Safety Control Requirements For Burner Of Gas-Fired Steam Boiler
Safety control requirements for burner of gas-fired steam boiler
As China is rich in natural gas and coal to gas (using 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 the market prospect and development prospects are very broad. However, in the development and design of the 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 state monitoring, ignition protection, flameout protection, high and low-pressure limit protection, and insufficient pressure protection, power-off 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 residual gas of the burner is unavoidable in the working furnace, there is no pre-blowing point, and there is a risk of explosion. 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 to 15-60 seconds, which is related to the furnace structure and soot blowing amount.
2. Automatic Ignition
Electric spark ignition shall be adopted for the burner to facilitate automatic control. The high-voltage ignition transformer can generate arc ignition, and its output energy is required to be greater than 3.5 kV, current ≥ 15 mA, and ignition time is generally between 2 and 5 seconds.
3. Combustion State Monitoring
The combustion state must be dynamically monitored. Once the flame detector detects the flameout signal, it must be fed back to the burner in a very short time. The burner will immediately enter the protection state and cut off the gas supply. The fire alarm can normally sense the flame signal, which is neither sensitive nor slow. Due to the high combustion sensitivity, if there is fluctuation, it is easy to generate misoperation, and the feedback flame signal lags, 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 the formation of an ignition temperature field in front of 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 entering the protection state shall be appropriate and shall not be too short or too long. If it is too short, there is no time to form a stable flame; If it is too long, 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 intake valve. If it does not catch fire, it will enter the protection state. If it catches fire, it will continue to burn.
5. Flameout Protection
The burner is accidentally flamed out during the combustion process, and the burner enters the 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 entry protection shall not exceed 1s.
6. Air Pressure High And Low 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 flow limiting gas is to ensure the stability of flame: limit the output heat of the burner and ensure the safe and economic operation of the equipment under the condition of no de ignition, no flameout, and no tempering. 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 switching 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 blowing. In case of air interruption or insufficient air caused by fan failure, the gas shall be cut off immediately, otherwise, the furnace will explode or backfire. Therefore, while improving the quality of the fan, 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 Off Protection
In case of sudden power failure of the burner during operation, the 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 closed automatically. Response time of solenoid valve closing ≤ 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 through the solenoid valve core end facing the furnace. Special attention shall be paid to the environment that may cause poisoning and explosion. First, ensure the pipeline is sealed, and regularly check the leakage of the pipeline. If the leakage of the pipeline must be eliminated, it can be used again; Secondly, to avoid the 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 provided; In addition, it is required to prohibit fireworks and explosion-proof electrical parts at the work site.
Leakage in the furnace may cause an explosion in the furnace. There are three ways to solve the leakage in the furnace: first, strengthen the pre-blowing time and the blowing quantity to remove or dilute the gas in the furnace; Second, the series structure of double solenoid valves is adopted to improve the safety of the system; Third, the pipeline leakage detection device is used to detect the gas pipeline before ignition. If the gas leakage reaches a certain amount, lock the burner.