Stopp_Watch_Editorial_June_2018 - Accidents in power plants and mitigation measures - Part II

Mon, 19/11/2018 - 18:02



A boiler is an enclosed vessel that provides a means for combustion and transfers heat to water until it becomes hot water or steam. Accidents in the boiler may occur due to de-aerator cracking, feedwater line erosion, economizer tubes, failure due to overheating, failure due to corrosion. These causes are explained in detail below.

Feedwater line erosion is caused by high-velocity water or water and steam.  Erosion occurs mostly at the hairpin bends of the economizer, the mixture of water and steam thins the elbows. This erosion can be prevented by maintaining water chemistry conditions, design, and operational changes.

External corrosion is caused by condensation of sulphur and moisture in the economizer. An economizer is a heat exchanger device used to pre-heat the fluid to reduce the energy consumption. It can either heat up the fluids or recover the heat from combustion product. It helps in saving fuel cost and applies the heat wherever required. This unit is subjected to both external and fireside corrosion. External corrosion can be avoided by keeping the temperature of tube metal below the acid dew point. Internal corrosion happens due to the presence of oxygen in the feed water and improper pH levels. This can be controlled by the de-aerator and mix appropriate chemicals into the feed water, which can reduce the oxygen or balance the pH.

Failure of boilers due to overheating can either be due to short-term or long-term over-heating. Overheating reduces the material strength. Overheating occurs when the temperature in the mid-wall is higher than the maximum temperature the material can bear. The heat generated from the burning fuel is transferred to the medium inside the tube. The temperature in the middle of the tube is called mid-wall temperature. The common reasons for such overheating may be due to sudden load rise, sudden fuel input etc. Long-term overheating happens over a period of time when the tube is subjected to not very high temperature, the outer surface develops a bulge or cracks creep along the axis of the tube.

Failure due to corrosion can be connected to many reasons such as stress corrosion cracking, caustic embrittlement, fatigue and corrosion fatigue, stress-induced corrosion etc. Caustic embrittlement is a type of corrosion which is a result of intercrystalline cracking of steel. The intercrystalline cracking happens when there are specific conditions of stress, a mechanism for the concentration such as leakage, and free NaOH in the boiler water. The boiler tubes fail from caustic embrittlement at points where tubes are rolled into sheets, drums or headers.

Further, accidents in thermal power plants can occur due to various reasons. A) A human error such as lack of attention at the workplace, lack of adequate training to handle a difficult situation and design conditions contribute to most of the accidents. B) Design: A small error in a design may lead to a bigger accident. All these could be avoided by factoring in different scenarios that the plant may experience, the design of equipment should be made according to that with relevant materials. C) Operating conditions contribute to accidents which could be avoided if the handling of operation is given to trained personnel who can work in different shifts to avoid lack of attention owing to tiredness. Investing time in planning the above will help save lives of innocent workers and contribute to the economic and social well being of the society.