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Posted by Gburns on February 13, 2002 at 06:42:05:
In Reply to: three elimante feed system posted by doug on February 07, 2002 at 18:34:11:
Three element feedwater control uses three inputs to control the drum level. The three elements are drum level, feedwater flow, and steam flow. Feedwater flow is basically being controlled based on STEAM DEMAND and not just drum level.
When operating, the steam flow signal acts as a feedforward signal to the drum level controller. This permits the feedwater control valve to respond when steam flow changes without having to wait for a change in drum level. The drum level controller then becomes, basically, a trim controller with the bulk of control action coming from the steam flow feedforward signal.
This type control will also somewhat counteract the affects of shrink and swell. With a rapid increase in steam flow the drum level may swell due to the drop in pressure. If there was no steam feedforward signal (single or two element) the drum level controller would decrease the feedwater flow even though it should be calling for an increase. With rapid decreases in steam flow the level may shrink due to an increase in pressure. Without the steam feedforward signal the controls would add feedwater instead of decreasing based on the true demand (steam flow).