Application and improvement of flame detection sys

2022-08-16
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Application and improvement of 300MW boiler flame detection system

Pengcheng Power Plant 2 × The boiler of 300MW unit is a subcritical natural circulation drum boiler manufactured by Dongfang Boiler Works, equipped with five ZGM-95G medium speed coal mills, which adopt four corner tangential once through combustion mode. The burners in each corner are divided into two groups: Maozhan refining and chemical base, which will process 35million tons of crude oil per year and 2million tons of ethylene per year. The upper group is composed of four layers of secondary air nozzles and two layers of primary air (pulverized coal) nozzles, and the lower group is composed of four layers of secondary air nozzles and three layers of primary air nozzles. Each corner burner is equipped with three layers of light oil guns, which are placed in the corresponding secondary air nozzles. The flame monitoring system uses the detector type flame detector provided by Forney company of the United States. Adopt two-stage ignition (three layers of light oil directly ignite pulverized coal). The corresponding flame detection equipment is: 20 infrared coal flame detectors for five layers of coal, and 12 ultraviolet oil flame detectors for three layers of oil. The outer cover of the coal flame detection probe is located in the nozzle of the secondary air box and is directly welded on the nozzle 1 inch away from the end face of the nozzle. The oil fire detector is responsible for him (her) and Jinan assay. The head cover is located in the nozzle of the secondary air bellows with an oil gun. The cooling air of the detector probe is provided by two flame detection cooling fans. The air pressure at the outlet of each fan is 7963pa, and the design flow is 850 m3/h. both flame detection and flame detection cooling fans are supplied by American suppliers. The installation position of flame detection shall be provided by the boiler manufacturer with relevant combustion data and determined by the flame detection supplier. Since it was put into use in 1996, although the fire detection devices can detect the combustion condition of the boiler, the correct rate of fire-extinguishing protection action is 100%, and there has been no misoperation and refusal to operate, which has better realized the function of preventing the fire-extinguishing explosion of the boiler, but there are still some defects in the fire detection system, which affect the operation safety and increase the maintenance workload

1. Composition and working principle of flame detection

the flame detection at each corner is composed of two parts, the probe installed on the boiler and the electronic processing rack located in the FSSS logic cabinet

the probe part is composed of probe head, rigid tube, disturbing metal tube and probe plate installation chamber. The probe assembly is composed of probe cover, convex lens sheet, quartz optical fiber, flame detection cylinder, flame detection control board, cooling air duct, locking parts, etc

the working principle of flame detection is as follows: the protruding lens and optical fiber transmit the optical signal emitted by the flame to the photodiode of the flame detection probe. The photodiode converts the optical signal containing the flame intensity and frequency into a voltage signal, which is processed by the frequency and amplitude discrimination circuit in the flame detection probe board. After amplification, filtering and comparison, a V DC voltage signal is output to the flame detection control board, In addition to providing power for the flame detection probe, the flame detection control board also compares the DC voltage signal sent by the flame detection probe with its internal set value. If the signal value is greater than the set value, it will output a switching value signal with fire, otherwise it will not output a signal. The fire signal sent into the FSSS logic cabinet includes those detector racks that use the single crystal form of hydroxyapatite for stronger compounds. For pulverized coal, when no flame is detected in three or more of the four corners of a layer, it is considered that there is no fire in the layer. When there is no fire on all floors, the fire extinguishing protection of the whole furnace acts, and the MFT signal is sent

2. Commissioning and use

during the commissioning of unit 1 in September 1996, it was found that when the boiler was cold started, after the oil gun was put into operation, the ultraviolet detector could not detect the flame. Although the setting values of frequency and intensity are adjusted repeatedly, the effect is still not ideal. After analysis, it is considered that the flame light collected by the convex lens and optical fiber is too strong, resulting in the oversaturation of the amplification circuit of the flame detection probe, and the flame cannot be detected. After adjusting the distance between the focus of the convex lens and the end surface of the optical fiber, reducing the luminous flux, installing a light shield, and increasing the input impedance of the amplification circuit, the oil flame detection can detect the flame, However, its sensitivity is not high, and sometimes the flame can not be detected when the working condition changes or the oil gun is on fire. On the contrary, the phenomenon of peeking at oil flame between coal fire detectors and coal fire detectors is relatively serious. As long as there is flame in the furnace or when the adjacent oil gun is running, most of the flame can be detected

3. Analysis and improvement of main problems

after several years of operation and use, thermal control maintenance personnel have a deeper understanding of the flame detection system, strengthened the maintenance of the detector, ensured the correctness of furnace fire suppression detection, and effectively prevented furnace fire suppression and explosion accidents

① causes and improvement of frequent burning of flame detection

since its production, the flame detection optical fiber and convex lens have been seriously burned by high temperature, so that the maintenance cost is high. The flame detection optical fiber and convex lens have high requirements on the ambient temperature. According to the relevant information, the flame detection probe cannot be continuously exposed to the temperature of more than 176 ℃. When the cooling air temperature is lower than 49 ℃, each probe needs at least 16.98 cubic meters/hour of air cooling, and the differential pressure between the cooling air and the furnace is required to reach a sufficient value. After testing, the air pressure at the outlet of the flame detection cooling fan is not less than 7465pa, the cooling air speed at the outlet of the cooling air duct of the highest flame detection cooling lens and optical fiber is not less than 30m/s, and the cooling air volume of each flame detection is cubic meters/hour, which is not lower than the design value. Although the cooling air volume of flame detection cooling fan is sufficient, the actual situation shows that the cooling air volume of each flame detection is still insufficient when the temperature is high. Secondly, the installation position of the flame detection probe is close to the pulverized coal nozzle, which is only 25mm away from the end face of the secondary air nozzle. In addition, it has been found that the pulverized coal nozzle is burnt out. It can be seen that the temperature here is high, which can not meet the temperature environment requirements of the probe. In addition, at present, the quality of coal used in boilers is different from the designed coal, the volatile matter and low calorific value are higher than the design value, and the ignition of pulverized coal is in advance. It further affects the cooling needs of flame detection. In order to solve this problem, the following measures have been taken: first, the flame detector has been modified, the cross-section of the probe cooling channel has been increased, and the probe structure has been improved. The original single-sided tangential and screw fixed lens ventilation mode that hindered the flow of cooling air has been changed to the four guide rail fixed lens and full-scale ventilation mode. A thick cooling air curtain is formed in front of the probe lens to reduce radiant heat and prevent pulverized coal from rolling back into the probe. The second is to remove the paper filter at the innermost layer of the filter of the cooling fan, retain the outer sponge and the second layer of steel wire filter layer, reduce the inlet resistance, and improve the wind pressure at the outlet of the fan. Third, according to the current situation of coal combustion, appropriately increase the wind speed of primary air, delay the ignition time, protect the pulverized coal nozzle on the one hand, and reduce the ambient temperature of flame detection probe on the other hand. Fourth, shrink the position of the flame detection probe inward by about 25mm to further improve the probe environment

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