Real time corrosion
monitoring for HRSG
Flow accelerated corrosion (FAC) can cause iron loss in steam piping and lead to
lethal accidents. As a result, programs are necessary to reduce metal loss and
to monitor the loss which does occur. A team from Danaher analyzed the problem
in a Power Engineering article earlier this year.
The reducing environment produced by oxygen scavengers is the prime ingredient
for single-phase flow-accelerated corrosion (FAC) of carbon steel. The attack
occurs at flow disturbances such as elbows in feedwater piping and economizers,
feedwater heater drains, locations downstream of valves and reducing fittings,
attemperator piping; and, most notably for combined-cycle heat recovery steam
generators (HRSGs), in low-pressure evaporators, where the waterwall tubes, aka
harps, have many short-radius elbows. In fact, FAC is typically the leading
on-line corrosion mechanism in HRSGs.
Based on the method of oxygenated treatment (OT) that arose in Europe in the
early 1970s, EPRI developed a program to replace AVT(R) for drum units, known as
AVT(O), which stands for all-volatile treatment oxidizing. If the condensate/feedwater
system contains no copper alloys, which is true for virtually all HRSGs, then
AVT(R) is not recommended, rather AVT(O).
In brief, with AVT(O) chemistry the oxygen scavenger feed is eliminated, and a
small residual concentration [5 to 10 parts-per-billion (ppb)] of dissolved
oxygen is maintained at the economizer inlet. Ammonia or an ammonia/neutralizing
amine blend is still utilized for pH control.
A combination of a simple colorimetric total iron laboratory analysis with a
sensitive laser nephelometric analyzer can also provide a method for cost
effective, quantitative, real-time corrosion monitoring. When properly
calibrated, the nephelometric units provided by a nephelometer can be correlated
to total iron concentration values. The iron concentration of the process water
is a direct indicator of steel corrosion.
As the process waters used in power generation are extremely pure, it can be
assumed that almost all insoluble matter present in a ferrous metallurgy process
stream is due to steel corrosion in the form of particulate or colloidal iron
oxides. Corrosion of steel components in power generation is generally found as
iron oxides and hydroxides, primarily, iron (II, III) oxide (magnetite), α-iron
(III) oxide (hematite), or dissolved iron. Each of these species produces a
different nephelometric response to visible light.