| Summary: | Boiler tube failures (BTF) are the leading cause of plant downtime and can cost utilities tens
of millions of dollars. One of the mechanisms of BTF is pitting corrosion in the steam side of
tubes of reheater and superheater tubes. Pitting corrosion in these tubes is a result of poor
shut down and layup practices of the boiler leaving stagnant oxygen rich water in the tubes
during the outage period.
Three methods of drying reheater tubes during the shutdown process exist within the South
African environment. These are forced drying, vacuum drying and boil drying. This research
looked at determining which of the three methods is the most effective, by analysing the
moisture content of the fluid inside the reheater tubes at the end of the drying procedure. This
was achieved by measuring the relative humidity and dew point of the air/water mixture
within the reheater tubes at seven power stations.
It was found that forced drying is the most effective method as it produced a fluid within the
reheaters dryer than ambient air. The fluid remaining in the reheater at Power Station A had a
relative humidity of 13.7% at 31.5°C and a dew point of 0.9°C as compared to the ambient
air of 47% relative humidity at 25.8°C with a dew point of 17.3°C. Forced drying allows a
large volume flow of air through the reheaters effectively replacing the steam with dry air.
Vacuum and boil drying leaves a fluid with 100% humidity within the reheater, which will
cool and condense and will result in pitting corrosion. Vacuum and boil drying do not
effectively replace the steam with dry air. The power stations making use of vacuum as well
as the one making use of boil drying had a fluid in the reheaters with a relative humidity of
100%.
This research has shown that forced drying while the boiler is hot is an effective method of
drying. Further, offline corrosion can be prevented by keeping the boiler in low humidity
conditions. This is achieved by circulating dehumidified air through the boiler tubes.
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