Recipe for repairs
Reducing excessive internal leakage in Westinghouse and Allis-Chalmers turbines may require taking any or all of the following steps:
- Replacing any HP-IP-LP dummy and reheat diaphragm packing seals with excessive clearance or broken teeth.
- Replacing bell seals or piston seal rings that are cracked or broken (Figure 7) or have excessive clearance/taper or erosion. The inner cylinder nozzle chambers or inlet pipes may require refurbishment if they are eroded or worn. Any unused bell seal retainer nut lock screw holes should be welded up as they can be a source of main steam leakage. Some piston seal ring designs (Figure 8) can produce excessive leakage if installed improperly, upside-down (one side is flat, and the other has alternating high and low sides).
- Checking inlet sleeves for cracks in the trepan radius using the turbine manufacturer's test method. In Westinghouse BB44s that are routinely found to have cracked bell seals, distorted inlet sleeves could be the problem (a consequence of age); the manufacturer offers a method for straightening the inlet sleeve that SCG has used successfully.
- Changing the usually distorted IP dummy ring on BB44s to an improved design offered by the manufacturer. There's another good reason to do this: BB44 IP dummy ring bolts broken by the distortion can enter the IP turbine and damage its blading.
- Repairing any broken equilibrium pipes or first-stage drain/pressure sensing lines. Sometimes, a equilibrium pipe can be eroded by a broken bell seal steam jet.
- Closing up or reducing in size any "extra" IP dummy ring cooling holes on IP1-HP-IP2 turbines. (Before doing this, consult with the manufacturer.)
- Inspecting the studs/shell threads of the HP inner shell horizontal joint for possible weld build-up caused by machining of the joint surface. The resulting leakage can actually flow up through the shell holes (Figure 9). The stud nuts should be "sounded" with a hammer to determine if any are loose prior to disassembly. For both studs and through-bolts, it is very important to know what they are made of because the material determines how much they can be tightened.
7. Yet another source. Cracked and broken main steam inlet bell seals on a Westinghouse BB44. Courtesy: Southern Company Generation
8. One-way street. The alternating (high- and low-side) piston seal rings of a 50-MW Westinghouse reheat turbine. Note that the rings are installed correctly, with the hi-lo side opposite the outer shell. Courtesy: Southern Company Generation
9. Loosey-goosey. Loose HP inner cylinder bolting on both sides of this BB44 turbine caused excessive HP to IP leakage, severely penalizing the unit's heat rate. Courtesy: Southern Company Generation
After a repair outage, the turbine must be retested to ensure that the work was successful. In SCG's experience with Westinghouse turbines, less than 6% of hot reheat flow leakage (the design level is 4%) can be achieved with good overall work. Although SCG's experience with Allis-Chalmers turbines is limited to three units, results indicate that less than 6% of hot reheat flow leakage (design is 3%) can be achieved.
Case study #1: Excessive HP to IP leakage in a 500-MW Westinghouse combined HP-IP drum turbine
In this extreme case, a bad bell seal was determined to be the cause of excessive leakage that raised the unit's heat rate. The turbine began producing an unusually loud noise following a temperature excursion. A temperature split test indicated that the measured IP efficiency to the LP crossover had decreased 5%, with a significant increase in the LP crossover pressure. Main steam flow had increased 6% although corrected load had only increased 3.2%. The test also revealed that total HP to IP leakage had increased from 6% of hot reheat flow to 13.5%. The temperature rise from the IP exhaust (deaerator/boiler feedpump turbine extraction) to the LP crossover suggested a possible leak through the bell seal or equilibrium pipe.
SCG engineers then decided to conduct the manufacturer's bell seal leakage performance test. Analysis of that data indicated that closing the No. 7 and 8 valves, which feed a common nozzle block, produced the biggest change in temperature.
Since the No. 7 and 8 valves on this unit are upper inlet valves (narrowing the scope of the problem), the decision was made to remove the upper half of the outer cylinder. Doing so revealed a broken bell seal on the No. 8 governor valve inlet sleeve (Figure 10). Fortunately, the broken portion of the seal could be extracted from the inner shell, minimizing the duration of the outage.
10. Untrained seal. A broken bell seal on the sleeve of the No. 8 governor valve of a 500-MW Westinghouse BB44 steam turbine. Courtesy: Southern Company Generation
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