By: Fred DeGrooth and Ulrich Woerz
Attemperators (aka desuperheaters) are used in fossil and combined cycle plants to protect boiler/HRSG components and steam turbines from temperature transients that occur during startup or load changes. The attemperator sprays water droplets into the superheated steam to ensure that the downstream, mixed, steam temperature will not adversely affect downstream components. While there are a number of attemperator designs and configurations (Figure 1 shows a schematic of a typical arrangement), all of them are potentially vulnerable to damage, making attemperators one of the most problematic components – particularly in combined cycle plants. If the causes of damage are not identified (and addressed) early, then cracking and steam leaks can occur leading to costly repairs and replacements.
The frequent cycling and wide operating range of combined cycle plants impose particular demands on attemperator functionality. Spraywater demand to the attemperator can fluctuate greatly within a startup where heat input to the boiler and steam flow are changing rapidly. At part load operation spraywater may be required continuously to moderate steam temperatures because of high exhaust gas temperature from the combustion turbine. Spraywater may also be demanded when duct burners are fired.