1 Overview:
Due to the characteristics of gas itself, which are flammable, explosive and easy to spread, gas boilers are more dangerous than coal-fired boilers, and have strict requirements on product quality and safety performance of the control system. This article only provides a preliminary discussion on the safety issues of medium and small gas boilers.
2.Safety device
"Safety is the life of the gas industry." Without safety, there will be no benefits and no development.
2.1 Automatic alarm device
When a gas boiler is running, the temperature and pressure of water and steam in the boiler, as well as the water level in the steam boiler, are monitoring parameters to ensure its safe operation. When these parameters are abnormal, the automatic alarm device should send out audible and visual signals.
⑴Over-temperature alarm
"Regulations" stipulates: When the rated outlet hot water temperature of a gas hot water boiler is higher than or equal to 120°C and the rated outlet hot water temperature is lower than 120°C but the rated power is greater than or equal to 4.2MW, an over-temperature alarm device should be installed and there should be Interlock protection, for small and medium-sized gas boilers, the temperature transmitter on the hot water outlet generally outputs a standard signal to the programmable logic controller (PLC). The standard value is compared with the given value. Once the temperature is overtemperature, the PLC immediately Emergency shutdown.
⑵Overpressure alarm
When the rated evaporation capacity of the steam boiler is greater than or equal to 6t/h, a steam overpressure alarm should be installed, and an interlock protection overpressure alarm should be installed. At the same time, the solenoid valve should be closed and the gas source should be cut off. The induced draft fan is still running. The boiler steam pressure is generally determined by the pressure The controller performs automatic control.
⑶Water level alarm
When the rated evaporation capacity of the steam boiler is greater than or equal to 2t/h, a high and low water level alarm should be installed, and the high and low water level alarm signals should be able to distinguish. When a low water level alarm occurs, close the solenoid valve, cut off the gas source, and stop the blower after 30 seconds. The induced draft fan Still running.
⑷Flow alarm
DC boilers should be equipped with an alarm device when the feed water flow rate is lower than the starting flow rate.
2.1.2 Cut-off device
Gas boilers should be equipped with interlocking devices. Once the situations described in (1) to (5) occur during boiler operation, the gas supply can be automatically cut off; for boilers equipped with blast burners, the air supply source can be automatically cut off.
(1) The pressure of the hot water boiler is reduced to the point where vaporization will occur or the water temperature rises to exceed the specified value;
(2) The steam boiler pressure exceeds the specified value or the water level is as low as the minimum value;
(3) When the gas pressure is too high, defire will occur, and even an explosion will occur in the furnace. Therefore, the burner must operate under the designed gas pressure. If the gas pressure is too high or too low, the cut-off device should be activated. At present, domestic gas boilers are equipped with imported burners with a high degree of automation. They are not only equipped with gas high and low pressure controllers, but also equipped with automatic leak detection controllers (leak detection control before ignition and during the furnace purging process). The device automatically detects leaks in the gas valve group system);
(4) The cutting device is activated when the air supply fan, induced draft fan, and circulating water pump of the hot water boiler suddenly lose power or stop running, or when a frequency conversion failure, contact point failure, or other fault occurs;
(5) The smoke temperature protection device is activated: The smoke temperature also reflects whether the boiler is operating normally. Once the smoke temperature exceeds the set value, the cut-off device should be activated.
2.1.3 Spray device
When the steam boiler is equipped with a reheater, it should be equipped with an automatic spray device. When the reheater outlet steam temperature reaches the maximum allowable value, emergency water spraying can be automatically activated for cooling.
2.1.4 Ignition control system and flameout protection device
Compared with coal-fired boilers, gas-fired boilers have very high explosion-proof requirements, especially for natural gas. The explosion limit of natural gas is 5% to 15%. If leakage occurs, it is easy to explode. When the flammable mixed gas explodes in a closed volume, the pressure generated , for different gases, it is about 0.75MPa to 1.65MPa. Therefore, before igniting a gas boiler, the furnace and flue must be carefully purged to eliminate flammable gas that may accumulate in the furnace. Reliable ignition program control and flameout protection are essential safety protection devices for gas boilers.
A reasonable ignition program control system should automatically control the ignition process. Only when the fan completes the task of cleaning the furnace, ignition can be carried out; only when the ignition flame has been established (confirmed by the flame detector) and after a predetermined time has passed, The control valve of the main burner can be opened; the ignition flame can automatically extinguish after a predetermined time; when the main flame fails to be ignited within the predetermined time, the main burner can automatically and quickly close when the ignition flame is extinguished. The ignition program is generally monitored and controlled by a program controller.
In the ignition program control, the total ventilation volume before ignition should not be less than 3 times the total volume from the furnace to the chimney entrance, and the ventilation time should not be less than the value specified in Table 1.