Why 3 elements instead of 2 elements in boiler?
Why Three-Element Control Is Preferred Over Two-Element Control in Boilers
Three-element boiler drum level control is better than two-element control in modern power plants because it gives steady, accurate, and safe drum level control under different load situations. The level of the boiler drum is very important, and if it is not controlled correctly, it can cause serious damage to the equipment, trips of the turbine, or even catastrophic boiler failures.
Two-element control usually employs the level of the drum and the flow of steam as control variables. This method works really well when the load is consistent, however it doesn’t function when the load fluctuates quickly. This is mostly because of the swell and shrink effects. When there is a rapid jump in steam demand, the pressure in the drum drops and the steam bubbles grow, which makes the water level look like it is rising (swell). When a two-element system reacts, it cuts back on feedwater, which makes things worse and could expose boiler tubes.
Three-element control gets around this problem by adding feedwater flow as the third monitored variable, along with steam flow and drum level. The system doesn’t wait until something goes wrong; it keeps the steam output and feedwater inflow balanced all the time. This makes three-element control work very well when the load changes, when the system starts up or shuts down, or when it runs at full capacity.
Another big benefit is that three-element control works as a cascade control system, with the drum level as the main controller and the feedwater flow as the slave controller. This construction makes the dynamic reaction better, reduces oscillations, and makes the boiler work better overall.
Three-element drum level control is necessary for high-pressure utility boilers that work at more than 30% load. It eliminates erroneous level tripping, lowers thermal stress, and makes sure that the boiler will work for a long time. Because of these things, three-element control is used instead of two-element control in most power plants and big industrial boilers.
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