Safety control requirements for burners of gas-fired steam boilers

Safety control requirements for burners of gas-fired steam boilers
Because China is rich in natural gas and coal-to-gas (coal as raw material) resources and belongs to clean energy, it has good social and economic benefits. The gas-fired boiler conforms to the industrial policy of our country, and its market prospects and development prospects are very broad. But in the development and design of gas burner, its combustion, explosion and toxicity are the main problems. According to the combustion characteristics of the gas in the furnace, the safety control requirements mainly include pre-blowing, automatic ignition, combustion status monitoring, ignition protection, flameout protection, high and low pressure limit protection, insufficient pressure protection, power failure protection, gas leakage prevention, etc.
1. Pre-blowing
Before ignition, the pre-blowing burner must have a period of pre-blowing to remove or dilute the residual gas in the furnace and flue. Since the burner residual gas inevitably has residual gas in the working furnace, there is a risk of explosion if there is no pre-blowing point. In order to ensure that the gas concentration is not within the explosion limit, it must be blown out or diluted. The pre-blowing time is generally set at 15-60 seconds, which is related to the furnace structure and soot blowing volume.
2. Automatic ignition
The burner shall be ignited by electric spark for automatic control. The high-voltage ignition transformer can produce arc ignition, and its output energy is required to be: voltage greater than 3.5 kV, current ≥ 15 mA, and ignition time is generally between 2-5 seconds.
3. Combustion status monitoring
The combustion state must be dynamically monitored. Once the flame detector finds the flameout signal, it must feed back to the burner in a very short time. The burner immediately enters the protection state and cuts off the gas supply. The fire alarm can normally sense the flame signal, which is neither sensitive nor insensitive. Due to the high combustion sensitivity, if there is any fluctuation, it is easy to cause misoperation, and the feedback flame signal lags behind, which is not conducive to safe operation. Generally speaking, the response time from flameout to flame detector shall not exceed 0.2 seconds.
4. Ignition protection
When the burner is on fire, the gas enters and ignites. The ignition process requires that the ignition temperature field be formed before the gas inlet to facilitate ignition and combustion. If there is no fire, the flame detector cannot sense the flame signal, and the burner will enter the protection state. The time from ignition to protection should be appropriate, not too short or too long. If it is too short, there is no time to form a stable flame; If it is too long, the ignition will not cause a large amount of gas, which will enter the furnace. Generally, the burner needs to judge the flame signal of the detector within 2-3 seconds of the inlet valve. If it is not on fire, it will enter the protective state. If it is on fire, it will continue to burn.
5. Flameout protection
The burner flameout unexpectedly during combustion, and the burner enters into protection state. Because the furnace is hot, the gas is easy to explode when entering, so it must enter the protection state in a very short time to cut off the gas supply. This process requires that the response time from flameout to entering the protection state shall not exceed 1 second.
6. High and low pressure limit protection
The gas burner burns stably within a certain range, and only the air pressure is allowed to fluctuate within a certain range. The purpose of low pressure of flow limiting gas is to ensure the stability of the flame: limit the output heat of the burner and ensure the safe and economic operation of the equipment without de-firing, flameout and flashback. When the gas pressure exceeds this range, the burner shall be locked. The burner design generally uses the air pressure switch to sense the pressure signal and output the switch signal to control the corresponding work of the burner.
7. Insufficient air pressure protection
The gas burner is designed with high heat intensity, and the combustion mode is forced by air blast. If the air is interrupted or insufficient due to the failure of the fan, the gas shall be cut off immediately, otherwise the furnace explosion or flashback may occur. Therefore, while improving the fan quality, the gas control must be interlocked with the air pressure. When the air pressure is insufficient, the gas supply should be cut off immediately. Generally, the air pressure switch is used to sense the air pressure signal, output the switch quantity signal, and control the corresponding operation of the air solenoid valve.
8. Power failure protection
In case of sudden power failure of burner during operation, gas supply must be cut off immediately to protect equipment safety. The air control solenoid valve must be closed frequently. In case of power failure, the air supply will be automatically closed. Solenoid valve closing response time ≤ 5s
9. Gas leakage prevention
Measures to prevent gas leakage accidents: gas leakage includes two aspects: one refers to the leakage of gas to the environment through the pipeline, and the other refers to the leakage of gas towards the furnace through the valve core end of the solenoid valve. Special attention should be paid to the environment that may cause poisoning and explosion. First of all, ensure that the pipeline is sealed, and check the leakage of the pipeline regularly. If the leakage of the pipeline must be eliminated, it can continue to be used; Secondly, to avoid gas concentration causing poisoning and explosion, it is required that the work site be well ventilated: permanent ventilation holes and forced ventilation devices shall be equipped; In addition, it is required to prohibit fireworks and explosion-proof electrical parts at the work site.
Leakage in the furnace may cause explosion in the furnace. There are three ways to solve the leakage in the furnace: one is to strengthen the pre-blowing time and the quantity of blowing gas, and remove or dilute the gas in the furnace; The second is to adopt the series structure of double solenoid valves to improve the safety of the system; The third is to use the pipeline leakage detection device to detect the gas pipeline before ignition. If the gas leakage reaches a certain amount, lock the burner to work.