Exam Question
Describe a three-element feed water controller (i.e., regulator) measuring steam flow, drum level and feed water flow and explain what relationship is maintained between the three variables and how? (16)
Class 2 (2nd Engineer)
MEKG
📅 Jul 2023
Reference Answer
### Principle of Operation
A three-element feedwater controller is an advanced automatic control system used for high-pressure, high-capacity water-tube boilers where stable drum water level is critical for safe and efficient operation. Its primary purpose is to overcome the limitations of single-element (drum level only) and two-element (drum level and steam flow) controllers, especially during rapid load changes which induce 'shrink' and 'swell' phenomena.
* **Shrink:** A sudden increase in steam demand causes a drop in drum pressure. This allows water in the downcomers and water walls to flash into steam, creating more bubbles. The volume of these bubbles causes the indicated water level to rise temporarily, even though the total mass of water in the drum has started to decrease. A simple controller would be misled and reduce feedwater flow, exacerbating the problem.
* **Swell:** A sudden decrease in steam demand causes a rise in drum pressure. This compresses the steam bubbles within the water, causing the indicated water level to drop rapidly. A simple controller would be misled and increase feedwater flow, potentially leading to a high-water condition once the load stabilizes.
A three-element feedwater controller is an advanced automatic control system used for high-pressure, high-capacity water-tube boilers where stable drum water level is critical for safe and efficient operation. Its primary purpose is to overcome the limitations of single-element (drum level only) and two-element (drum level and steam flow) controllers, especially during rapid load changes which induce 'shrink' and 'swell' phenomena.
* **Shrink:** A sudden increase in steam demand causes a drop in drum pressure. This allows water in the downcomers and water walls to flash into steam, creating more bubbles. The volume of these bubbles causes the indicated water level to rise temporarily, even though the total mass of water in the drum has started to decrease. A simple controller would be misled and reduce feedwater flow, exacerbating the problem.
* **Swell:** A sudden decrease in steam demand causes a rise in drum pressure. This compresses the steam bubbles within the water, causing the indicated water level to drop rapidly. A simple controller would be misled and increase feedwater flow, potentially leading to a high-water condition once the load stabilizes.
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