MODELING, SIMULATION AND CONTROLOF A DRUM BOILER

A Thesis Report Submittedinpartialfulfilmentoftherequirements ofthedegreeof

MASTER OF ENGINEERING

ByDIPANKAR KALITA(2012H132143P)

Under the supervision of Dr. HARE KRISHNA MOHANTA (Assistant Professor)

DEPARTMENT OF CHEMICAL ENGINEERINGBIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANIMay 2014

CERTIFICATE

This is to certify that the Dissertation work entitled Modeling, Simulation and Control of a Drum Boiler by Dipankar Kalita (ID - 2012H132143P) in partial fulfillment of requirement of BITS G629T Dissertation is a bonafide research work carried by him under my supervision and guidance.

Date: Dr. Hare Krishna MohantaPilani. Assistant Professor

ACKNOWLEDGEMENTS I would like to take this opportunity to respectfully acknowledge the guidance and support given by Dr. Hare Krishna Mohanta throughout this project work. I express my sincere thanks and deep sense of gratitude for his valuable mentorship and encouragement at every stage of this Dissertation work. Whenever I have approached him to discuss ideas for my project or any problem, I have always found an eager listener.

I would like to express my gratitude to Mr. Ajaya Kumar Pani without whom this project work would not have been completed within the time frame. With his deep insight and knowledge in the field of process control, he has always been a constant support throughout this work. My thanks also goes to all my other batch mates who have made the atmosphere in my classes lively and interesting. Last, but most importantly, Im grateful to my family for their love, blessings and support throughout this endeavour.

ABSTRACTSince boilers are so common there are many modelling efforts. There are complicated models in the form of large simulation codes which are based on finite element approximations to partial differential equations. Although such models are important for plant design, simulators, and commissioning, they are of little interest for control design because of their complexity. The objective of this work is to obtain a simplified nonlinear dynamic model for natural circulation drum-boiler system which can be used for control. The model should describe the complicated dynamics of the drum, downcomer, and riser components. It is to be derived from first principles, and is to be characterized by a few physical parameters. The model should be validated using the available plant data.This project involves modelling, simulation and control of a drum-boiler system using MATLAB and SIMULINK. For efficient control of the process various advanced controllers such as ANN- and Fuzzy Logic based controllers have been used along with the traditional PID controllers.

Keywords: Drum-boiler, Drum, Downcomer, Riser, MATLAB , SIMULINK, ANN, Fuzzy logic, PID

TABLE OF CONTENTS

ACKNOWLEDGEMENTS (i)ABSTRACT (ii)1Introduction 12Modeling and Simulation 32.1Global mass and energy balance 32.2Distribution of steam in risers and drum 62.3Mass and energy balance for riser section 82.4Distribution of steam in drum 92.5The Model 102.6Summary 132.7 Steady state solution of the model 132.8The solution142.9Step response..14

3Control schemes 183.1PID Controllers 183.1.1Tuning methods 193.2Fuzzy Logic Controller (FLC) 203.2.1Optimization of membership functions 233.3Neural Network Predictive Control 25

4Results & Discussions 314.1PID Controller324.1.1Regulatory Problem 324.1.1Servo Problem 354.1Fuzzy Logic Controller374.1.1Regulatory Problem 384.1.1Servo Problem 40

5Conclusions 42References43Appendix A: MATLAB Code 44Appendix A: Optimiztion Code 47

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Chapter - 1IntroductionThere are dramatic changes in the power industry because of deregulation. One consequence of this is that the demands for rapid changes in power generation is increasing. This leads to more stringent requirements on the control systems for the processes. It is required to keep the processes operating well for large changes in the operating conditions. One way to achieve this is to incorporate more process knowledge into the systems. The goal is to develop moderately complex nonlinear models that capture the key dynamical properties over a wide operating range. The models are based on physical principles and have a small number of parameters; most of which are determined from construction data. Particular attention has been devoted to model drum level dynamics well. Drum level control is an important problem for nuclear as well as conventional plants. In Parry, Petetrot and Vivien (1995) it is stated that about 30% of the emergency shutdowns in French PWR plants are caused by poor level control of the steam water level. One reason is that the control problem is difficult because of the complicated shrink and swell dynamics. This creates a nonminimum phase behaviour which changes significantly with the operating conditions. Since boilers are so common there are many modelling efforts. There are complicated models in the form of large simulation codes which are based on finite element approximations to partial differential equations. Although such models are important for plant design, simulators, and commissioning, they are of little interest for control design because of their complexity. The model presented here is adapted from K.J Astrom and R.D. Bell (1998). A nonlinear dynamic model for natural circulation drum-boilers is adapted. The model describes the complicated dynamics of the drum, downcomer, and riser components. It is derived from first principles, and is characterized by a few physical parameters. A strong effort has been made to strike a balance between fidelity and simplicity. Since the model is derived from first principles it can describe the system for a wide operating range. Simulation of the model is done using MATLAB R2010a and results have been verified with plant data presented in K.J Astrom and R.D. Bell (1998). The conventional PID controller is applied when the boiler is operating at medium load for both servo and regulatory problems. Advanced controllers such as Fuzzy logic controllers(FLC) and Neural Network(NN) predictive controllers have been also tried with satisfactory results. The details of these controllers are discussed in the report within.The drum boiler is a simple boiler which consist of a drum, downcomer and riser components. A simple schematic of the drum boiler have been shown in Figure 1. The heat Q, supplied to the risers causes boiling. Gravity forces the saturated steam to rise causing a circulation in the riser-drum-downcomer loop. Feedwater qf , is supplied to the drum and saturated steam, qs, is taken from the drum to the superheaters and turbine. The presence of steam below the liquid level in the drum causes the shrink-and-swell phenomenon which makes level control difficult. In reality the system is much more complicated than shown in the figure. The system has a complicated geometry and there are many downcomer and riser tubes. The outflow from the risers passes through a separator to separate the steam from the water. In spite of the complexity of the system it turns out that its gross behaviour is well captured by global mass and energy balances. Fig 1: Schematic of a drum boiler[1].

The shrink and swell phenomena in a drum boiler makes the level control very difficult. Under boiling conditions, steam supporting field products such as bubbles exist below the water/steam level interface. These bubbles have volume and therefore displace water to create a misrepresentation of the true water level in the drum. Another effect upon drum level is pressure in the drum. Because steam bubbles compress under pressure the steam bubbles expand or contract respective to these pressure changes. A higher steam demand will cause the drum pressure to drop, and the steam bubbles to expand to give the appearance of a water level higher than it truly is.This fictitious higher water level causes the feedwater input to be shut down at a time when more water is really required. A surge in water level as a result of the drum pressure decreasing is called 'swell'. A water level decrease due to drum pressure increase is called 'shrink'.

Chapter - 2Modeling and Simulation2.1 Global mass and energy balance equationsThe global mass balance equation isRate of accumulation of mass in the system = Rate input of mass Rate output of mass..(1)whereS= density of saturated steam (kg/m3)Vst= total volume of steam in the boiler system (m3)W= density of saturated water (kg/m3)Vwt= total volume of water in the boiler system (m3)qf= feed flowrate (kg/s)qs=steam flowrate(kg/s)The global energy balance isRate of accumulation of energy in the system = Rate input of energy Rate output of energy...(2)us= specific internal energy of steam (J/kg).uw= specific internal energy of saturated water (J/kg).mt= total mass of metal tubes and the drum.(kg)Cp= specific heat of metal (J/kg 0C)tm= temperature of the metal (oC)Q= heat supplied to the risers (W)hf= specific enthalpy of feedwater (J/kg)hs= specific enthalpy of steam (J/kg)From first law of thermodynamicsH=U+PVwhich can be rearranged asu = h - p/.Now we write the global energy balance in terms of enthalpy instead of internal energy by using the first law of thermodynamics....(3)The total volume of the drum, downcomer and riser is.(4)Equation (1), (3) and (4) combined with saturated steam tables yields a simple boiler model. We will however make manipulations of the model to obtain a state model. There are many possible choices of state variables. Since all parts are in thermal equilibrium it is natural to choose drum pressure ,p as one state variable. This variable is also easy to measure. Using saturated steam tables, the variables S , W , hS and hW can be expressed as functions of steam pressure. The second state variable can be chosen as the total volume of water in the system VWt. Using equation (4) VSt can be eliminated and replaced by VWt.Equation (1) can be written as.....Equation (3) can be written as.....

So.(5).Where.=(6)..Steam tables are required to evaluate ,,,,,,,, and . The results are based on approximations of steam tables with quadratic functions...........

The condensation flow rate is given by a simple energy balance done within the system.where . Here the term on the left hand side of the equation accounts for the thermal energy change within the system and the term accounts for the pressure energy change within the system. The negative sign appears before since condensation is taking place.The term on the right hand side accounts for the physical changes taking place within the system due to total change in stored energy within the system.So it can be rewritten as)..(8)2.2 Distribution of steam in risers and drumSaturated mixture quality in a heated tubeConsider a vertical tube with uniform heating. Let be the density of the steam-water mixture. Furthermore let q be the mass flow rate, A be the area of cross section of the tube, V the volume, h the specific enthalpy, and Q the heat supplied to the tube. All quantities are distributed in time, t and space, z. Assume for simplicity that all quantities are same in a cross-section of the tube.The mass balance for heated section of tube can be derived from the mass continuity equation. The continuity equation can be stated as.u,v and z are velocity in x,y and z directions respectively. Since mass is only entering and leaving in z- direction. So we have...(9)The energy balance can be derived as follows. Let us consider a infinitesimally small block of cube through which the fluid is flowing. The cube is a control volume. We will do the energy balance around this cube. The cube has three coordinates x, y and z. SoEnergy flow into the cube- Energy flow out of the cube= Rate of change of energyEnergy flow into the cube=Energy flow out of the cube=..Dividing both sides by dxdydz we get.....(10)The specific internal energy of mixture of steam and water is.(11)Where denotes the mass fraction of steam in the flow i.e. the quality of the mixture.In steady state we get...from (10) and (11). So.Let be the normalized length coordinate along the risers and be the steam quality at riser outlet. The steam fraction along the tube. , 0