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FLOW ACCELERATED CORROSION
What Should You Tell Your Boss?
Congratulations, you and most of your plant staff have survived construction,
start-up and “the warranty period” of a $500 million dollar power plant. Now
your company expects you to keep the HRSG in good condition, with a skimpy
budget, minimal staff, and short maintenance outages, all while the plant cycles
on/off more than anyone ever imagined. Meanwhile, your boss just learned at a
User Meeting that FAC (flow accelerated corrosion) is one of the biggest sources
of HRSG tube leaks. Now you need to explain if “your HRSG” is at risk to FAC
damage….. What should you say?
- I need to budget $100,000 and 2 weeks of outage time next year for
scaffolding and NDE of all the potential FAC areas.
- We have no risk because the HRSG OEM advised that it was built with
components designed to resist FAC.
- We just commissioned an independent team of HRSG specialists to perform
a FAC Risk Assessment that will look at water chemistry strategy and
history, pressure part metallurgy and HRSG steam/water circuit velocities.
From the resulting report, we anticipate having our usual HRSG inspector
spend an extra day or two during future maintenance outages to perform UT
readings in the higher priority risk areas.
If answer (C) sounds good to you… then keep reading!
The risk of FAC is influenced by the combination of water velocity,
turbulence, temperature, chemistry and pressure part metallurgy. Determining
where these five factors add up to the greatest risk is the key to prioritizing
where to look for FAC thinning. Because FAC is most common in evaporators, a
thorough circuit-by-circuit circulation assessment is necessary. Circulation
assessment requires accurate HRSG thermal modeling, because as the turbine
exhaust passes through each row of tubes, it gets cooler, producing less steam
in subsequent rows. The steam produced in each tube row impacts circulation. The
design of the feeders and risers connected to each tube panel also impacts
circulation. HRSG OEMs vary the quantity and size of risers and feeders, so the
tube panels with the hottest turbine exhaust may not have the highest
velocities. Because HRST has both thermal and circulation modeling expertise, we
can perform accurate modeling and assessment. Accurate modeling only works with
the proper inputs, so part of our task is to sift through drawings and technical
data sheets to properly set up our computer models. If needed, we can determine
or verify pipe sizes and layouts as part of an overall HRSG inspection.
Water chemistry is part of the problem and solution. So we also need to know
your history and strategy for minimizing water chemistry conditions that
increase the risk of problems. If the strategy needs improvement, or the
chemistry targets are not proper, we will include recommendations for action as
part of our assessment. Water chemistry history may also influence the urgency
to inspect high risk areas identified during the FAC Risk Assessment. (Continued
on pg 3) is a prioritized list of HRSG inspection areas and water chemistry
recommendations. Ideally, the recommended inspection areas and inspection cost
can be tackled in small bites each scheduled outage, rather than one big bite
every several years. The biggest benefit, of course, is if the assessment helps
you avoid pressure part damage completely. The cost of an HRST Risk Assessment
can be recovered in one outage from the savings of unnecessary scaffold
installation and unnecessary insulation removal in low priority FAC areas!
Lester Stanley, P.E.
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