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AMMJ Lessons Learnt In 45 Years of Condition Monitoring 26 Case study 4 The new graduate engineer hooked up the accelerometer via the long cable reel to the signal conditioning/ readout instrument. He reported that turbine vibration was 55mm/s rms – over 10 times greater than what might be expected! Before panic set in, we found that he had used the cable to connect the accelerometer to the instrument. It was an ordinary shielded co-axial type, intended to be used from the instrument to an analyser. Low noise cables are required from charge output accelerometers to avoid tribo-electric boosting of the output to give a spurious high vibration reading. Case study 5 Lesson #13 Check, and recheck, critical data values if any look to be unusual. Using an innovative approach, site trim balancing was conducted on a 120MW generator rotor. The coupling between turbine and generator was unbolted, and faces held apart. The exciter was connected to run as a motor, with the rolling torque provided using the overhead crane and a rope wrapped around the rotor. (Appropriate design checks had been made). After reassembly, run-up proceeded as normal, until when nearing normal service speed generator bearing vibration suddenly jumped so much that the floor shook and dust fell from the rafters! The operator tripped the machine. Subsequent attempts at run-up were no different. A challenge for the vibration team! The gear was set up with the analyser set to PEAK HOLD mode. The extreme vibration was revealed as at 19 Hz – the first critical speed of the rotor. It was noticed that the vibration started soon after the auxiliary oil pump was stopped, so it was left running and the unit was eventually put into service. The 19Hz vibration was still evident, and could be varied in amplitude by changing the oil temperature. Bearing dimensions and clearances were found, and the bearing wedge pressures (giving shaft loading) and oil temperatures noted to calculate the Load Parameter. The resulting plot on a bearing stability assessment chart showed that the operating range was well outside the “recommended” area (ESDU 1966), as shown in Figure 5. The only variable that could be changed permanently was the length of the bearings (to increase the specific loading). Surprisingly, the spares in the store were found to be shorter, as were those on the adjacent “identical” machine! Bearing changeover was the cure. This was a strange case, as this machine had operated 17 years without this problem. The vibration team gained superhero status for this success. (Beebe, 2002). CM by performance analysis – the big bucks Case study 6 Fig 5 Bearing stability chart showing effect of oil viscosity and bearing length Load parameter W’ Original operation Operation when bearings modified I had developed performance tests for both types of 200MW machine at Hazelwood with useful outcomes and continued this at Yallourn W. The methods followed had since been published (ASME, 1970). On one unit, our tests were run before the official acceptance tests. Routine tests on a 350MW unit showed a small but significant decline in performance. Prior to a planned outage, a steam forced cool was conducted. This procedure is used to bring the machine to standstill more quickly by cooling the turbine metal, rather than allowing slow natural cooling. The inlet steam temperatures were slowly decreased over some hours during offloading. Testing after return to service showed that the performance has returned to its initial level. Close examination of the data concluded that there was some restriction in the intermediate pressure section, deduced to be from blade deposits (Beebe, 1978). Soon afterwards, the OEM site manager met with the plant manager to tell him that as the first unit had reached 2 years of service, it was time to arrange a major outage and stripdown. When asked the reason for the recommendation he was told that an inspection after two years was standard practice in the OEM’s country. The manager had been my boss and mentor in my initial job, so was well versed in CM! He did not support an overhaul given our vibration and performance condition assessment, and the machine continued to operate for 17 years before its high pressure section was opened. Lesson #14 Take OEM recommendations into careful consideration, but do not follow them blindly. 10 1.0 0.1 Oil 71°C Oil 40°C Lines of increasing constant b/d Increased risk of halffrequency whirl 0.1 0.5 0.9 Eccentricity ratio Recommended area Bearing too short Vol 24 No 2
AMMJ Lessons Learnt In 45 Years of Condition Monitoring 2 Case study 7 In 1995, tests run on a 500MW turbine at the latest plant (Loy Yang B) led to an overhaul to remove metal debris carried from the boiler. Long experience elsewhere had shown that accurate special tests were needed to obtain CM data, as plant instruments were not sufficiently accurate nor repeatable. As this plant had a DCS, opportunity had been taken at each accurate (and high cost) Valves Wide Open test to extract data from the plant historian and compute the same condition parameters. Although the same values were not obtained, a directly comparable trend was clear as shown in Figure 6, which shows the VWO trend over its life from its initial acceptance test. Lesson #15 Assess whether the plant instruments can be used to give a usable trend for CM. If a DCS exists, then try data extraction and utilisation. Case study 8 460 31-Jan-93 28-Oct-95 24-Jul-98 19-Apr-01 14-Jan-04 The superheater tubes in a series of large coal boilers of the same natural circulation drum type design leave the furnace through spaces between roof tubes, and connect to later sections, often via headers. There are several superheater sections in series. The platen superheater at the top of the furnace has 30 sections, each with 16 tubes in a U-shaped pendant loop, hanging through the roof tubes of the furnace. Unlike other designs where the leading tube down has a kink so that it becomes an inner tube in the up direction, these pendants are laid out such that the assembly would be flat, i.e the inner tubes are progressively shorter than the outer tube. The platen is heated mainly by radiation, so the longest tubes on the outside of the array take up more heat than those on the inner side. Eliminate unnecessary failures by using a systemic approach to problem solving. Teach your personnel how to identify the "root cause" of failures. Increase productivity Reduce downtime Increase profits RCA Rt provides training & coaching programs to grow a culture of defect elimination & continuous improvement. Public workshops Ongoing coaching Onsite workshops Instruction for internal trainers Focus Find Causes Fix Forever Step 1 Define the problem Step 2 Select the target Step 3 Assess & monitor Step 4 Confirm containment Step 5 Practical look Step 6 Patterns & comparisons Step 7 Brainstorm for causes Step 8 Build a cause tree RCA Rt facilitators are expert incident investigators available to lead your team onsite Step 9 Select a change Step 10 Prepare and execute Step 11 Change review Step 12 Make it last www.rcart.com.au +61 3 9697 1100 Copyright Figure 6 Comparison of accurate tests (upper plots) with results calculated using DCS data (lower more numerous points) Corrected VWO Output MW 530 520 510 500 490 480 470 www.rca2go.com online problem solving Process Maps 5 Why, RCA, FMECA, 6 Sigma's DMAIC Date of test The R Manufacturing Game RCA Rt Home of The Manufacturing Game The Manufacturing Game workshops are known throughout industry as an extremely effective tool for increasing team learning, cross-departmental communications and decision making that lead to eliminating plant defects and improving operations. The Manufacturing Game is a strategic simulation of a manufacturing plant. Originally introduced at the DuPont Chemical Company, The Manufacturing Game has been used by a host of manufacturing companies worldwide to hone their operations. Time and time again, the workshop has effectively shown employees how their jobs impact every facet of both the manufacturing process and the plant's profitability. By providing the tools for change, and by launching cross-functional action teams, The Manufacturing Game® is able to accelerate the journey from reactive to proactive Operations Want to play the game? contact: melissa@sirfrt.com.a Contact: 03 9697 1100 melissa.cameron@sirfrt.com.au www.rcart.com.au
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