According to the analysis of the opening action characteristics of the spring type safety valve, it can be concluded that under the action of the external force FW, the change rule of the external additional force FW during the whole process from the closing to the opening of the safety valve and then from the opening to the closing, When the additional force is gradually increased from zero, and the sum of the internal pressure PL×S is just the spring preload, the valve is slightly opened, increasing the medium acting area S, so that the internal pressure acting to overcome the spring preload is sharp. As a result, the result is an external additional force that is reduced in an instant. Thus, there are * characteristic peaks A. When the external additional force gradually decreases and reaches the closing point, since the area of ​​the medium is suddenly reduced, in order to maintain the balance of the force, at this time, the external additional force will appear to rise instantaneously, that is, the second characteristic peak point B. The above two characteristic peak points A and B are the technical basis for detecting the opening pressure and the return pressure of the safety valve under the on-line condition. Spring-type safety valves are divided according to the structure, and are divided into vertical hammer type, lever type, spring type and pilot type (pulse type); according to the valve body structure, they can be divided into closed type and non-closed type. Enclosed safety valves, ie, excluded media, are not leaked and are all discharged along the outlet to a designated location, typically used in toxic and corrosive media. For safety valves for air and steam, non-closed safety valves are often used. The selection of safety valve products should be determined according to the actual sealing pressure. For spring-loaded safety valves, springs with several working pressure levels within a nominal pressure (PN) range. In addition to the safety valve type, name, medium and temperature, the valve body sealing pressure should be noted. Otherwise, it is supplied according to the large sealing pressure of zui. All safety valves should be installed vertically. There should be no resistance at the exit of the safety valve to avoid pressure. The safety valve should be specially tested before installation and checked for its official seal. The safety valve in use should be inspected regularly. Safety valve installation should meet the following requirements: (l) For boilers with a rated evaporation of more than 0.5t/h, install at least two safety valves: boilers with a rated evaporation of less than or equal to 0.5t/h, with at least one safety valve. Dividable Safety valves must be installed at the exit of the economizer and at the outlet of the steam superheater. (2) The safety valve should be installed vertically in the high position of the boiler and the box. Between the safety valve and the drum or header, the outlet pipe and valve for steam should not be installed. (3) The lever type safety valve shall have a device for preventing the weight from moving by itself and a guide frame for restricting the deviance of the lever, and there shall be a device for lifting the handle and preventing the adjusting screw from being twisted. (4) For boilers with rated steam pressure less than or equal to 3.82 MPa, the safety valve throat diameter shall not be less than 25 mm: for boilers with rated steam pressure greater than 3.82 MPa, the safety valve throat diameter shall not be less than 20 mm. (5) The connecting pipe of the safety valve and the boiler shall have a sectional area not less than the inlet cross-sectional area of ​​the safety valve. If several safety valves are installed together on a short pipe directly connected to the drum, the cross-sectional area of ​​the short pipe should not be 1.25 times that of all safety valves. (6) Safety valves should generally be equipped with exhaust pipes. The exhaust pipes should be straight to a safe place and have sufficient cross-sectional area to ensure unobstructed exhaust. The bottom of the safety valve exhaust pipe should be filled with a drain pipe that is connected to a safe place, and no valve is allowed on the exhaust pipe and the drain pipe. Comparison of main manufacturers of domestic safety valves and selection of connection sizes There are many manufacturers of safety valves in China, and the connection sizes are mostly not uniform. Mainly divided into the following major categories: (1) A general class based on JB/T2203-1999 "Structure Length". At present, most domestic safety valve manufacturers are designed and produced according to this standard. However, this standard is not perfect, the specifications are not complete, the micro-open safety valve Zui large nominal diameter is D NI 00, full-open safety valve zui large nominal diameter DN200, the lack of DN65, DN125 two specifications. According to the specifications of the safety valve produced by our factory and the information we have mastered, the nominal diameter of the micro-opening safety valve is DN250, and the nominal diameter of the full-open safety valve is DN400. According to my research, the connection sizes of various manufacturers are not uniform. For example, DN150 full-open safety valve, in order to have a unified standard, the user can exchange the same specifications during the selection and installation. It is recommended that Hefei General Machinery Research Institute JB/T2203 -1999 "Structure Length" was revised. It is recommended that the design institute and users select according to the standard, and the safety valve manufacturer designs and manufactures according to the standard. 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The following should be noted for the installation and maintenance: