Information Notice No. 91-58: Dependency of Offset Disc Butterfly Valve's Operation on Orientation with Respect to Flow

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                               September 20, 1991



All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission is issuing this information notice to 
alert addressees to the possibility that offset disc butterfly 
motor-operated valves (MOV) may not function properly under design 
differential pressure and flow conditions because the orientation of the 
valve to the direction of flow can affect the operating characteristics of 
the valve.  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 February 27, 1991, three service water MOVs that isolate the 
recirculation spray heat exchangers (RSHXs) at the North Anna Power Station, 
Unit 1, failed a special test when they did not fully open on demand.  The 
Virginia Electric and Power Company (the licensee) was conducting the test 
under high flow and differential pressure conditions to respond to NRC 
Generic Letter (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and 
Surveillance."  The unit was in cold shutdown for refueling when the 
failures occurred.

The valves are 16-inch, 150-pound offset disc butterfly MOVs.  The licensee 
determined that the valves had been installed in an orientation such that 
the service water flow was toward the curved (shaft) side of the disc 
instead of toward the flat side of the disc as intended (see Figure 1).  
This orientation resulted in the need for a much higher torque than 
anticipated in order to open the valves under the conditions established for 
the test.


The licensee had recently performed modifications that reduced the torque 
switch settings on the valves to values that ensured that the maximum torque 
achieved would be within the actuator's rating.  The licensee also 

                                                       IN 91-58
                                                       September 20, 1991
                                                       Page 2 of 3

the required torque values for the system application using the American 
Water Works Association (AWWA) Standard for Rubber-Seated Butterfly Valves 
(C504-80).  The calculation indicated that the expected unseating torque was 
greater than the dynamic torque (letter from W. L. Stewart, VEPCO, to NRC 
Document Control Desk, "Reverse Installation of Safety Related Motor 
Operated Offset Disc Butterfly Valves," May 31, 1991).  However, the 
vendor-supplied valve coeffi-cients used in the calculation were for a disc 
of a different type than that installed, leading to an error in the 

The affected MOVs are normally shut, and the intended safety function of the 
valves is to open during an accident to supply cooling water to the RSHXs.  
Under normal conditions, these valves should not experience significant flow 
or differential pressure during the opening stroke because of another set of 
isolation valves upstream.  However, responding to Generic Letter 89-10, the 
licensee developed a test to demonstrate that the valves could reopen under 
worst-case flow and differential pressure conditions.  On February 27, 1991, 
the licensee conducted the tests and challenged each valve individually to 
open against a maximum differential pressure of approximately 75 pounds per 
square inch (psid) and a maximum flow of approximately 9000 gallons per 
minute (gpm).  The three valves in question failed to fully open because the 
torque switches opened at roughly mid-position.  The licensee noted that the 
orientation of the three failed valves was such that the service water 
flowed toward the curved (shaft) side of the disc (see Figure 1).  Five 
other similar valves in the system which were properly oriented with respect 
to the service water flow were tested successfully.

For this type of valve disc, when flow is toward the flat side of the disc, 
the maximum torque on the opening stroke occurs during valve unseating, and 
the actuator rating and torque switch setting should be selected to achieve 
this torque.  However, if the service water flows toward the curved (shaft) 
side of the disc, the hydrodynamic torque developed as the flow increases 
acts against the opening motion, and, thus, a much higher actuator torque is 
required to fully open the valve (see Figure 2).  The actuators were not 
adequately sized and rated for this situation, and the torque switches 
operated to prevent damage to the actuators before the valves reached the 
fully open position.

The licensee indicated that the valves were probably installed correctly by 
the architect/engineer with service water flow toward the flat side of the 
disc.  The licensee concluded that a lack of maintenance controls most 
likely caused the reversal in the orientation.  The licensee has also 
identified weaknesses in the torque calculation method such as poor modeling 
assumptions and a lack of consideration of the differences between the 
coefficients for symmetrical and offset discs.

The licensee removed the three valves and reversed them so that the service 
water would flow toward the flat side of the disc.  The licensee marked the 
valves and the associated piping to indicate the direction of flow.  
Subsequent testing of the valves was satisfactory.  The licensee is also 
reviewing and evaluating the models used for the service water system and 
the related calculations.

                                                       IN 91-58
                                                       September 20, 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 project manager.

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contacts:  P. L. Campbell, NRR
                     (301) 492-1311

                     M. Lesser, RII
                     (703) 894-5421

1.  Butterfly Valve Figures 1 and 2
2.  List of Recently Issued NRC Information Notices

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