FROM THE FEBRUARY ISSUE: Plants built in the ethanol construction
peak are approaching 10 to 15 years of operation, and many are exploring
whether to repair or replace. HRST Inc. has some advice.
The thermal oxidizer (TO) heat recovery steam generator (HRSG) is a
staple system for many ethanol plants built during the mid-2000s
construction boom. As plants near the 10- to 15-year operating mark,
performance and efficiency might be the deciding factor between
repairing or replacing equipment. Routinely inspecting and quantifying
performance can improve the reliability and operation of an existing
HRSG, extending the life of the system. hrsg boiler design
Assessing the condition of the HRSG is a critical first step in
determining its overall health. Having a comprehensive water- and
gas-side inspection helps establish a baseline condition of the HRSG
that can be compared to future inspection findings. Comparing can be
qualitative and simple, “The tubes appear more oxidized than last year;”
or more quantitative and complex, “The material thinning in this
location is 1/16 of an inch and last year it was at 1/32 of an inch.”
A variety of locations should be observed on the water and gas side.
Knowing at-risk areas can help determine where to look and when.
Inspection areas for a typical TO/HRSG configuration include the HRSG
inlet from the TO, the evaporator section of the HRSG, steam drum
connections and internals and the packaged economizer. Important items
and components to observe in these locations are: liner and baffle
condition; tube-to-header connections/welds; external tube fouling,
corrosion and oxidation; internal tube deposits or material loss; steam
separation equipment and configuration; and overall as-built design
(pipe size, configuration, material).
Temperature is a key factor in how efficiently the HRSG components
operate. In HRSGs that are fired harder (e.g., a TO burner that is
pushed to its maximum firing rating), metal components often exceed the
recommended design limit temperature. Liners, baffles and tubes are all
susceptible to overheat.
Overheated liners might oxidize, warp or break off studs when thermal expansion causes them to distort. As the liner sheets distort and expose insulation, the high temperature turbulent gas flow can wear away the insulation and cause hot spots on the HRSG.
Baffles become brittle and degrade over time as they overheat, eventually allowing the flue gas flow to bypass the tube bank, resulting in lower HRSG performance. In the case of bypass along the sidewalls of the evaporator, the increased flow along the panel end tubes will disproportionately cause tubes to overperform and generate more steam than the tubes toward the center of the panels.The evaporator tubes are less susceptible to overheat damage, but the fin material will begin to oxidize and break off until the fin tip temperature is at the material threshold. While this is not a reliability concern, it can lower the performance of the evaporator. Tube leaks or failures often occur at the tube-to-header connection. This can be caused by high stress at the weld joint, often related to the geometry of the tube-to-header connection, but sometimes because of flow conditions on the water or gas side. Performing a gas-side inspection at the beginning of a shutdown can allow more time for tube leaks to be identified and repaired. Depending on the location within the bundle, this could be a time-consuming process. When tube failures occur, a root cause failure analysis (RCFA) should be performed to determine the failure mechanism, in addition to implementing a preventative operation or maintenance procedure to mitigate future failures. Don’t just weld the crack; involve someone who can review the location, failure and maintenance history.
