Boiler Controls And Boiler Interlocks 30291b

BOILER CONTROLS AND BOILER INTERLOCKS Introduction Main automatic control of water tube type, drum boilers can be classified into 5 major control items  Automatic combustion control  Feed water control  Superheated steam temperature control  Reheated steam temperature control  Furnace draft control PURPOSE The drum-type watertube boiler is the fundamental steam generator for both industrial and utility applica-tions. The steam generated by a boiler may be used as a heat transfer fluid for process heating, or it may be expanded in steam turbines to drive rotating machinery such as fans, compressors, or electric generators.Figure 1 is a schematic representation of a drum-type boiler. The steam drum and mud drum are mounted in a furnace and are interconnected with watertubes called risers and downcomers. The furnace includes one or more burners for the combustion of an air and fuel mix-ture. The heat of combustion is transferred to the watertubes to generate steam. Steam bubbles form in the tubes (risers) closest to the burner and rise to the steam drum where they are separated from the water. The steam in the risers is replaced by water in the downcomers to provide natural circulation in the watertubes. A continuous supply of feedwater is necessary to replace the steam leaving the boiler. In most cases, the saturated steam leaving the steam drum is returned to the furnace for superheating as shown in Figure 1.

A forced draft (FD) fan provides combustion air to the windbox from which it is delivered to the burners. An induced draft (ID) fan draws the flue gases from the fur-nace and drives them up the stack. Heat from the flue gas is used to preheat the combustion air to improve efficiency. The primary purpose of any boiler control system is to manipulate the firing rate so that the supply of steam remains in balance with the demand for steam over the full load range. In addition, it is necessary to maintain an adequate supply of feedwater and the correct mix-ture of air and fuel for safe and economical combustion. Automatic combustion control Steam pressure is one of the most important control items of the boiler primarily due to the following reasons.  Safety of the boiler operation :- the main structure of the boiler is a high pressure container with a certain allowable limit. If the steam pressure exceeds the main structure may be damaged and high pressure water may evaporate at one time causing explosion of the boiler. To guard this a safety valve is provided. But if the safety valve opens the steam will be sent to atmosphere causing huge heat loss. Hence it is mandatory that the boiler pressure be controlled below the maximum operating pressure.  Convenient for tracking of load change :- when the boiler is in a balanced state, the input energy and the output energy of the boiler is equal and the entire boiler system is in equilibrium state. when the balanced state is disturbed, energy will be build up in the boiler or the total amount of energy stored in the boiler will be

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reduced. When this has occurred since the amount of water and steam held with in the boiler do not largely vary the levels of energy stored in the water and steam vary. In general most part of the energy fed into the boiler is used for evaporation of water and only a minor part of the energy is used for superheating and preheating. There fore most part of the input and output heat energies of the boiler can be balanced by controlling the amount of heat energy fed to the boiler in response to the load changes. Thus control of steam pressure is an effective way of controlling the amount of water in response to load changes. Answering a requirement by load :- when the load of a boiler is a turbine, change in output pressure of the boiler is undesirable as it will cause external disturbances to the turbine and degradation of turbine efficiency.

Due to the above reasons automatic combustion control of water tube type drum boiler is done by detecting the boiler steam pressure (generally in the steam header), controlling the fuel fed to the boiler furnace and controlling the combustion air at a certain fuel air ratio for efficient combustion of the fuel. General Combustion Control Here load change is detected by the master steam pressure transmitter/ controller is used as the set point signal for the fuel flow controller in order to cascade control of the fuel flow. On the other hand the flow signal detected by the air flow transmitter is multiplied by a fuel air ration setter for the complete combustion of the fuel and then the signal is applied to the air flow controller. The set point value for this is provided based on the fuel flow and the air flow is cascade control. The fuel flow is controlled by a control valve and air flow by Forced Draft Fan (FDF) vane or damper. Master steam Fuel Flow Air Flow Pressure Transmitter Transmitter Transmitter

Master steam Pr.controller Fuel oil flow controller

RR

Fuel air ratio setter

FC

Combustion air flow controller

PC FC

FDF control valve Fuel flow control valve When the load has varied in this system, the fuel oil flow varies first and air flow varies next and this results in decrease in O2 % . when load decreases the reverse will happen and O2% increases.

One method to avoid this is to maintain a large air fuel ratio to guard against shortage of air (which is dangerous to the boiler) which could result in high fuel consumption. Hence a modified system is used. Master Steam Pr. Transmitter Master Steam Pr.controller

PC

Low selector

Lo

Fuel oil flow Controller

FC

Fuel flow control Valve

Fuel Flow Transmitter

Air Flow Transmitter

RR Fuel Air ratio setter

BS Bias Setter Hi High Selector

FC combustion Air Flow controller FDF damper

Assume that the boiler load is steady and the automatic control system is in a balanced state. in this the following are in equilibrium state.  Master steam pressure controller output signal  Fuel flow controller set point and PV signal  Air flow controller set point and PV signal Therefore the output signal of the bias setter, which is applied to the low selector is higher by the amount of bias than the output signal of the master steam pressure controller. When the load is varied from this balanced state the control actions are as follows. When load is increased As the master steam pressure falls lower than the set point pressure, the output signal of the master steam pressure controller rises to increase the fuel flow. The increased signal is applied as an set point signal to the fuel flow controller. It can be increased by an amount of bias by means of the low signal selector. On the other hand regarding the set point of air flow controller, the signal from the steam pressure is more than the fuel flow and hence this is the set point. Hence the air flow increases first. Due to this the set point of fuel flow increases slowly after an increase in the air flow. The bias is set at a level which is safe to have an amount of increase in fuel flow without an increase in air flow. Thus we can conclude that the air flow increase leads the fuel flow increase. When load is decreased

When the load is decreased the master steam pressure becomes higher than the set point and the output of the pressure controller falls. When this has occurred as the output signal of the bias setter at the low selector is always higher than the output signal of the bias setter at the low selector is always higher the master steam pressure controller, the output signal of the controller has the priority. The air flow decreases following the decrease of the fuel flow. If the response of the air flow system is slow, the fuel flow is limited at the bias level. Here the air flow decrease lags the fuel flow decrease. Problems regarding instrumentation and control To obtain a low excess air combustion system , various factors related to the boiler itself such as the structure of the boiler main body (especially of the heat conduction area and air ingression), burners, the structure of the wind box, and the factors related to the instrumentation such as locations of detectors, accuracies of instruments, measuring time lags (especially of analyzers), time lags of control end devices and the type of control system (electronic or pneumatic). O2 analyzer Performance of the low excess air combustion system is evaluated on the basis of %O2 contained in the flue gas. The ratio of O2 contained in the flue gas is 1% when the excess air ratio is 5%. Thus the analyzer should have a range of approximately 0-5% and accuracy of 0.02% of full scale. The O2 % indicated for the sampled gas must be of a truly representative one. As flow and concentration of the flue gas are not uniform, sampling should be done at least at 2 points (upstream and down stream) of the fuel economizer at least. Attention must be paid also to location and installation method of the sampling loop. CO analyzer No CO gas is generated when fuel is burning normally. When the excess air ratio has become lower than 1.5% ( flue gas less than 0.3%), CO gas starts to be generated. Hence for a low O2 operation, a CO gas analyzer must be provided. O2 trimming For boilers normally a continuous O2 analyzer is installed and the fuel air ratio is automatically controlled with its output signal. The desired excess air ratio with respect to the load is non linear.

Optimal O2 percentage

Boiler load (%) The basic control system is as follows. The output signal of the main steam flow transmitter is fed as a set point signal to the O2 % controller through a function generator. The O2 % controller compares the measured signal with the set point signal and if any deviation exists the controller performs a P+I action on the deviation signal and provides a correction signal for the fuel air ratio. The O2 % control system must be designed with a provision to prevent application of a wrong signal to the O2 % control system and upsetting of the fuel air ratio. This is done by using a high and low signal limiters in the output circuit of the O2 % controller.

Master steam Pressure tran.

PC

fuel flow Transmitter

Air flow Transmitter

O2 % Analyzer

Fuel O2 % O2C air ratio setter Contr.

Main steam Flow tran.

Functiof(x) Generator

High limiter Fuel air ratio Correction unit Low limiter low selector

Bias setter

Fuel FC Flow cont.

High selector

Fuel flow control valve

Combustion air flow controller

FDF control valve

AUTOMATIC FEED WATER CONTROLLER BOILER DRUM LEVEL (FEEDWATER) CONTROL BENEFITS Maximizes steam quality. Maintains proper drum level to prevent damage to boiler. BACKGROUND The cylindrical vessel where the water-steam interface occurs is called the boiler drum. Boiler drum level is a critical variable in the safe operation of a boiler. A low drum level risks uncovering the water tubes and exposing them to heat stress and damage. High drum level risks water carryover into the steam header and exposing steam turbines to corrosion and damage. The level control problem is complicated by inverse response transients known as shrink and swell. Simply put, shrink and swell refer to a decreased or an increased drum level signal due to the formation of less or more vapor bubbles in the water, and not a change in the amount of water in the drum. This condition produces level changes during boiler load changes in the opposite direction of what is expected with a particular load change. Although only temporary, this can cause severe control system overshoot or undershoot. MEASUREMENT Several different measurements are required for feedwater control depending on the control strategy being used. The following measurements are marked as 1E, 2E and 3E to indicate they are required for single, two or three-element feedwater control strategies. Boiler Drum Level [1E 2E 3E] The boiler drum level has a typical span of 30 inH2O, often with some suppression required depending on the physical location of the transmitter. The boiler will be pressurized so the transmitter must be able to operate with a static pressure of up to several thousand PSI. In addition, chemicals injected into the feedwater need to be considered when specifying the transmitter’s materials of construction. Drum Pressure [1E 2E 3E]

Since the density of steam and water at the saturation temperature change with pressure, the drum level calibration will only be accurate at a single boiler drum pressure. The drum level signal can be compensated for all pressures by using a drum pressure transmitter. Generally, both industrial and utility boilers run at constant drum pressure. Pressure compensation is typically employed with utility boilers which operate at much higher pressures than industrial boilers. Steam Flow [2E 3E] Steam mass flow is required to compensate for shrink and swell during load changes. This measurement can be made using a multi-variable differential pressure type flow meter and an orifice plate to provide a mass flow output. Feedwater Flow [3E] Feedwater flow is typically used in a cascade control strategy to ensure that proper valve characteristics are maintained. CONTROL Typically one of three control strategies is employed to control the boiler drum level. Single-Element Drum Level Control The single element system is the simplest type used for controlling packaged firetube and watertube boilers. In this strategy, control is based on the boiler drum level measurement only. This does not allow for compensation of any shrink or swell and, therefore, is not usually an acceptable control strategy. However, for small boilers with slow load changes this may be acceptable. Drum level transmitter

Drum level controller feed water control valve when the steam evaporation area in the drum is reduced in relation to the evaporation amount, the drum level responds in the reverse manner. This is because the capacity and amount of steam bubbles in the drum water vary and when the boiler load is increased drum water swells or rises. If the single element control system is used to control the drum level which has the reverse response as above, as the drum water level rises when the boiler load is increased, the feed water is controlled in the decreasing direction. Consequently the drum level rapidly falls from the equilibrium state and the loop will start hunting. This is very dangerous and will lead to shutdown.

Two-Element Drum Level Control In two-element control, steam flow is measured along with boiler drum level. The steam flow signal is used in a feed forward control loop to anticipate the need for an increase in feedwater to maintain a constant drum level. This strategy requires the differential pressure across the feedwater control valve to remain constant as well as the control valve signal vs. flow profile. Boilers with moderate load changes can usually be controlled with this strategy. When the load is increased, the drum level will become higher than the set point drum level due to the reverse response and the drum level controller will generate a correction signal in the direction of reducing the feed water. On the other hand the steam flow signal which is on an increasing trend is applied to the totaliser so its output balances out or slightly overcomes the controller output, thereby guarding the feed water flow against reduction. Steam flow Steam flow transmitter Drum level transmitter

controller

bias Boiler feed water

Three-Element Drum Level Control Three-element drum level control adds a feedwater flow signal to the steam flow and boiler drum level signals used in two-element feedwater control. The drum level controller manipulates the feedwater flow setpoint in conjunction with feedforward from the steam flow measurement. The feedforward component keeps the feedwater supply in balance with the steam demand. The drum level controller trims the feedwater flow setpoint to compensate for errors in the flow measurements or any other unmeasured load disturbances (e.g. blowdown) that may effect the drum level. Three-element control is used in boilers that experience wide, fast load changes. Steam flow Steam flow transmitter A

+50% B Controller -50%

Drum level tran.

bias C Feed water flow controller

BFW transmitter Boiler feed water The biasing function is (steam flow) + (water level controller output signal)- (50%signal) is the set point of BFW controller. When the drum level is at the set point (normally at 50%), the controller output will be 50%, hence the feed water flow is controlled at the same value with the steam flow. When the drum level is shifted from the set point level a correction signal is applied.

This control system is effective for boilers the drum level of which has large reverse response characteristics. This bias 50% signal is true when the boiler drum level to be maintained at –50% to +50%. Let us suppose the level is to be maintained is –15% to +15%, the bias signal can be calculated as follows B= -15 + 30B’ (-15 is lowest point and 30 is –15 to +15=30) A= 100 A’ C=100 C’ C= A + B : 100C’=100 A’ +30 B’-15 C’ = A’ + 30(B’-0.5) and –0.5 or 50% signal is the bias. As mentioned earlier the level to be maintained between –50% to +50%. In this case 100C’ = 100A’ + (-50 + 100B’) C’= A’ + B’- 0.5 PLANT MASTER CONTROL Steam pressure is the key variable that indicates the state of balance between the supply and demand for steam. If supply exceeds demand, the pressure will rise. Conversely, if demand exceeds supply, the pressure will fall. Figure 2 shows a single-loop control dia-gram that manipulates the firing rate demand to control steam pressure at the desired setpoint.

Plants may experience fluctuations in demand due to batch processes or other process changes. In this case, a steam flow feedforward signal is used with steam pressure control.

The term Plant Master is most applicable to the situation in which more than one boiler supplies a common steam header. In this case, there are multiple boiler masters but only one plant master. The plant master generates the master firing rate demand signal that drives the individual boilers in parallel. With multiple boilers, the Plant Master is typically configured with a variable gain, based on the number of boilers in automatic mode. BOILER MASTER CONTROL With several boilers supplying a common header in parallel, it is generally desirable to provide a way to adjust the load distribution among the boilers. Depending on the load and the performance of the individual boilers, the most efficient operation may be achieved with some boilers shut down, some boilers base loaded (constant firing rate), and the remaining boilers allowed to swing with the load (variable firing rate). Figure 3 shows a boiler master control diagram to pro-vide these adjustments. Each boiler master provides a bias adjustment and an auto/manual transfer switch. In manual, the operator can reduce the firing rate to a low fire condition for shutdown, or hold the firing rate at any appropriate base loading condition. In auto, the boiler master follows the master firing rate demand signal except as altered by the bias adjustment. The operator can ad just the boiler master bias up or down to increase or decrease its share of the load.

FURNACE PRESSURE CONTROL A basic boiler has a steam water system and a fuel-air-flue gas system. In the fuel-air-flue gas system, the air and fuel are mixed and ignited in the furnace. Air and fuel flow into the furnace and flue gas flows out. The force driving this flow is the differential pressure between the gases inside the furnace and those out-side the furnace. Furnace pressure is commonly referred to as draft or draft pressure. The draft is maintained slightly negative to prevent the combustion products and ash from being discharged from the furnace into surrounding areas through inspection ports, doors, feeders, etc. For greatest efficiency, the controlled pressure should be as close as possible to atmosphere, thereby minimizing the ingestion of "tramp air" or excess air drawn through the openings in the furnace duct work that cool combustion gases. Furnaces are classified by the method for moving air and other gases through the system. Natural Draft A natural draft furnace uses the stack (chimney) effect. Gases inside the stack are less dense than those out-side the chimney. The gases in the stack will rise, creat-ing a vacuum (suction) which will draw the combustion air into the furnace and combustion gases or flue gas out of the furnace. Natural draft furnaces naturally oper-ate below atmospheric pressure. Induced Draft An induced draft fan draws the gases through the fur-nace and the combustion air into the furnace. An in-duced draft fan makes high stacks unnecessary. Control is accomplished by regulating the fan speed or damper operation. An induced draft furnace is operated slightly below atmospheric pressure. Forced Draft A forced draft furnace uses a fan or blower to force combustion air through the system. Control is accomplished by regulating the fan speed or damper operation. This type of furnace is operated slightly above atmospheric pressure. Balanced Draft Furnaces equipped with both FD and ID fans are called balanced draft systems. To control furnace pressure, it is necessary to maintain a balance between the flow in and flow out of the furnace. Balanced draft furnaces operate at slightly negative pressures to prevent flue gas leakage to the surroundings. However, too low a pres-sure must also be avoided to minimize air leakage into the furnace and, in the extreme, to prevent furnace implosion.

As shown in Figure 10, the FD fan damper is generally manipulated by the air flow controller, and the ID fan damper is manipulated by the furnace pressure controller. When the air flow controller manipulates the flow into the furnace, the pressure will be disturbed unless there is a corresponding change to the flow out of the furnace. An impulse feed forward connection couples the two dampers to minimize the furnace pressure disturbance on a change in air flow. As the impulse decays, external reset to the furnace pressure controller drives the integral component to maintain the new steady state ID damper position. The furnace pressure controller trims the feed forward compensation as required to control the pressure at set point.

DRUM PRESSURE COMPENSATION When the design steam pressure exceeds 500 psig, pressure compensation of the drum level signal is rec-ommended. This is necessary to provide an accurate drum level signal over the full operating range, from am-bient conditions up to normal operating pressure and temperature. The level measurement is a hydrostatic measurement based on the differential pressure devel-oped between top and bottom pressure taps in the drum. The differential pressure developed is a function of both the water level and the densities of the water and steam in the drum. Over a wide range of pressure and temperature, the change in density has a significant effect on the differential pressure. Pressure compensation corrects the differential pressure measurement to represent the actual liquid level in spite of changes in density. Figure 13 shows the derivation of the drum level equa-tion as a function of the differential pressure and the specific gravities of the liquid (G f ), vapor (G g ), and filled reference leg (G r ). All of the specific gravities are refer-enced to the density of water at 68°F, since this is the standard reference

temperature used to calibrate differ-ential pressure transmitters in inches of water. The deri-vation is based on inches of level (L) relative to the bot-tom pressure tap. However, it is conventional practice to define the level range (L 0 ) with zero representing the normal water level. Therefore, the equation derived for L must be biased an amount X B to shift the zero point of the L 0 range. Notice that the portion of the range above (X A ) and below (X B ) the normal water level are not nec-essarily equal and are, therefore, treated as indepen-dent variables.

The differential pressure transmitter is shown with the bottom tap connected to the high pressure connection and the top tap connected to the low pressure connec-tion via the filled reference leg. This connection arrange-ment provides for an output signal that varies directly with level. However, with this arrangement, it should be noted that the pressure at the high pressure connection will always be less than or equal to the pressure at the low pressure connection. This results in a differential pressure range that varies from -X to 0 inches of water. Some s may prefer to reverse these connections, in which case the transmitter output signal must be re-versed prior to compensation with these equations. The first compensation equation assumes the input sig-nal from the level transmitter represents actual differen-tial pressure. If the

differential pressure signal has been configured to represent uncompensated level (instead of differential pressure), the second compensation equation should be used. As shown in Figure 14, the specific gravities of the liq-uid and vapor in the drum are characteri ed functions of pressure based on data available in the steam tables. The reference leg data is treated as a constant and is also determined from steam table data using the actual temperature of the reference leg and normal operating pressure conditions.

L = (P - Gg X + Gr X)/(Gf - Gg) NOTE: All specific gravities (G) are referenced to water @ 68°F L = Lo + Xb Lo = L - Xb Lo = (P - Gg X + Gr X)/(Gf - Gg) - Xb If the -DP input is configured using the uncompensated level range, then: for DPT calibration: -X to 0 and (Lo)unc range: -Xb to Xa [P - (-X)]/X = [(Lo)unc - (-Xb)]/[Xa - (-Xb)] but X = Xa + Xb P + Xa + Xb = (Lo)unc + Xb

P = (Lo)unc + Xb - Xa - Xb P = (Lo)unc - Xa Lo = [(Lo)unc - Xa - Gg X + Gr X)]/(Gf - Gg) – Xb BURNER MANAGEMENT SYSTEM The burner management system includes the following functions.  Interlock systems  Master fuel trip systems (MFT)  Individual fuel trip system  Flame monitoring and trip systems  Ignition and burner sub system control BMS system is designed to manage the control of the following operating boiler conditions.  ESD interlock systems  Burners firing with say fuel gas only, fuel oil only and both  Alarms systems  Monitoring Burner front valves The burners system is designed and constructed with necessary equipments to prevent malfunctions and control boiler safety conditions according to NFPA 8502 General logic The sequences controlled by the general logic are  Boiler shut down conditions  Boiler furnace purge  Natural draft  Main igniter gas valves  Main fuel oil valves  Main fuel gas valves  Fuel gas vent valves  Light off and shut down of gas igniters  Fuel gas burner light off  Fuel gas burner shut down  Fuel oil burner light off  Fuel oil burner shut down  Air

Boiler shut down conditions The primary function of the safety system shall be to monitor pre determined boiler conditions and to initiate a master fuel trip (MFT) when these conditions are not inside the prefixed limits. A second function of the safety system shall be to inform the operator which specific condition initiates the MFT. The cause of the MFT are  Boiler shut down push button operated at local , central control room(CCR) and burner platform.  Low low steam drum level  Failure of both Forced Draft (FD) fans  Low low combustion air flow  Low low fuel gas pressure *  High high fuel gas pressure *  Low low fuel oil pressure *  Low low instrument air pressure  Low low atomizing steam pressure (in case of fuel oil burner only)  High high furnace pressure  High high steam temperature  Loss of all flames  Burner shut off valves failure with at least 1 burner on **  All fuel inputs zero  Main vent valve not close after control time expired. (fuel gas burner firing only) *the MFT is carried out if the boiler is not firing simultaneously both fuel oil and fuel gas. These conditions are enabled 20 sec after the opening of relevant invoice main shut off valves ** shut off valves not closed without flame detection. The MFT sequence will include the following steps  Close all main fuel safety shut off valves  Close all burners fuels shut off valves  Open all vent valves  Close all igniters shut off valves and open vent valves  De energize all sparks  Close fuel oil circulating valves Boiler Furnace purge

The boiler purge system requires a furnace air purge for the following cases  Before any light off of the first igniter after boiler stop  When the flame of the first main burner is not proven within 5 seconds after the main shut off valve leaved the position close  When any condition required for purge listed below is lost during purge operation  When the last burner is taken out off service Before to initiate a boiler purge sequence the following purge permissive conditions must be satisfied.  No shut down cause present  All igniters fuel gas automatic shut off valves closed  Main fuel gas valves closed  Fuel gas main vent valve open  All burners fuel gas shut off valves closed  Main fuel oil valve closed  Fuel oil re circulation valve closed  All burners fuel oil shut off valves closed  No flames detected  No natural draft in progress  No waiting time for air trip in progress  At least 3 air s opened  Combustion air flow >= 25% of MCR  Igniter main valve closed When all the above purge permissives are satisfied “Purge permissive” indication is on, then purge start can be initiated after this “Purge in progress” indication will be on. After 5 minutes of purge time “Purge Completed “ will be on. The boiler purge is always carried out after a MFT, in case the MFT is caused by a loss of both FD fan or the combustion air flow low low the sequence shall be anticipated for the former case by natural draft and for the later case by waiting for air purge sequence. Natural Draft The natural draft sequence shall follow a MFT caused by the loss of both FD fans. The natural draft lasts at least 15 minutes. During this period of time all dampers (burner air s and FD fan inlet/ outlet vane) in the air ages of the unit shall be opened fully open position in order to create as much natural draft as possible to ventilate the unit. At the

end of the natural draft period one of the fan shall be manually started by the operator to initiate the boiler purge sequence. Waiting for air trip This is performed when the combustion air flow is below purge rate at the time of trip. The FD fans shall be continued at the existing rate for 5 minutes (waiting for air trip) and then gradually increased (by operator) to purge rate air flow. During the waiting for air trip air flow should not be increased immediately by deliberate manual or automatic control actions. Gas igniter safety shut off valve The conditions to open the igniter safety shut off valves are  Main fuel gas or main fuel oil open  Igniter shut off valves closed. The conditions to close the igniter safety shut off valves are  No igniter flame detected in the last 5 minutes  Master trip  Low igniter gas pressure or high igniter gas pressure. Main fuel gas valves When the purge is completed the BMS system requests to open the main fuel gas valve, which is selected by operator as initial light off fuel. The main fuel gas valve can be opened by DCS system in CCR. The main fuel gas valve can be open if the following conditions are satisfied.  Boiler furnace purge complete  All burners fuel gas and shut off valves closed.  Fuel gas control valve close to light off position The main fuel gas valve is closed when one of the following conditions occur.  By DCS system in CCR  Master fuel trip  Low low fuel gas pressure  High high fuel gas pressure  The first main gas burner flame is not proven with in 5 minutes after the main fuel gas valve has been opened  Main fuel gas valve not open (after the opening control time expired 20 secs)  Shut off valves of the last burner in operation closed  Burner shut off gas valves failure (shut off valves not closed when flame is not proven)

Fuel Gas vent valve The fuel gas vent valve is automatically closed when are satisfied the following conditions  If the main fuel gas valve is opened and the flame of the burner is proven  During fuel gas leak test sequence The conditions for the automatic opening of the fuel gas vent valve are  Main fuel gas shut off valve closed  Fuel gas vent valve not close after the closing command and control time (5 seconds) expired. BURNER LOGIC Air s Each burner is equipped with one air . The opening and closing of air s are driven by pneumatic actuator by means of 2-3 ways solenoid valve. During the normal operation the position of the air s are automatically controlled by BMS logic. It is possible to control each s manually by local and by push button of DCS. The air s are opened at least one of the following conditions  Manual opening requested by operator  Relevant burner shut off valves not closed  Boiler purge  Natural draft The air s are closed  Manual closing request by operator  Igniter light off sequence (automatic operation)  Burner shut down (automatic operation)  At boiler stop after post firing purge (manually by operator) Light off and shut down of gas igniters (for each burner) The conditions to light off the gas igniter are  No shut down cause present  Associated igniter shut off valve closed  Associated gas burners shut off valves closed  Flame not detected  Main igniter light off valve closed When these conditions are met “ignition permissive” indication will be on, press “igniter start push button” or by requesting gun scavenging, the following sequence starts.  Start the ignition control time of 10sec.  Close air s  Opening igniter shut off valves and closing the relevant igniter vent valve

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Pilot transformer is energized during start igniter control time ( it is de energized 3 sec after the “flame on “ is detected). At the end of the igniter control time, if the flame is present (flame rod), the igniter is “on”.

The causes for igniter shut down are the following  Boiler shut down conditions  Burner stop push button pressed, local or remote  Igniter flame failure (after the control time expired)  Burner ignition control time expired and burner not light with in 3 minutes  Igniter light up failure (1 min of relight interval)  Burner light up failure  Relevant burner on  Igniter safety shut off valve closed  Gun scavenging complete Boiler Drum Level Compensation Used in Boiler7 (MAA) The boiler Drum level measured by the level transmitter will be inaccurate and wide variation can be noticed due to the Boiler Drum Pressure variation. Hence pressure compensation is a must for correct level measurement. From the steam table we will get the variation of Saturated steam density with pressure and this is implemented in DCS by using a Characterizer block. The measurement input to this block is Boiler drum pressure. The density at various pressures such as 0, 10,20, 30, 40, 50, 60, 70, 80 , 90, 100 Bar is mentioned as X1 to X11 and the corresponding density 0.5889, 5.6317, 10.5341, 15.5095, 20.6147, 25.8979, 31.39, 37.1139, 43.1142, 49.4269, and 56.077 are mentioned as Y1 to Y11. The block output will be the saturated steam density at the measured drum pressure and let it be Ys. Similarly the saturated water density is measured by another characterizer block and the block output will be the saturated water density at the measured drum pressure. Let it be Yw. The actual drum level can be calculated by using the equation ‘hm = h * transmitter range - (100 * Ys) H Yw – Ys Where ‘hm = Actual drum level

h Ys Yw H

= = = =

level measured by transmitter in % saturated steam density saturated water density wet leg height

In boiler 7 the transmitter range is 686mm WC and wet leg height is 0.7mtrs. This equation is implemented in DCS by using a Calculator Block. The real inputs to the block are measured Level , the saturated steam and saturated water density at the measured steam drum pressure from the characterizer blocks. The values stored in Memory 1 and Memory 2 are 686 and 0.7 and memory 3 to 24 are zeroes. The following steps are followed to implement the equation. STEP1 IN RIO1 (ex. 40%) STEP2 LAC M1 STEP3 MUL STEP4 LAC M2 STEP5 DIV STEP6 STM M3 STEP7 IN 100 STEP8 IN RI02 density , for ex.

Level measured by the transmitter load accumulator memory 1 = 686 multiply ( 40 * 686) load accumulator memory 2 =0.7 divide ( (40 * 686) / (0.7)) store in memory 3 real input 02 ie. saturated steam The saturated steam density at 60 bar is

31.39 STEP9 MUL STEP 10 STM M4 STEP 11 LAC M3 / (0.7)) STEP 12 LAC M4 31.39) STEP 13 SUB (100*31.39) STEP 14 IN RI03 for ex. STEP 15 DIV 728 STEP 16 OUT R001 level

multiply ie. ( 100 * 31.39) store in memory 4 load accumulator memory 3 (ie. (40*686) load accumulator memory 4 (ie. 100* subtract

(ie,

((40*686)/(0.7))

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real input 03 ie. Saturated water density , At 60 bar this is 728 divide ie. ((40*686)/(0.7) – (100*31.39) / output real output. Ie. The compensated At the measured steam drum pressure.

Note :- the equation implemented in DCS is

‘hm = h * transmitter range - (100 * Ys) H Yw Instead of ‘hm = h * transmitter range - (100 * Ys) H Yw – Ys