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In large capacity boilers, the superheater and reheater temperature at the boiler outlet is controlled by many methods. Water spraying in both superheater and reheater is a very commonly adopted method. In using a spray water system, there are many points to be addressed as otherwise serious failures can result. The superheaters and reheater are sized in such a way that the full steam temperatures are achieved from 60 % boiler load. This means the steam temperature will be much higher than the rated steam temperature. Hence, to control this increase, water spraying is adopted. These desuperheaters are designed in such a way that the water is split into fine particles and allowed to mix with the steam, ensuring no water particles remain as it enters the superheater.
Superheater spray system
The superheater spray system consists of feed water tapped after the feed pump and sent through a set of control and isolation valves to the desuperheater. This is arranged between the primary or low temperature superheater and the high temperature radiant superheater section. Some designers adopt two stage desuperheater systems. In this case the second stage of the desuperheater comes between two stages of high temperature superheaters. The control on the main-steam outlet temperature is done suitably by a feed forward and fine tune control philosophy. Over spraying in superheaters results in long term overheating failures in sections preceding the desuperheater due to the metal temperature going higher. This mainly occurs at the material transition point in the superheater section. In superheater sections immediately after the desuperheater, the water particles deposits salts when they evaporate fully and a probability for long term overheating, corrosion, and thermal fatigue failures results.
While over spraying in superheaters, the steam flow through the preceding sections will be reduced to the extent of the span being given. This reduced flow increases the metal temperature at each point along the length of the tube in the coils. If the slope of this increase is more than the limit to which it can exceed the margin in allowable metal temperature at the transition point of the material, then it can lead to long term over heating failure.
Depending upon the quality of feed water being maintained by the boiler operator and the quantity of spray the deposition inside the section of superheater just after the desuperheater will have a high probability for overheating failure, corrosion, and thermal fatigue.
Reheater spray system
Reheater spray is normally provided at the inlet of reheater in the cold reheat pipe. This is used in boilers only if the first control system provided could not reduce the temperature to the required level. Hence this is also called as emergency injection by many boiler designers. Using the emergency spray in reheater reduces the heat rate as the injection water is tapped before the high pressure heaters in the power generation cycle. Other effects of over spraying in reheater are the same as superheater except that of metal temperature. In reheater the overall steam flow increases with injection in all the sections and so the metal temperature gets controlled.
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About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
Water Quality in High Pressure Boilers - The process for water treatment is decided dependent on the source of water of the power plant and its quality. The levels of water quality to be maintained depend upon the drum operating pressure.
Superheater Temperature Control in Boilers - Steam temperature control is carried out in all boilers to get a constant temperature within a range. Methods like desuperheating by water injection, flue gas flow diverting, burner tilting are few systems adopted. Main steam and reheat outlet temperature are controlled by these methods in boiler.
How Long Term Overheating Tube Failures Happen in Boilers - Long term overheating tube failures are due to operating metal temperature of the boiler tubes going above the allowable limit. These types of failure are seen in steam cooled tubes like superheaters and reheaters and in water cooled tubes of waterwalls.