March 15, 2006

Designing HRSG desuperheaters for performance and reliability

Dr. Sanjay V. Sherikar, PE, Control Components Inc., and Peter Borzsony, CCI International Ltd.

Desuperheaters—often called attemperators—in heat-recovery steam generators (HRSGs) are located between the primary and secondary superheaters and reheaters, and sometimes after the final stage of superheating (Figure 1). They are responsible for controlling steam temperature in accordance with start-up and steam-turbine-inlet requirements. The attemperators also prevent thermal damage to superheater and reheater tubes, and to outlet steam piping and downstream equipment.

 

  

1. Where you'll find them. Attemperators for heat-recovery steam generators (HRSGs) are located between the primary and secondary superheaters and reheaters, and sometimes after the final stage of superheating. Courtesy: CCI International Ltd.

 

Interstage attemperators for superheaters typically see pressures up to 1,900 psig, temperatures to 1,020F, and steam flow rates to 660,000 lb/hr. Reheater attemperators experience similar temperatures and flow rates, but pressures normally go to only 450 psig.

Problems with attemperators

The addition of unwanted water to the steam line due to improper operation of an attemperator, or to the inability of its control element to remain leak-tight, is a major concern of operators. Failure of the attemperator to control the injection of water into interstage lines often produces thermal-shock damage to hardware and piping. In severe cases, the unatomized water will even erode piping. In most cases, the usual consequences are a forced outage and an expensive repair bill.

Another problem seen with interstage attemperators, though not as catastrophic, is their inability to control final steam temperature within specified limits. This occurs when the attemperator lacks sufficient turndown, or if installation is poor, or when the leakage across the attemperator's control element exceeds the demand for spray water. Such failures reduce the steam turbine's efficiency and electrical output.

Root causes of these problems include poor design of the attemperator and/or spray-water control element, poor installation, and improper control instrumentation.

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