How do swell and shrinkage affect a single element feedwater controller?

Swell and shrinkage in a boiler refer to the increase and decrease in water level that can occur due to changes in steam production and pressure. In the context of a single element feedwater controller, these phenomena can have significant effects on its operation.

When swell occurs, the steam bubbles in the water increase, leading to a higher apparent water level due to the foam-like behavior of the water. Conversely, during shrinkage, the water level appears lower than it actually is because of the reduction in steam bubbles. A single element feedwater controller relies on the water level measurement and adjusts the feedwater flow accordingly.

However, swell can trick the controller into thinking that the boiler has an adequate water level when it is actually less than necessary, prompting the controller to reduce feedwater flow when more is needed. Similarly, during shrinkage, the controller may increase feedwater flow when the actual demand is lower. Therefore, swell and shrinkage can indeed lead to the controller making opposite flow adjustments than what is truly required for maintaining proper water levels in the boiler.

This understanding highlights the importance of using multi-element controllers in some applications, which account for steam flow, water levels, and other parameters to provide a more accurate control response, compensating for issues like