1, Introduction Safety valve is a very important protection valve, widely used in various pressure vessels and piping systems, when the pressure in the pressure system exceeds the specified value, it can automatically open, the excess media emissions To the atmosphere to ensure the safe operation of pressure vessels and piping systems to prevent accidents, and when the system back to the working pressure or slightly lower than the working pressure can automatically shut down. Safety valve work is directly related to the reliability of equipment and personal safety, it must be given attention.   2, the common cause of the safety valve failure analysis and solutions   Safety valve leakage in the normal working pressure of the device, the valve flap and valve seat sealing surface occurred more than allowable degree of leakage, not only will cause leakage of safety valve Loss of media. In addition, the continuous leakage of media will make the hard sealing material is damaged, but the sealing surface of the commonly used safety valve is a metal material on the metal material, although strive to be smooth and smooth, but in the case of media pressure To the absolute leak is also very difficult. Therefore, the working medium is a steam safety valve, at the specified pressure value, if the naked eye at the exit can not see the leak, it is considered the sealing performance is acceptable. Leakage of the valve body junction surface refers to the phenomenon of leakage at the joint surface between the upper and lower valve bodies. The main causes of such leakage are as follows: First, the bolts on the joint surface are not tight enough or tightly pressed, resulting in the joint surface Poor sealing. Elimination method is to adjust the bolt tightness, the tight bolt must be diagonally tight manner, it is best to measure the tightness around the gap, the bolt tight so far, and the junction of the gap around Consistent. Second, the safety valve body with tooth-shaped gasket seal does not meet the standard. For example, the radial seal groove slightly grooved, poor parallelism, tip too sharp or over-the-slope and other defects can cause seal failure. So that the valve body surface leakage. In the maintenance of good spare parts quality, the use of standard toothed gaskets can prevent this from happening. Third, the flatness of the valve body with the flatness of the valve is too poor or hard impurities caused by gasket failure seal. The valve body due to the flatness of the joint surface caused by poor bonding surface leakage, the elimination of the valve is to disintegrate the valve to re-ground the joint until the quality standards. As a result of impurities caused by gasket failure seal in the valve assembly carefully clean the surface to avoid impurities fall into. Main safety valve does not move after the impulse safety valve is actuated This behavior is often referred to as the main safety gate's reflex action. The main security door refuse to move to the boiler in danger is very large, is a major hidden dangers of equipment, seriously affecting the safe operation of equipment, and pressure in the pipeline when the medium pressure exceeds the rated value, the main safety gate No action, so that equipment over-pressure operation can easily cause equipment damage and major accidents. Before analyzing the reason why the main safety valve refuses to move, first analyze the action principle of the main safety valve. As shown in Figure 1, when the pressure in the pressure vessel rises to the full pressure of the relief valve, the impulse safety valve moves. The medium flows from the container through the pipe to the main relief valve piston chamber. There will be a slight expansion in the piston chamber Step-down, if at this time the pressure inside the piston chamber P1, piston throttle area Shs, then acting on the piston f1: f1 = P1 × Shs ........................ (1) If this time commitment The pressure of the medium in the pressure vessel is P2 and the area of ​​the valve spool is Sfx, then an upward force f2 of the medium on the valve core at this time is: f2 = P2 × Shx (2) Usually, the safety valve Of the piston diameter larger than the spool, so the formula (1) and (2) Shs> Sfx  P1 ≈ P2 If the spring through the valve stem upward pull force is set to f3 and between the moving part and the fixed part Friction (mainly the friction between the piston and the piston chamber) is set to fm, the precondition for the action of the main safety gate: only the force f1 acting on the piston is slightly larger than the force acting on the valve core to make it upward And spring through the valve stem upward pull force f3 and moving parts and fixed parts Friction (mainly the friction between the piston and the piston chamber) fm sum, that is: f1> f2 + f3 + fm the main security gate to start. Through practice, the main safety valve safety valve refused to move mainly with the following factors: safety valve valve moving parts have jamming phenomenon. This may be due to improper assembly, dirt and impurities mixed or parts corrosion; poor surface finish of the piston chamber, surface damage, there are defects such as groove marks caused by defects. This makes the moving parts and fixed components between the friction fm increases, in other conditions remain unchanged f1
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