Role of Boiling Mode and Rate in Formation of Waterside Deposits in Heat Recovery Steam Generator Evaporator Tubes

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Waterside Deposits in evaporator tubes have been an issue in steam generators as long as boilers have been used. Substantial experience in deposit formation and management has been gained in conventional goal and oil/gas boilers over time. The role of boiling modes in the steam generator tubes is very critical to areas of deposit formation. 

Incipient boiling, nucleate boiling and convective boiling modes all have different deposition behavior. When Gas Turbine Combined Cycle (GTCC) power plants of larger size (> 100 MW) began operation in the 1990’s, deposits in evaporator tubes were not considered a significant issue. Operating boiler pressures were low (500–900 psig) as were flue gas temperatures, use of supplemental firing was limited. Other than known problems with feedwater contamination such as operation with leaking seawater-cooled condensers, deposits were not found to be forming. The rapid increase in size and operating pressures in HRSG’s raised the likelihood of waterside deposits developing. Both Vertical and Horizontal Gas Path HRSG designs are considered. Drawing on field observations, the morphology and location of HRSG deposits are reviewed, as are changes in deposit formation with the mode and rate of boiling.

When Gas Turbine Combined Cycle (GTCC) power plants of larger size (> 100 MW) began operation in the 1990’s, deposits in evaporator tubes were not considered a significant issue.    Operating boiler pressures were low (500-900 psig/ 35-60 barg) as were GT exhaust gas temperatures.  The use of supplemental firing, which significantly raises gas temperatures, was limited.  Other than the consequences of feedwater contamination, such as operation with leaking seawater-cooled condensers, deposits were not found to be forming.

The rapid increase in HRSG unit size with concomitant increase in operating pressures and temperatures made the likelihood of waterside deposits increase. Current pressures are all subcritical but can be up to 2400 psig/163 barg.  Although the underlying deposition mechanisms are similar, the details of boiling in the HRSG are different from a conventional coal/oil/gas boiler.  This leads to some differences in deposition rates and the type of deposits found in a HRSG.  In particular it is important in defining the areas of the HRSG evaporators at risk.

Figure 1 shows an example of the extent to which surface deposition can develop in an HRSG evaporator tube; Figure 2 shows a typical deposit cross-section. 

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Figure 1 - Heavy Deposits in HRSG Tube – Horizontal HRSG near bottom of HP Evaporator

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Figure 2 - Deposit Cross Section – HP Evaporator Tube with Deposits - Horizontal

The full paper was published in the 2014 ASME Power Conference Proceedings and can be found here

For more information about how to manage waterside deposits contact Dave Moelling This email address is being protected from spambots. You need JavaScript enabled to view it. (Americas) or James Malloy This email address is being protected from spambots. You need JavaScript enabled to view it. (EMEA).

 

 

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