Application note - Further advances in monitoring Iron transport


Dokument-ID PR125


Veröffentlichungsdatum 16.05.2022
Application note - Further advances in monitoring Iron transport
Produkt Überblick
Further advances in monitoring Iron transport measuring dissolved and particulate Iron and differentiate by analysing the different Iron oxides Magnetite & Hematite in a water steam cycle.
In an earlier paper, it was discussed how a laser Nephelometer could be used to detect the transfer of corrosion products in a HRSG power plant at ppb levels with particles in the sub-micron size range. Also, a modified trace level iron test was documented using colorimetric methods on a lab spectrophotometer to measure total iron levels to 1ppb.
In this paper, further advances will be outlined which improve the accuracy and repeatability of the lab method, while providing further insight into the level of protection being provided to steam cycle components against FAC and other corrosion mechanisms. Focus will be in understanding the chemistry of the measurement so as to provide more specific data to plant personnel regarding the level and species of iron being observed.

Background and History:
The traditional colorimetric method for dissolved iron is based on the extremely sensitive ferrozine ferrous iron complex described by Stookey (1). Ferrozine complexes with dissolved ferrous iron to form an intensely colored purple complex. The dissolved ferrous iron concentration may be determined by measuring the absorbance of this complex colorimetrically. Modifications of this traditional method allow for the determination of both dissolved iron and particulate iron oxides at very low concentrations.
Particulate and colloidal iron may be monitored with surrogate techniques, but a total iron analysis is necessary for measuring particulate as well as dissolved iron. Flow accelerated corrosion proceeds through the reduction and dissolution of a protective magnetite boundary layer into the bulk stream flow. The majority of this ferrous iron is re-oxidized and present as particulate magnetite, but the mechanism for this transformation is unclear (2). As dissolved ferrous iron is known to exist as a corrosion product, and its oxidation mechanism is unknown, it is prudent to monitor this form as well as the oxidized form.

The International Association for the Properties of Water and Steam (IAPWS) offers some good guidelines to assure that a representative iron sample is being collected (5). The optimum place to sample for corrosion products is from a stream that has moderate to high velocity, is downstream, and as close as possible to the component of interest in the loop.

Improvements to the Ferrozine Method of analyzing total iron have now been made, bringing sensitivity down to the 1ppb level. By altering the digestion process, it is now possible to differentiate between the hematite and magnetite forms of iron in the sample.

Even with improved analytic capabilities, sampling remains a concern and an area that needs attention. Drawing samples at strategic locations in the loop will help insure that the readings alert the operator to potential concerns within the system. Yet for those samples to be representative, flows and linear velocities within the pipe need to be selected that keep iron particles suspended. Good laboratory techniques will further insure that the sample taken is not contaminated or altered in the analytical portion of the measureme

Further Advances in Monitoring Low Level Iron in the Steam Cycle-PPChem 2015_DOC040.53.10063

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