Kazanların incelenmesi ve kontrol tipleri

Abstract

Kazan verimi kazanda suya transfer edilebilen ısının yakılan yakıtın ısıl değerine oranı olarak tarif edilmektedir. Kazan verimi birçok etkene bağlıdır ve bu etkenlerin değişimine bağlı olarak farklılık gösterir. Kazanlarda optimizasyonun amacı kazan yükü, yakıt, çevre ve kazan şartları değiştiğinde kazan verimini maksimumda tutmaktır. Gerçek kazan verimi genellikle % 65-90 arasında değişir. 45 ton/saat buhar üretim kapasiteli bir kazanın veriminde % 1'lik bir kayıp yıllık üretim maliyetlerinde yaklaşık 20.000 $'lık bir artışa neden olur. Yukarıda verilen bigiler ışığında günümüzde sanayide karlılığı belirleyen en önemli etkenlerden teknolojik gelişmişliğin yanında enerji tasarrufu yapmak amacı gözönüne alınarak bu tez kapsamında sanayi'de geniş bir uygulama alanına sahip buhar kazanlarının optimizasyonu için gerekli kontrol tipleri örnekleriyle açıklanmış; genel olarak buhar kazanı tipleri ve bunların çalışma şekilleri anlatılmıştır. Bir buhar kazanının maksimum verimde çalıştırılabilmesi için vazgeçilmez kontrol tipi olan hava fazlalığı ve yakıt kontrolü hesaplan bilgisayar akış şeması ile birlikte verilmiş, hesaplamalarda kazandaki buhar üretimine bağlı olarak ihtiyaç duyulan yakıt ve hava miktarının hesaplanması, ve hesaplanan değere bağlı olarak bir kazan kontrol sistemi detaylarıyla açıklanmıştır.

The efficiency "of a steam generator is defined as the ratio of the heat transferred to the water (steam), to the higher heating value of the fuel. The purpose of the optimization is to continuously maximize boiler efficiency, boiler variations occur in the load, in the fuel, and in the ambiend and boiler conditions. Boiler efficiency is influenced by many factors. A fully loaded large boiler that is clean and properly tuned (with blow down loses, and pump and fan operating costs disregarded) is expected to have the following efficiencies 88% on coal with 4% excess oxygen. 87% on oil with 3% excess oxygen. 82% on gas with 1.5% excess oxygen. Actual boiler efficiency seldom exceeds 90% or drops below 65%. Efficiency will tend to vary with individual design and with loading. Efficiency will also vary as a function of excess air, fluegas temperature and boiler maintenance. A 1% loss in efficiency on a boiler (45.000 kg/h) will increase its yearly operating cost by about $ 20.000. Such a 1% efficiency loss can result from a 2% increase in excess oxygen. Because of the above information the subject of this thesis is chosen about boiler optimization. Boiler types, their operation systems and the control types that are needed for boiler optimization are explained with their models; boiler types and their operation systems are also explained. İn the other section of this thesis an example is given about the excess air and fuel control that is essential to keep boiler efficiency at max level. IX Instruments and controls are an essential part of all steam generating installations. They serve to assure safe, economic, and reliable operation of the equipment. They range from the simplest manual devices to the complete automatic control of the boiler, or nuclear steam supply system, and all associated apparatus. Various types of boiler control systems for fossil-fuel boilers include: 1- Boiler instrumentation systems. 2- Combustion control systems. 3- Steam temperature control for superheated and reheater outlet. 4- Drum level control. 5- Burner sequence control systems. 6- Once through control systems. 7- Data processing and display. 8- Plant automation. Basic Boiler Control Theory Boiler control is the regulating of the boiler outlet conditions of steam flow, pressure, and temperature to their desired values. In control terminology, the boiler outlet steam conditions are called the output or controlled variables, and the desired values of the outlet conditions are the set points or input- demand signals. The quantities of fuel, air, and water are adjusted to obtain the desired outlet steam conditions and are called the manipulated or controlled variables. The boiler is referred to as the system, plant, or process, and disruptive influences on the boiler, both internal and external, such as variations in heat content of fuel or cycle efficiency are the disturbance inputs. The controller or control system has the function of "looking at" the desired (set points) and actual values (output variable) of the output steam conditions and adjusting amounts of fuel, air, and water (manipulated variables) to make the outlet conditions match their desired values. The controller can be manuel with an operator making the adjustments or it can be automatic with a pneumatic or electronic analog computer or a digital computer making the adjustments. While it is theoretically possible to operate a boiler satisfactorily with manuel control, the operator must maintain a tedious constant watch for the occurrence of a disturbance. Time is required for the boiler to respond to a correction, and this can lead to over correction with further " upset "to the boiler. An automatic controller, on the other hand, does not experience tedium and, once properly adjusted, will always make the proper adjustment to reduce the upsets to the boiler and, therefore, will control the system more accurately and reliably. Open -Loop Control The simplest control mode is the open - loop, feedforward, or non - feedback control, where the manipulated variables of fuel, air, and water are adjusted only from the input - demand signals without monitoring the outlet conditions or output variables. As an example, Figure 3.3 illustrates an open- loop control system to accomplish the control function illustrate by Fig. 3.2. Fig. 3a is a block diagram, representing the action to be taken, which is "feedforward" only. Fig 3b is a calibration curve, expressing fuel-valve position as f(d), a function of steam demand. This calibration curve, established by manually determining the fuel-valve position required to obtain the desired steam flow with a constant steam pressure, is entered the controller. The response of this open-loop control is very fast and depends only on the accuracy of the calibration curve. Close-loop control If the system requirements cannot be met by open-loop control, then a closed-loop or feedback control must be used. In closed-loop control mode, the actual output of the system is measured and compared to the input demand signal with the difference between the signals (the error signal) used to reduce the difference between the demand and output signals to zero. Proportional Control The simplest type of closed-loop system is proportional control where the manipulated variable or controller output is proportional to the controlled variable from its desired or setpoint value. The deviation of the controlled variable from its set point is called the error signal. Depending on the arrangement of controller, the output signal of a proportional control system will always be either directly or inversely proportional to the controlled variable. There may be a certain amount of cycling or hunting, depending on the proportional gain of the controller, before the steam pressure stabilizes. It can be noted in Fig 3.6 that the steam pressure does not stabilize at its set point, but is offset to a value below the set point. A characteristic of "proportional only" control is that an error or offset is necessary to provide a steady-state fuel valve opening which will support the desired load, except for a single load condition. Integral or Reset Control This offset may be eliminated by the addition of the integral or reset mode of control to the proportional control system, as illustrated in Fig 3.7. The response of the hypothetical boiler to a step increase in load when using a proportional-plus-integral control system is shown in Fig 3.8. The steam pressure is returned to its set point without the offset that is present the proportional-only control. However, the system may be less stable as it takes a longer period for the steam pressure to stabilize with the offset eliminated. Derivative Control The stability and response of the system can be improved still further by XI adding a third mode of control action to the controller called derivative or rate control. Derivative depends on change of the controlled variable from its set point, as shown Fig 3.9. The addition of the derivative control mode to the controller-boiler is shown in Fig 3.10. As soon as the step change is made, the pressure starts to drop and the proportional mode begins to open the fuel valve. The derivative mode will also open the fuel valve further as a function of the rate at which the pressure is changing, providing anticipation of where the fuel valve should be positioned. When the rate of change of the steam pressure decreases, The derivative control has less effect and the proportional and integral modes do the final positioning of the fuel valve. Feedforward-feedback Control In a closed-loop control system, the controlled variable always has to deviate from its set point before any corrective action is initiated by the controller. In this respect the open-loop system has a faster response since it takes corrective action before the controlled variable starts to change. When the open-loop or feedforward system is combined with the close-loop or feedback system, the result is a system with fast responsethat is able to compansate for changes in the calibration curve. Feedwater Control Systems The purpose of the feedwater control is to regulate the flow of water to drum-type boiler so as to maintain the level in the boiler drum between the desired limits. The control system will vary with the type and capacity of the boiler as well as the charecteristic of the load. Most shop-assempled boilers in the lower capacity range are equipped with self-contained feedwater control systemsof the thermo-hydrolic or thermostatic types. The thermo-hydrolic type is generally applied to the boilers having an operating pressure in the range between 60 and 600 psi and capacities not exceeting 75.000 to 100.000 Ib/hr under steady load conditions. Combustion Control Systems The function control system is to control the fuel and air input or firing rate to the furnace in response to a load index representing a demand for the level of fuel input. The demend for firing rate is, therefore, a demand for energy input to inthe system to match a withdrawal of energy at same point in the cycle through increased steam flow and, in turn increased power generation in an electric generating plant. For boiler operation and control systems, variations in the boiler outlet pressure are often used as an index of an unbalance between fuel energy input and energy with drawal in the output steam. Fuel Management Finally a degree of fuel system automation can be achieved that will permit fuel equipment to be placed in service without supervision by the xii operator. A fuel-management system can be applied that will recognize the level of fuel demand to the boiler, will know the operating range of fuel equipment in service, will reach a decition concerning the need for starting up or shuttingdown the next increment of fuel equipment, and will select the next increment based on the firing pattern of burners in service. Such demands for start-up or shutdown of fuel preperation and burning equipment can be initiated by the management system without the immediate knowledge of the operator and, in fact, his attention may be deverted from the firing and fuel properation equipment at this time.