The hottest question on the safety of atmospheric

  • Detail

Query on the safety of atmospheric and natural circulation heating hot water boilers

classification number: tk229 document identification code: a

article number: (2000) query to safety of atmospheric pressure boiler and

natural circulating hot water boiler Ke jia

(Beijing Fengtai boiler works, Beijing 100083) Abstract: the paper put solution is to check whether the power line connected to the experimental machine is connected properly; Check whether the emergency stop switch is screwed up; Check whether the power supply voltage connected to the experimental machine is normal; Check whether the fuse on the machine socket is burnt forward some different viewpoint on safety of atmospheric pressure and natural circulating hot water boiler

Key words:Hot water boiler; Safety technology; Design; Operation; Query ▲ 1. There are two "big breakthroughs" in the design of hot water boilers for heating in China. One is the promotion of natural circulation availability in the 1980s, which was later popularized on a large scale; Another is the atmospheric pressure boiler in the 1990s, which has also been widely promoted. The author believes that it may be feasible to make a very conditional recognition of the two, but it is problematic that the two "breakthroughs" are almost unconditionally appreciated and recognized, which needs to be discussed again. 2. Analysis of the operation safety of atmospheric pressure boilers in terms of atmospheric pressure boilers, we want to ask: why does the National Bureau of standards stipulate that the rated pressure of a 95/70 boiler with an outlet water temperature of only 95 ℃ is 0.4MPa (less than 0.7mw) and 0.7MPa (more than 0.7mw)? Why "Article 127 of the safety and Technical Supervision Regulations for hot water boilers stipulates that" the outlet pressure of steel hot water boilers should not be lower than the rated outlet hot water temperature plus the corresponding saturation pressure of 20 ℃? "? The rated pressure is the design pressure, while the "safety regulation" refers to the operating pressure. In short, there should be pressure, and the promotion of atmospheric pressure cannot be said to be in contradiction with the above provisions

those who advocate atmospheric pressure boilers think that without pressure, there is no need to worry about safety (without high-pressure blasting), and the strength problem can be basically eliminated. Thin materials and relatively inferior materials can be used to save material costs. However, the author believes that generally speaking, the establishment of atmospheric pressure boilers only improves the mechanical safety (reduces the strength requirements), but reduces the thermal safety - it is more prone to saturation boiling, especially surface boiling, and is more prone to hard scaling and tube explosion on the hot wall

now with  φ fifty-one × 3. Taking the furnace water wall tube as an example, the heat load intensity of the furnace tube is about 60000kcal/m2. Now the internal heat transfer coefficient under various water pipe flow rates, the temperature difference between the tube wall and water, the temperature of the inner wall of the tube and the saturation temperature at this temperature are calculated. The calculation results are shown in Table 1. Table 1 water velocity W, (m/s) heat transfer coefficient α, (kcal/m2℃)696. Wall temperature and water temperature difference, Δ t. ℃ 90.451.937.5729.82521.6 wall temperature, tw=95+ Δ t,℃185.4146.9132.57124.. 6tw corresponds to the saturation pressure, and the heat transfer coefficient in mpa1.1390.440.2930.2330.1980.108 is calculated according to the following formula: (1) the reason why the wall temperature is taken as 95+ Δ T is to take the rated outlet water temperature as the bottom temperature from the perspective of safety

it can be seen from table 1 that the wall temperature has a great relationship with the water flow rate in the pipe. If the fast installed hot water boiler adopts pure natural convection circulation, the flow rate in the pipe (riser) is difficult to reach 0.2m/s (calculated that the flow rate in the riser of many boilers is mostly less than 0.1m/s). At this time, if it is required that there is no surface boiling, the water pressure is required to be 0.44Mpa, and the gauge pressure is required to be 0.4MPa. It is assumed that the forced circulation of the riser flow rate of 0.6m/s in the former Soviet standard is adopted, If the water pressure is 0.1MPa, the surface can be prevented from boiling, but the detection of self-adhesive is controlled by those of us who do experiments

even if the design pressure is 0.7MPa, the actual pressure in operation depends on the system flow resistance. The flow resistance of the thermal system corresponding to the small hot water boiler often makes the boiler operating pressure 0.4 to 0.5MP. Establish a regional coal storage center a (the gauge pressure is 0.3 to 0.4MPa). If the flow rate of the rising pipe can ensure more than 0.2m/s, it is possible to prevent surface boiling. In addition, in operation, it is often shown that the low temperature difference between inlet and outlet water is 70 ℃ ~ 80 ℃ or 65 ℃ ~ 85 ℃. At this time, there will be a lower density difference, resulting in smaller lift and smaller natural circulation flow rate, making surface boiling easier to occur

some people may think that if water does not contain saline alkali, surface boiling is not terrible. The problems are: (1) the area of water containing calcium and magnesium salts in China is quite wide, and more and more groundwater is used. The wells are deeper and deeper, and the salt concentration is higher and higher; (2) The softened water used is cation exchange type, and the replacement of calcium with sodium is not complete. Due to the poor training of the operators who use small hot water boilers, they often do not restore the working capacity of water treatment equipment with salt water, and do not often remove the scale mechanically and remove the acid and alkali. The above two reasons have shortened the actual service life of atmospheric and natural circulation boilers by many times

another factor that should also be considered is that now, atmospheric pressure boilers can be operated by anyone, and labor departments basically can't manage them. Many units that completely don't understand boiler design, production, installation and operation can be operated, and become "mushrooms", which will actually form a situation of far more frequent accidents

in the design and manufacture of atmospheric pressure boilers, the use of materials is not much saved, and the pipes are the same φ fifty-one × 3. As for the drum, head or tube plate, there is a provision of not less than 6mm in the design. How much can 6mm save compared with 8mm or 10mm? Compared with the overall burning after frequent tube explosion and scaling in the future, it is not necessarily economical. Conclusion the author believes that the normal pressure and natural circulation heating hot water boilers should be promoted conditionally, such as: ① household electric heating boilers and oil (steam) burning boilers, with a power of 5 × Below 104kcal/h; ② No matter what kind of hot water boiler, it should be carefully calculated, and the circulation mode used will not cause saturated boiling and surface boiling under working pressure

in short, in order to be safe, the hot water boiler must have both circulating flow rate and pressure, and must take the non occurrence of saturated boiling and surface boiling as the necessary conditions in the design. For the incompleteness of the natural circulation of small hot water boilers, see "Engineering

Copyright © 2011 JIN SHI