A meticulously performed chemical analysis is of little value if a bad sample is used. Problems with sample withdrawal, transport, collection and handling are often major sources of errors that can lead to incorrect or unnecessary corrective actions by operators. Proper design of the sampling systems is critical in order to produce samples and analytical results that are representative of the sampled stream (3-9). To monitor systems for the ingress of impurities and for the production and transport of corrosion products, several cycle streams are sampled and analyzed, either continuously or periodically. This article outlines the principles that must be considered when designing and operating water- and steam-sampling systems. These errors, as well as data inconsistencies and concentration swings in the analytical results, become commonplace and are often ignored by plant personnel, preventing the timely identification of actual chemistry excursions. As a result, operating decisions are often based on data that can have sampling errors as high as 1,000%. In fact, in water chemistry and corrosion control audits, sampling problems are found in roughly 70% of all plants. Unfortunately, many utility and industrial steam plants do not have properly designed and operated sampling systems to monitor water and steam chemistry. Typical target concentrations are in the range of <1 part per billion (ppb) to several parts per million (ppm) (1), (2). In steam plants, the chemical parameters of interest include: pH conductivity sodium calcium magnesium chloride sulfate fluoride phosphate acetate formate propionate total organic carbon (TOC) silica copper and dissolved and suspended iron (oxides). To reduce the risk of corrosion and deposition in water and steam systems, the standard practice is to monitor cycle chemistry and control impurity levels within industry-and manufacturer-recommended limits for the equipment. Corrosion-related failures can result in outages ranging from a few days to several months, depending on the affected systems, and can potentially cost tens of millions of dollars. Deposits and scale buildup on heat-transfer surfaces reduce efficiency, and when allowed to accumulate on steam turbines, such buildup can reduce the capacity. Corrosion and deposition in boilers, steam turbines and many types of process equipment are among the most expensive causes of outages in utility and industrial steam plants.
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