Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 20, 1991
NRC INFORMATION NOTICE 91-58: DEPENDENCY OF OFFSET DISC BUTTERFLY VALVE'S
OPERATION ON ORIENTATION WITH RESPECT TO FLOW
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
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.
Discussion
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
calculated
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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
calculation.
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.
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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
Attachments:
1. Butterfly Valve Figures 1 and 2
2. List of Recently Issued NRC Information Notices
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