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.
The cycling and thermal shocking of valves and attemperator components can lead to wear-out and leak-by, resulting in poor spraywater atomization and inadvertent ingress of water. This can be compounded by poor attemperator piping arrangements with insufficient upstream or downstream straight lengths to provide proper mixing and evaporation of the spraywater droplets.
Poor control logic can also contribute to problems with hunting of valves, inappropriate timing of spraywater, or spraying with insufficient temperature head for evaporation. These conditions often result in pooling of spraywater on the bottom of the pipe or impingement of spraywater droplets on downstream elbows or bends, both of which cause large temperature differentials resulting in high thermal stresses and consequently thermal fatigue damage and/or warping and distortion of
the piping. Figure 2 shows common causes and effects of malfunctioning attemperators.
In January of 2017 a power plant in the Northeast was experiencing recurring attemperator issues.
The 620 MW Combined Cycle Electric Generating Facility consists of two combustion turbines, two heat recovery steam generators, and one steam turbine. Upon inspection, the High Pressure final stage attemperators had cracked at the weld from the attemperator nozzle to steam piping and some of these cracks migrated into the bore of the piping.
The liners were showing signs of cracking at the locations where they attach to the piping. Additionally,
the spray water block valves showed signs of leakage. These and other attemperator issues forced the plant to shut down operations several times over the later part of 2016 and first part of 2017.
While plans were made to repair the damage and address the design issue, the plant personnel contemplated what else could go wrong with the attemperator and became determined to take a proactive approach to maintaining the health of the system. Being well aware of the issues described above, they desired a system that could monitor for damage downstream of the attemperator in case of leaking, inappropriate control logic, etc.
Because much of the damage to attemperator systems results from spraywater pooling or impingement it is effective to install thermocouples on the top and bottom (if the piping is nominally horizontal) of the piping downstream of the attemperator, and on the extrados of the elbow downstream of the attemperator.
Such thermocouples installed on the OD of the piping have proven effective to detect leak-by, inappropriately timed spraywater, or damaged attemperator spray heads. To aid in detection of events and the magnitude of damage caused, SI has developed a real-time Attemperator Damage Tracking Application which processes the data from the thermocouples to determine the severity of events and incorporates a fatigue cycle counting algorithm to track the cumulative damage.
In the combined cycle plant described above, thermocouples were installed at three locations as shown schematically in Figure 3. This allows for detection of water pooling and impingement events. The thermocouples were mounted on the outside of the pipe and the signal is routed through a signal converter into the plant’s data historian. From there, the Attemperator Damage Tracking App accesses the data remotely through the historian’s web API. This setup allows for minimal installation effort and investment for plant operators.
The plant users access the App online through SI’s web-based PlantTrack software to view data trends. Figure 4 is an example of one of the App screens, showing temperature differentials measured by the installed thermocouples.
Based on the configured settings, the App filters these differentials and translates them into different event categories (e.g. critical vs. warning), which can then be analyzed in conjunction with other plant data to determine the cause and define mitigating actions. The software can also be configured to provide email alerts when certain events occur, or based on trends in damage accumulation. This allows early detection of potentially damaging events so that appropriate mitigations (maintenance, logic updates, etc.) can be performed before costly repairs are required. A major advantage of PlantTrack’s Online Attemperator Damage Tracking App is that it can detect temperature excursions regardless of the cause. This is helpful since the causes can be multifold, as described above.
The alternative approach of monitoring specific causes (e.g. block valve settings), would oftentimes reveal only part of the picture and could lead to undetected damaging events. The attemperator tracking solution is one of multiple Online Damage Tracking Apps SI has developed to help plant operators track critical components conditions. The Apps have been integrated into SI’s PlantTrack software.