ML18135A210

Meeting Industry Response to Flowserve Part 21 on Anchor Darling Double Disk Gate Valves and NRC Staff Next Step

ML18135A210
Person / Time
Issue date:	05/16/2018
From:	Stewart Bailey NRC/NRR/DE/EMIB
To:
Farnan M
References
Download: ML18135A210 (17)

Text

Industry Response to Flowserve Part 21 on Anchor Darling Double Disk Gate Valves and NRC Staff Next Steps Public Meeting May 16, 2018 1

Overview

• Operating Experience has identified failures of Anchor/Darling (A/D) Double Disk Gate Valves (DDGVs)

• Significant progress has been made

- Industry has developed guidance

- All licensees have submitted information on the affected valves, including commitments for valve repairs

• It appears that industry is taking steps to address the issue

• The NRC staff is preparing to inspect licensees corrective actions

• The NRC staff continues to assess the need for a Generic Communication, but does not plan one at this time 2

Background

• Failure of A/D DDGV at Browns Ferry in 2013 revealed that threaded stem-to-wedge connection had not been properly torqued

• Flowserve Part 21 notification February 25, 2013

- Recommended assessing wedge pin susceptibility to shear and rework the valve if needed

• BWROG developed guidance to address Part 21 to include:

- Prioritization and Screening Criteria

- Evaluation Methods

- Inspection and Diagnostics

- Repair Methods 3

Background (cont.)

• NRC staff evaluated 2013 Part 21 and determined the issue would be monitored with no generic communication

• Additional failures occurred at LaSalle Unit 2 and Columbia

• LaSalle event elevated to NRC special inspection

• Information Notice (June 2017)

• Flowserve updated Part 21 (July 2017)

• BWROG updated guidance to Rev. 4 (August 2017)

• NRC staff considered need for generic communication due to larger population of failures and limited information readily available to the staff 4

Progress to Date

• NRC staff held public meetings on guidance and licensee corrective actions

- Staff requested clarification of guidance (October 2017)

- NEI provided clarification (November 2017)

• All licensees submitted information (December 2017)

- Valve population

- Valve characteristics (susceptible, non susceptible, risk category)

- Rework status

- Commitments for future repairs

- Public Data Compilations are in ML18053A023 and ML18053A904

• NRC staff held public meeting February 15, 2018

- Staff discussed guidance document, licensee corrective actions, and future plant inspections 5

Scope of Meeting

• Discuss the draft Temporary Instruction (TI) developed to inspect and assess industry progress on addressing Flowserve Part 21 issue on A/D DDGV

• Discuss inspection plan and schedule 6

NRC Temporary Instruction Scope

• Identify

- Verify licensee properly identified valves population (2 inch valves and larger with a stem-to-disc threaded connection design and an actuator that applies torque to the stem)

• Evaluate

- Evaluate actuator force for impact on valve integrity

- Evaluate torque/shear on pin to determine whether valve susceptible

• Review

- Review history of valve operation and plans for monitoring possible over torque events if licensee does not use maximum torque

- Review risk categorization

- Evaluate planned corrective actions 7

Identify

• Verify licensee has identified the A/D DDGV population

- Valve size

- Motor and Actuator type and size

- Plant system

- Valve function

- Normal valve position

- Valve operational requirements (open, closed, or both, cycles)

- Stem/Disc connection (threaded or T-head)

- Stem collar type (pressed on fit or integral)

- Safety risk category (high, medium, or low)

- Previous test and/or repair results 8

Evaluate

• Calculate maximum load stall torque applied

- Standard industry motor stall torque equation

- Motor stall torque value obtained from motor curve. If unavailable, motor stall torque estimated value will be 110% of rated motor torque

- Actual voltage at the motor needs to be considered

• Evaluate maximum test torque applied

- NRC staff recognizes that a large percentage of actuators have maximum stall torque values that are greater than the capability of the valve components

- If test max torque is used, evaluate how licensee will manage possible future motor stall events 9

Evaluate (cont.)

• Considerations for managing possible motor stall events

- Wedge pin shear capability has been included in the weak link analysis

- Procedures have been updated as necessary

- Valve has been modified to lower actuator capability and/or valve components replaced with stronger materials

- Valve drawings have been updated with Part 21 information regarding pressed fit collars and/or updated to reflect pressed fit collar being replaced with an integral stem

- Preventive maintenance (PM) activity has been updated to monitor and assess motor control center (MCC) contactor mechanical performance and/or have a periodic contactor replacement strategy 10

Evaluate (cont.)

• Evaluate applied force

- Torque, thrust, stem factor, coefficient of friction (COF)

- If COF is unknown, a value of .08 shall be assumed. This value is based on historical test data

• Calculate stem and wedge thread shear capability

- ASME B1.1-2003 Unified Inch Screw Threads (UN and UNR thread form)

• Compare applied force vs stem and wedge thread shear

• Calculate wedge pin shear capability

• Compare applied force vs wedge pin shear

• Stress values based on BWROG guidance 11

Evaluate (cont.)

• Evaluate credit for thread resistance

- BWROG guidance allows reasonable engineering judgement

- NRC staff notes there is no accepted guidance available regarding how to credit thread friction to resist applied torque

- NRC staff considers thread resistance should only be used to determine valve operability for the short term and should not be relied upon for a long term fix

- NRC staff considers 0.1 friction factor as a reasonable value for the short term.

- NRC staff considers use of higher COF values warrants additional attention until the valve can be reworked (e.g., stem rotation check during each quarterly valve exercise) 12

Review

• History

- Review susceptible A/D DDGV maintenance and test history

• Risk

- Review methodology used to apply risk

- Review valve risk ranking

• Actuator Capability vs Valve Components

- Review licensee calculations

- Calculations must use conservative factors such as applying actual voltage to motor and coefficient of friction on the stem/stem nut interface. Licensees may use actual test data with conservatism for expected variation and measurement uncertainties 13

Review (cont.)

• Weak link analysis

- Review valve structural capability and weak link calculations

• Corrective action plan

- Review adequacy and the timeline to complete repairs. Justification is required for the schedule that exceeds the specifications of the stated valve categories in BWROG guidance

• Reworked Valves

- A sample will be selected and reviewed to determine if the rework meets Flowserve Part 21 recommendations

• Process

- Review licensee process to ensure susceptible A/D DDGV provides reasonable assurance that the valves will be able to perform their safety function 14

NRC Inspection Plans

• NRC draft Temporary Instruction will be finalized following todays public meeting

• NRC staff is developing an inspection sample

- Initial inspection plan is to select two or three plants per region

- Evaluation of the first plants inspected will determine if additional inspections are needed

- Estimated inspection time is 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br /> (2 inspectors for one week) 15

NRC Next Steps

• Finalize TI (May 2018)

• TI inspections (2018 into 2019)

• Continue to update NRC assessment on need for generic communication 16

QUESTIONS?

Future Questions Stewart.Bailey@nrc.gov 301-415-1321 Michael.Farnan@nrc.gov 301-415-1486 17