ML18153D241
| ML18153D241 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna, Surry, North Anna  |
| Issue date: | 02/15/1993 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 92-330, GL-89-10, IEB-85-003, IEB-85-3, NUDOCS 9302220126 | |
| Download: ML18153D241 (9) | |
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e VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 February 15, 1993 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 SURRY POWER STATION UNITS 1 AND 2 GL 89-10 RESPONSE REVISION Serial No.
MPW/MAE:
Docket Nos.
License Nos.92-330 R13 50-338 50-339 50-280 50-281 NPF-4 NPF-7 DPR-32 DPR-37 SAFETY-RELATED MOTOR-OPERATED VALVE TESTING AND SURVEILLANCE FOR DIFFERENTIAL PRESSURE AND/OR FLOW The Virginia Electric and Power Company (Virginia Power) Motor Operated Valve (MOV) Program was implemented to address the requirements of IEB 85-03, "Motor-Operated Valve Common Mode Failures During Plant Transients Due To Improper Switch Settings". The scope of affected systems was increased by Generic Letter 89-10, "Safety-Related Motor-Operated Valve Testing and Surveillance" and committed to by our letter dated December 26, 1989 (Serial No.89-530).
In subsequent evaluations, we concluded thE.t design basis testing of butterfly valves, pursuant to Generic Letter 89-10, is not required to ensure valve operability or function and, therefore, is considered to be a significant regulatory burden without commensurate safety benefit. Consequently, Virginia Power is revising our response to Generic Letter 89-1 O, Recommended Action c, for MOV testing of butterfly valves at design basis differential pressure and/or flow.
Differential pressure testing at design basis conditions is used to validate the calculational methodology used to predict design basis valve operational requirements. This is justified for valve designs where the calculation or its constants have not been empirically determined, as in the case of the valve factor for gate valves. Because the equations used to predict butterfly valve torque requirements utilize coefficients that were initially determined through manufacturers testing, it is of marginal benefit to reproduce those results for the Generic Letter. Design adequacy of butterfly valves is therefore ensured by calculational verification of valve torque requirements. Continued operability is ensured by our current maintenance program 190025 __ ~-- ----~
,/ -~ -9302220126 930215 PDR ADOCK 05000280 p
and ASME Section XI required valve testing, i. e., the lnservice Testing Program. A detailed discussion -of our exception to design basis differential pressure and/or flow testing of butterfly valves is presented in the attachment.
Furthermore, qualified test equipment with sufficient accuracy to validate the torque requirement calculations, is not presently available for in-place testing. Prototypical testing in a laboratory environment is a viable means of acquiring accurate data. The estimated cost to test a representative sample of butterfly valves in this manner is at least $475,000.00.
The Virginia Power MOV Program will continue to ensure that motor-operator switch settings (torque, torque bypass, position limit, overload) for butterfly MOVs are selected, set and maintained so that the MOVs will operate under design basis conditions for the life of the plant. The remaining commitments previously made to Generic Letter 89-1 O will continue to be implemented for butterfly valves.
If you have any questions or require additional information, please contact us.
Very truly yours,
~R'~~\\
\\.\\[~\\
W. L. Stewart Senior Vice President - Nuclear Attachment
cc:
U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, N.W.
Suite 2900 Atlanta, Georgia 30323 Dr. Thomas Murley, Director Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Mr. M. S. Lesser NRC Senior Resident Inspector North Anna Power Station Mr. M. W. Branch NRC Senior Resident Inspector Surry Power Station Mr. R. Clive Calloway Nuclear Management And Resource Council 1776 Eye Street, N.W. Suite 300 Washington, D. C. 20006 e
ATTACHMENT EXCEPTION FROM GL 89-10 REQUIREMENTS FOR DIFFERENTIAL PRESSURE TESTING OF BUTTERFLY VALVES NORTH ANNA AND SURRY POWER STATIONS VIRGINIA ELECTRIC AND POWER COMPANY
e EXCEPTION FROM GL 89-10 REQUIREMENTS FOR DIFFERENTIAL PRESSURE TESTING OF BUTTERFLY VALVES NORTH ANNA AND SURRY POWER STATIONS 1.0 HISTORY In 1985 the NRC issued IE Bulletin 85-03 which required licensees to develop and implement a Motor Operated Valve (MOV) Program. The program would ensure motor operator switch settings were determined, set, and maintained so the MOV would be capable of performing its design basis function for the life of the plant.
The priority systems identified in IEB 85-03 were the high pressure safety injection, and the emergency feedwater systems. The valves in service in these systems are typically the rising stem type, gate or globe valve.
During the IEB 85-03 program implementation, deficiencies were recorded !hroughout the industry that included degraded material condition of the valves and motor operators, and incorrect torque, torque bypass, and limit switch settings.
Additionally, concurrent testing by the Idaho National Engineering Laboratory, as part of the resolution of Generic Issue 87 "Failure of HPCI Steam Line Without Isolation",
raised questions regarding the ability of industry sizing equations to conservatively determine thrust requirements for rising stem gate valves that are required to close to isolate a high pressure line break.
Collectively, this information resulted in the issuance of Generic Letter 89-10, which expanded the scope to include all safety related or position changeable valves.
2.0 BACKGROUND
Butterfly valves equipped with motor operators are inherently different from rising stem motor operated valves in at least three distinct areas: valve characteristics, power transmission, and operator control scheme.
Valve Characteristics The equations used to analytically determine the torque requirements for butterfly valves are more exact than the equations used to determine the thrust requirements for rising stem valves. This is largely due to the design of the valve and the resultant forces present during valve operation.
A butterfly valve isolates flow by sweeping across a sealing surface and remaining in a position that is perpendicular to the flow. For the range of motion prior to seating, the frictional forces internal to the valve are limited to the shaft bearings. These frictional forces can be accurately estimated knowing the bearing material.
The torque requirements of the valve are determined using equations with empirically determined coefficients. These coefficients are constant over the life of the valve and depend on Page 1 of 5
the valve seat design, the valve body design, the disk shape, and the location of the shaft to the disk edge. As such, the coefficients used in the torque requirement equations are very specific and are determined by the manufacturer through testing for each size and style of valve.
Rising stem valves control flow by positioning the valve disk in the flow stream. In the case of a wedge or parallel disk this is accomplished by sliding the disk against valve guides located in the body of the valve. To isolate flow, a rising stem valve presses or wedges the disk against the seating surface. Because of the sliding motion of the valve disk against the seating area, the standard friction factor used in the equation to calculate thrust requirements to close the valve has been found to depend on the interface material, its surface condition, and the differential pressure across the valve.
The variability of the factors that can affect the frictional forces internal to the valve requires that the thrust equation employ a generic friction factor that bounds expected operation. To date, this factor has been dependent on valve type (gate, globe, parallel disk) alone and has not necessarily considered the effect of valve size or manufacturer.
Power Transmission In butterfly valves, torque of the operator is transmitted through a gear box to torque in the valve. The fact that the valve requirements are expressed in the same terms (torque) as the operator output simplifies the coordination of the two devices and minimizes the assumptions made in the equations used to compare valve requirements to operator capability. The calculation used to determine the torque produced by the operator and available to the valve, utilizes well understood motor outputs and gear efficiencies.
Rising stem valves require the torque of the operator to be converted to thrust in the valve. This conversion utilizes assumptions regarding the coefficient of friction that exists at the point of interface where torque is converted to thrust, namely the valve stem to operator stem nut interface. Because this interface is not bathed in lubricant, the surface and consequently the friction factor is subject to changes over time. The approach to date has been to use a friction factor that bounds valve operation. This approach tends to make a design determination of operator output less certain.
Operator Control Scheme Butterfly valves use a gear driven limit switch as the primary control switch for deenergizing the motor on Limitorque operators. Rising stem valves typically use a torque switch to accomplish this task. Experience hqs shown that the gear driven limit switch is more predictable, repeatable and reliable than the torque switch. Utilizing the limit switch allows the full capability of the motor to be available as needed.
Butterfly valves in many cases, have no mechanical stops in the valve body that dictate the point of maximum sealing. Rather, the final resting place of the valve disk is determined by manually placing the disk in its desired location and setting the gear driven limit switch in the operator. From that point on, the limit switch depends on the number of rotations of the motor to dictate the open and closed position of the valve.
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The magnitude of the thrust exerted on a rising stem valve is frequently determined by the operator torque output, and controlled by the torque switch setting. The ability to set the torque switch setting at a point which assures the required thrust is delivered to the valve is a function of the valve frictional forces, the stability of the relationship between the torque-thrust interface, and the ability of the test equipment to accurately correlate torque to thrust.
Gear driven limit switches have historically been used in the majority of rising stem applications to de-energize the motor in the open position. This limit switch typically provides the contacts for a light indicating both the open and closed positions.
Position indication is routinely used during the performance of ASME Code Section XI stroke time tests. Limit switches have performed-reliably in this function.
3.0 DIFFERENTIAL PRESSURE TESTING Differential pressure testing at or near design basis conditions is a viable means of validating the calculation methodology used to predict a valve's operational requirements. This is especially important, and justified, when the calculation or its constants have inherent uncertainties due to valve designs as in the case of the valve factor for gate valves.
Differential pressure testing is of marginal benefit for butterfly valves since the factors used in the torque requirement equation were initially determined through testing.
Specifically subjecting a plant to abnormal valve line-ups in order to create near design basis conditions in an effort to validate what has previously been empirically determined by manufacturer testing is considered to be a significant regulatory burden without commensurate safety benefit.
Test equipment has been developed that can be used to monitor the thrust output of a motor operator joined to a rising stem valve. The test equipment has undergone rigorous testing to quantify its capabilities and uncertainties.
This equipment is
- available and has been independently qualified. Equipment with similar qualification for in place testing has yet to be developed for use on butterfly valves. Without equipment to accurately quantify test results, validation of the calculational methodology cannot take place.
4.0 ALTERNATIVES TO DIFFERENTIAL PRESSURE TESTING In the Virginia Electric and Power Co"mpany MOV Program, differential pressure testing is used to validate the calculational methodology used in determining rising stem type valve requirements.
This validation is important in rising stem valves because of the uncertainty introduced by various design features of the valves which affect terms used in the thrust requirement calculation.
With empirically determined coefficients used in the equation, the same level of confidence of the butterfly valves ability to perform its intended function can be gained Page 3 of 5
through calculational evaluation without confirmational differential pressure testing.
As with all valves in the MOV program, further assurance of valve performance is provided by preventative maintenance inspection of the operator and valve and periodic stroke testing.
The adequacy of this approach is substantiated by the experience at the Surry Power Station. Surry Power Stations' service water and circulating water systems are gravity fed from an intake canal. Eighty percent (48) of the safety related butterfly valves are located in these two systems. Eight of these valves are stroked at near design basis conditions during normal plant operations. Another 14 were tested at near design basis conditions during the performance of the recirculation spray heat exchanger service water flow testing performed in response to Generic Letter 89-13. Full flow testing of a new valve that is representative of another four of this population was conducted in a flow loop as part of the procurement process. In every case the MOVs performed reliably. In the instance where the torque requirements were quantified during full flow testing in a laboratory environment, it was shown that the actual torque required versus the torque used for operator sizing was conservative by forty-five percent.
The material condition of these valves has been preserved by performing routine preventative maintenance.
In the last five years 67% of these valves have been replaced due to environmental effects on the material condition of the valve. The decision to replace these valves was arrived at using data gathered from standard maintenance processes.
North Anna butterfly valves are also associated with the service water system.
Because the service water system is a pumped system, a computer model has been developed to characterize the flow and pressure effects. To date, nine cases have been run which consider various equipment and valve line-ups in order to simulate worst case design basis conditions.
Information gained from this conservative analysis is used with manufacturers' empirically determined flow coefficients to calculate the torque requirements on 66% of the 85 valves. This analysis has resulted in the recommendation to increase the size of 16 motors in order to improve the margin between the motor operator capability and the valve torque requirement. The remaining 34% are in the process of undergoing the same analysis.
Efforts to monitor and improve material condition of the valves at North Anna have been mad~ as demonstrated by the fact that 57 of 85 valves have been completely refurbished in the last two years, while another 12 have been replaced within the last
. five years.
Preventative maintenance frequencies have been adjusted based on actual experience to identify degradation and take corrective action before it becomes significant.
5.0 CONCLUSION
The approach and regulatory requirements identified to date have been largely in response to problems identified with rising stem gate valves.
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The inherent differences between rising stem and butterfly valves in the areas of valve characteristics, power transmission, and operator control scheme tend to contribute to an increased confidence in the ability to analytically determine the design basis requirements of butterfly valves and coordinate that with motor operator output.
Differential pressure testing of butterfly valves is of marginal benefit when the coefficients used in the equations have been previously determined empirically by the manufacturer.
Finally, there is presently no qualified in-place test equipment that can accurately quantify the results for use in validation of the calculational methodology for butterfly valves.
We conclude that analytical evaluation backed by a solid maintenance program is sufficient to provide assurance that butterfly valves will perform their intended safety function, thus satisfying the intent of Generic Letter 89-1 O. Accordingly, we are taking exception to Recommended Action c of Generic Letter 89-10, which requires design basis differential testing, as it applies to butterfly valves. Remaining commitments to Generic Letter 89-1 O for butterfly valves will continue to be met.
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