ML18044A983

Meeting Slides for February 15, 2018, Public Meeting Regarding Industry Response to Flowserve Part 21 on Anchor Darling Double Disk Gate Valves and NRC Staff Next Steps

ML18044A983
Person / Time
Issue date:	02/15/2018
From:	Stewart Bailey NRC/NRR/DE/EMIB
To:
Bailey S
References
Download: ML18044A983 (21)

Text

Industry Response to Flowserve Part 21 on Anchor Darling Double Disk Gate Valves and NRC Staff Next Steps Public Meeting February 15, 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 and is correcting the issue

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

• The NRC staff and Industry continue to discuss the issue

• The NRC staff is preparing to inspect licensees corrective actions

• The NRC staff continues to assess the need for a Generic Communication 2

Background

• Failure of Anchor/Darling (A/D) Double Disk Gate Valve (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 the 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 the 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 5

NRC Observations on BWROG Guidance 6

Summary

• Allowance to use engineering judgement for key assumptions that determine whether a valve is susceptible

- Credit for thread friction

- Maximum actuator torque

• Limited effectiveness of testing and diagnostics 7

Engineering Judgement on Credit for Thread Friction

• Rev. 4 guidance implies that friction should only be used for borderline cases (small negative margin) and low levels of friction should be assumed

• Stem-to-Wedge thread friction has a wide range of possible values that could change over time and system conditions (.02 to .78 steel on steel)

• Staff questions crediting of stem-to-wedge thread friction to declare a valve non susceptible. Thread friction is acceptable for short term operability until the valve can be reworked to Flowserve Part 21 recommendations. A reasonable thread friction value to use for interim evaluations is 0.101.

1 Staff letter dated October 31, 2017 8

Engineering Judgement on Maximum Applied Torque

• Rev. 4 guidance provides little direction on actuator torque

• Many plants are using as-tested values instead of full actuator capability

• Other licensees are using limiting value of the spring pack capability or maximum torque from the valve/actuator weak link analysis.

• Staff questions use of less-than-maximum actuator torque, which is stall torque and stall efficiency as appropriate, to bound potential over-torque events 9

Hidden Over-Torque Events and Reliance on Spring Pack

• Licensees have experienced motor stall events caused by a sticky contactors, such as those in the motor control center

• Excessive force may be been applied in the early test years, such as when addressing GL 89-10

• Valve may have been subjected to a pressure locking event

• Test equipment in early test years was not as accurate as current test equipment (GL 89-10 Supplement 5)

• Spring pack does not limit torque on stem during over torque event, it prevents over travel of torque switch assembly 10

Diagnostics for Evaluating Stem-to-Wedge Connection

• NRC SIT team for LaSalle Unit 2 MOV failure concluded that diagnostic testing and stem rotation checks are inconclusive in determining active stem-to-disc connection degradation1

• Diagnostics can be useful to help plan the schedule for rework

• Diagnostic testing and stem rotation checks will identify gross failure of the stem-to-wedge connection2 1 Staff inspection report dated August 31, 2017 2 Staff letter dated October 31, 2017 11

Example of Diagnostic Test Inability to Conclusively Determine Degradation of Stem-to-Wedge Connection 12

LaSalle Unit 2 Anomaly 2015 - Example used in BWROG Guide as Active Stem-to-Wedge Connection Degradation 13

LaSalle Unit 2 As Left Thrust/Torque Trace After Rework 14

Recent MOV Rework As-Found-Left Thrust Trace 15

LaSalle Unit 2 Anomaly Compared to Recent MOV Post Rework Trace Recent MOV Post Rework Unit 2 Anomaly As Left Torque & Thrust Trace 16

Summary of A/D DDGV Population from Licensee Submittals

• 584 - Total # of valves reported

• 119 - Total # of valves repaired

• 425 - Total # of valves not repaired

• 40 - Total # of valves N/A (T-Head design, not always reported)

• 106 - Total # of high risk valves

• 163 - Total # of medium risk valves

• 305 - Total # of low risk valves

• 182 - Total # of valves that use thread friction > 0.10

• 59 - Total # of valves that use thread friction < or = 0.10

• 38 - Total # of valves that are non-safety

• 225 - Total # of valves that require further NRC review 17

A/D DDGVs Requiring Further Review

• 113 - Total # of valves considered not susceptible using thread friction > 0.10 with no plans to repair

• 51 - Total # of valves considered not susceptible using thread friction < or = 0.10 with no plans to repair

• 39 - # of High Risk valves using thread friction with no plans to repair

• 14 - # of High Risk valves with no plans to repair and provided data not clear

• 13 - # of valves that have been repaired and using thread friction to justify final repair

• 35 - # of non-safety related valves. Need to verify failure does not affect other systems.

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NRC Inspection Plans

• NRC staff is developing an inspection sample

• NRC draft Temporary Instruction is in internal review

• Focus of inspection:

- Verify licensee properly identified valves population

- Evaluate thrust for impact on valve integrity

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

- Evaluate history of over torque events and plans for identifying future over torque events if licensee does not use maximum torque

- Review risk categorization

- Evaluate planned corrective actions 19

NRC Next Steps

• Public Meeting (April 2018)

• Finalize TI (May 2018)

• TI inspections (2018 into 2019)

• Continue to assess need for generic communication 20

QUESTIONS?

Future Questions Stewart.Bailey@nrc.gov 301-415-1321 Michael.Farnan@nrc.gov 301-415-1486 21