One element control using only the drum level is applied during the low load (< 20-25% boiler load).
before we talk about the control theory of one element control , we have to show the (SHRINK and SWELLING) phenomena which is happened at boiler drum ,whenever you start firing the boiler:-
To illustrate the shrink and swell effect in a steam drum let us consider a sharp increase of steam consumption. With the sudden increase in the steam consumption the steam drum pressure drops immediately. With the sudden drop in the pressure the steam bubbles in the water wall and the drum swell and results in a sharp momentary increase in the drum level. After the pressure stabilizes the drum levels behaves in a conventional manner. This initial increase in the level is called swell and it is unique to the steam drum.
Similarly, when the steam consumption reduces suddenly, the drum pressure rises immediately and the steam bubbles in the water shrink. This leads to a sharp momentary decrease in the drum level. This initial decrease in the steam drum level is called shrink.
In a conventional one-element control strategy the output of a level controller cascades into a flow controller. Consider now the use of a conventional one-element control strategy to control the steam drum level. As the drum level increases the controller reduces the feed water supply. And similarly, if the drum level decreases the controller increases the feed water supply. Let us assume that the steam consumption increases suddenly. Due to the swelling effect the steam drum level will rise initially and then decrease. The controller will initially reduce the feed water supply. This will in effect reduce the water inventory and after the swell effect the water drum level will drum significantly.
This is a disadvantage of one element control system , therefore we use only this system structure at (20 to 25 % load) , and we have to take care of drum level by controlling the start up blow off valves. So, to eliminate such this problem , another control scheme is applied to eliminate the drawbacks of one-element control at higher loads.