Over the years, the demand for automated, high quality and greater efficiency boilers has significantly increased in power plants and industries. In a competitive environment where generation companies are keen to reduce costs and optimise operations, outages due to boiler faults can lead to significant revenue loss. Therefore, it is important for plant operators to undertake regular monitoring and maintenance exercises for boilers and associated systems. In boilers, the basic maintenance criteria essentially remains the same regardless of their design, application and size.
To improve the performance and thermal efficiency of boilers, it is important to periodically remove the deposits and keep the heating surface clean. The most common cause of corrosion in boilers is dissolved oxygen entering the system via feedwater. It causes localised corrosion in the form of pitting. The pits are deep pinpoint holes that eventually penetrate tube walls and cause their failure. This kind of corrosion is normally controlled by removing oxygen from the feedwater through a deaerating heater or by chemically removing it with an oxygen scavenger such as sodium sulphite.
Most oxygen scavengers contain a catalyst, which speeds up the reaction of sulphite with oxygen. In systems equipped with a deaerator, sulphite should be fed into either the storage tank of the deaerator or the suction or pressure side of the feedwater pump. In systems that do not have a deaerator, sulphite can be fed at any point in the feedwater system, including the condensate tank. Another common cause of corrosion in boiler systems is low pH levels, which may result from carbon dioxide infiltration or contamination by other chemicals.
During the start-up phase of the boiler, it is important to ensure proper water chemistry, load ramp rate, oil/gas consumption, metal temperature rise, process control stability, and control of spray valves. After boiler trip-out, a furnace purge is required to expel all unburnt fuel particles and gases from the boiler to minimise the risk of explosion when the boiler is lighted. Moreover, precise and concise operating procedures need to be followed with strict compliance with operations and maintenance (O&M) manuals provided by the original equipment manufacturer. Systems for tracking and recording performance deviations should be implemented to facilitate proper monitoring, analysis and record of operating data. Monitoring the availability of spares is also important to reduce the shutdown time and increase plant availability. Further, plant operators should engage experienced contractors for quality maintenance.
Key O&M requirements of supercritical boilers
The sliding-pressure operation mode should be utilised for supercritical boilers. Some of the key advantages of sliding-pressure operation are low thermal stress in the turbine during load changes, extension of the control range of RH temperature, and reduced pressure level at lower loads. This prolongs the life-span of the components, leads to an overall reduction in power consumption, and improves the heat rate. Also, constant monitoring of the furnace wall outlet temperature is necessary to ensure that superheating in a furnace wall is within acceptable limits.
Water chemistry also plays a very important role in the smooth operation of a supercritical boiler. Supercritical boilers faced a lot of failures at the early stages of implementation due to improper water chemistry, leading to failures in water wall tubes, reduction of heat transfer area due to high water side deposition, and corrosion issues. Oxygenated treatment (OT) came as a breakthrough to deal with these issues in supercritical boilers. In OT, high purity oxygen is injected into the outlet of the condensate polishing unit. In the presence of oxygen, the pores in the magnetite protective layer are blocked and covered with lower solubility ferric oxide hydrate and/or ferric oxide. In this way, iron concentration in the feedwater is reduced. OT gained popularity owing to its ability to prevent flow accelerated corrosion and reduce iron content in the feedwater.
If boilers are not properly maintained by power plant operators, frequent equipment failures and subsequent plant outages could occur, resulting in undue expenses for power plant operators. Therefore, apart from regular maintenance and monitoring practices, a quality improvement plan and a result assessment system based on testing and inspection of boilers should be formulated.