Boiler Terminology Explanation (Part 20)

Boiler Terminology Explanation (Part 20)

191. Chemical cleaning of boiler is a technique that uses chemical methods to remove various deposits, metal oxides, and other impurities from the water-steam system of a boiler, while forming a protective film on the metal surface. It is an important technical measure to reduce corrosion, poor heat transfer, and water-steam contamination caused by scaling and deposits on the heating surface, thereby ensuring the safe and economical operation of the boiler. The cleaning process generally uses acidic media, also known as pickling. For newly built units, it mainly removes high-temperature oxide scales formed inside the heating surface pipes during rolling, and clears lubricants introduced during machining as well as rust products, welding slag, grease, sand, and other impurities generated during storage, transportation, and installation. For boilers in operation, it mainly removes deposits such as calcium-magnesium scale, iron oxide scale, copper scale, silicate scale, and oil deposits from the metal heating surface on the water side. The scope of chemical cleaning for new boilers is mainly determined based on boiler parameters, structural characteristics, and the degree of rust inside the pipes. Generally, for drum boilers with a superheater outlet pressure of 9.8MPa and above, chemical cleaning must be carried out before commissioning. For severely corroded superheaters and reheaters, chemical cleaning can also be performed, provided that measures are taken to prevent gas pockets in vertical pipes and the accumulation of corrosion products in bent pipes. If the corrosion of the condensate system and high-pressure feedwater system is minor, chemical cleaning is unnecessary — chemical desalinated water flushing is sufficient. Typically, this includes the boiler's water-steam system, with cleaning cycles determined by the service life and the amount of deposits in the pipes. When deposit levels exceed limit values, chemical cleaning should be considered. The cleaning interval should also be adjusted according to boiler type, working pressure, fuel type, deposit thickness, composition and characteristics, abnormal water quality incidents during operation, and internal inspection results during overhauls. In some systems, components such as the ASTM B148 UNS C95800 casting ball valve play a critical role in ensuring the safe and effective flow control during these processes.

192. Combustion in boiler furnace is the process of feeding prepared fuel and air into the furnace, where, under certain temperature and time conditions, intense oxidation occurs, emitting light and heat, and producing combustion products. Boiler combustion involves not only chemical reactions, but also physical phenomena such as flow, heat transfer, and mass transfer, which interact with chemical reactions.

193. Adjustment of boiler combustion ensures that fuel fed into the furnace is burned in a timely, complete, stable, and continuous manner through various regulation methods. While meeting unit load requirements, it aims to optimize combustion conditions. The quality of boiler combustion conditions significantly impacts the economic efficiency, operational safety of the boiler equipment, the entire power plant, and atmospheric environmental protection. In modern large-scale coal-fired power units, improving boiler efficiency by 1% can reduce the plant's standard coal consumption by 3–4g/(kW·h).

194. Boiler minimum load for stable combustion (boiler turndown ratio) is the minimum steam generation that a boiler unit can maintain continuous and stable combustion without auxiliary fuel. For coal-fired boilers, it refers to the lowest evaporation rate, expressed as a percentage of the rated load, at which stable combustion can be maintained without oil support. If the load drops below this point, the furnace flame temperature decreases, leading to flame instability or even flameout.

195. Testing of unit interlock protection involves adjusting and testing the characteristics of interlock protection devices to confirm the proper operation of interlock protection circuits. It ensures that when abnormal operating conditions occur during the start, stop, or running of unit equipment, the relevant equipment and personnel are protected through corresponding interlock protection actions. Thermal power plants have systems such as Main Fuel Trip (MFT) and interlocks, turbine interlock protection, unit fast load shedding (FCB), load rejection (RB), and linkage systems for major auxiliary machines. Interlock protection devices perform detection, judgment, setting, and execution functions, typically composed of hardware like relays, though software-based systems using microcomputers have emerged in recent years.

196. Unit performance test involves determining the actual operational performance of the unit, which can be categorized into design performance and actual operating performance. The purpose of unit performance testing is to obtain the actual running efficiency under various load points as a check on the design performance, and for equipment acceptance, model finalization, improvement, operational guidance, and economic dispatching. Unit performance tests include boiler performance tests and turbine generator performance tests. According to the content and number of test items, performance tests can be divided into single-item and comprehensive performance tests. Single-item performance tests are conducted to obtain specific performance data, evaluate individual parameters, or solve particular performance problems. However, unit performance tests generally refer to comprehensive performance tests.

197. Sliding parameter start-up is a combined machine-boiler start-up method used for unitized systems. In this method, as boiler steam parameters gradually rise, the turbine is rolled, accelerated, and loaded. When the boiler steam parameters reach the rated values, the turbine also achieves its rated load. Some units use shaft turning preheating before rolling, and during rolling, relatively high parameters are selected, around 4.0–5.0MPa and 300–350°C, hence it is also known as medium parameter start-up.

198. Sliding parameter shutdown is often used for planned shutdowns of large boilers and turbine generator sets for maintenance. Its main feature is that, with the regulating valves fully opened, the boiler reduces steam pressure and temperature while the turbine reduces load. As steam parameters and load decrease, unit components cool down more quickly and uniformly, shortening the cooling time after shutdown and allowing for earlier maintenance work. The specific operation typically involves first reducing the unit load to 80–85% of the rated value, adjusting the boiler steam parameters to the lower limit of operational allowance, fully opening the turbine regulating valves, running stably for a period, conducting shutdown preparations and system switching, and then lowering temperature, pressure, and load according to a prescribed sliding shutdown curve. The rates of steam temperature and pressure decrease vary throughout the process — slower at high loads and faster at lower loads. When the load drops to a very low level, the boiler is shut down, the turbine is tripped, and the generator is disconnected.

199. Theoretical combustion temperature refers to the temperature achieved under adiabatic conditions when combustible materials (mainly carbon and hydrogen) in the fuel react completely with the theoretical minimum required amount of air to form stable oxides such as CO₂ and H₂O.

200. Thermal power refers to the heat released by the combustion of fuel in the furnace, expressed as Q = BQr, where B is the fuel consumption (kg/s), and Qr is the lower calorific value of the fuel (kJ/kg).