Boiler Drum Level Control In Thermal Power Plant

Transcription

1 International Avance Research Journal in Science, Engineering an Technology National Conference on Emerging Trens in Engineering an Technology (NCETET 16) Loures Matha College of Science & Technology, Thiruvananthapuram Boiler Drum Level Control In Thermal Power Plant Shiji S.R 1, Anish.S 2, Swapna.M 3 PG Stuent, Department of EEE, Loures Matha College of Science an Technology, Kuttichal, Kerala, Inia 1 Engineer, Department of CIG, Centre for Development of Avance Computing, Vellayambalam, Kerala, Inia 2 Associate Professor, Department of EEE, Loures Matha College of Science an Technology, Kuttichal, Kerala, Inia 3 Abstract: The boiler is a funamental part of almost all inustries. It varies in application from low pressure to high pressure. The proper an safe control of the steam rum water level is the most crucial operation of the boiler. The boiler may get amage ue to overheating, if the level is too low, an also, ue to overflow of water, if the level is too high. Therefore, there exists an optimum interface level between steam an water within the boiler rum. The steam boiler rum level is characterize by a high extent of nonlinearity, uncertainty, isturbance an overshoot.. The cascae-three parameters water level control system inclues boiler rum water level, steam flow an water supply. Boiler rum water level is controlle by using cascae Proportional Integral Derivative(PID) controller techniue an then optimization is one by using Moel Preictive Controller(MPC).Simulations are one in 210MW thermal power plant. The comparative analysis shows the better result when an avance controller such as MPC is use. MPC has a comparatively less overshoot than PID controller an much faster settling occurs for an MPC, which shows the superiority of MPC. Keywors: PID Controller, MPC Controller, Process Variables (PV) I. INTRODUCTION The water level of boiler rum is one of the crucial control parameters for any process inustries, which reflects the control of mainly boiler loa an fee water inirectly. If the water level is critically high it results in low rum pressure causing swelling an results in water carryover into the steam piping.. If the water is low then results in high rum pressure causing shrinkage which is severe enough to trip the boiler. The feewater control system is a three element type, esigne to monitor changes in steam flow, water flow an rum level. Steam flow is the rate of steam leaving the boiler - the eman. Water flow is the rate of feewater flow into the boiler - the supply. Drum level reflects the amount of water in the boiler - the inventory. With changes in boiler loa (steam flow), steam an water flow become unbalance an water level conseuently eviates from the normal position. In such an event, the level controller will ajust the boiler fee water valve position to restore the flow to its set point before the boiler rum liui level is even affecte. The level controller is the primary controller, (sometimes referre to as the master controller) in this cascae, ajusting the set point of the flow controller, which is the seconary controller (sometimes ientifie as the slave controller). or measurement of rum level following points must be taken care of:- (a)elimination of errors which are attribute in measurement ue to fluctuating water level or changing rate of water inflow an steam outflow. (b)thermoynamics of pressure an temperature, geometry of the steam rum, an euation of continuity are the parameters to efine transmitter calibration. (c)the zero elevation an span against transmitter specifications must be checke to ensure proper calibration of selecte transmitter. Boiler operating conitions that alter the total volume of water in the boiler cannot be correcte by fee forwar control strategy. or example, force circulation boilers may have steam generating sections that are place out of service or in service intermittently. The level controller itself must be correct for these unmeasure isturbances using the normal feeback control algorithm. eewater flow an steam output flow transmitters are reuire to fin out the ifference between the two flows. A rum level transmitter reference is use to control the boiler fee water control valve. II. TYPES O BOILER EED WATER CONTROL There are ifferent types of control techniues for a boiler fee water control. A. Single Element Control Single Element Drum Level Control is the simplest an least effective metho to regulate the water level. It is mainly use for boilers with moest change reuirement an start-up operation. The process variable coming out from the rum level transmitter is compare to the set Copyright to IARJSET DOI /IARJSET 139

2 International Avance Research Journal in Science, Engineering an Technology National Conference on Emerging Trens in Engineering an Technology (NCETET 16) Loures Matha College of Science & Technology, Thiruvananthapuram point an a eviation value is prouce. This signal is fe into the controller an make corrective action. ig1: Single Element Control B. Two Element Control Two Element Drum Level Control inclues steam flow as a fee forwar element Two element control system has two variables, rum level an steam flow to manipulate the fee water control valve. This strategy compensate the imbalance between fee water mass flow an steam mass flow out into the rum. It is aeuate for loa changes of moerate spee an magnitue. It has some rawbacks i.e. it can use on a single boiler with a single fee water pump using a constant fee water pressure, it cannot ajust for pressure or loa isturbances in the fee water system an it cannot eliminate phasing interaction between various portions of the process. III. PROBLEM ORMULATION A boiler of a thermal power plant is taken an the rum level control of the boiler is achieve using conventional PID controller an compare with MPC. The comparison of both the controller performance is analyze IV.MATHEMATICAL MODELLING O BOILER DRUM Mass an energy conservation euations are written for the rum in its entirety. The entire contents of the rum are assume to be in the saturation state. A. Mass an Energy Balance The boiler rum level l respons in an integrating manner to the steam flow an fee water flow ew. Basically, the rum moel escribes the ynamics of rum water volume Vw in terms of the mass an energy conversion euations. A separate relationship between Vw an l is erive then from the geometry of the rum an this euation is solve using the Newton-Raphson metho to obtain l. The mass an energy balance euations for the boiler rum are erive as follows : Mass Balance: Rate of change of mass of steam an water in the rum = ee water flow to the rum steam flow from the rum t V V V (1) w w w ew ig 2: Two Element Control C. Three Element Control Three element control strategy is the commonly use metho to control the rum water level. This metho is ieally suite for a boiler plant, which consists of multiple boilers with multiple fee water pumps. By using cascae control mechanism, level element act as primary loop, flow element acts as the seconary loop an steam flow acts as the fee forwar controller. This strategy attempts to compensate the changes or isturbance in steam flow an fee water flow. Energy Balance: Rate of change of energy of steam an water in the rum = Total energy of steam an water mixture after leaving the water walls - Energy of rum water - Energy of rum steam t V V h V h h h h (2) w w w w D w D ew w D The ifferential euations obtaine are t V t w w ew w X 1 3 ew w where X is the steam uality. D w X The rum level l is compute using Newton-Raphson Metho. [3] D (3) (4) ig3: Three Element Control l f l k k 1 l k (5) f ' l k Copyright to IARJSET DOI /IARJSET 140

3 International Avance Research Journal in Science, Engineering an Technology National Conference on Emerging Trens in Engineering an Technology (NCETET 16) Loures Matha College of Science & Technology, Thiruvananthapuram V. BOILER DRUM LEVEL MODEL WITH CONVENTIONAL CONTROL SYSTEM ig4: ee Water low Control System Most of the meium an high pressure boilers toay use a 3-element boiler control strategy. The term 3-element control refers to the three process variables(pv s) that are measure to effect control of the boiler fee water control valve. These measure PVs are: -Liui level in the boiler rum -low of fee water to the boiler rum -low of steam leaving the boiler rum ig 4 shows a cascae control for 3-element boiler rum level control. The fee controller will immeiately sense any variations in the supply conitions which prouce a change in fee water flow. The flow controller will ajust the boiler fee water valve position to restore the flow to its set point before the boiler rum liui level is even affecte. The level controller is the primary controller also known as master controller in this cascae, ajusting the set point of the flow controller, which is the seconary controller, referre as slave controller. The thir element is the flow of steam leaving the steam rum. The variation in eman from the steam heaer is the most common isturbance to the boiler level control system in an inustrial steam system. The boiler level control scheme o not fee the steam flow signal irectly. Instea, the ifference between the outlet steam flow an the inlet water flow is calculate. The ifference value is irectly ae to the set point signal to the fee water flow controller. Therefore, if the steam flow out of the boiler is suenly increase by the startup of a turbine, for example, the set point to the fee water flow controller is increase by exactly the amount of the measure steam flow increase. VI. MODEL DEVELOPMENT AND SIMULATION USING MATLAB / SIMULINK The ifferential euations (3) an (4) are solve an simulation stuies are carrie out with the help of the nominal values of the boiler rum variables an parameters collecte from the 210 MW thermal power plant boiler uring the stuy. The ata pertaining to these rum variables an parameters are represente in Table1. Open loop step response tests are conucte on the moel in orer to etermine its suitability to control application. Table 1 Nominal values of Boiler Drum variables Variables Description Value Unit Density of rum steam x 10 Kg/cm3 w Density of water in rum x 10 Kg/sec ew eewater flow Kcal/kg hew Enthalpy of feewater Kg/sec Steam flow from rum Kg/sec l Drum level Cm p Steam pressure in the rum Kg/cm2 T Saturate steam temperature 354 o C X Steam uality h Graient of rum steam enthalpy Kcal-cm3/Kg2 hw Graient of water enthalpy at rum Conitions Kcal-cm3 /Kg2 Graient of rum steam ensity w V Volume of rum cm Vw Drum water volume cm l Length of rum 1500 cm r Raius of rum 88.9 cm Copyright to IARJSET DOI /IARJSET 141

4 International Avance Research Journal in Science, Engineering an Technology National Conference on Emerging Trens in Engineering an Technology (NCETET 16) Loures Matha College of Science & Technology, Thiruvananthapuram Input, output an state variables of the boiler rum moel is represente in igure 5 ew BOILER DRUM MODEL l Vw p ig 5: Block iagram of boiler rum moel screen an on a black-an-white harcopy ig 8: Regulation of boiler rum level using PID controller Using the escribe controller esign, boiler rum water level control of boiler rum is implemente using SIMULINK. We get the output response shown in fig 7, an fig.8. rom the response of PID Control, it is foun out that the settling time an overshoot is high. To improve the response of PID controller, here we use an avance controller MPC. ig 6: Boiler Drum Moel Simulink VII. SIMULATION RESULTS The Boiler Drum water level control system is moele in this paper, an is simulate with conventional PID controller an an avance control techniue such as MPC using MATLAB/SIMULINK environment. The simulations are carrie out with the nominal values of a 210MW thermal power plant. In both cases fee water flow an steam flow is consiere an the corresponing variations in rum level an rum water volume is analyze. ig:9 Regulation of boiler rum rum water volume using MPC ig:10 Regulation of boiler rum level using MPC ig 7: Regulation of boiler rum water volume using PID controller Above fig 9 an ig 8 shows the simulation of boiler rum level an volume using MPC. A better response is obtaine with reuce settling time an overshoot compare to PID controller. Copyright to IARJSET DOI /IARJSET 142

5 International Avance Research Journal in Science, Engineering an Technology National Conference on Emerging Trens in Engineering an Technology (NCETET 16) Loures Matha College of Science & Technology, Thiruvananthapuram VIII. CONCLUSION In this paper, PID controller an MPC are esigne for the regulation of boiler rum level. This work basically evaluate the performance of PID an MPC for boiler rum level control system. The simulation result shows the performance of propose control system an the comparison between both the control strategies are stuie. rom Simulation response the performance of MPC is more accurate an better than PID controller. With MPC a faster settling time is obtaine without any overshoot. REERENCES [1] Damian lynn: Thermal Power Plant Simulation an control, The Institution of Electrical Engineers (2003). [2] Rominus Valsalam S: Moelling, simulation an aaptive optimal control of a boiler, PhD thesis (2003) [3] Davi, A. C: An introuction to genetic algorithms for scientists an engineers, Worl Scientific Publishing Co. Pte. Lt (1999). [4] Wei, J. L. Wang, J. H. an Wu, Q. H.: Coal mill moelling using evolutionary computation techniue an its on-line implementation, Control 2004,University of Bath U.K,ID-080 (2004). [5] Astrom K.J an Bell R.D: Drum boiler Dynamics, Automatica, No.36, pp , (2000) [6] lymn ME an Malley M.J.O: A rum boiler moel for long term power system ynamic simulation, IEEE transactions on Power Systems, Vol 14, No.1 (1999) Copyright to IARJSET DOI /IARJSET 143