Three element control using the followings is applied during the normal operation (> 20-25% boiler load )
- Drum level
- Main steam flow
- Feed water flow
In order to handle the situation, the steam flow rate should also be considered for drum level control. It can be done by adding the steam flow rate as a feed forward signal to the output of the level controller. Hence, the supply of the feed water flow is compensated for changes in the steam flow rate demand. With this strategy as the steam flow rate changes the demand for the feed water flow rate also changes in the right direction and minimizes the effect of shrink and swell on the drum level.
Now, let us assume that the steam consumption increases suddenly. As the steam consumption increases the feed forward signal increases the feed water supply to the steam drum. Due to the swell effect the level controller reduces the feed water supply. The net effect of the three-element level control scheme changes the feed water supply appropriately and reduces the effect of swell on the drum level. Thus, the three-element level control strategy provides a more stable drum level control.