Process Monitoring Solutions for Boiler-Feed and Cooling Water Streams in Power Plants
Ritesh N Vyas
Product Manager - Applikon &
Electrochemistry Division
Metrohm India Limited

Nearly 50 percent of unplanned downtime in power plants is caused due to contaminants or problems in the chemistry of the water-steam circuit, with corrosion being the primary factor. Every cooling circuit has a unique design and thus its own unique challenges. The specifics of the water che -mistry to be regulated(and therefore the applicable limits) depend on the type of power plan, the cooling circuit design, and construction materials. Timely and effective monitoring of power plant water chemistry is critical for maintaining efficiency and safety. By using online analyzers, operators gain the information they need to accurately identify trends and address operational issues before costly problems arise. In this review, we highlight the importance of such real-time analyzers in boiler-feed and cooling water streams to optimize safety and plant efficiency.

The rapid increase in the Earth's population, which is growing by about 80 million every year, has also led to rising energy consumption. Calculations by the International Energy Agency (IEA) predict that the global energy demand will increase by about 65% by 2035. A major fraction of the required energy will continue to be provided by fossil fuel-fired and nuclear power plants.

Various guidelines define the permissible operating ranges for water chemistry used by power plant operators, including standards provided by the Electric Power Research Institute (EPRI), the Association of Large Boiler Operators (formally known as VGB - Vereinigung der Grosskesselbesitzer e.V .), the European Power Plant Suppliers' Association (EPPSA), and the International Association for the Properties of Water and Steam (IAPWS). Nuclear power generation is governed by the safety standards of the Nuclear Regulatory Commission (NRC) and the International Atomic Energy Agency (IAEA ).

Corrosion chemistry monitoring helps to minimize loss of efficiency and protect the components being exposed to steam and water against damage. The mechanisms and associated root causes responsible for most chemistry-related damage and efficiency loss are now very well understood, but the precise conditions at which corrosion and deposition activity begin are still not known. Past and present chemistry guidelines all serve to provide the operator with a warning as to when corrosion and/or deposition activities that place the power generation unit at risk may begin.

Flow-accelerated corrosion (FAC) of the metal components in power plant watersteam circuits reduces the lifetime of water-exposed carbon steel pipework and copper heat exchangers. Zinc ions, phosphates, and phosphonates are commonly used as corrosion inhibitors in steel piping. Chloride causes pitting corrosion on turbine blades and rotors. In combination with sulfate, it also leads to corrosion fatigue and stress corrosion cracking (SCC).

























Water Circuits in Thermal Power Plants

Thermal power plants use the heat generated by combustion or nuclear fission to produce steam, which is fed into a turbine driving a generator that converts the mechanical energy into electrical energy. Downstream of the turbin, the steam is condensed to water in a condenser. This water is fed into a feed tank from where it is pumped back into the steam boiler. Cooling water flows through the condenser in a separate circuit and removes the heat of condensation released by the steam via a heat exchanger. Nuclear power plants with pressurized water reactors have an additional water circuit known as the primary circuit.


























All thermal power plants use water as a central (operating) medium. As a liquid, it is used for cooling and as a gas, it drives the turbines. In nuclear power plants, it also moderates the fission neutrons and thus controls nuclear fission. A welldevised water chemistry ensures safe and efficient power plant operation. The water chemistry depends on the type of ;power plant, the cooling circuit design, and the construction materials.

Dedicated Online Analyzers

Effective cycle chemistry programs depend upon the selection of treatments customized to the specific unit and its characteristics. Treatment control and optimization, in turn, requires rapid, accurate sampling and analytical capabilities. While all power plants can benefit from online analysis of critical parameters such as corrosion indicators and inhibitors for optimum chemistry control, monitoring of diagnostic parameters such as chloride, sodium, sulfate, ammonia, hydrazine, silica and TOC can also be highly advantageous for protection and process optimization.

Not only do dedicated online analyzers help to safeguard plant operation and efficiency, but they also provide a continuous record of plant operating conditions for increased plant uptime and to facilitate long-term improvements in productivity. A strong requirement for integrated online analytical systems based on lab-proven methods in titration, spectroscopy, electrochemistry, photometry, TOC, ion chromatography, and ion selective measurements is clearly visible that contribute towards a reliable and sustainable operation of power plants to minimize costly downtimes.

Nuclear Power Plants

The most common types of nuclear reactors are the boiling water reactor (BWR) and the pressurized water reactor (PWR). The primary circuit of a PWR circulates cooling water at high pressure (up to 160 bar) through the reactor core, where boric acid dissolved in the primary circuit acts as a moderator for the nuclear reaction. Determination of boric acid concentration in the primary circuit is thus extremely important in controlling reactivity for reactor efficiency and safety. Fuel assemblies cannot be exchanged during operation of a PWR due to their use of light water, so a fuel reserve must be in place prior to the start of an operating cycle. The associated excess activity in the reactor is controlled by using a higher boric acid concentration.

Process Based Analysis of Boric Acid

The Boric Acid Process Analyzer provides fast and reliable values via potentiometric titration for continuous monitoring of boric acid concentration throughout the fuel cycle, both during the process and in the spent fuel pool. Efficient use of an automated titration system eliminates the tedious pipetting of the sample, distilled water, and mannitol solution needed for boric acid determination through manual methods. Automated analysis improves accuracy, eliminates the potential for operator error, and provides more rapid process feedback to maximize operational efficiency.

Fossil Power Plants

Fossil power plants consume fuel such as coal, natural gas, or oil, generating electricity via a steam turbine. The steam generation process requires cooling water, either in a closed loop or open loop system, so that the steam can be condensed for either re-use or discharge depending on the type of system. Cooling water composition must be monitored to optimize power generation, ensuring plant efficiency and safety.

Cooling Water Cycle

Cooling water is used to condense the exhaust steam from the turbine to water, traveling through kilometres of piping in the condenser before being sent back to the water-steam circuit as feed water. The cooling water itself is cooled either by once-through cooling, in which heat is transferred to water taken from a river or in a circuit in a wet cooling tower via heat dissipation into the atmosphere. Continuous circulation of the cooling water in either system increases the concentration of contaminants, and requires water analysis to control corrosion and deposition processes taking place in the cooling water circuit. Thus, cooling water composition must be monitored to optimize power generation, ensuring plant efficiency and safety.

In-line Monitoring of Silica

Build-up of silica from boiler feed and cooling water can lead to reduced boiler efficiency and eventually blockage and rupture of pipes. Monitoring of the silica content in influent and cooling water can provide valuable information that is useful in preventing scaling and operational efficiency losses. A single parameter silica analyzer can monitor silica levels from low ppb to high ppm level, which is critical for maintaining the required low levels of silica and preventing costly scaling and related issues.

In-line Monitoring of Phosphate

Trisodium phosphate is used for corrosion prevention in boiler water systems , leading to the need to monitor phosphate levels at several points within the process. Phosphate levels are ideally kept low to prevent any precipitation and solids transport, yet must remain high enough for corrosion prevention. The phosphate used for this purpose can sometimes be carried over into other system, particularly the steam system, leading to similar precipitation and scaling issues and thus a real-time analyzer proves to be a highly efficient solution.

In-line Nitrogen Species Analyzer

Ammonia is added to cooling water streams to help maintain a slightly basic pH, thus slowing corrosion. Other nitrogen containing corrosion inhibitors such as azoles are sometimes used, which can degrade to produce nitrogen species such as ammonia, nitrite, and nitrate in water streams. A Nitrogen Species Analyzer delivers rapid, accurate measurements of ammonia, nitrite, and nitrate species simultaneously, and can be configured to handle multiple streams.

Conclusion

Compared to an expensive yet bench top based ICP-OES or GC-MS systems, such real-time based process analyzers are cost-effective and efficient solutions to a variety of process analytical needs in power-plants. Not only are they capable of providing 'real' data for control parameters, they can alzso be recognized as the future guiding platforms for overall process sustainabilit.