip because subsequent restarts were successful. However, during a followupinvestigation, the licensee found that a procedural deficiency had resulted inan isolation valve for a steam trap remaining closed after a previous outage.The licensee modified the procedure to ensure that the valve was properlyaligned and added a program to check the steam drain system periodically.At Crystal River, the AFW system actuated automatically. The turbine-drivenAFW pump started as required but immediately tripped on overspeed. Themotor-driven AFW pump started normally and supplied feedwater to the steamgenerators. The licensee later found that the warmup line for the turbinethrottle valve had been isolated for unknown reasons and had allowed the steamsupply line to cool. Condensate formed in the steam supply line and caused theturbine to trip on overspeed. The licensee revised procedures to ensure thatthe warmup line was not inadvertently isolated.The NRC issued IN 86-14 and its supplement to alert addressees to the possibil-ity that turbine-driven pumps could trip on overspeed and to summarize theresults of AEOD report C602. However, the staff believes some licensees arenot fully aware of the problem or may have inadequate programs to control theproblem. AEOD is continuing to study the reliability of safety-related steamturbine-driven standby pumps to address the continuing repetitive failuresof turbine assemblies. Further information will be issued to addressees ifappropriate.This information notice requiresyou have any questions about theof the technical contacts listedno specific action or written response. Ifinformation in this notice, please contact onebelow or the appropriate NRR project manager.Arles`E~.os~sei, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical Contacts:Thomas F. Stetka, Region IV(817) 860-8247Michael F. Runyan, Region IV(817) 860-8142William T. LeFave, NRR(301) 492-3285

List of Recently Issued NRC Information Notices

• ,81AttachmentIN 86-14, Supplement 2August 26, 1991 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICESInformation Date ofNotice No. Subject Issuance Issued to91-5191-5091-4991-48Inadequate Fuse ControlProgramsA Review of Water HammerEvents After 1985Enforcement of SafetyRequirements for Radiog-raphersFalse Certificates of Con-formance Provided by West-inghouse Electric SupplyCompany for Refurbished Com-mercial-Grade CircuitBreakersFailure of Thermo-LagFire Barrier Material toPass Fire Endurance TestQuestionable Certificationof Material Supplied tothe Defense Department byNuclear SuppliersDegradation of EmergencyDiesel Generator Fuel OilDelivery SystemsPossible Malfunction ofWestinghouse ARD, BFD, andNBFD Relays, and A200 DCand DPC 250 Magnetic Con-tactors08/20/91 All holders of OLs or CPsfor nuclear power reactors.08/20/91 All holders of OLs or CPsfor nuclear power reactors.08/15/91 All Nuclear Regulatory Com-mission (NRC) licenseesauthorized to use sealedsources for industrialradiography.08/09/91 All holders of OLs or CPsfor nuclear power reactors.08/06/91 All holders of OLs or CPsfor nuclear power reactors.07/19/91 All holders of OLs or CPsfor nuclear power reactors.07/18/91 All holders of OLs or CPsfor nuclear power reactors.07/05/91 All holders of OLs or CPsfor nuclear power reactors.91-4789-56,Supp. 291-4691-45OL = Operating LicenseCP = Construction Permit