IntroductionAbout Kapco:KAPCO(Kot Addu Power Company limited) is Pakistan’s largest independent power producer (IPP) with a name plate capacity of 1600 MW. In April 1996, KAPCO was incorporated as a public limited company under the companies’ ordinance, 1984 with the objective acquiring the power plant from WAPDA. The power plant is situated in District Muzaffargarh, Punjab, and 90KM north west of Multan of the left bank of the Indus River by a distance of 16KM from Taunsa Barrage. The area is surrounded by agricultural land on the north and West Side of Kot Addu. The principal activities of KAPCO include ownership, operation, and maintenance of the power plant. Share Holders:On June 27, 1996, following international competitive biding by the privatisation commission Government of Pakistan. (The ―Privatisation Commission‖ ), the management of KAPCO was transferred to National Power (now International Power ) of the United Kingdom, which acting through its subsidiary National Power (KOT ADDU) Limited (―NPKAL‖), bought shares representing a 26% stake in KAPCO. Later, NPKAL bought a further 10% share holding in KAPCO, increasing its total share holding to 36%. The other majority shareholder in KAPCO is WAPDA with present share holding of 46%. Following the successful completion of the offer for sale by the Privatisation Commission (on behalf of WAPDA) in February 2005, 18% of KAPCO’s share holding is now held by General Public. On April 18, 2005, KAPCO was formally listed on all three stock exchanges of Pakistan. VISION AND MISSION:Vision Statement To be a leading power generation company, driven to exceed our shareholder’s expectations and meet our customer’s requirements. Mission Statement To be a responsible corporate citizen. To maximize shareholder’s return. To provide reliable and economical power for our customer. 1 Plant Management:To run the power plant in an efficient way the whole organization is divided into following main departments. I&C Block 2 Department:Hierarchy Senior Manager Principal engineer Instrument engineer Instrument engineer Foreman GT Foreman STG Test Inspector Technician Test Inspector Technician 2 Plant Overview Kot Addu power plant comprises of ten multifuel-fired gas turbine and 5 steam turbines installed in 5 phases between 1985 and 1996. These turbines are divided into 3 energy blocks with each block having a combination of gas and steam turbines. The power plants combined cycle technology enables KAPCO to use the waste heat from the gas turbine exhaust to produce steam in the heat recovery steam generator which in turn is used to run the steam turbines thereby resulting in fuel cost, efficiency and minimum wastage. The power plant is a multifuel gas turbine power plant with the capacity of using three different fuels to generate electricity, namely: natural gas, low sulphur furnace oil and high speed diesel to generate electricity. The power plant is also the only major plant in Pakistan with the ability to self-start in case of a countrywide blackout. Plant Distribution:- Block 1 GT 1,3 STG 9 GT 2,4 STG 10 Block 2 GT 5,6 STG 11 GT 7,8 STG 12 Block 3 GT 13,14 STG 15 Block 1:This block comprises of four Gas Turbines (GT1, GT2, GT3, GT4) and two Steam Turbines (STG 9, STG10). GT 1 &2:Manufacturer......................................... Siemens (Germany) Model........................................................………..V 94.2 3 Starting Device.................................................... Generator runs as motor initially Starting time up to 3000RPM..............................4 minutes Turbine stages..........................................4 Flue gas mass flow...................................426 Kg/sec Control....................................................Iskamatic Capacity (MW) IDC...............................95MW GT 3&4:Manufacturer............................. FIAT M/s GIE(Italy) Model......................................................TG-50 Starting Device.......................................11KV_1915kW Starting time up to 3000RPM..............25 minutes Turbine stages.........................................4 Flue gas mass flow..................................322 Kg/sec Control....................................................conventional relay type; FIAT HI Tech Capacity (MW) IDC...............................82MW STG 9&10:Manufacturer.........................................ABB, Germany Model......................................................DK2056 Rated Power...........................................112.2 MW Vacuum (Bar)........................................0.091 HP Steam Inlet Press/Temp..................47.9bar/495°C LP Steam Inlet Press/Temp..................3.99bar/190.6°C Capacity (MW) IDC...............................87MW (STG 9) Capacity (MW) IDC...............................97MW (STG 10) Block 2:This block comprises of four Gas Turbines (GT5, GT6, GT7, GT8) and two Steam Turbines (STG 11, STG12). GT 5 -8:Manufacturer...............................ALSTOM France 4 Model...................................................... MS 9001 E Starting Device......................................... 6.6kV-1000kW Starting time upto 3000RPM..................10 minutes Turbine stages...........................................3 Flue gas mass flow...................................406 Kg/sec Control....................................................Mark four Speedtronic Capacity (MW) IDC...............................79MW (GT5) Capacity (MW) IDC...............................82MW (GT6) Capacity (MW) IDC...............................77MW (GT 7) Capacity (MW) IDC...............................79MW (GT8) STG 11&12:Manufacturer......................................... RATEAU, France Model...................................................... VEGA209 110B Rated Power...........................................103.4 MW Vacuum (Bar)........................................0.091 HP Steam Inlet Press/Temp..................40bar/510°C Capacity (MW) IDC...............................76MW (STG11) Capacity (MW) IDC...............................82MW (STG12) Block 3:This block comprises of two Gas Turbines (GT13-14) and one Steam Turbines (STG15). GT 13-14:Manufacturer...............................Siemens (Germany) Model...................................................... V-94.2 Starting Device.........................................Generator runs as motor Starting time up to 3000RP....................4 minutes Turbine stages...........................................4 Flue gas mass flow...................................471 Kg/sec Control....................................................TELEPERM Capacity (MW) IDC...............................106 MW 5 STG 15:Manufacturer......................................... Siemens, Germany Model........................................................030-16, N30-2X5-B-9 Rated Power...........................................148.6 MW Vacuum (Bar)........................................0.091 HP Steam Inlet Press/Temp....................57bar/528°C LP Steam Inlet Press/Temp....................5.78bar/221°C Capacity (MW) IDC...............................120MW Combined Cycle Power plant:The combined cycle power plant consists of two gas turbine-generator units, a steam turbine – generator complete with a condenser and condensate/ feed water system and all required auxiliaries. Two gas turbines that drive their own generators exhaust into a special boiler called a Heat Recovery Steam Generator (HRSG) that generates steam for use is Steam Turbine. One of the principal reasons for the popularity of combined cycle power plant is their high thermal efficiency. Combined cycle plant with thermal efficiencies as high as 52% have been built. Combined cycle plants can achieve these efficiencies because much of the heat from the gas turbines is captured and used in the Rankine cycle portion of plant. The heat from the exhaust gases would normally be lost to the atmosphere in an open cycle gas turbine. Another reason for the popularity of combined cycle plant is that it requires less time for their construction as compared to a conventional steam power plant of same output. Although it takes longer time to built the combined cycle plant than a simple gas turbine plant. Thermodynamic Cycles: Gas turbine Cycle..................................Joule-Brayton cycle Steam-water Cycle................................Rankine cycle 6 Joule-Brayton Cycle:It consists of four processes Compression of Air(Work is done on the air by the compressor, the air stores this energy in the form of temperature and pressure) Addition of heat at constant Pressure(by burning of fuel) Expansion (energy of hot pressurized gas is used to perform work) Cooling of hot gases that exhaust to the atmosphere. Compression Cooling Heat addition Expansion Brayton Cycle Rankine cycle:A simple rankine cycle of conventional steam power plant consists of five processes. Increase in pressure of condensate from condenser(Boiler-feed pumps) Addition of heat to convert water into steam at constant Pressure(Boiler) Additional energy is added to steam(Super heater) Steam is expanded and cooled as it passes through turbine(energy is used to perform work) Condensation of steam that exhausts from the turbine 7 Boiler feed pumps condensor Boiler Steam turbine Superheater Process control system & Instrumentation Why Automatic control? The need for automatic control system has become steadily more and more important in the modern power plant. It is quite impossible to build and operate the large power plants safely without automatic control system. It is fairly clear that the control of the bigger power plants is too much complicated and cannot be left solely in the hands of operator. It is therefore obvious that one of the main objectives of the process instrumentation is the automatic control of measured process variable (temperature, flow, level, pressure etc). Process control system:It is defined as the function and operation necessary to change a material either physically or chemically to maintain a process. Control can be discrete or analog, manual or automatic, and periodic or continuous The automatic control systems have four basic elements. They are primary element, the measuring element, the controlling element and the final control element. The brief description of all these elements is given here under Primary elements This is device that detects the change in the measured variable. For example the thermocouple, Borden tube, flow orifice etc. are considered as primary elements. 8 Measuring Element This is the device that receives the signal from primary element. It measures the amount of measured variable that has drifted from the set point. It receives the signal from sensing element and sends it to controller. For example the pressure transmitter, pneumatic transmitter, thermocouple transducer and level transmitter can be considered as measuring element. Controlling Elements The controlling element (controller) uses changes in the value of measured variable to alter the mechanical, pneumatic or electrical source of power. The controlling element can actuate the source of power, increase or decrease its output, or turn it off, depending upon its setting. Final Control Element The final control element is a device that varies the energy supplied to the process. This energy is manipulated variable so that the value of the measured or controlled variable is maintained within the desired range e.g. control valve, actuators etc. Control system can be classified as open loop or closed loop control system. Open loop control system:The process is controlled by inputting to the controller the desired set point believed necessary to achieve the ideal operating point for the process and accepting whatever the output results. Since the only input to the controller is the set point, it is apparent that an open loop system controls the process blindly. That is, the controller receives no information concerning the present status of the process or the need for any corrective action. The effect of the disturbance is never seen in the process output. Open loop control systems are considerably cheaper and less complex than their closed loop counterparts. Set point Controller Output signal conditioning Output actuator disturbance Process Feed-back control system:A closed loop control system is one in which the output of a process affects the input. The system measures the actual output of the process and compares it to the desired output. Adjustments are made continuously by the control system until the difference between the desired and actual output is as small as practical. Disturbance signals are all those signals, which 9 influence the control value in an unwanted way. Feed back control system looks to these disturbances and control the process according to given set points. Set point is the input that determines the desired operating point for the process. It is normally provided by a human operator, altough it may also be supplied by another electronic circuit. Process variable is the signal that contains the information about the current process status. Error amplifier determines whether the process operation matches the set point. The magnitude and polarity of the error signal will determine how the process will be brought back under control. Error amplifier Set point + Input signal conditioning Input sensors Controller Output actuator Process disturbance Process variable signal Basic Process control system Block diagram:In short, the major components of process control system are Sensors Signal conditioning Operator-machine interface Controller Actuators Operator-machine interface External sensor External actuators Input signal conditioning Controller Output signal conditioning Process sensor Controlled process Process actuators 10 Sensors: A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. Sensors convert physical phenomena to measurable signals, typically voltages or currents Sensors are also called transducers. This is because they convert input physical phenomena to an electrical output. Provides input from the process and from the external environment. There are many types of sensors used in power industry. In KAPCO, following parameter sensors are extensively used. Temperature sensors Pressure sensors Level sensors Flow sensors For gas turbine and generator protection, following sensors are used. Speed sensors Vibration sensors Proximity sensors Flame detectors Temperature sensors:There are four basic types of temperature sensors. Thermocouple Resistance temperature detector (RTD) Thermistor pyrometers Thermocouples:Construction:A thermocouple is constructed of two dissimilar metal wires joined at one end. When one end of each wire is connected to a measuring instrument, the thermocouple becomes a sensitive and highly accurate measuring device. Thermocouples may be constructed of several different combinations of materials. 11 Basic Principle:―If a temperature gradient is present in electric conductor, the heat flow will create the movement of electrons and an electromotive force will be generated in that region. Direction of emf will be dependent on the magnitude and direction of temperature gradient and material forming the conductor.‖ This is called thermoelectric effect. Operation:Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit. Heating the measuring junction of the thermocouple produces a voltage which is greater than the voltage across the reference junction. The difference between the two voltages is proportional to the difference in two temperatures and can be measured on the voltmeter (In milli volts). Types of thermocouples:In KAPCO,K types are extensively used. Type Temperature Range K T J E 0 to 1100 -185 to 300 20 to 700 0 to 800 Advantages: They have wide temperature range. They are self powered. Disadvantages: Voltage-temperature graph nonlinear Low voltage Reference required 12 Resistance Temperature detector:The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases Decrease in resistance as temperature decreases. RTDs act somewhat like an electrical transducer, converting changes in temperature to voltage signals by the measurement of resistance. RTD elements are normally constructed of platinum, copper, or nickel. These metals are best suited for RTD applications because of their linear resistance temperature characteristics, their high coefficient of resistance, and their ability to withstand repeated temperature cycles. In KAPCO, RTD are installed on thermal oil heaters in the forwarding skid. Construction:RTD elements are usually long, springlike platinum wires surrounded by an insulator and enclosed in a sheath of metal. The insulator prevents a short circuit between the wire and the metal sheath. The change in temperature will cause the platinum wire to heat or cool, resulting in a proportional change in resistance. RTD use protective well. The well protects the RTD from damage by the gas or liquid being measured. Protecting wells are normally made of stainless steel, carbon steel or cast iron, and they are used for temperatures up to 1100°C. Advantages: More linear than thermocouples More stable Disadvantages Expensive Power supply required Small change in resistance Self heating 13 Thermistor:It is a thermally sensitive resistor that usually has a negative temperature co efficient. As Temperature increases, its resistance decreases Temperature decreases, its resistance increases Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistor typically achieve a higher precision within a limited temperature range [usually -90C to 130C]. They are not extensively used in KAPCO. Advantages: Fast response High output Disadvantages: Nonlinear Limited temperature range Power supply required Pyrometers:A pyrometer is a non-contacting device that intercepts and measures thermal radiation. This device can be used to determine the temperature of an object's surface. There is no need for direct contact between the pyrometer and the object, as there is with thermocouple and Resistance temperature detector (RTDs). Pyrometers are suited especially to the measurement of moving objects or any surfaces that cannot be reached or cannot be touched. Temperature detection circuit:The bridge circuit is used whenever extremely accurate resistance measurements are required (such as RTD measurements). The basic bridge circuit consists of: Two known resistors (R1 and R2) that are used for rationing the adjustable and known resistances One known variable resistor (R3) that is used to match the unknown variable resistor. In balanced bridge configuration, we have to change the resistance of the R3 in such a way that ammeter shows zero current. This change in resistance is used to measure temperature. In unbalanced bridge the value of R3 is fixed and the amount of current flowing through a balanced circuit is calibrated according to temperature change. 14 One unknown resistor (Rx) that is used to measure temperature A sensing ammeter that indicates the current flow through the bridge circuit Pressure sensors:Many processes are controlled by measuring pressure. Pressure is defined as a force per unit area, and can be measured in units such as Psi (pounds per square inch), millimeters of mercury, Pascal (Newton per square meter) or bar. In industry Bar is normally common. Gauge pressure:It is equal to absolute pressure minus atmospheric pressure. Gauge pressure is measured relative to prevailing atmospheric pressure (approximately 14.7 psi).Hence, if we check a pressure gauge and it says 30 psi, the pressure being measured is 30 psi above atmospheric pressure. Absolute pressure:It is equal to gauge pressure plus atmospheric pressure. If one completely evacuates from a container, then it has an absolute pressure of 0 (zero) psi (or any other unit of pressure). So, in the previous example of 30 psi gauge pressure, the absolute pressure would be about 47.7 psi. Differential pressure:Differential pressure is the difference in pressure between two points. 15 There are numerous types of pressure sensors. Some of the pressure sensors are Bellow type sensors Bourdon tube type sensors Bellow type sensors:The need for a pressure sensing element that was extremely sensitive to low pressures and provided power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressure sensing element. Pressure range:The metallic bellows is most accurate when measuring pressures from 0.5 to 75 psig. However, when used in conjunction with a heavy range spring, some bellows can be used to measure pressures of over 1000 psig. Construction:The bellows is a one-piece, collapsible metallic unit that has deep folds formed from very thin-walled tubing. The elastic elements in bellows gauges are made of brass, phosphor bronze, stainless steel, beryllium-copper, or other metal that is suitable for the intended purpose of the gauge. The diameter of the bellows ranges from 0.5 to 12 in. and may have as many as 24 folds. System pressure is applied to the internal volume of the bellows. As the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows is connected to a mechanical linkage assembly. As the bellows and linkage assembly moves, either an electrical signal is generated or a direct pressure indication is provided. Bourdon tube type sensors:The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. In KAPCO, temperature controllers have bourdon type tube in it. Mostly pressure sensors in KAPCO are of Bourdon type. 16 Construction and operation:The bourdon tube consists of a thin-walled tube that is flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal to indicate the value of the applied internal pressure. In short, in a bourdon tube type sensor System pressure is applied to the inside of a slightly flattened arc shaped tube. As pressure increases, the tube tends to restore to its original round cross-section. This change in cross-section causes the tube to straighten. Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. The tip movement can then be used to position a pointer or to develop an electrical signal. 17 Pressure detection circuitry:Any of the pressure detectors previously discussed can be joined to an electrical device to form a pressure transducer. Following detection circuitry is used. Resistance type transducer Inductance type transducer Capacitive type transducer Resistance type transducer:1. First method:In this method strain gauge is used. A strain gauge measures the external force (pressure) applied to a fine wire. The fine wire is usually arranged in the form of a grid. The pressure change causes a resistance change due to the distortion of the wire. The value of the pressure can be found by measuring the change in resistance of the wire grid. 𝐿 𝑅 = 𝑘 𝐴 R = resistance of the wire grid in ohms K = resistivity constant for the particular type of wire grid L = length of wire grid A = cross sectional area of wire grid As the wire grid is distorted by elastic deformation, its length is increased, and its cross-sectional area decreases. These changes cause an increase in the resistance of the wire of the strain gauge. This change in resistance is used as the variable resistance in a bridge circuit that provides an electrical signal for indication of pressure. An increase in pressure at the inlet of the bellows causes the bellows to expand. The expansion of the bellows moves a flexible beam to which a strain gauge has been attached. The movement of the beam causes the resistance of the strain gauge to change. The temperature compensating gauge compensates for the heat produced by current flowing through the fine wire of the strain gauge. 18 2. Second method:Other resistance-type transducers combine a bellows or a bourdon tube with a variable resistor. As pressure changes, the bellows will either expand or contract. This expansion and contraction causes the attached slider to move along the slide wire, increasing or decreasing the resistance, and thereby indicating an increase or decrease in pressure. Inductive type transducer:The inductance-type transducer consists of three parts: a coil, a movable magnetic core, and a pressure sensing element. The element is attached to the core, and, as pressure varies, the element causes the core to move inside the coil. An AC voltage is applied to the coil, and, as the core moves, the inductance of the coil changes. The current through the coil will increase as the inductance decreases according this formula (𝑓𝑙𝑢𝑥) 𝐿 = 𝑁 𝑖 Another type of inductance transducer, utilizes two coils wound on a single tube and is commonly referred to as a Linear Variable Differential Transformer. Operation of Differential transformer:The primary coil is wound around the center of the tube. The secondary coil is divided with one half wound around each end of the tube. Each end is wound in the opposite direction, which causes the voltages induced to oppose one another. A core, positioned by a pressure element, is movable within the tube. When the core is in the lower position, the lower half of the secondary coil provides the output. When the core is in the upper position, the upper half of the secondary coil provides the output. The magnitude and direction of the output depends on the amount the core is displaced from its center position. When the core is in the mid-position, there is no secondary output. The excitation for an LVDT varies. Typical values range from 50HZ 19 to 30 kHz. If the transducer must accurately track rapidly changing displacement, the higher frequencies are advantageous. The voltage applied to the primary is usually around 10V. Displacement of as little as 50 inches is detected by LVDT’s. Capacitive type transducer:In KAPCO, few pressure sensors have capacitive type transducer. Capacitive-type transducers consist of two flexible conductive plates and a dielectric. The Dielectric used in KAPCO is fluid. As pressure increases, the flexible conductive plates will move farther apart, changing the capacitance of the transducer. This change in capacitance is measurable and is proportional to the change in pressure. Piezoelectric When a crystal undergoes strain it displaces a small amount of charge. In other words, when the distance between atoms in the crystal lattice changes some electrons are forced out or drawn in. This also changes the capacitance of the crystal. This is known as piezoelectric effect. The charge generated is a function of the force applied and the strain in the material. When using piezoelectric sensors charge amplifiers are needed to convert the small amount of charge to a larger voltage. Its current force relationship is 𝑑𝐹 𝐼 = 𝜀𝑔 𝑑𝑇 Where g is material constant and e is dielectric constant. This is not commonly used in KAPCO. Level sensors:Level sensors detect the level of liquid in the tank. The level measurement can be either Continuous values Point values. Continuous level sensors:It measure level within a specified range and determine the exact amount of substance in a certain place Point level sensors:Point-level sensors only indicate whether the substance is above or below the sensing point. Generally it detects levels that are excessively high or low. Following level sensors are used in KAPCO. Gauge glass Ball float Chain float Magnetic bond 20 Bubbler gauge Conductive probes Differential pressure Gauge glass:In the gauge glass method, a transparent tube is attached to the bottom and top (top connection not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in the tube will be equal to the height of water in the tank. The gauge glasses in the form of ―U" tube manometer where the liquid seeks its own level due to the pressure of the liquid in the vessel is commonly used in KAPCO. Gauge glasses made from tubular glass or plastic are used for service up to 450 psig and 400°F. If it is desired to measure the level of a vessel at higher temperatures and pressures, a different type of gauge glass is used. Refraction gauge glass is used in the dark areas. Lights are usually used in refraction gauge. Operation is based on the principle that the bending of light, or refraction, will be different as light passes through various medium. Light is bent, or refracted, to a greater extent in water than in steam. The portion of the gauge containing water appears green; the portion of the gauge from that level upward appears red. This type is used in the region of steam turbine auxiliaries to measure the level of water. Ball float:The ball float method is a direct reading liquid level mechanism. The most practical design for the float is a hollow metal ball or sphere. The design consists of a ball float attached to a rod, which in turn is connected to a rotating shaft which indicates level on a calibrated scale. The ball floats on top of the liquid in the tank. If the liquid level changes, the float will follow and change the position of the pointer attached to the rotating shaft. The travel of the ball float is limited by its design to be within ±30 degrees from the horizontal plane which results in optimum response and performance. The actual level range is determined by the length of the connecting arm. These are mostly used in KAPCO steam turbine unit. Chain float:The operation of the chain float is similar to the ball float except in the method of positioning the pointer and in its connection to the position indication. The float is connected to a rotating element by a chain with a weight attached to the other end to provide a means of keeping the chain taut during changes in level. These are not used much in KAPCO. Magnetic Bond method:The magnetic bond mechanism consists of a magnetic float which rises and falls with changes in level. The float travels outside of a non-magnetic tube which 21 houses an inner magnet connected to a level indicator. When the float rises and falls, the outer magnet will attract the inner magnet, causing the inner magnet to follow the level within the vessel. Conductivity probe method:It is normally used for two types of alarm Low level High level It consists of one or more level detectors, an operating relay, and a controller. When the liquid makes contact with any of the electrodes, an electric current will flow between the electrode and ground. The current energizes a relay which causes the relay contacts to open or close depending on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control valve, or all three. This is used in the tank where the fluid inside is conductive. Bubbler gauge:In this case, liquid level is determined by bubbling air through the liquid. The amount of pressure required to force the air out of the bottom of the dip tube depends on the level of the liquid. Differential pressure level detectors:Differential pressure level detectors are extensively used in KAPCO. They are used in steam drum, feed water tank, lube oil tank level detection etc. Operation:The differential pressure detector method of liquid level measurement uses a differential Pressure detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the DP detector. A differential pressure (D/P) transmitter which consists of a diaphragm with the high pressure (H/P) and low pressure (L/P) inputs on opposite sides. As the differential pressure changes, the diaphragm will move. The transducer changes this mechanical motion into an electrical signal. If tank is not exposed to atmosphere:When the tank is not exposed to atmosphere, then both the sides of differential pressure transmitter are connected to the tank. The high pressure connection is connected to the tank at or below the lower range value to be measured. The low pressure side is connected to a "reference leg" that is connected at or above the upper range value to be measured. The reference leg is pressurized by the gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must be maintained dry so that there is no liquid head pressure 22 on the low pressure side of the transmitter. The high pressure side is exposed to the hydrostatic head of the liquid plus the gas or vapor pressure exerted on the liquid’s surface. The gas or vapor pressure is equally applied to the low and high pressure sides. Therefore, the output of the DP transmitter is directly proportional to the hydrostatic head pressure, that is, the level in the tank. Drawback of differential pressure level measurement:When the liquid density changes with temperature then it will give us a wrong level measurement because of the pressure difference increase or decrease due to density changes. We know that ∆𝑃 = 𝜌𝑔 Where ρ is the density of the fluid The density of steam (or vapor) above the liquid level will have an effect on the weight of the steam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vapor increases, the weight increases and causes an increase in hydrostatic head even though the actual level of the tank has not changed. As the temperature of the fluid in the tank is increased, the density of the fluid decreases. As the fluid’s density decreases, the fluid expands, occupying more volume. If the fluid in the tank changes temperature, and therefore density, some means of density compensation must be incorporated in order to have an accurate indication of tank level. Density compensation is accomplished by using either: Electronic circuitry Instrument calibration Ultrasonic level measurement:An ultrasonic sensor operates by sending sound waves toward the target and measuring the time it takes for the pulses to bounce back. The time taken for this echo to return to the sensor is directly proportional to the distance or height of the object because sound has a constant velocity. The echo signal is electronically converted to a 4-20 mA output. Density factor doesn’t affect the reading of the sensor as in DP level transmitter. These are installed in main fuel oil tanks in KAPCO. They are very accurate as compared to DP level transmitter. Speed sensors:For the over speed protection of the turbine we use speed sensors KAPCO. Normally shaft speed is monitored for operational, control, and protection of the machine. Speed is normally monitored at the turning gear section or at the exciter end of the turbine. Operation:A tooth wheel of magnetic material is mounted on the turbine shaft. Speed pickups are installed in perpendicular to the tooth surface. When a teeth passes through the pick up a pulse signal is generated. There are two types of magnet speed sensors. 23 Induction type sensor Eddy current type sensor Induction type sensor:A magnet is attached to a shaft. As a tooth of wheel passes the sensor (coil). It induces voltage in it and gave pulse each time. The voltage output of the pickup is usually very small and requires little amplification to be measured. Eddy current type sensor:Voltage output of the probe is changed due to the eddy current losses in the teeth, when it passes the sensor. Pulses of output signal from speed sensor is counted in a set time interval by the electronic circuit and is converted into speed signal. As the speed increases, the frequency of the pulses increases. Therefore the number of pulses counted in a fixed time interval increases. 𝑃𝑢𝑙𝑠𝑒 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = 𝑛𝑜 𝑜𝑓 𝑡𝑒𝑒𝑡 ∗ 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑡𝑒 𝑚𝑎𝑐𝑖𝑛𝑒 60 Tachometer generators:Tachometers coupled to motors are commonly used in motor speed control application to provide a feedback voltage to the controller that is proportional to motor speed. A Tachometer normally refers to a small permanent magnet dc generators. When the generator is rotated, it produces a dc voltage directly proportional to speed. Vibration sensors:―Vibration is a response to some form of excitation‖ The free movement of shaft in a journal bearing will cause it to vibrate when a forcing function is applied. In kapco vibration sensors are used to protect the turbine if vibrations are too much. When vibration is out of the range it will shut down the machine. Vibration measurement:o Displacement measurement o Velocity measurement o Acceleration measurement Displacement measurement: Measurement of total movement in relation to a reference point. Sensors used in this type of measurement are Proximity probes. Velocity measurement: Measurement of rate of movement. Units are mm/sec or inch/sec Measurement of distance covered by a sine wave in unity time is velocity measurement. 24 Velometers are used. Acceleration measurement: Rate of change in velocity measurement. Accelerometers are used to measure rate of change of velocity of vibrating body. Velocity sensors:It consists of an electric coil mass suspended by a spring in a field of permanent magnet. The magnet vibrates in direct accordance with the machine case. The inertial mass coil senses the magnet movement and induces electric voltage that is proportional to the machine case velocity. Velocity has amplitude in proportion to the vibration velocity and frequency equal to frequency of vibration. Sensor output= 150mV/inch per sec Proximity sensors:These types of sensors are used to in KAPCO to measure the position of the damper which are BYD (bypass damper) and BID (Boiler inlet damper). They are pilot devices that detect the presence of an object without any physical contact. They are solid state electronic devices that are completely encapsulated to protect against excessive vibration, corrosive agent found in industrial environment. They have high switching rates. There are generally two types of proximity sensors. Inductive proximity sensor Capacitive proximity sensor Inductive Proximity sensors:When energy is supplied, the oscillator operates to generate high frequency field. There must not be any conductive material in high frequency field. When a metal object enters high frequency field, eddy currents are induced in the surface of the target. This result in the loss of energy in the oscillator circuit, this causes smaller amplitude of an oscillator. Detector recognizes a specific change in amplitude and generates a signal that will turn the solid state output turn ON or OFF. When metal object leaves sensing area, the oscillator regenerates allowing a sensor to return to the normal state. Capacitive proximity sensors:It is a sensing device that is actuated either by conductive or non conductive materials. Instead of a coil, active faces of a capacitor sensor are formed by two metallic electrodes-rather like an opened capacitor. They are in a feedback loop of high frequency oscillator that is inactive with ―no target‖ present. As target approaches the face of the sensor, it enters in the electrostatic field formed by the electrodes. This cause an increase in the coupling capacitance and circuit begin to 25 oscillate. The amplitude of these oscillations is measured by evaluating circuit that generates a signal to turn the solid state output ON or OFF. Flame sensors:Flame detectors are used to sense the presence or absence of flame. In KAPCO, during the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system. There are four flame sensors. The ultraviolet flame sensor consists of a flame sensor, containing a gas filled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation which is emitted by a hydrocarbon flame. A DC voltage supplied by the amplifier is impressed across the detector terminals. If flame is present, the ionization of gas in the detector allows conduction of current in the circuit which activates the electronics to give an output defining flame. Conversely the absence of flame will generate an opposite output defining ―no flame‖. In KAPCO if a loss of flame in the combustion chamber is indicated by two flame detector sensor, the control circuitry will cause an annunciation only of this condition. 26 Signal conditioning:Input signal conditioning involves converting input and output signal to a usable form. In other words signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing. It includes following techniques. Amplification Attenuation Filtering A/D converters D/A converters Current to voltage converter Voltage to current converter Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage and current, frequency and electric charge. Sensor inputs can be accelerometer, thermocouple, thermistor, resistance thermometer, strain gauge or bridge, and LVDT or RVDT. Outputs for signal conditioning equipment can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer. Filtering:Filtering is the most common signal conditioning function, as usually not all the signal frequency spectrum contains valid data. Electronic filters are used in filtering. Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones. Filters can be analogue or digital, active or passive, low pass or high pass. Amplification:Signal amplification performs two important functions: increases the resolution of the input signal, and increases its signal-to-noise ratio. For example, the output of an electronic temperature sensor, which is probably in the millivolts range is probably too low for an controller to process directly. In this case it is necessary to bring the voltage level up to that required by the controller. Normally used amplifiers are log amplifiers, antilog amplifiers, simple op amp etc. Multiplexers:It is a device that performs multiplexing; it selects one of many analog or digital input signals and forwards the selected input into a single line A/D converter:Analogue to digital converter converts continuous quantity on to discrete quantity. Sampling is done in order to convert analogue in to digital signal. 27 Attenuation:It is the opposite of the amplification. An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform. It provides gain less than 1.Simple attenuators are voltage divider circuit. In measuring signals, attenuator pads are used to lower the amplitude of the signal a known amount to enable measurements Digital-to-analog converter It converts digital input in to analogue output. This can be done through R-2R ladder. Operator Machine Interface:In modern automatic control system, few parameters can be changed by the operator. All modern control systems have operator machine interface. This facility is in all the LCR,CCR of KAPCO. It Allows input from a human to setup the starting condition or alter the control of the process Allows human input through various type of switches, controls and keypads Operates using supplied input information that may include emergency shutdown or changing the load, speed, the type of the fuel it use, to gave set points to the controller. 28 Controller Controller is a device which makes system’s decisions based on the input signals. They generates output signal which operate actuator to carry out the decision. Controllers may be open loop or closed loop as described above. There are two types of control. Discrete control Analogue control Digital control:Discrete control deals with systems in which each element can only exist in certain defined states. This type of control is implemented with logic diagrams and circuits. Analogue control:Analog control deals with systems in which variables can have a continuous range of values, rather than simply discrete states. Basic analog control consists of the process of measuring the actual output of a system, comparing it to the desired value of that output, and taking control action based on the difference to cause the output to return to the desired value. The four most popular types of control response used in the process industry are ON/OFF Proportional Integral Derivative ON/OFF:With ON/OFF control, the final control element is either ON or OFF-one for the occasion when the value of the measured variable is above the set point and the other when the value of the measured variable is below the set point. The controller will never keep the final control element in the intermediate position. E.g. we are using ON/OFF controller when a liquid is heated by the steam. If the liquid temperature goes below the set point the steam valve opens and the steam is turned ON or vice versa. This type of controller is usually inexpensive but not very accurate to be used in accurate control applications. Drawbacks: It can position a valve in only two different settings It treats a large offset and small offset the same way It allows oscillation to the process and cannot give control for steady states for the process. Proportional:These controllers are designed to eliminate the hunting or cycling associated with the ON/OFF controller. Proportional (P) controllers produce an output that is directly proportional to the error signal. 29 It allows the final control element to take the intermediate positions between ON or OFF depending on the error signal. This permits the analogue control of the final control element to vary the amount of energy to the process, depending upon how much the value of the measured variable has shifted from the desired variable. Proportional controllers are tune able. Their response to process changes can be adjusted to suit the time constants of specific process system. In theory, a proportional controller should be all that is needed for process control any change in the system output is corrected by an appropriate change in controller output. Unfortunately the operation of a proportional controller leads to a process deviation known as offset. This steady state error is the difference between the attained value of the controller and the required value. To compensate the steady state error it is used in conjunction with the integral or derivative controller. Process with the long time lags and a large maximum rate of rise (e.g. heat exchanger) require wide proportional bands to eliminate oscillation. Integral action:It is sometimes called reset action respond to the size and the time duration of the error signal therefore the output signal from an integral controller is the mathematic integral of the error. An error signal exists when there is difference between the process variable and the set point, so the integral action will cause to change and continue to change until the error no longer exists. Integral action eliminates steady state error. The amount of integral action is measured as minutes per repeat. Derivative action:The derivative mode controller respond to the speed at which error signal is changing-that is greater the error change , the greater the correcting output. The derivative action is measured in term of time. Proportional-Integral controller:PI controller combines the characteristics of both types of control. A step change in the measurement causes the controller to respond in a proportional manner followed by the integral response, which is added to the proportion response. Because the integral mode determines the output changes as a function of time, the more the integral action in the control, the faster the output changes. It is used To eliminate the offset error, the controller needs to change its output until the process variable reaches to zero Reset (integral) control action changes the controller output by the amount needed to derive the process variable back to the set point value. Proportional- derivative controller:It is used in process control systems that have errors that change very rapidly. By adding derivative control to proportional control, we get a controller output that responds to the measurement’s rate of change as well as to its size. 30 PID controller:Proportional integral derivative controllers produce outputs that depend on the magnitude, duration and rate of change of the system error signal. Sudden system disturbances are met with an aggressive attempt to correct the condition. A PID controller can reduce the system error to zero faster than any other controller because it has an integrator and a differentiator. During start up the set point, proportional band, reset, and rate as well as the output limits are specified. All these can be changed during operation to tune the process.PID controllers are used in the PLC’s of KAPCO. In KAPCO, Pneumatic PI controllers are installed in the fuel and gas lines of heat exchanger in the filtering skid and forwarding skid. They sense the temperature with the help of fluid which increases the pressure of the bourdon tube which tells the pneumatic PI controller to decide the flow of air to open the valve which will try to follow set points given by operator. Actuators: An actuator is any device that converts an electric signal in to mechanical movement. There are many types of actuators. A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve. Actuators can create a linear motion, rotary motion, or oscillatory motion. That is, they can create motion in one direction, in a circular motion, or in opposite directions at regular intervals. Pneumatically operated control valve actuators are the most popular type in use, but electric, hydraulic, and manual actuators are also widely used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide high stem force output for demanding service conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are available for direct installation on rotary-shaft control valves. Electric and electro-hydraulic actuators are more complex and more expensive than pneumatic actuators. 31 They offer advantages where no air supply source is available. Pneumatic operated diaphragm actuator:Pneumatically operated diaphragm actuators use air supply from controller, positioner, or other source. Various styles include Direct acting (increasing air pressure pushes down diaphragm and extends actuator stem); reverse-acting (increasing air pressure pushes up diaphragm and retracts actuator stem, Direct-acting unit for rotary valves (increasing air pressure pushes down on diaphragm, which may either open or close the valve, depending on orientation of the actuator lever on the valve shaft. In KAPCO, they are used in the filtering skid. Piston Actuators Piston actuators are pneumatically operated using high-pressure plant air to 150 psig, often eliminating the need for supply pressure regulator. Piston actuators furnish maximum thrust output and fast stroking speeds. They are used for pneumatic trip valves and lock-up systems. In KAPCO, these types of actuator are used in speed ratio valve and gas control valve. Valves: A valve is a device that regulates the flow of a fluid (gases, liquids or slurries) by opening, closing, or partially obstructing various passageways The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. There are many types of control valves like sliding stem and rotary shaft control valve. 32 Sliding stem control valve: Single ported valve body: This valve contains one plug means it allows only one path for fluid to flow i.e. one inlet and one outlet. It is simple in construction and valve plug can be moved in or out by applying pressure pneumatically and electrically through the actuator. Three way valve body: They are designed specifically to blend (mix) or split two flowing streams. There are two inlet ports and one output port for blending while one inlet port and two output port for diverting (splitting). Rotary shaft control valve:Butterfly Valve Bodies It consists of a shaft supported disc which rotates within cylindrical body. They are now being designed for high and low pressure drops high static pressure and tight shutoff. Tight shutoff is accomplished using soft seating as rubber lining. They are being used for low pressure drop requirements, high capacity and minimum space for installation. Butterfly valve bodies might require high-output or large actuators if the valve is big or the pressure drop is high, because operating torques might be quite large. Ball type valve:It consists of a ball in place of plug to open and close the path of fluid. Its design provides self-cleaning; it provides tight shutoff and wide range ability for accurate controlling of flow. 33 Control Valve Accessories Positioner:Pneumatically operated valves depend on a positioner to take an input signal from a process controller and convert it to valve travel. Pneumatic Positioner—A pneumatic signal (usually 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position. Analog I/P Positioner—This positioner performs the same function as the one above, but uses electrical current (usually 4-20 mA) instead of air as the input signal. Digital Controller—Although this instrument functions very much as the Analog I/P described above, it differs in that the electronic signal conversion is digital rather than analog. Supply Pressure Regulator Supply pressure regulators commonly called air sets; reduce plant air supply to valve positioner and other control equipment. Common reduced-air-supply pressures are 20, 35 and 60 psig. The regulator mounts integrally to the positioner or nipple-mounts or bolts to the actuator. 67CF series filter regulators are used to regulate air pressure in various Gas turbine units in KAPCO. Their specifications are Maximum inlet pressure…..250 psi Outlet pressure ranges………0 to 35 psig/0 to 60 psig/0 to 125 psig 34 Solenoid valve:A solenoid valve is a combination of two basic functional units: A solenoid (electromagnet) with its core or plunger. A valve body containing an orifice in which a disc or plug is positioned to restrict or allow flow. A solenoid is a device used to convert an electrical signal or an electric current into linear mechanical motion. It is basically an actuator. The solenoid is made up of a coil with a moveable iron core. When the coil is energized, the core is pulled inside the coil. The amount of pulling or pushing force produced by the solenoid is determined by the number of turns of copper wire and the amount of the current flowing through it. Now coming back to solenoid valve, flow through an orifice is OFF or allowed by the movement of the core and depends on whether the solenoid is energized or de energized. Solenoid valves are available to control hydraulics, pneumatics Symbols:- Normally Closed Inlet Normally open Inlet Electro hydraulic servo valve:Servo-controlled hydraulic systems provide exceptional control over very large forces. The most critical element of the system is the electro hydraulic controller or servo. Common servo-valves consist of a two stage spool whose position is controlled by electromagnetic coils. Energizing the coils allows fluid flow in one direction or the other depending on the input signal. The basic servo-valve produces a control flow proportional to input current for a constant load. Two stage servo-valves may be further divided into nozzle-flapper and jet pipe types. Nozzle-flapper type servo-valves are currently by far the most common in high performance servo applications. So we will discuss nozzle type servo valve. In KAPCO, we are using Moog G77XK series industrial servo valve are used. Construction and Operation:It consists of a polarized electric torque motor and two stages of hydraulic power amplification. The motor armature extends in to air gap of the magnetic flux circuit and is supported in this 35 position by a flexture tube member. The flexure tube act as a seal between the electromagnetic and hydraulic sections of the valve. The two motor coils surround the armature, one on each side of the flexure tube. The flapper of the first stage hydraulic amplifier is rigidly attached to the midpoint of the armature. The flapper extends through the flexure tube and passes between two nozzles crating two variable orifices between the nozzle tip and the flapper. The pressure controlled by the flapper and the nozzle variable orifice is fed to the end areas of the second stage spool. The second stage is a conventional four way spool design in which output flow from the valve, at a fixed pressure drop, is proportional to the spool displacement from the null position. Input signal induces a magnetic charge in the armature and causes a deflection of the armature and flapper. This assembly pivots about the flexure tube and increases the size of one nozzle orifice and decreases the size of the other. This action creates a differential pressure from one end of a spool to the other and result in spool displacement. Spool movement continues until the feedback wire force equals the input signal force. A is the inlet of the hydraulic oil, B is the controlled output hydraulic oil, T is drain and P is to control the movement of spool. Servo controller:In servo controller the error amplifier continuously monitors the input reference signal (Ur) and compares it against the actuator position (Up) measured by a displacement transducer (LVDT) to yield an error signal. The error is manipulated by the servo controller according to a pre-defined control law to generate a command signal (Uv) to drive the hydraulic flow control valve. 36 Relays:Relays are used primarily as switching devices in a circuit. In KAPCO, relays are used in SPEEDTRONIC control system of GTs for switching. In world there are various type of relays are used but some relays which are used in KAPCO with respect to process control or switching are following Electromechanical relays Solid state relay Timing relay Latching relay Electromechanical relay:It is magnetic switch. It turns a load circuit ON or OFF by energizes an electromagnet. A relay usually has one coil, but it has any number of different contacts. They contain a stationary and the moving part. The moving contact is attached to the plunger. Contacts are referred as normally open (NO) and normally close (NC). Action of this field, in turn, causes the plunger to move through the coil closing the NO contact and opening NC contacts. NO contacts are open when coil is de energized NO contacts are close when coil is energized Control relays are used as auxiliary device to switch control circuits and load such as small motors, solenoid and pilot lights. They can be used to control a high voltage load circuit with a low voltage control circuit. Coils and contacts are well insulated. Some important terminologies of relays:Pick up voltage:Level of voltage at which relay coil is energized, resulting in contact switching. Drop out voltage:Level of voltage at which relay coil is de energized. Inrush current:When the coil is energized the plunger is in an out position. Because of an open gap in the magnetic path, the initial current in the coil is high. This high current is called in rush current. Sealed current:As plunger moves in to the coil, closing the gap, the current level drop to a lower level. The lower value is called sealed current. 37 Solid state relays:They are not used in KAPCO. EMR and SSR both perform the same function but with different mechanism. They use bipolar transistor, MOSFET, SCR (silicon controlled devices) or triacs. Solid state relay has no moving part. They are resistant to shock and vibration. Like EMR, SSE found its application in isolating a low voltage control circuit from a high power load circuit. Optically coupled SSR:LED glows when condition are correct to actuate the relay. LED shines on phototransistor, which then conducts, causing the trigger current to be applied to the triac. Thus output is isolated from the input by simple LED and phototransistor arrangement. Timing relays:It is used in industry when time delay is required e.g. machine in which start of an event must be delayed until another event has occurred ( a machine must be on gas until the fuel has been heated up to 100°C temperature). Timing relays are conventional relay that are equipped with an additional hardware mechanism or circuitry to delay the opening or closing of load contact. There are two types of timing relays. ON delay OFF delay They are further divided in to NO or NC contact. Let us take the example of OFF delay NO timing relay. S1 (switch) TD NO lamp L1 When S1 opens, TD de energized, TD opens, and L1 is OFF S1 close, TD close instantaneously and L1 is switched ON S1 opens, TD de energized, timing period start, TD is still closed and lamp is still on After 10s or any timer set value, TD open and lamp is switch OFF Latching Relay:- 38 Electromechanical latching relays are designed to hold the relay closed after power has been removed from the coil. The latching coil is momentarily energized to set the latch and hold the relay in latched position Advantages: In control circuit to have to remember when particular event take place and not permit certain functions once this event occurs. E.g. Shutdown. Power failure of the control system 39 System architecture:Control system architecture can range from simple local control to highly redundant distributed control. SCADA systems. Local control It describes a system architecture in which sensors, controller, and controlled equipment are within close proximity and the scope of each controller is limited to a specific system or subsystem. Local controllers are typically capable of accepting inputs from a supervisory controller to initiate or terminate locally-controlled automatic sequences, or to adjust control set points, but the control action itself is determined in the local controller. Centralized control Centralized control describes a system in which all sensors, actuators, and other equipment within the facility are connected to a single controller or group of controllers located in a common control room. Locating all controls, operator interfaces and indicators in a single control room improves operator knowledge of system conditions and speeds response to contingencies. This type of system architecture was common for power plants. In this if the main controller fails then the whole machine trips. No exchange of controller status or data is sent to other controllers. Distributed control Distributed control system architecture offers the best features of both local control and centralized control. In a distributed control system, controllers are provided locally to systems or groups of equipment, but networked to one or more operator stations in a central location through a digital communication circuit. Control action for each system or subsystem takes place in the local controller, but the central operator station has complete visibility of the status of all systems and the input and output data in each controller, as well as the ability to intervene in the control logic of the local controllers if necessary. 40 There are a number of characteristics of distributed control architecture which enhance reliability: Input and output wiring runs are short and less vulnerable to physical disruption or electromagnetic interference. A catastrophic environmental failure in one area of the facility will not affect controllers or wiring located in another area. Types of DCS:Plant distributed control system (DCS):While the term DCS applies in general to any system in which controllers are distributed rather than centralized, in the power generation it has come to refer to a specific type of control system able to execute complex analog process control algorithms at high speed, as well as provide routine monitoring, reporting and data logging functions. In most applications, the input and output modules of the system are distributed throughout the facility, but the control processors themselves are centrally located in proximity to the control room. 41 Direct digital control:They consist of local controllers connected to a network with a personal computer (PC) based central station which provides monitoring, reporting, data storage and programming capabilities. They are normally used in HVAC plants. Remote terminal unit (RTU) based SCADA: RTU-based systems are common in the electric distribution industries where monitoring and control must take place across large geographical distances. The RTUs were developed primarily to provide monitoring and control capability at unattended sites such as substations, metering stations. They communicate with a central station over telephone lines, fiber-optics, radio or microwave transmission. Monitored sites tend to be relatively small, with the RTU typically used mainly for monitoring and only limited control. Programmable logic controller (PLC) based systems: PLCs can be networked together to share data as well as provide centralized monitoring and control capability. Control systems consisting of networked PLCs are supplanting both the plant DCS and the RTU-based systems in many industries. They were developed for factory automation and have traditionally excelled at high speed discrete control, but have now been provided with analog control capability as well. The recommended controller for SCADA systems is the programmable logic controller (PLC). PLCs are general-purpose microprocessor based controllers that provide logic, timing, counting, and analog control with network communications capability. They provide high speed processing, which is important in generator applications. A PLC consists of the required quantities of the following types of modules or cards, mounted on a common physical support and electrical interconnection structure known as a rack. Power supply: The power supply converts facility electrical distribution voltage, such as 120 VAC or 125 VDC to signal level voltage used by the processor and other modules. Processor: The processor module contains the microprocessor that performs control functions and computations, as well as the memory required to store the program. Input/output (I/O): These modules provide the means of connecting the processor to the field devices. Communications: Communications modules are available for a wide range of industrystandard communication network connections. These allow digital data transfer between PLCs and to other systems within the facility Redundancy: Many PLCs are capable of being configured for redundant operation in which one processor backs up another. S3 PLC is used for fuel dosing pump in the KAPCO. 42 Mark four SPEEDTRONIC:This type of control systems are used in the local control room of the GT5-8. It is a plant distributed control system. It controls the operation and monitoring of the GT 5-8. It employs three computers, identified as controller R, S and T which performs all calculations necessary to keep the gas turbine running after it has reached a complete sequence and also for the shutdown of the unit. Each computer is designed such that it drives its output in a defined direction on loss of power or failure of any computer. The two out of three logic is provided in this system configuration. A fourth computer called as ―communicator‖ or ―human machine interface‖ supervises the three controllers and initiates an audible alarm where there is any disagreement between any control parameter or logic signal in the three controller. The turbine however continues to run because the control is responding to the median value. The field trip contacts are wired to the two contact input modules. From here the signals are paralleled to the three optical couplers which feed them to the separate digital input cards in R, S and T. the C computer monitors the input seen by R, S and T and performs the majority vote and feed the voted values to the CRT for display. Channel 1 1&2 2&3 Machine emergency trip Channel 2 Channel 3 3&1 For analogue control system the temperature input signal are connected to the I/O module where they are filtered prior to being fed to the controller R, S, T or C. These incoming signals are then multiplexed on the computer card dedicated to specific functions and then wired to another input analogue card for second stage of multiplexing and the final analogue to digital conversion. 43 The thermocouple inputs are processed similar to other analogue inputs. Here the trip and essential control thermocouples are connected to separate input modules. Two types of relays are used soft relays and hardware relays. Soft relays are used for soft contacts in the ladder diagram while hardware relays are used for switching. 256 alarm messages are available and additional alarms are dedicated to internal diagnostics. The status of all the contact inputs can be displayed simultaneously to assist in trouble shooting. STG 11-12 control system:STG 11-12 has Distributed control system. It is a very huge and complex control system. The very brief introduction of the STG control system is as following. T20:T20 cards controllers are used for steam turbine auxiliary control e.g. boiler, feed water tank, steam drum etc. They are open loop control systems. The detail of the open loop system is mentioned above. Micro z controllers:Micro z controllers are also used in steam turbine auxiliary. They are closed loop control systems. Turbine control:These control cards are used to control the steam turbine. STG controller DIGIREC 920 STG safety REC 920 Temperature rack Vibratory monitoring and protection Stress calculator ALSPA C100 PLC STG controller DIGIREC 920:DIGIREC920 control following parameters Speed control Acceleration control HP pressure control Valves position control Gland steam control (gland steam is used to stop the lost of vacuum generated in condenser) Safety REC 920: Over speed protection Lube oil pressure low 44 Lube oil tank level low Condenser pressure high Generator failure Internal tripping orders (vibration rack, bearing temperature high, generator excitation temperature high) Functions of ALSPA C100 PLC: Logics for fundamental safeties test Start up checks Stress calculator: Monitoring of inlet steam pressure, temperature and turbine casing temperatures. Calculate turbine rotor and casing stress 45 Power plant safety rules:Electric and electronic circuit can be dangerous. Safe practices are necessary to prevent electric shocks, fires, explosions, mechanical damage and injuries resulting from the improper use of tools. Perhaps the greater hazard is electrical shock. A current through the human body in excess of 100 milli amperes can paralyze the victim and make it impossible to let go of a ―live‖ conductor or component. High voltage can force enough current through the skin in order to produce a shock. The danger of shock increases with the increase in voltage. Any voltage above 30V is considered dangerous. The pathway through the body is another factor influencing the effect of electric shock. E.g. a current from hand to foot, which passes through the heart and part of the central control system, is far more dangerous than a shock between two points on the same arm. Safety in the workplace:Many statistics shows that 98 percent of all accidents are avoidable. So following things must be kept in mind while working in KAPCO or any other power plant. Red is used to designate fire protection equipment Yellow is used to designate caution and physical hazards Orange is used to designate dangerous part of machines Purple is used to designate radiation Hazards Green is used to tell the location of first aid equipments Personal safety:The clothing worn at work is important for personal safety. The following things should be observed. Hard hats Safety shoes Goggles Metal jewelry should not be worn while working on energized circuit; gold and silver are excellent conductor. Grounding:It refers to the deliberate connections of parts of a wiring installation to a common earth detection. Grounding guards us against the shock hazard. It arises when there is a little or no leaking current but the potential for the abnormal current flow present exists. E.g. if the exposed live wire touched the metal frame of an ungrounded piece of electrical equipment , the voltage of the live wire would charge the metal plate. If we touch the charged metal frame, our body can provide it a current path and we suffer a serious shock. For this reason in KAPCO every machine is grounded. When there is abnormal current flowing through the metal body it will blow a fuse or trips a circuit breaker to immediately open the circuit. Grounding has nothing to do with the operation of electrical equipment. Its sole purpose is the protection of life and property. 46 Electrical lock out:They are normally known as permit. They are necessary so that someone will not inadvertently turn the equipment to the ON position while it is being worked. 47