Three Element Drum Control System in Boiler Water Level Control

Boiler Feed Water Control System  

        In marine boilers , the feed water is controlled by measuring 3 variables mainly, water level in boiler drum, Steam flow and feed water flow. So this system is called Three Element Boiler Water Level Control System .

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Why 3 element boiler water control system is preferred in marine boilers?. 
         Certain boliers with high water volume, and low steam production requires only one element boiler water level control system ie, only by measuring feed water level, according to which the level is adjusted . 
          Unlike other boilers marine boilers are produce large amount of steam with high temperature and pressure in relatively low water volume so that the effect of swell and shrinkage is more in marine boilers.

Swell
        You know that when boiler is stopped, level gauge reading shows a lower value than operating. This higher reading while operating is because of steam bubbles in boiler drum water. If there is a situation with a high steam demand, the boiler pressure will fall and hence some quantity of water present in the drum at high pressure will flash off and become steam. Theses bubbles of steam will increase the water level inside the drum. Actually the mass of water is reduced in the drum which also result in more steam production which will further increase the water level. This effect is called ‘swell’. If you are using a system that only measures the water level, it will not allow feed water valve to open in this situation. 

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Shrinkage
         When the boiler steam demand comes back to normal condition, the drum pressure will be raised and bubble formation get reduced , which result in a decrease in water level. Because of decreasing level , feed water valve gets open and these incoming cold water will further reduces the steam bubble formation. This will also reduce the drum level, this effect is known as “shrinkage”.
          Both these effects can only be reduced by 3 element control system, by measuring water level in boiler drum, Steam flow and feed water flow.

Three Element Boiler Water Level Control System

            Boiler 3 element feed water level control system is shown in fig. Signal from the steam flow and feed flow is compared in the differential relay. The differential relay output and drum level signal goes to controller and comparator. Any deviation from the actual drum level or steam and feed water flow results in controller action . To adjust the feed water control valve, make the drum level normal.

You Can Answer Following Questions
> Explain Boiler Water level control system
> How boiler water level is controlled by three parameters?
> How boiler maintain water level even in sudden load changes?

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