To maintain consistent power generation, today’s power plants must continuously monitor corrosive ionic impurities in various plant water streams, such as cooling waters, boiler waters, feed waters and steam condensates. Why? The presence of impurities above a specified concentration can result in stress-corrosion cracking and other corrosion mechanisms—potentially having a significant negative impact on power generation.
Interested to learn more about monitoring and analyzing corrosive ions? Check out these helpful questions and answers:
How can corrosive ions damage power plants? Why is monitoring valuable?
Damage from corrosive ions can cause forced outages and component failures, costing millions of dollars in lost revenue. The continuous monitoring of ionic species provides valuable information regarding the source of the contamination, assists power plants in understanding corrosion mechanisms, allows recommendations for appropriate treatment to prevent corrosion impacts, and produces concentration trends for individual ions over a period of time. This information can be used to minimize corrosive damage.
Which corrosive contaminants should be monitored?
Corrosive contaminants — particularly sodium, chloride and sulfate — have been implicated as a major source of corrosion and deposition-related plant shutdowns in nuclear and fossil-fueled plants.
How can ion chromatography (IC) help?
Ion chromatography (IC) can measure these contaminants and has been implemented in several U.S. power plant water chemistry monitoring programs. This methodology allows analysts to achieve parts per trillion (ppt) detection limits for individual anionic and cationic species, and operate on-line to effectively measure and eliminate corrosive hideout, thereby reducing the plant’s operating costs.
What’s one example of a challenging contaminant?
The measurement of ultratrace levels of sodium in boiler waters treated with amine additives, such as ethanolamine, is a particularly challenging analytical problem. The purpose of this all-volatile treatment (AVT) is to provide a high-pH and high-purity environment to minimize corrosion of metal surfaces.
This environment is accomplished by adding the amine to the boiler water at concentrations typically in the range of 0.5-10 mg/L. In effect, the high amine concentration can preclude the separation and detection of sodium typically present in the low-ppt range. However, many of these challenges have been overcome by the development of efficient, high-capacity cation-exchange columns capable of tolerating high-ionic-strength matrices.
What other cationic species are of interest to power plants?
There are several. For example, the determination of calcium and magnesium may be used to calculate water hardness and reveal the presence of condenser leaks and water polisher failures. These and other cationic species must be detected at the sub-μg/L concentrations in power plant samples. The lowest detection limits are achieved by suppressed conductivity detection.
Is there an alternative to suppressed conductivity detection?
Yes, an alternative to suppressed conductivity detection is analysis without the suppressor (i.e., nonsuppressed conductivity). In this detection mode, the column effluent flows directly into the conductivity cell. Consequently, lower-capacity columns using dilute acidic eluents are required to achieve a reasonable background signal. Weakly acidic complexing agents, such as tartaric acid, pyridine-2,6-dicarboxylic acid (PDCA), and oxalic acid, are also commonly used with nonsuppressed IC columns.
When is nonsuppressed conductivity detection the preferred choice?
Although suppressed and nonsuppressed conductivity can be used for the detection of alkali and alkaline earth metals, only nonsuppressed conductivity is suitable for the detection of transition metals. The formation of insoluble metal hydroxides from the suppressor reaction precludes the user of a suppressor for this analysis. However, in comparison to photometric detection, nonsuppressed conductivity does not provide the required specificity for many transition metals in high-ionic-strength matrices because all cations are detected.
Why is IC a great technique for detecting transition metals?
IC provides a convenient and reliable methodology to separate and detect transition metals on-line ant low- to sub-ppb concentrations. The separation of these metals requires the formation of a complex with a weak organic acid — such as citric acid, oxalic acid, tartaric acid, or PDCA — to reduce their effective positive charge. This complexation allows a change in selectivity of the metal ions that cannot be accomplished using only a monovalent eluent ion, such as hydronium. The most common mode of detection used for this analysis involves derivatization of the column effluent with 4-(2-0pyridylazo) resorcinol (PAR) and subsequent photometric detection at 520-530 nm. PAR provides a very broad selectivity and sensitivity for transition metals.
To learn more about the comparison of suppressed to nonsuppressed conductivity detection for the determination of sub-ppb concentrations of sodium in simulated power plant matrices, read this application note.