Monitoring Iron in Feedwater
Most of this article focuses on iron, because it is the predominant corrosion product in virtually all steam/water cycles. Because iron is predominantly present as suspended oxides, exact analyses of grab samples requires acid-washed sampling apparatus and glassware and digestion before analysis. The traditional Millipore iron test was developed to meet these needs more than 40 years ago as part of the start-up of the first supercritical boiler in the U.S.
Tests found that nearly all of the iron was present in suspended form and was removed by filtration through a 0.45-micron filter. Some have used finer filters (0.20 to 0.22 micron) for slightly improved accuracy. The Millipore test probably will continue to be used for rapid assessment of grab samples during start-ups.
The Millipore test has the added benefit of indicating the presence of all types of suspended matter that may be present during start-up of a unit, including dirt, grit-blasting material, and copper and nickel oxides. One study found that zinc was completely soluble in condensate, copper can be present in suspended and/or soluble forms, and nickel can be present as mixtures of suspended oxides and soluble forms. Because there can be soluble forms of corrosion products, plants with a significant amount of nonferrous (copper, nickel, and zinc) alloys may need to perform additional tests to obtain an indication of the soluble component.
Traditional colorimetric methods for iron and copper analyses can be adapted to provide detection limits in the low-ppb range. Following digestion, graphite furnace atomic absorption (GFAA) spectrophotometry is still considered the benchmark analytical method by most power plant chemists. Digestion and inductively coupled plasma with a mass spectrometer (ICP-MS) is claimed to have equal or better detection limits, although the extremely low detection limits claimed by instrument suppliers are orders of magnitude lower than those offered by most commercial laboratories with ICP-MS instrumentation.
X-ray fluorescence (XRF) also has been developed for online, low-ppb monitoring of iron in feedwater. A study in a nuclear plant found that over 80% of the iron was transported into the unit in less than a two-hour period. XRF analyzes the suspended iron oxides that accumulate on a Millipore filter. The filter pad is changed every few days (Figure 5).
5. Online corrosion monitoring. This online corrosion products monitoring data, collected by an X-ray fluorescence detector for iron in a supercritical boiler, shows large spikes of iron oxides during plant start-up and cycling loads. The results cover a 65-hour, 42-minute period. Source: Sheppard T. Powell Associates LLC
For start-ups, other analytical methods are available for rapid assessment of suspended iron. Turbidimeters calibrated with 100 nephelometric turbidity units (NTU) = 2 mg/L of formazin were found to be directly proportional to suspended iron oxide (100 to 2,000 ppb = 5 to 100 NTU). This relationship corresponds to 20 ppb of iron oxide per NTU of turbidity, although the correlation is more variable at lower (<100 ppb) concentrations.
Indications of particulate matter also can be provided by particle monitors and particle counters (PCs). Particle monitors measure fluctuations in light intensity and provide a particle index value. There are two types of particle counters: those that rely on light blocking and those that rely on light scattering. Light-blocking PCs count the number of times shadows are cast by particles 1 or 2 µm to 100 µm. Light-scattering PCs determine the number of particles (down to 0.05 or 2.0 µm, depending on the instrument) based on the amount of light scattered. In addition to a total particle count, PC units usually can provide a breakdown of the number of particles in several particle size ranges, although total counts are often the most useful.
—Robert D. Bartholomew (rdb@stpa.com) is an associate with Sheppard T. Powell Associates LLC.
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