Feedwater Regulator for Drum Boilers

Keeping accurate and stable drum level control is vital to optimal plant operation, but there are many challenges that are inherent with this process. Innate to level control on a drum style boiler is a dynamic called “shrink and swell”, which is caused by the formation of vapor bubbles in the boiler evaporation tubes as steam demand changes. On increase of steam demand, the drum pressure decreases which, in turn, causes additional steam to be created through water evaporation, and causes expansion of the vapor bubbles below the surface of the water. This phenomenon actually causes the drum level to rise initially, instead of the drop that would be expected upon more steam leaving the drum (swell). Likewise, on a decrease of steam demand, the pressure in the drum increases and the drum level initially drops (shrink). In order to control drum level accurately, the effects of “shrink and swell” are typically compensated for in the control system with a cascade/feed-forward control strategy that utilizes steam flow and feedwater flow transmitters in conjunction with the drum level and drum pressure transmitter measurements.