Development of best practice for the use of Type R thermocouples for creep testing

Transcription

1 Development of best practice for the use of Type R thermocouples for creep testing Andrew Mills, Group Leader Materials Testing Siemens Industrial Turbomachinery Ltd., Lincoln, UK Restricted Siemens AG 2016 siemens.com/energy

2 Development of best practice for the use of Type R thermocouples for creep testing Creep in Industrial Gas Turbines Creep Testing Background Project Background Changes Implemented & Impact of Changes Page 2

3 Creep in Industrial Gas Turbines Gas turbines are a very challenging environment to select materials to operate in. Components are expected to operate for long periods at high temperature and high loads, which cause metals to creep. Industrial gas turbines especially are susceptible to creep problems as they maybe expected to operate for long durations without stopping. Creep properties of selected material dictate overhaul regime. Page 3

4 Creep in Industrial Gas Turbines Considering these conditions, creep cannot be ignored and hence precise testing needs to be performed to enable accurate life predictions to be made. Creep tests are performed isothermally, thus accurate temperature measurement and control is essential to produce reliable results. Tests are performed to match turbine operating conditions Temperatures C Durations up to 4 years. Output of the tests is a strain vs. time curve. Page 4

5 Anatomy of a creep testing machine Lever arm system to multiply load Furnace providing heat Test specimen inside furnace 3x Thermocouples attached to specimen. Dead weights provide load Extensometer measuring displacement Furnace control system Page 5 Image Zwick Roell Group

6 Anatomy of a creep testing machine Image Eurotherm Page 6

7 Creep in Industrial Gas Turbines Often, type-k thermocouples (chromel-alumel) are used for creep testing. These have relatively poor drift characteristics, and can lose calibration after only a few 100 hours continuous operation at high temperatures > 800 C. Precious metal thermocouples such as type-r (Platinum-Plat13%Rhodium) are much better in this respect, but still suffer from long term drift due to ageing. Page 7

8 Creep in Industrial Gas Turbines Drift in the order of single digit C can be enough to half the creep life of the specimen on test. This could result in excessive scatter in results, based on something that was assumed to be a constant. Hence it is very important for us to understand the limitations of thermocouples, and select wisely for testing. Page 8

9 Project Background - Previous Process Materials testing laboratory owned a large number of type-r thermocouples more than were required. Thermocouples were a standardised design, but various different lengths. Thermocouples were being used anywhere they were required, on any machine type including for fatigue testing. Limited considerations had been made to handling & storage. Page 9

10 Project Background - Previous Process Thermocouples were used indefinitely. - Recalibrated every year. - If they broke they were repaired & recalibrated. Thermocouples would regularly fail calibration, creating unnecessary costs. Calibration requirements being driven by price rather than quality. Limited considerations were made impact of prior thermocouple history to drift. Page 10

11 Project Background - Consequences Temperature control during tests was poor. Manifested as fluctuating temperature during test, or apparent temperature gradient along the length of the specimen. Fluctuating temperature resulted in cycling expansion / contraction of the specimen. Causing noise in strain reading, which made analysis of the data difficult. Unquantified uncertainty included in results. AREAS FOR IMPROVEMENT IDENTIFIED Page 11

12 Changes Implemented All existing thermocouples were retired once off test. A standard thermocouple design for each type of test rig was defined. - Observe constant immersion depths Once a thermocouple was issued to a test rig, it was not moved to a different test rig. - Maintain constant immersion depth. Moved to a reel calibration system, rather than individual calibration. - Reduces calibration requirements. Page 12

13 Changes Implemented Searched for a calibration partner to allow focus on quality. - Able to build a two-way relationship. Storage/transit cases purchased for thermocouples not in use to protect from damage. - Prevent accidental strain damage and contamination. No thermocouple is used for more than one year, unless test is predicted to last longer. New thermocouples are used for these tests and retired once complete. - Better drift considerations. Thermocouples colour coded to identify them to their intended test rig. - Reduce operator errors. Page 13

14 Results Vast improvement in quality of testing. Problems with temperature variation and fluctuation eliminated. Aborted starting of tests due to temperature problems greatly reduced. Time wasted dealing with thermocouple calibration related issues also greatly reduced. Financial benefits from: Reduced calibration requirements. Reduced thermocouple quantity requirements. Reduced time problem solving. Page 14

15 Acknowledgements NPL Graham Machin Jonathan Pearce Claire Elliott CCPI Trevor Ford Peter Mitchell Page 15

16 Thank you for your attention Any questions? Andrew Mills Group Leader Materials Testing Siemens Industrial Turbomachinery Ltd. Ruston House Waterside South Lincoln LN5 7FD Phone: siemens.com Page 16