Fixing tube leaks
I read with interest your two recent articles on benchmarking boiler tube failures (POWER, October and November 2005). While I do not disagree with any of the statements or data in the article, I would like to clarify a couple of points.
The article states, "the three worst-performing large units (with 123, 188, and 221 total tube leaks over the study period) are powered by cyclone-type boilers." That's true. But one of the best-performing large units—with only two tube leaks over the study period—also was powered by a cyclone-type boiler.
Finally, if you look at hours of generation lost to boiler tube leaks, the three worst-performing large units are all wall-fired units (with 2,158, 2,546, and 2,570 hours of generation lost to boiler tube leaks over the study period). A statistical analysis does show some correlation between "burner type" and "number of hours of generation lost due to boiler tube leaks. However, the correlation isn't very strong (P value = 0.061).
I think it's important for anyone involved with boiler tube leaks to not just to look at the study's raw data, but also to analyze it statistically for the performance measurements of interest to them. That, of course, assumes you have access to all of the data. Accordingly, I would encourage all utilities to participate in the upcoming 2006 EUCG survey.
—George Mues, Ameren UE (St. Louis)
Keeping cooling towers cool
I read with interest the article on plastic cooling towers in your November/December 2005 issue ("Graduating to plastics," on p. 18). At FM Global, we have seen the same recent move to plastic cooling towers by many of our Fortune 500 property insurance clients. The advantages they cite are many of the same ones mentioned in your article, including reduced maintenance and increased cooling capacity.
However, one important factor that your article did not address is the higher combustibility of plastic cooling towers. The use of plastic as the tower's material and/or tower fill necessitates an additional level of fire protection. The protection could be provided by automatic sprinklers or a deluge system. On our web site (www.fmglobal.com), data sheets provide detailed guidance on how to prevent plastic cooling towers from going up in flames.
Perhaps the easiest way to reap the benefits of a plastic cooling tower without having to worry that it will catch fire is to select one that is "FM Approved." We have had several on the market evaluated and tested, and our web site identified the ones that do not support the spread of fire throughout the cooling tower structure.
—J.C. Harrington, Assistant Vice President, FM Global (Johnston, R.I.)
Correction
An article in our April 2006 issue ("Steam turbine upgrading: Low-hanging fruit") has a section on p. 36 describing the Unit 3 steam turbine overhaul completed by KeySpan Corp. at its Northport Power Station on Long Island. KeySpan has asked us to provide some amplifying comments.
KeySpan engineers note that the rationale for the overhaul was degraded performance (peak capacity and heat rate) due to internal seal leakage. Before the overhaul, the unit was load-limited by 15 MW, with the main boiler feed pump at maximum speed. We incorrectly stated that the pump is rated at 15 MW.
The article also stated that "main steam temperature also was limited—by increasing hot reheat temperatures and by the steady falloff in first-stage pressures since the unit's last overhaul." Clearly, the falloff in first-stage pressure is directly related to seal leakage, but only indirectly to limiting steam temperatures.
Finally, the post-upgrade performance improvement was noted as "465 Btu/kWh (net)—almost twice the predicted 257 Btu/kWh gain." The performance improvement was indeed 465 Btu/kWh (net), but some 317 Btu/kWh of it was attributed to the turbine, even greater than the predicted 257 Btu/kWh gain from that unit.
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