May 15, 2007

Focus on O&M (May 2007)

COMBINED-CYCLE PLANTS

O&M problems not caused by cycling

The steam system (the steam turbine and the heat-recovery steam generator) of a combined-cycle facility is an essential component of the overall plant. It is also the system that is least standardized because every facility has different thermodynamic requirements. Although gas and steam turbines are highly standardized and benefit from shared fleet experience, the designs of the HRSG and supporting systems at combined-cycle plants are almost always site-specific.

The first large combined-cycle plants featuring modern industrial gas turbines, such as the GE Frame 7 and Westinghouse 501F, entered commercial operation about 12 years ago. Newer plants, including those with steam-cooled gas turbines, built during the ordering boom of 1999–2004 have now accumulated enough operating hours to teach valuable lessons.

Most combined-cycle plants were designed for baseload service or limited seasonal operation. But changing economics of the wholesale power industry have dictated that the plants be run (and not run) in unintended ways, such as:

• Two-shift (daily) cycling.
• Long periods of operation at partial load.
• Use of alternate or lower-grade fuels.
• Extended lay-ups.
• Increased use of wastewater for cooling.
• The need to meet tighter air quality rules.

New modes, new problems

The consequences of frequent (daily) cycling of combined-cycle plants have been on the industry's radar screen for several years. Operators have done a good job of solving problems such as thermal transients during start-ups and shutdown, boiler water chemistry excursions, and inadequate condensate drainage.

What the industry also needs to address, however, is the increasing damage and failures of pressure parts caused by accumulated low-level stress to which tubes and piping are subjected as plants age. The number of failures of these components, and the cost to repair them, are on the rise. Failures of superheater and reheater tubes at tube-to-header joints are becoming more frequent, as are leaks in economizers and preheaters.

Quality control problems with pressure-part welds were common during the 1999–2004 boom. The earlier defects did not reduce plant reliability, but now they are, because combined-cycle plants are being run in unanticipated ways. The more recent problems affect tube-to-header welds, welded stubs on headers, header end caps, and smaller bore piping. These have the potential to be major cost items in the future. A problem peculiar to plants with air-cooled condensers (ACCs) is that increased cycling can substantially increase the transporting of iron from the ACC to the tubes of the boiler/economizer.

Partial load operation

Running a combined-cycle plant for long periods at partial load lowers pressures in the low- and intermediate-pressure sections of the evaporator, lowers temperatures in high- and intermediate-pressure loops, and makes steam temperature harder to control. Some plants have experienced greater gas-side corrosion and deposition, increased flow-accelerated corrosion and erosion-corrosion, higher gas-side pressure drops (due to more deposits), and significant degradation of their heat rate.