Case Study #1: Boiler Control 锅炉控制 Lei XIE Institute of Cyber-Systems & Control, Zhejiang University, Hangzhou, P. R. China

Contents Process Diagram and Control Problems of Boiler (锅炉设备的生产流程与控制问题) Characteristic Analysis & Three-element Control for Drum Level (汽包水位特性分析与三冲量控制) Cross-limiting Air/Fuel Ratio Combustion Control (双交叉空燃比燃烧控制)

Simplified Process Diagram of a Boiler Main equipments ? Decompose the process into several subsystems ?

Boiler Control Problem Select the best pairing of MVs & CVs ? System decomposition: (1) drum-level control; (2) combustion control; (3) steam-temperature control.

Drum-level Control Problem Controlled variable: drum level, in H (s) Manipulated variable: feedwater flow, in F (s) Main disturbances: Steam flow, in D (s) Please explain the reason why it is a difficult problem

Drum-level Characteristics Disturbance path characteristic (Why?) Control path characteristic (Why?) Models ?

Non-minimum phase Characteristics There will be a zero in right side of the complex plane if The process will be called “non-minimum phase system (非最小相位系统)”

One-element (单冲量) Control Simple PID Control

Two-element (双冲量) Control Problem discussion: (1) Point out the kind of control methods ? (2) Obtain control diagram of the scheme. (3) Select the controller action, the symbol and the value of C2, if the valve is a fail-open valve and C1=1.

Three-element (三冲量) Control Control Diagram ?

Two Simplified Connections of Three-element Systems Which differences between two connections ?

Combustion Control Problem MVs and CVs Pairings: Flue gas →Furnace pressure, Air → Excess Air (O2% in flue gas), Fuel → Steam pressure

Simple Control Scheme for Combustion Control Existing main problem ?

Cross-limiting Air/Fuel Ratio Combustion Control Analyze the control scheme Advantage: Decoupling design Protect against excess of fuel under all conditions

Feedforward/Feedback Control for Furnace Pressure Analyze its design principle

Steam-temperature Control Scheme Which kind of control methods ?

Problem Discussion 《工业过程控制工程》思考题与习题 23-2~23-4, P.420

Exercise 9.1 To control drum level, the three- element control scheme shown in left figure is common used. If the feedwater valve is failed-open, then (1) describe the complete control diagram, and note the inputs and outputs for each block; (2) determine the symbol and the value C1, C2, C3 as well as the action of level controller LC.

Next Topic: Distillation Control (精馏塔控制) Description of Distillation Column Distillation Control Objectives Steady-state Characteristic Measurement of Composition Decomposition of Control Problem Basic Control Schemes

Next Topic: Problem Discussion CVs (被控变量): P, LD, LB, TR/xD, TS/xB MVs (操作变量): D, B, L, QH, QC, DG DVs : F, xF, TF To design your multi-loop control system to satisfy the above requirements and reduce interaction