Explosion proof control system of boiler furnace i

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The explosion-proof control system of boiler furnace in power plant

the traditional thermal control device adopts the assembled instrument with discrete components, which has a large number of hardware and imperfect system design functions. With the development of thermal control devices of large thermal power units, the control system has the characteristics of reliable hardware, large memory capacity and strong software function, which greatly improves the automatic control function of the unit and the furnace explosion-proof control system is also becoming more and more perfect

the traditional furnace pressure control system is a simple single loop control system, which uses the furnace pressure signal to directly control the opening of the moving blade or guide vane at the inlet of the induced draft fan to maintain the furnace pressure. The modern control system uses the opening of the moving blades of the forced draft fan representing the total air volume as the feedforward signal, and the furnace pressure as the main regulating signal to control the opening of the moving blades or guide vanes at the inlet of the induced draft fan to maintain the furnace pressure at the desired set value. The traditional automatic regulation system only regulates the furnace pressure without protection function. When the measured value of furnace pressure deviates greatly from the set value, the automatic regulation system will switch to manual mode and send an alarm signal to the operator for manual processing. The modern furnace pressure control system based on computer unifies and coordinates the operation procedure, pressure regulation, interlocking and protection, providing a reliable safety assurance system for the equipment. When the furnace pressure shows signs of accident, the control system can automatically take appropriate measures to control the furnace pressure, prevent or reduce accidents, and avoid expanding accidents due to the untimely operation of operators

1 classification and cause analysis of furnace explosion

furnace explosion can be divided into furnace external explosion and furnace internal explosion

1.1 furnace explosion

the basic cause of furnace explosion is to ignite the combustible mixture accumulated in the furnace or in the limited space of the channel or smoke exhaust system connected to the boiler. When the dangerous combustible mixture accumulated in the furnace is fully mixed with the air in a certain proportion, if the fire source exists, it will lead to rapid or uncontrollable combustion, resulting in huge explosion, adding a little additive force, resulting in furnace damage

the factors causing furnace explosion are mostly related to the operation of boiler furnace. Experience shows that the following conditions may cause furnace explosion:

(1) when the interruption of fuel or air or ignition source is enough to cause the instantaneous loss of flame, the polymer in the furnace shall be ignited immediately or delayed

(2) fuel leaks into the shutdown furnace and ignites the accumulated polymer in the furnace with electric spark or other ignition sources

(3) repeated unsuccessful ignition without sufficient purging leads to the accumulation of explosive mixture

(4) some burners lose flame or incomplete combustion, which will cause the explosive mixture of fuel and air to accumulate in the furnace

(5) the loss of flame in the whole furnace leads to the accumulation of explosive mixture of fuel and air in the furnace. Without sufficient purging, spark or other ignition sources are used to ignite the accumulated polymer in the furnace

(6) blow with excessive air volume, causing smoldering of combustibles

(7) when the conditions in the furnace are disordered or the control system fails, it may cause the fuel/air mixture to extinguish the fire. When the combustible fuel/air ratio is re established, it will ignite again. Then, the combustible mixture accumulated in the furnace or in the dead zone of other parts of the boiler under the condition of furnace disorder will explode during ignition

(8) most furnace explosions occur after the boiler is extinguished and re ignited without sufficient purging, caused by titanium alloy, stainless steel, aluminum alloy, etc., or some burners of the boiler are extinguished or some burners are not fully burned

1.2 Furnace Implosion

Furnace Implosion refers to the phenomenon of equipment damage caused by low flue gas side pressure. Furnace explosion can often attract everyone's attention and prevention, but furnace explosion is easy to be ignored

1.2.1 cause of Furnace Implosion

(1) the equipment regulating the gas flow of the boiler (including air supply and flue gas removal) malfunctioned, resulting in the furnace bearing excessive induced draft head

(2) due to the rapid reduction of fuel input or MFT, the gas temperature and pressure in the furnace drop sharply

1.2.2 analysis of the causes of Furnace Implosion

(1) human error is one of the important causes of furnace implosion, which is generally manifested in the following aspects:

① lack of understanding of the correct operation procedures and wrong use methods of safety devices and equipment

② the equipment or its control characteristics are not easy to operate

③ various elements of boiler combustion system and their control are lack of coordination

(2) poor design of control function

1.3 measures to prevent boiler explosion

1.3.1 design and install fire extinguishing protection devices

modern large-scale thermal power plant boilers should be equipped with fire extinguishing devices (FSSS). For example, when the furnace pressure is too low or the corner flame is lost (when three or more coal feeders are running, all burners in any corner of the furnace cannot monitor the flame) or the critical flame is lost (within 15s, 50% of all burners in operation cannot monitor the flame), or the whole furnace flame is lost, the boiler MFT acts. After the MFT acts, it must meet the purging conditions to prevent the accumulation of combustible mixture in the furnace, such as the completion of the fuel leakage test, the closure of the outlet baffle of the coal mill, the closure of the hot air isolation valve of the coal mill, and the 25% - 30% rated air flow. Only after the furnace is continuously purged for 5min, can the MFT relay be reset and the boiler be allowed to ignite

1.3.2 improve control function design

the design and installation of furnace pressure control system, as well as the functional objectives and control of the whole system components should be integrated. Its basic operation objectives are:

(1) the operation procedure with the least manual operation should be established

(2) all operating procedures shall be standardized. Adopt interlocking mode to minimize improper operation procedures. When the condition continues to be incorrect, the running program should be interrupted. It is particularly important to set up purging and start-up procedures that are strictly implemented and interlocked as necessary

(3) for boilers that adopt the ventilation ignition procedure, the furnace ventilation volume shall be maintained equal to or greater than the air volume flow of furnace purging during all operation periods

2 protection, interlocking and signal of furnace explosion prevention

2.1 furnace high positive pressure protection circuit

explosion during furnace ignition is more serious than explosion during operation, which can be explained by the following thermodynamic law. Assuming that the fuel entering the furnace is B (kg), its heating capacity is Q (kj/kg), the furnace volume is V (M3), the constant volume specific heat in the furnace is CV (kj/m3 ·℃), and the temperature rise is △ t (℃), the following equation can be obtained:

at the moment of explosion, assuming that the heat transfer process in the furnace is a constant volume adiabatic process, according to the laws of Thermodynamics:

P1/P2 = T1/T2 = T1/(T1 + △ T)

P1, P2 in the formula - before explosion Rear furnace pressure

t1, T2 - furnace temperature before and after explosion

can be obtained from the above two equations:

p2 = P1 [1 + bq/cvvt1]

it can be seen that the lower the temperature T1 before explosion, the greater P2 after deflagration. At the beginning of ignition, the furnace temperature is low, and the calorific value of the fuel used is high. However, in normal operation, T1 is high, and the calorific value of the fuel is low. Therefore, the explosion during ignition is more destructive, sometimes even causing damage to the whole boiler. The explosion during ignition is commonly known as cold shooting, and the explosion during operation is commonly known as hot shooting. Cold blasting mostly damages the lower furnace, while hot blasting generally damages the furnace top and horizontal flue. In order to prevent explosion accidents in the furnace, the furnace pressure control system is designed with control, interlocking and protection functions for high furnace pressure

the high positive pressure of furnace pressure is often caused by the failure of regulating system, actuator or induced draft fan. Therefore, when the furnace pressure is too high, the control of the induced draft fan moving blade is automatically switched to manual, and it is restricted to turn down the induced draft fan moving blade and prohibit to turn up the forced draft fan moving blade. When the required value of the furnace pressure and the opening of the movable blade of the induced draft fan is the largest, the unit coordinated control system will reduce the load at the rate of 10mw/min until the furnace pressure is normal. When the required value of furnace pressure or the movable blade of induced draft fan is at the maximum opening, the coordinated control system will restrict the unit from increasing load

2.2 furnace high negative pressure protection circuit

when the furnace negative pressure is too low, measures should be taken to automatically prevent the further expansion of the fault. Generally, the following schemes should be adopted:

(1) when the furnace pressure is -500pa, the induced draft fan control is automatically switched to manual, and it is forbidden to open the inlet moving blade of the induced draft fan and to close alpha+ beta; The two-phase area is forged into a slab, and the inlet moving blade of the small forced draft fan

(2) when the furnace pressure is -150pa, the difference between the furnace pressure measurement signal and the given value (-150pa) passes through the negative deviation proportional controller and directly turns down the inlet moving blades of the induced draft fan

(3) when the required value of furnace pressure (-200pa) or the opening of moving blades at the inlet of induced draft fan is at the minimum, the load reduction is limited. The furnace pressure is too low, which is often caused by the failure of the regulating system or actuator, resulting in the excessive output of the induced draft fan. Therefore, if conditions permit, it is necessary to appropriately increase some loads or open the inlet moving blades of the forced draft fan

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