United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 82-08: Check Valve Failures on Diesel Generator Engine Cooling System

                                                            SSINS No.:  6835
                                                            Accession No.: 
                                                            8202040113 
                                                            IN 82-08 

                                UNITED STATED
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C.  20555

                               March 26, 1982

Information Notice No. 82-08:  CHECK VALVE FAILURES ON DIESEL GENERATOR 
                                  ENGINE COOLING SYSTEM 

Description of Circumstances: 

On October 23, November 19, and December 1, 1981, the Dresden Station 
experienced events concerning degradation of the Unit 2/3 and Unit 3 diesel 
generator (DG) cooling water systems.  The Unit 2/3 DG is a single diesel 
generator that can be electrically aligned to serve either Unit 2 or Unit 3. 
The Unit 3 DG serves only Unit 3. 

At approximately 0217 on October 23, 1981, a monthly DG surveillance test 
was commenced on the Unit 2/3 DG.  The diesel was started normally from the 
control room.  At approximately 0224, the Unit 2/3 DG tripped on high engine
temperature.  Cooling water flow to the DG heat exchanger was found to be 
inadequate.  Attempts to restore adequate water pressure to the DG heat 
exchanger were unsuccessful and Unit 2/3 DG was declared inoperable.  At 
approximately 0400, surveillance was commenced on the Unit 3 DG.  
Indications of insufficient cooling water flow were observed and a DG 
shutdown was commenced by the control room operators at about 0407.  A few 
seconds later, the diesel tripped on high engine temperature.  The Unit 3 DG 
cooling water pump (DGCWP) was cycled several times, during which time the 
pump was vented. At about 0417, immediately after one of the pump starts, 
Unit 3 DGCWP discharge pressure increased and the observed DG heat exchanger 
pressure returned to normal.  The pump was cycled several more times without 
incident. A hot restart of the Unit 3 DG was then conducted, and the DGCWP 
and DG functioned normally.  The Unit 2/3 DGCWP was then tested, found to 
operator satisfactorily and returned to service.  Because of the unusual 
nature of the event, the licensee agreed to conduct daily surveillance tests 
on the Unit 2/3 and Unit 3 DGCWPs for 7 days while investigating the event. 

At about 0453 on November 19, during a surveillance test of the Unit 3 DG, 
the diesel tripped on high engine temperature.  The Unit 3 DGCWP was 
declared inoperable and the Unit 3 DG was removed from service.  Dresden 
Unit 3 then operated under a Technical Specification limiting condition for 
operation while the event was investigated.  On November 23, 1981, a broken 
check valve on the discharge of the Unit 3 DGCWP was found and replaced.  
The valve disc had broken free of the pivot arm and was lodged in the 
discharge side of the valve, restricting nearly all flow.  The valve was 
replaced and DG 3 was returned to service.  The failure of the 2/3 DGCWP on 
October 23 was not yet explained. 
.

                                                            IN 82-08 
                                                            March 26, 1982 
                                                            Page 2 of 2 


On December 1, 1981, the Unit 2/3 DGCWP exhibited a slow decrease in 
indicated discharge pressure accompanied by increasing noise and vibration 
levels at the pump.  This decrease in indicated pressure and the increase in 
noise and vibration levels were later determined (through visual 
inspections, testing, and determination of actual bearing clearance) to have 
been caused by excessive wear of the DGCWP bearings.  The pump was replaced.  
During the pump replacement, the licensee inspected the Unit 2/3 DGCWP 
discharge check valve and found it was broken.  As was the case with the 
Unit 3 pump discharge check valve, the disc had broken free of the pivot 
arm.  In the instance of the Unit 2/3 pump, however, the disc had not lodged 
into the body of the valve, but was free to move in any direction within the 
valve body.  Although the Unit 2 diesel had functioned properly throughout 
the foregoing events, the discharge check valve for the Unit 2 DGCWP was 
inspected, found to be broken, and replaced.  In the case of the Unit 2 
DGCWP discharge check valve, the pivot arm remained attached to the valve 
disc, but was broken at the hinge to the valve body. 

All three DGCWP systems at Dresden Units 2 and 3 involved check valve 
failures which were discovered during a short period of time.  These 
failures were not adequately characterized by operator observations and 
instrument readings during diesel generator surveillance tests, but were 
discovered by direct inspection of the internals of the check valve.  It is 
not known how long these check valve discs were free to move within the 
valve bodies and may have been that way for some time before coming to rest 
in a position which would restrict flow enough to cause the diesel to trip 
on high engine temperature. The subject check valves are horizontally 
mounted Crane, 8-inch, tilting-disc check valves, Type 373, and have a 
pressure rating of 125 psi. 

This information is provided as notification of a potentially significant 
matter.  It is noted that these check valves are not routinely covered by 
inservice testing programs or routine surveillances to verify valve 
operability.  It is expected that recipients will review the information for
possible applicability to their facilities.  No specific action or response 
is requested at this time.  If you have questions regarding this matter, 
please contact the Regional Administrator of the appropriate NRC Regional 
Office. 

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