Water and Steam Cycle

Below is a typical diagram of a water and steam cycle for a Heat Recovery Steam Generator (HRSG). To prevent unexpected shutdowns or damage to cycle components, water and steam quality must be controlled at all times. The VGBE standard specifies a range of quality parameters for critical lines, such as feed water, boiler water, and steam.

Due to the variety of lines with different characteristics, the quality parameters to be monitored vary as needed, for different reasons. The most common parameters controlled by an online system are:

  • pH
  • Conductivity
  • Dissolved Oxygen
  • Sodium
  • Silica

Generally, these controls are intended to prevent corrosion and solid material deposits in the boiler (scaling) that cause irreparable damage and loss of steam generation efficiency, as illustrated in the summary below.

pH

pH is an important measurement parameter in all lines of the cycle and is related to the reactivity of water. By its definition, each pH unit represents a factor of 10 times the concentration of hydrogen ions. It is monitored in virtually all lines of the cycle, as its control prevents the corrosion of metal alloys and the dissolution of the protective iron layer. For an accurate measurement of this value (essential for this application for efficiency and safety reasons), sensors with automatic temperature compensation referenced at 25 °C should be used. If the sample has a specific conductivity below 10 µS/cm, some precautions must be taken, such as using a neutral salt before measurement or calculating pH through conductivity (knowing the alkalizing agent). Dr. Thiedig offers pH analyzers for all possible conditions in the water and steam cycle, such as the analyzer with a glass electrode for different sample chemical conditions with the 6 m pH illustrated below and the Catcon 6 Delta conductivity analyzer with pH calculation, which will be mentioned in the conductivity section.

Conductivity

Electrical conductivity is the measure of the sum of all dissociated substances (salts, acids, bases, and some organic substances) in liquids and serves as a substitute for measuring total dissolved solids (TDS) when it is below 1 mg/l. Like pH, conductivity is an important parameter in all lines of the steam and water cycle, giving us an idea of water purity to prevent scaling or carryover of substances to the steam. Conductivity should also be referenced at 25 °C and can be divided into two types: specific conductivity and cation conductivity (also known as acid conductivity). Specific conductivity is common conductivity, without altering the sample’s chemical characteristics. Cation conductivity is measured after the sample is treated by a strongly acidic ion exchanger (a cation resin) that retains the sample’s cations and replaces them with H+ ions. In this case, the sample’s salts become acids, which have a 3 to 4 times higher conductivity, enhancing ion detection sensitivity for salt traces. For conductivity measurements, we offer three devices:

  • Con 6 m SC – Single-channel conductivity meter for specific conductivity measurement
  • Con 6 m CC – Single-channel conductivity meter with a cation column for cation conductivity measurement
  • Catcon 6 Delta – Dual-channel meter with a cation column for specific cation conductivity and pH calculation, as shown in the image below. Note: We also offer the EDI module as an optional addition to the cation columns, with the benefit of automatic resin regeneration, drastically reducing equipment maintenance time.

DAC – Degassed Cation Conductivity

Degassed cation analysis is the measurement of conductivity after treatment with the cation column and a system for removing dissolved gases from the sample (mainly carbon dioxide). It can be interpreted as cation conductivity without CO₂ interference, which is not harmful to the turbine but contributes to cation conductivity value. The DAC ultimately analyzes the conductivity of dissolved solids in the sample, excluding gases, enabling faster detection of steam quality suitable for turbine use. This analysis is particularly useful for the steam line during plant startup. Though it has a specific purpose, this analysis saves time and costs, as illustrated in the following graph.

When analyzing the graph, we see that the greater the gas removal efficiency from the sample, the faster it is detected that steam quality is ideal for beginning power production. With this in mind, Dr. Thiedig developed the Digox 602 DAC, which achieves over 96% carbon dioxide removal efficiency without heating the sample. The method used to expel gases is purified atmospheric air bubbling.

Dissolved Oxygen

Measured in feedwater, dissolved oxygen must be removed as it can cause pitting corrosion at very low concentrations under high pressure, temperature, salt concentration, and pH conditions. To remove dissolved oxygen, oxygen-sequestering chemicals or, more commonly today, deaerators are used. This parameter analysis assesses the efficiency of dissolved oxygen removal, protecting the water and steam cycle equipment from corrosion. Dr. Thiedig offers three solutions for this measurement: the Digox Optical, an optical analyzer that is the most commonly used method in thermal power plants and small to medium-sized boilers today, and two amperometric analyzers: Digox 6.1 KS (online) for samples requiring a very low detection limit (LD = 0.2 ppb) and Digox 6.1 K-LC (portable) for leak detection and maintenance assistance.

Sodium

Sodium must be analyzed in different lines for different reasons. In boiler water, it is essential to control agents like sodium hydroxide or sodium triphosphate dosage when used in the process. In the condensate, it is the most important parameter for detecting condenser leaks, indicating that part of the cooling water is contaminating the cycle. In steam, it is measured to prevent deposits on superheaters or turbines, which require costly repairs. This measurement is made using an ion-selective electrode. Among the possible ions in the water and steam cycle, H+ is the only one that interferes with the reading, so a pH controller agent must be used to keep this influence constant, allowing it to be eliminated in the equipment calibration curve. Another interference that needs to be well-controlled to a constant value is the sample pressure. Dr. Thiedig’s Digox 602 sodium analyzer is ideal for online sodium measurements in the steam and water cycle, eliminating all previously mentioned interferences for the highest analysis accuracy possible. More details about this instrument are below.

Silica

Silica is commonly monitored in various stages, such as at the outlet of the mixed bed column in demineralized water production. For this application, the analysis serves to detect resin exhaustion in ion exchange columns since silica has very low conductivity, going unnoticed by conductivity analysis. Additionally, silica is the first ion to escape from the exhausted column due to the higher selectivity of columns for stronger ions. The difference in conductivity and silica analyses after treatment and the selectivity of an anion column can be seen below. In the boiler cycle itself, silica is typically monitored in feedwater, boiler water, and steam to prevent deposition in turbines. As the most volatile contaminant among possible dissolved substances in the steam and water cycle, silica is a key parameter of cycle quality. In the condensate line, it serves as an indicator of polishing efficiency. Dr. Thiedig offers the Digox 602 silica as a solution for all these lines, as it has up to six channels with an integrated sample sequencer in its electronics, high precision, reagent economy, and low maintenance with its microdosing pumps. Check it out below!