Early detection of Tube Support Degradation in Feedwater Heaters

1. Early detection of Tube Support Degradation inFeedwater Heaters Presentation to the 2013 Feedwater System Reliability Improvement Meeting Presented By: Steven Schaefer Authored By: Steven Schaefer & Jacob Endres January 21-24, 2013

2. INTRODUCTION • What is ‘Tube Support Degradation’ ? • Erosion of the carbon steel tube support(s) in Feedwater Heaters • What causes Tube Support Degradation ? • Steam passing between the tube OD and the TSP hole ID • Why is important that it is identified early? • Large numbers of tubes can be left unsupported which can lead to multiple failures from fatigue or a plant excursion • Involves Costly and Timely design review, procurement and replacement • Can identify associated damage to FWH shell which could lead to catastrophic failure • How is it typically first identified? • When the TSP signals are almost totally missing • What are the precursors that can be identified early? • TSP signals at the same axial location that appear to be smaller then normal • Low Frequency absolute deposit signals on one side of the TSP

3. Goals of Tubing Inspection • Primary Goals • Condition Assessment • Condition of each tube inspected • Condition of the overall HX • Component Reliability & Operability • Identify potential failures before they occur to ensure continuous operation between inspections • Not only for the next cycle but more importantly for long term operation • Challenges • Probability of Detection (POD) • Sizing Accuracy • Schedule • Cost

4. Standard Inspection Technique - Bobbin • Multifrequency Bobbin Coil Inspection • Advantages • Inspection Speed (as high as 80ips) • Cost • Basic equipment and software • High sensitivity & POD for ‘Typical’ damage mechanisms • Dis-Advantages • Detection of Circumferential Cracking • Accurate sizing of certain volumetric conditions

5. Detection Of TSP Degradation • Early Signs That Identify TSP Degradation/Erosion May Be Present • Changes from Nominal TSP Signatures on Low Frequency Absolute Channel • A Change or Shift of the phase angle of the TSP signal • Lower Amplitude (voltage) TSP signatures identified on Low Frequency Absolute Low Sensitivity strip chart • In extreme cases, a completely missing TSP signal where one is expected • Deposit like drift on either side of the TSP signal • Any of the above occurring repeatedly at the same axial TSP location in one area, usually near an extraction steam inlet or drain cooler inlet • May be found in the condensing zone or the drain cooler • May be found in Low, Intermediate or High Pressure FWH’s • Eroded pieces of internals, such as tie rod shrouds, found in the heater drain or down steam from the FWH drain • FWH shell erosion near the extraction steam inlet(s)

6. Accurate Sizing Of TSP Degradation • Sizing of TSP erosion with Bobbin Probe Technique • There is no accurate sizing technique possible with Standard Bobbin Probe Technique • This is due to the amount and proximity of the remaining TSP material to the tube OD, the type of erosion (straight cut or tapered) and the amount of deposits near or under the TSP • In extreme cases a piece of the remaining ligament can be all that is in contact giving a false indication of actual ‘Support’ from a TSP • The TSP signal amplitudes can be documented and categorized by voltage for comparison with future inspections but only for general trending • Array Coil Technology is being developed for a more accurate measurement technique • While this could give a more accurate sizing technique it will mean additional cost, analysis and schedule time that must be built into the budget and outage schedule

7. Typical Bobbin Coil

8. Bobbin Coil Sensitivity

9. Eroded Areas Around Tubes

10. Eroded Areas Around Tubes

11. Eroded Areas Around Tubes

12. Tubesheet Map of Erosion at TSP Horizontal U-Tubed single zone FWH with extraction steam inlets at 12 o’clock. The red tubes indicate tubes recording possible TSP degradation while the blue tubes were inspected with no damage of any type recorded.

13. Internals Drawing of Erosion at TSP

14. ET Response of Nominal TSP

15. ET Response of Eroded TSP

16. ET Signal Response Variations Proximity of Tube OD and actual TSP hole

17. ET Signal Response Variations Remnants of TSP ligaments that can give small but false signals of supports

18. ET Signal Response Variations Large amounts of deposits or magnetite can alter ET response

19. Early Detection is Critical • Early Detection is Critical • Allows for early planning of FWH replacement or temporary repairs • Plan more frequent inspections to trend TSP signals • Indicates present FWH design may not be adequate for steam flow • Review FWH specifications vs. design steam flow • Review flows associated with power up-rate (past, present & future) • Indicates higher potential for tube failure(s) • Unsupported spans can lead to tube failure from fatigue over time • Unexpected event like a plant trip or water hammer can lead to multiple tube failures at one time • Indicates FWH shell has high potential for erosion and failure • Shell should be inspected for thinning with UT in areas of TSP erosion and near extraction steam inlets • If TSP erosion is in the drain cooler • FWH level may be operating below desired level • Review of operational data to determine typical operational level • Verify level indicator is operating correctly and is accurate • Drain cooler snorkel could be damaged or improperly designed • Drain cooler end plate holes could be eroded and allowing steam intrusion

20. Reporting of TSP Degradation • As stated earlier, TSP hole sizing is inaccurate with standard bobbin probe technique • Signal amplitude changes with proximity to TSP or remnants of TSP • Simple trending by percent of full nominal response by 100%, 80%, 60%, 40%, 20% and <20% of signal voltage • This gives a simple signal voltage % to track overall TSP condition • Example: Nominal TSP somewhere near the middle of the bundle is set to 10.00 volts then: 9.00 volts and greater = 100%, 7.00 to 8.99 volts = 80%, 5.00 to 6.99 volts = 60%, 3.00 to 4.99 volts = 40%, 1.00 to 2.99 volts = 20% & <1.00 volts = <20%; this allows a tolerance and gives a general condition • Again, this is a general condition and your actual mileage may vary, but it allows for trending and assessing the overall stage of progression and the ability to set a timetable for replacement or immediate repairs until a replacement can be completed

21. TSP Erosion Sized by General Percent Actual Inspection with TSP degradation quantified by general percent

22. Autopsy of FWH - Inverted

23. ET Response vs. Actual TSP Actual ET results utilizing general voltage percent Actual removed TSP from Drain Cooler

24. Tube-Pro 3D Modeling Single Zone Feedwater Heater with two Extraction Steam Inlets Impingement Plates Below Extraction Steam Inlets

25. Tube-Pro 3D Modeling Single Zone Feedwater Heater with two Extraction Steam Inlets Tubes with TSP Degradation Identified on Tubesheet

26. Tube-Pro 3D Modeling Single Zone Feedwater Heater with two Extraction Steam Inlets Locations at Tube Supports that indicated TSP Degradation

27. Tube-Pro 3D Modeling Single Zone Feedwater Heater with two Extraction Steam Inlets Close up of area showing TSP Degradation

28. Summary • What is ‘Tube Support Degradation’ ? • Erosion of the carbon steel tube support(s) in Feedwater Heaters. • What causes Tube Support Degradation ? • Steam passing between the tube OD and the TSP hole ID. • Why is important that it is identified early? • Large numbers of tubes can be left unsupported which can lead to multiple failures from fatigue or a plant excursion • Involves Costly and Timely design review, procurement and replacement • Can Identify associated damage to FWH shell which could lead to catastrophic failure • How is it typically first identified? • When the TSP signals are almost totally missing. • What are the precursors that can be identified early? • TSP signals at the same axial location that appear to be smaller then normal. • Low Frequency absolute deposit signals on one side of the TSP.

29. References • Some pictures and slides were used with permission of the author from a previous presentation Titled: “Comparison of Degraded Baffle Signal Methodology to Actual Degraded Baffles – Results in: a Better Methodology”,  Perry Holzman,  2010 EPRI BOP HX NDE Conference

30. For Additional Information

31. Early detection of Tube Support Degradation inFeedwater Heaters Presentation to the 2013 Feedwater System Reliability Improvement Meeting Presented By: Steven Schaefer Authored By: Steven Schaefer & Jacob Endres January 21-24, 2013