Applications – Boiler Drum Level Control

The control systems commonly used in the process industry are based on the single-loop and multi-loop techniques described in Control Loop Foundation – Batch and Continuous Processes. Some combination of these techniques is often required to meet product specification and plant throughput targets. The application examples included in Chapter 16 of Control Loop Foundation are designed to illustrate the various ways these techniques may be used to automate plant control. The six(6) workshop exercises included in this chapter allow the reader to explore the different features of the control design using a dynamic simulation of the associated processes.  The first workshop in Chapter 16 addresses boiler drum level control. When the boiler steam demand changes over a wide range, boiler drum level control may be improved through the use of multi-element control as illustrated below.

Power boilers often use three element drum level control. Also, three element drum level control is standard on most large utility and waste fuel boilers. Changes in the boiler steam flow resulting from variations in demand are a disturbance to the level control process. Thus, variations in the drum level may be reduced by introducing the steam flow measurement as a feedforward input to the level control. Also, the impact of any non-linear installed characteristic of the feedwater valve on the level control may be minimized by measuring the feedwater flow rate and structuring the level control as a level-flow cascade loop.

The control module provided for the three element drum level control workshop is shown below.

By accessing the book’s web site, you may complete this three element drum level control workshop using your web browser. The viewer below may be used to see the solution to this exercise.