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AMMJ Lessons Learnt In 45 Years of Condition Monitoring 2 These tubes are then led out of the furnace space through gaps in the roof tubes into the dead space, where they connect to primary superheater inlet tubes. There are 80 of these superheater sections across the gas path, so each has 6 tubes. Excessive metal temperatures lead to considerable reduction in creep rupture life. At these temperatures, an increase of only 11 C° can halve the life, so operational monitoring is important. Manufacturers use thermocouples installed in tube walls, sometimes in special sections (BEI) to try and measure the maximum metal temperature. Such sophistication was not available when these boilers were built, so at several sections, 5 thermocouples were fixed across the gas path to primary superheater outlet tubes in the dead space, and the limits for operation derived by calculation. Two boilers built almost at the same time exhibited quite different temperature behaviour at otherwise similar operating conditions. One was often close to alarm limits, and operation was adjusted to keep within them. The other showed no such high temperatures. After some years of service the “good” boiler suffered a spate of superheater tube failures due to overheating and creep rupture, and the complete superheater had to be replaced. Why should two identical boilers be so different? Close investigation and painstaking tracing of tube path layouts showed that the hottest tubes from the outside of the platen array led mostly to leading tubes, but sometimes to the tube behind it in the primary superheater, as the number of platen tube banks is less than the number in the primary superheater. The monitoring thermocouples were installed on leading tubes. Unfortunately, in the “good” boiler, the thermocouples were installed on leading tubes that did not come from the hottest tubes out of the platen. Presumably, the installer was given set distances in from the furnace wall rather than specific tube numbers. The lesson here is to check such points in detail if two “identical” plant items show quite different behaviours. Lesson #16 In critical cases, do not believe everything you read in the control room without verification of labels and actuality at and inside the plant. Conclusions and final lessons Figure 7 Boiler cross-section above furnace. Condition monitoring can be a key contributor to higher reliability and availability when set up properly and run by trained and dedicated people. Some investment is needed in equipment but much useful work can be achieved with simple instruments. Lesson #1 Training is essential before starting CM work, followed by regular reinforcement via courses, conferences. Consider getting certification to verify capability. Lesson #1 Share your learning via on-line forums, conferences, articles in engineering magazines. Lesson #19 Make recommendations clear and concise: put the technical complexity in appendices. Lesson #20 THE MAJOR ONE. Condition monitoring is not an end in itself, and should be applied along with other maintenance strategies as decide by an RCM or similar analysis. Vol 24 No 2
AMMJ Lessons Learnt In 45 Years of Condition Monitoring 29 References ASME (1970) Simplified procedures for routine performance tests of steam turbines ASME PTC 6S Report (reaffirmed and revised 2003) Beebe, Ray (1978) Recent Experience with Condition Monitoring of Steam Turbines by Performance Analysis - IEAust Mechanical Engineering Transactions, 1978 pp 42 – 49 Beebe, Ray (2001) Machine condition monitoring MCM Consultants, Hazelwood Beebe, Ray (2002) Diagnosis and solution of resonant whirl on a steam turbine generator Proceedings ICOMS2002, Brisbane. Beebe, Ray (2003) Predictive maintenance of pumps using condition monitoring Elsevier, London Beebe, Ray (2008) Is your control room data telling you what you think it is? Proceedings MARCON2008 (and several journals) BEI (British Electricity International) Modern power station practice: incorporating modern power system practice. 3rd ed. Oxford Pergamon Press, 1990-1992 ESDU 66023 (1966) Calculation methods for steadily loaded pressure fed hydrodynamic journal bearings Engineering Sciences Data Unit, IMechE London About Ray Beebe Senior Lecturer and Co-ordinator, Monash University School of Applied Sciences and Engineering Ray Beebe has a passion for condition monitoring from 28 years in power generation, followed by 18 years at Monash University, where he led the postgraduate programs in maintenance and reliability engineering (off campus learning) up to 2011. Since retiring from tenured service, he continues teaching and speaking involvements and is working on a third book. He was awarded Engineers Australia’s 2004 George Julius Medal for his second book Predictive maintenance of pumps using condition monitoring. For 30 years, he has spoken at conferences worldwide, and many papers have appeared in technical magazines. Note: papers are available on request. A complete list of my papers can be found on: http://www.gippsland. monash.edu.au/science/aboutus/people/academics/raybeebe.shtml
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