May 1, 2009

How to Measure Corrosion Processes Faster and More Accurately

Daniel C. Sampson and Peter D. Hicks, Nalco

Maintaining reliable and efficient plant operations requires good control of corrosion and corrosion product transport in power plant water systems. The Electric Power Research Institute recommends oxidation-reduction potential for passivator control in feedwater systems, as do many industry experts. Here’s how to turn that recommendation into a robust feedwater monitoring program.

The Electric Power Research Institute (EPRI) has for some time advocated the use of online room-temperature (RT) oxidation-reduction potential (ORP) measurements to monitor boiler feedwater corrosion chemistry and thereby minimize corrosion and reduce the frequency of boiler tube and other steam cycle failures. Unfortunately, current corrosion control and monitoring methods located upstream of a boiler can be unreliable, unpredictable, and impractical. The right instrument, improperly installed and operated, will return poor or misleading results.

Most plants attempt to monitor dissolved oxygen (DO), ORP, and reductant residuals (if reductant is used). Their monitoring programs, however, are often deployed in few locations with poor or little corrosion product transport monitoring, which can leave large portions of a plant unprotected. Even when corrosion product monitoring is performed, there are often questions related to the validity and interpretation of the numbers produced.

Fortunately, new, more accurate and responsive ORP measurement instruments are available that enable plant operators to significantly reduce steam cycle corrosion.

Dynamic Corrosion Effects

Power plant operators and chemists tend to perceive ORP as analogous to conductivity — a relatively static measurement that, taken in one location, provides a good indication of conductivity throughout the steam cycle. In fact, ORP is much more analogous to temperature, which is dynamic and varies widely throughout the steam cycle. ORP is really an ever-changing kinetic indicator of system health at both the point of measurement and throughout the steam cycle.

Corrosion stress, both general and localized, occurs often in a power plant, but traditional monitoring programs often fail to detect or react to such events. Local damage may cause failure, and corrosion product transport provides a secondary failure mechanism where corrosion deposits accumulate. Detecting these events may require major upgrades to the monitoring tools available at most plants. DO, reductant feed and consumption, and corrosion product transport would have to be monitored at multiple locations and with great accuracy. In most plants, this isn’t practical (because of instrumentation cost and the lack of installed sample points) or possible (given limitations in real-time corrosion product transport measurements, data validity, and interpretation, as well as the plant’s use for direct control).

Many plants focus control of corrosion stress at a single location (EPRI recommended the economizer inlet in earlier work and the deaerator inlet in its later recommendations), with the assumption that control at a single point provides protection throughout a circuit. This approach is relatively straightforward and can be effective in stable systems or when the location of the corrosion (reduction/oxidation) stress site is known or is close to the point of measurement.

Most systems, however, are not stable, and potential corrosion stress is widespread and often dynamic. The corrosion "space" changes, and single-point monitoring with slow response can miss short-lived changes in chemical demand or the corrosion environment. Traditional corrosion monitoring and control programs miss the dynamic stresses by assuming that slow response is "good" and a "good" measurement in one location indicates adequate protection in all locations.

The ever-changing ORP space requires a rapidly responding, real-time monitoring and diagnostic measurement that reacts immediately to the changing corrosivity of the system. Such a tool gives operators and controllers time to respond to the cause of the variation, thus minimizing any of its negative impacts.