| For air heater corrosion and fouling problems, some provision must be made to maintain a corrosion specimen temperature that is within the ranges typically found in the operating air preheater. Corrosion probe methods (see Table 22-1) are used to simulate corrosion rates in an operating air preheater. Figure 22-4 shows a multipoint corrosion probe. The temperature to be maintained on the specimen is determined by calculation of average cold-end temperature and measurement of dew point.
Table 22-1. Comparison of corrosion rate assessment methods
| Method |
Variable
Measured |
Time
Required |
Instrument
Cost |
Advantages |
Limitations |
| Corrosion coupon installed on surfaces |
loss of weight of coupon |
1-4 week |
low |
simple; measures corrosion directly |
not clear if location of coupon represents entire surface |
| Air-cooled corrosion probe for flue gas test |
loss of weight of coupon |
1-4 week |
low |
simple; can determine corrosion as a function of temperature |
temperature cannot be closely controlled without relatively high expense |
| Multipoint corrosion probe |
short-range loss of iron |
5-7 hour |
low to moderate |
can determine corrosion over a wide temperature range; rapid indication |
difficulty extrapolating short-range to long-range results |
| Sampling of flue gas |
flue gas sulfur trioxide |
4-5 hour |
high |
direct measurement of SO3; then dew point temperature is accurate |
more technical knowledge required to operate equipment; not a direct indication of corrosion |
| Electrical conductivity, dew point and rate of acid buildup meter |
temperature at which acid condenses on probe; rate of acid deposition below the dew point |
2-4 hour |
high |
quick; gives some idea of corrosion problem |
low sulfur trioxide gives inaccurate and nonreproducible results; high dust loadings interfere with rate of acid buildup measurements; does not measure surface coating additive effect |
The electrical conductivity dew point meter is useful in problem assessment work and some results monitoring. In addition to corrosion monitoring, this meter provides an indication of the deposition rate by measuring the increase in the conductance of an acid-containing film with respect to time. The electrical conductivity meter is acceptable for results monitoring only where a sulfur trioxide removal additive is used. The dew point meter is shown in Figure 22-5.
Normally, direct measurement of sulfur trioxide is used only for Environmental Protection Agency tests because of its cost and complexity. All of these evaluation techniques and tools are used by suppliers of proprietary cold-end treatments. They enable the engineer to define the problem and measure results properly.
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| (Chapter 21 Boiler Fireside Deposit And Corrosion Control) |
(Chapter 23 Cooling Water Systems-Heat Transfer) |
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