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The ZS60-4.0/420-Q gas/fuel boiler is a water-tube gas (oil) steam boiler with a vertical double-drum arrangement and a ‘D’ type combustion configuration. Two fully automatic gas and oil dual-fuel burners are installed on the front wall of the boiler furnace. The main components of the boiler body include: upper and lower drum, furnace membrane wall, convection tube bank, superheater, and economizer. 1. System description: The steam and water system of gas/fuel boilers consists of a feed water pump, feed water control valve, economizer, steam drum, and superheater, etc. The boiler feed water passes through flow meter FT29100 and then splits into two paths: one path goes through TV29100 and reaches the temperature reduction valve where it is used as cooling water ； The other route passes through the economizer and then reaches the steam drum. Gas/fuel and air burn in the furnace in a certain proportion; the heat generated causes the water in the steam drum to vaporize. The resulting saturated steam enters the superheater where it is further heated to become superheated steam, which then moves to the steam main. Combustion system: Gas/fuel enters the furnace through control valves. Air is supplied by a blower, preheated in an air preheater, and then sent into the furnace via primary air control valves. The high-temperature flue gases generated during combustion are used to heat steam in a heater; they also heat the feed water in the boiler’s drum. After being preheated in the air preheater, the air is sent into the boiler, and finally, the exhaust gases are drawn into the chimney by an exhaust fan and released into the atmosphere. Temperature and pressure reduction system: Superheated steam passes through a temperature and pressure reducer, where its temperature and pressure are adjusted to those of the desired steam. 2. System Configuration: This system is equipped with 2 monitoring operation stations, 1 backup manual controller console, and 1 field control station. The main controllers operate in hot-redundancy mode, and an online 3kVA uninterruptible power supply is also provided. On the backup manual controller console, there are ammeters for the supply air fan, feed water pump, and circulation pump ； Electrical contact level gauge (drum level) ； Manual operator for make-up air control valve, manual operator for primary air control valve, manual operator for boiler feedwater control valve, manual operator for desuperheating water control valve, manual operator for mixed header inlet control valve, manual operator for gas/fuel flow control valve. 3. Control scheme: Control scheme for the boiler section: Set furnace temperature, pressure, and flame detection ； Detection of main feedwater flow rate, temperature, and pressure ； Detection of the flow rate, temperature, and pressure of superheated steam ； Detection of gas/fuel flow rate, temperature, and pressure. Automatic control device: Drum water level control valve ； Combustion system control valve ； Superheated steam temperature control valve. ① Steam drum water level control: An excessively high steam drum water level can affect the separation of steam and water inside the drum; this leads to an excessive amount of water being carried in the saturated steam, thereby reducing the temperature of the superheated steam ； When the water level is too low and the load increases, the rate of water vaporization accelerates, causing the amount of water in the drum to decrease rapidly. If this is not adjusted in time, all the water in the drum will vaporize, posing a threat to the safety of the boiler. When the load is unstable, disturbances in the feedwater flow cause a significant delay in the drum water level. Changes in steam flow lead to false water levels, making it difficult to operate using the three-input control method. Therefore, it is first necessary to operate using the single-input control method to stabilize the operating conditions before switching to the three-input control method. This requires that the two control methods can be switched over smoothly without any disruptions. ② Control of the combustion system: Purpose of control – to stabilize the boiler steam pressure ； Ensure the economic efficiency of the combustion process ； Maintain a constant negative pressure in the furnace chamber. Control scheme: Fuel and air are controlled using a ratio control method ； Steam pressure control adopts a cascade control approach using steam pressure and fuel flow ； The furnace negative pressure control employs a feedforward-feedback control system composed of an exhaust air control valve and an intake air control valve. Note: a. After the main feedwater control valve, the system is equipped with a mixing header feedwater control valve and a desuperheater feedwater control valve. Considering the stability and reliability of the system control valves, the steam temperature is regulated by the feedwater control valve of the desuperheater, while the feedwater control valve of the mixing header is kept fully open with no regulation applied. b. The control valve for the steam pressure in the combustion system is distributed evenly among two primary air control valves and two gas/fuel control valves. ③The temperature control valve at the desuperheater outlet uses a single-loop control system. ○4. The temperature and pressure reduction section is equipped with temperature and pressure control valves, operating on a single-loop control principle. 4. Boiler protection interlock for shutdown: The boiler is equipped with an emergency shutdown button. Shutdown protections include: excessive and insufficient drum water level ； Furnace pressure is excessively high ； The drum pressure is excessively high ； Gas/fuel pressure is too low ； The furnace has gone out. When the flame detector switch is closed, the emergency gas/fuel shut-off valve is closed. 5. Operating parameters of the SZS60-4.0/420-Q gas/fuel boiler are shown in the table below: Name, Symbol, Data; Design fuel: Dry gas, light fuel oil; Evaporation capacity, D: 60 t/h; Operating pressure, P: 4.0 MPa; Superheated steam temperature, t: 420°C; Flue gas temperature, t: 160°C; Calculated fuel consumption, Bj: 6380 m3/h; Design efficiency, η: 92.2%; Feedwater temperature, t: 105°C. a. Operating mode: The boiler is designed for operation at constant pressure. b. Furnace pressure: The boiler operates with balanced ventilation and under negative pressure. Under normal operation, the furnace pressure is -20 Pa. The alarm limits for furnace pressure are ±500 Pa, while the protection action limits are ±2000 Pa. c) Drum water level: The normal water level in the drum is 50 mm below the center line of the drum. The alarm limit for the drum water level is ±50 mm, and the protection action limit is ±70 mm. d) Steam temperature variation: The temperature of the superheated steam varies within the range of 420±10°C