United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-85: Potential Failures of ThermostaticControl Valves for Diesel Generator Jacket Cooling Water

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              December 26, 1991


NRC INFORMATION NOTICE 91-85:  POTENTIAL FAILURES OF THERMOSTATIC CONTROL 
                               VALVES FOR DIESEL GENERATOR JACKET COOLING 
                               WATER


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 the potential for failure of thermostatic 
control valves for diesel generator jacket cooling water.  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 10, 1991, at the Catawba Nuclear Station, Unit 2, the Duke 
Power Company (the licensee) removed emergency diesel generator (EDG) 2A 
from service for minor corrective and preventive maintenance.  The next day, 
the engine was successfully tested for its performance in a no load 
condition for 5 minutes and was shut down.  Shortly afterwards, the engine 
was restarted for a 1-hour operability test.  After operating the EDG for 15 
minutes at full load, operators observed that the engine cooling water and 
lube oil temperatures were increasing abnormally.  The engine oil level was 
checked and found to be within normal range.  After about 20 minutes, alarms 
for high lube oil inlet and outlet (175�F) and high jacket water (175�F) 
temperatures were received.  Responding to these alarms, an operator 
verified that the nuclear service water supply valve was open and that the 
flow was within normal range.  After operating for about 28 minutes, the 
engine tripped on a high lube oil outlet temperature (200�F).  After the 
trip and while the engine was coasting to a stop, an explosion occurred in 
the crankcase.  Upon examining the engine, the licensee determined that a 
thermostatic valve in the jacket cooling water system had failed resulting 
in the engine overheating, which resulted in significant engine damage.

Further details of the event may be found in Licensee Event Report 
50-414/91-10 or NRC Inspection Report 50-414/91-21. 


9112190286 
.

                                                       IN 91-85
                                                       December 26, 1991
                                                       Page 2 of 3


Discussion

At Catawba, the cooling water system for the Transamerica Delaval DSRV-16-4 
(16 cylinder) diesel generator engine supplies cooling water to the engine 
jacket, the engine lube oil cooler, the combustion air after-coolers, and 
the governor lube oil cooler.  An engine-driven circulation pump circulates 
cooling water through the closed loop system that includes a three-way 
thermostatic control valve (AMOT Model 8D).  When the jacket water 
temperature is low, the AMOT valve, which works similarly to the thermostat 
in an automobile engine, diverts diesel jacket cooling flow from the heat 
exchanger, which is cooled by nuclear service water.  The AMOT valve 
modulates open to control the temperature of the diesel jacket water to 
about 165�F as the engine reaches operating temperature.  Heat is 
transferred through the heat exchanger to the plant's nuclear service water 
system.

The licensee's investigation revealed that the AMOT thermostat valve had 
malfunctioned and caused the engine to overheat.  During the operability 
test, the AMOT valve only partially opened from the bypass position, thus 
allowing the jacket and lube oil temperatures to rise above normal operating 
temperatures.  With the engine fully loaded, the temperature of the oil and 
water rose to 200�F.  At these elevated temperatures, the oil emitted an 
increased amount of vapors.  The elevated temperatures also affected the 
clearances between the moving parts and reduced the oil's lubricating 
qualities.  The heat generated by the friction between the piston and liner 
ignited the oil vapors.  The ignition of the oil vapors caused the rapid 
pressure increase and the explosion in the crankcase.  

The AMOT valve malfunctioned because two of the four "power elements" 
(sensing elements) in the valve had failed.  Licensee personnel examined 
these elements at the licensee's metallurgical laboratory and attributed the 
root cause of the failure to slow growing intergranular stress corrosion 
cracking.  The cracking caused small openings that allowed the thermally 
active medium of the power element to leak out.  This loss of thermally 
active medium directly affects the actuating rod travel (valve stroke).  The 
failed elements were those originally provided with the engine in 1979.  The 
licensee had also found degraded power elements during a recent inspection 
of the Unit 1 diesel engines.  The vendor's documentation indicates that the 
power elements have a 15-year shelf life and should be inspected at 
intervals of 2 to 3 years to detect and make provision for normal wear.  

The licensee's corrective actions include changing the AMOT power element 
replacement schedule and evaluating changing the engine oil to a type with 
increased film strength and greater stability at elevated temperatures. 

Related Generic Communications

A similar failure of a thermostatic control valve, but resulting from a 
different failure mechanism, was discussed in NRC Information Notice 82-56, 
"Robertshaw Thermostatic Flow Control Valves," December 30, 1982.

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                                                       IN 91-85
                                                       December 26, 1991
                                                       Page 3 of 3


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:  Robert Martin, NRR
                     (301) 504-1493

                     William Orders, RII
                     (803) 831-2963

                     John Zeiler, RII
                     (803) 831-2963


Attachment:  List of Recently Issued NRC Information Notices  
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