Where to Sample?
Most coal-fired plants monitor and control the oxidizing environment at two or three locations, typically at the condensate pump discharge, deaerator inlet and outlet, and economizer inlet (Figure 2). Combined-cycle plants usually monitor and control the oxidizing environment at just two locations: the condensate pump discharge (usually after chemical feed) and the deaerator or low-pressure (LP) drum outlet. The sample locations themselves might be appropriate, but the AT ORP sampling should be made as close as possible to the actual sample take-off point.
2. Popular sample points. These are typical steam cycle water and steam sample point locations. Source: Nalco
It’s important to remember that steam cycle corrosion occurs at temperature and pressure. Any measurement taken to estimate corrosion should be made as close as possible to the conditions at which the critical corrosion processes are occurring. The following are additional suggestions for improving the quality of your samples.
Don’t Quench the Sample. The active species in your sample will be quenched in your sample cooler. That means you forfeit the opportunity to observe any correlations that exist at actual operating temperatures and pressures. In some cases, the wrong conclusions are reached by measuring ORP at temperatures that are unrepresentative of actual operating temperatures.
Minimize Sample Lag Time. Sample water often travels hundreds of feet from the sample connection to the steam sample panel. Sample water often passes through large-volume coolers and filters at the sample panel before contacting the panel’s low-temperature probes. This circuitous path adds to sampling lag time and dilutes real system effects. Also, further corrosion reactions can occur as the sample passes through the long sample lines and large coolers before it contacts the room-temperature probes.
Accurately Measure Reductant Activity. Like corrosion stressors and corrosivity, reductants (oxygen scavengers/passivators) are more active at temperature. This is particularly true of many of the passivating scavengers such as hydrazine, carbohydrazide, diethylhydroxylamine, and the like. The reductant impact on corrosion stressors is enhanced at temperature, so control based on reductant residual is more sensitive and realistic at temperature. A cooled sample might not show how much "power" (in reductant) is present in the system, but an at-temperature sample (AT ORP) will show this.
Figure 3 illustrates the impact of reductant feed on both RT ORP and AT ORP. Reductant was added to a solution initially containing 140 ppb DO. More reductant was added over time, and DO declined. The range of ORP movement increases significantly with the AT ORP measurement. The AT ORP probe is more responsive to changes in DO and/or reductant. This increased sensitivity has important implications for system control based on an ORP setpoint. Greater sensitivity provides better response to changes in the corrosion environment. RT ORP probes are completely insensitive to some of the macro corrosion changes occurring at temperature.
3. Some like it hot. The at-temperature oxidation-reduction potential (ORP) probe, with water samples at operating temperatures and pressures, produces superior results. Source: Nalco
Carefully Measure Dissolved Oxygen. RT ORP can correlate with DO, but it tends to be a poorer indicator of reductant excess in boiler feedwater systems. However, even as a DO "indicator," the AT ORP probe response and magnitude of movement to DO upsets is far superior to that of RT ORP.
Figure 4 presents laboratory data showing ORP response as measured after making short-term changes in DO concentration in a system. The immediate, relative, and greater magnitude of the AT ORP response is evident. It’s especially important to note the resolution of the AT ORP response at extremely low (<10 ppb) DO concentrations. RT ORP response is especially poor at low dissolved oxygen concentrations, yet this is the very zone where power plants typically operate.
4. At-temperature measurements are more responsive. Note the difference between at-temperature (AT) ORP and room temperature (RT) ORP response to adding various amounts of air-saturated water at 400F. Source: Nalco
The greater response of AT ORP technology to changes in the oxidizing environment makes it an excellent tool for plants that do not feed reductant but that remain justifiably concerned about corrosion stress. The extremely sensitive response of the AT ORP probe to low-level DO concentrations indicates that this technology can be used in both reducing (AVT-R) and oxidizing (AVT-O) regimes.
Correctly Measure Corrosion. AT ORP technology will also respond to any species present in the water that will affect the corrosion space and is not limited to DO and oxygen scavenger feed. Figure 5 illustrates the AT ORP probe response as compared to the RT ORP probe as a result of corrosion processes occurring within a feedwater heater. The additional soluble corrosion products, such as iron and/or copper, are sensed by the AT ORP probe but go unnoticed by the RT ORP probe.
5. AT ORP sees more. AT ORP is much more responsive than RT ORP to corrosion occurring in the outlet of a feedwater heater. Source: Nalco
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