Water and steam cycle
Understanding the water and steam cycle is essential to ensuring the efficient operation of heat recovery boilers (HRSG). This cycle involves several steps, during which the quality of the water and steam must be continuously monitored. If the control fails, the equipment suffers serious damage, and unexpected shutdowns can compromise the entire process.
For this reason, the VGBE standard defines strict parameters for critical system points, such as feedwater, the boiler, and the generated steam. Although requirements vary depending on the application, some parameters are essential in most cases.
Essential parameters for the water and steam cycle
Key indicators include pH, conductivity, dissolved oxygen, sodium, and silica. Proper analysis of these elements prevents problems such as corrosion and scale buildup—the accumulation of solids in the pipes, which compromises heat exchange and reduces the system's lifespan.
pH
pH plays a fundamental role in all stages of the cycle. It directly influences the chemical reactivity of water, interfering with the formation and maintenance of the protective iron layer on metal surfaces. Small variations can have significant impacts, as each pH unit corresponds to a tenfold difference in the concentration of H⁺ ions.
To ensure reliable readings, the sensors used must offer automatic temperature compensation, standardized at 25°C. Furthermore, if the sample conductivity is less than 10 µS/cm, it is recommended to use a neutral salt before measurement or calculate the pH based on the conductivity, provided the alkalizing agent is known.
Dr. Thiedig offers instruments such as the With 6 m pH and Catcon 6 Delta, which perform measurements with high precision in various chemical conditions.
Conductivity
Electrical conductivity measures the concentration of dissolved substances in water, such as salts, acids, and bases. When this concentration is below 1 mg/L, conductivity replaces total dissolved solids (TDS) analysis. Thus, it becomes an essential parameter for indicating water purity and preventing contaminants from being carried into the steam.
Measurements should be taken at 25°C to ensure comparability. There are two main methods: specific conductivity, which analyzes the sample directly, and cationic conductivity, which passes through an H⁺ ion exchange resin, increasing the accuracy of residual salt detection.
Dr. Thiedig provides equipment such as the Con 6 m SC, With 6 m DC and Catcon 6 Delta, which combines the two analyses and also calculates the pH. In addition, the module EDI allows automatic resin regeneration, optimizing maintenance.
DAC – degassed cation conductivity
During the commissioning of thermal power plants, degassed cation conductivity (DAC) analysis becomes essential. It measures conductivity after removing dissolved gases—primarily carbon dioxide (CO₂)—which increase values without posing a real threat to the turbine.
By eliminating CO₂, the operator gains a more accurate view of the dissolved solids present in the steam. As a result, they can quickly assess whether the steam meets required standards, reducing costs and improving efficiency.
O Digox 602 DAC, developed by Dr. Thiedig, removes over 96% of CO₂ without the need to heat the sample. It uses purified atmospheric air, which makes the process simpler and more economical.
Dissolved oxygen
Dissolved oxygen, present in feedwater, can cause pitting corrosion even at very low concentrations. This occurs more frequently in environments with high temperatures, salinity, and high pH. To prevent this type of damage, thermal deaeration or the application of sequestering agents are used.
Monitoring this parameter reveals the efficiency of oxygen removal and protects metal components from localized corrosion. Dr. Thiedig offers solutions such as Digox Optical, Digox 6.1 KS, with online amperometric reading, and the portable Digox 6.1 K-LC, ideal for rapid leak detection.
Sodium
Sodium must be controlled at different points in the cycle for different reasons. In the boiler, for example, it guides the dosage of products like sodium hydroxide. In condensate, its presence indicates leaks and contamination of the cooling water. In steam, sodium can form deposits in turbines and heaters, causing irreversible damage.
The measurement uses ion-selective electrodes. To ensure accuracy, it is necessary to stabilize the sample's pH with a regulating agent and control its pressure. The analyzer Digox 602 sodium performs online measurements with high reliability, eliminating interference.
Silica
Silica analysis is common at the outlet of a mixed-bed column, as it indicates the exhaustion of ion-exchange resins. Because of its low conductivity, silica can go undetected in conventional analyses. However, it is the first ion to escape from the exhausted resin.
Furthermore, silica control is important at various stages of the cycle, especially in feedwater, boiler, and steam. This is because silica volatilizes easily and can deposit on turbines, reducing their efficiency. In condensate, this parameter reflects the performance of the polishing system.
The analyzer Digox 602 silica, features up to six channels and an automatic sequencer. It offers high precision, reduced reagent consumption, and minimal maintenance, thanks to its microdosing system.
Accurate monitoring for maximum efficiency
Maintaining control over water and steam quality is a technical requirement that directly impacts the performance, safety, and durability of thermal systems. By investing in continuous analysis of critical parameters, operators prevent failures, reduce energy consumption, and extend equipment life.
Dr. Thiedig provides complete solutions for all stages of the water and steam cycle. With robust equipment and cutting-edge technology, the company helps optimize processes and ensure compliance with the most demanding energy sector standards. Contact us to discover the ideal analyzers for your plant.
