NEIL Turbine Generator Risk Assessment. Todd Raymond Loss Risk Analysis Manager Nuclear Service Organization April 16-17, 2013

Transcription

1 NEIL Turbine Generator Risk Assessment Todd Raymond Loss Risk Analysis Manager Nuclear Service Organization April 16-17, 2013

2 Program Origin Large Losses due to Turbine Generator Failures in recent years Board direction to evaluate the fleet to determine if: The Risk across the mutual is homogenous The Risk is greater than we thought Program developed in 2010 with help of TG Advisors and representatives from the industry Defined 38 Problem Areas and ran as a pilot at 10 units

3 Program Refined The 38 problem areas were narrowed down to 16 failure modes expected to cause at least: $10 Million 8 Weeks of lost generation

4 Scoring Probability x Consequence

5 Problem Areas Overview Aeroelastic Vibration New LP Turbines with freestanding last stage blades or in higher untested flow conditions for the first operating cycle Stall Flutter Units with challenges in maintaining sufficient margin to OEM backpressure limitations Blade Attachment SCC Based on several factors, including chemistry, inspection history and actions taken, and yield strength

6 Problem Areas Overview LP Disc Burst Based on several factors, including inspection frequency, turbine design, steam chemistry, and inspection results HP Rotor Burst Based on rotor design and boresonic inspection frequency and findings LP Rotor Burst Based on rotor design and boresonic inspection frequency and findings

7 Problem Areas Overview Torsional Vibration Proximity of system modes to double line frequency Calibration of models Mixed OEM Analysis only basis Design history Negative sequence history and events

8 Problem Areas Overview Torsional Vibration (Cont d) 2009 ISO Standard

9 Problem Areas Overview Water Induction Based on whether the unit meets ASME TWDPS 1 Part 2, or has some water induction protection equipment installed Generator Stator Winding Factors include: Age without a rewind, spare windings, leak detection, water chemistry, and electrical test results Generator Stator Core Driven by age, ELCID results, and visual indications of arcing or burning

10 Problem Areas Overview Generator Rotor Forging Factors include boresonic inspection frequency and results, TIL 1292 inspections, and tooth top cracking Generator Rotor Winding Unit rewind history, rotor windings with deteriorated insulation (resistance and PI values), copper dusting, units with flux probes and PdM testing programs, units without field ground detection Generator Retaining Rings Essentially a 18/18 vs. 18/5 material rating

11 Problem Areas Overview Rotor Slot Wedge Failure Severe negative sequence events, evidence of surface damage, units with over 30 years of service without an inspections, units with aluminum wedges Generator Blower Blades and Vanes NDE methods, corrective actions for know issues, frequency of inspections Turbine Trip Protection Turbine valve inspection and testing frequencies, recent upgrades in first cycle of operation

12 Observations Most Units Have Experienced Back Pressure Excursions Some Units are Load limited in Summer Months Significant Variations in Back Pressure Conditions Low load restrictions not always recognized in alarm/trip setting Wide variations based on design type Aeroelastic Vibration hard to address in design process - first of a kind testing is critical Boresonic inspection intervals varied significantly HP, LP and Generator

13 Observations Large Percentage LP Shrunk On Discs Designs Replaced Significant SCC issues Bore and Attachments 50,000 to 100,000 Hr. threshold for initiation Missile analysis restrictions Advanced Disc Design LP s Completed First Inspections with no SCC issues Significant Variations in valve testing and overhaul schedules Overspeed Protection System Upgrades

14 Interesting Findings Resurgence of Torsional Vibration Issues Lack of field and shop testing Relying on analysis only Analysis inaccuracies found > 10 Hz Units with pending modifications Increase if design criticality Increasing field modifications Most generator rotors have been rewound (some twice)

15 Observations Generator Fan/Blower Cracking Varied NDE methods based on design type Cracking found on varying designs Robotic generator inspections not widely used Re-wedging - some level needed every 2-3 majors based on design type Most units have plans to rewind stator Driven by hotspots and aging concerns Significant number of stators approaching or exceeding 35 years of service Stator cores requiring tightening

16 TGRA Generator Results Torsional Vibration Stator Windings Stator Core Generator Rotor Forging Rotor Windings Retaining Rings Rotor Slot Wedges Blower Blades and Vanes

17 TGRA Turbine Results

18 Number of Units Industry Results - Property 25 Aggregate Risk Points Above Baseline All 3 s and High 2 s Risk Points

19 Todd Raymond Steve Reid Nuclear Service Organization TG Advisors Manager Loss Risk Analysis President & Principal Engr Questions?