Failure Analysis of Boiler Safety Valve

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 to the atmosphere Go, to ensure the safe operation of pressure vessels and piping systems to prevent accidents, and when the system pressure 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.  Safety valve failure analysis and solutions   1, the valve leakage Under the normal working pressure of the device, the valve flap and valve seat sealing surface occurred more than allowable degree of leakage, leakage of the safety valve will not only lead to 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. Common causes of valve leakage mainly in the following three cases: a situation is that dirt impurities fall on the sealing surface, the sealing surface pad, resulting in a gap between the valve core and the valve seat, so the valve leakage. To eliminate this failure is to remove the dirt and impurities falling to the sealing surface, the general size of the boiler to be shut down when the repair, the first to do safety valve running test, if the leaks are found out after the disintegration of maintenance, if Is the point after the run furnace test runner leaks found in the test, it is estimated that this situation caused by cooling after running can run 20 minutes after the rudder once again on the sealing surface erosion. Another case is the sealing surface damage. Seal surface damage caused by the following main reasons: First, poor sealing surface material. For example, in the No. 3 to No. 9 furnace main security door due to years of overhaul, the main safety valve spool and seat sealing surface has generally been studied very low, so that the sealing surface hardness is greatly reduced, resulting in decreased sealing performance, eliminating this Phenomenon is the best way to turn the original sealing surface down, and then re-welding the drawings required processing to improve the sealing surface hardness. Note that in the processing must ensure the quality of processing, such as the sealing surface cracks, trachoma and other defects must be machined back after processing. Newly processed valve seat must meet the drawing requirements. Currently using YST103 common steel welding rod sealing surface sealing effect is better. Second, the poor quality of maintenance, valve seat grinding can not meet the quality standards, the way to eliminate this failure is based on the degree of injury using grinding or turning grinding method to repair the sealing surface. Another reason for the leak of the safety valve is due to improper assembly or inappropriate parts dimensions. In the assembly process, the valve seat is not fully aligned or the surface with light transmission phenomenon, or the valve seat sealing surface is too wide, which is not conducive to sealing. Elimination method is to check the spool around the size and uniformity of the gap to ensure that the valve tip hole and the sealing surface with a positive, check the Ministry of the gap does not allow the lift valve; .  2, valve body surface leakage refers to the upper and lower valve body surface leakage at the junction, resulting in leakage of the main reasons for the following aspects: First, the binding surface of the bolt tightness or tight bias, resulting in a combination Surface seal is not good. 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 valve body with the tooth surface seal does not meet the standards. 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 poor or hard padded impurities caused by seal failure. 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. 3, Impulse safety valve action does not move after the main safety valve This phenomenon is often referred to as the main security door refused to move. 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 reasons why the main security gate refuses to move, first analyze the action principle of the main security gate. 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, this time 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) Normally, the piston diameter of the safety valve is larger than the diameter of the valve plug. Therefore, Shs> SfxP1≈P2 in Equation (1) and Equation (2). Suppose that the pull force of the spring through the valve stem toward the valve plug is set as f3 And the friction between the moving part and the fixed part (mainly the friction between the piston and the piston chamber) is set as fm, the precondition of the action of the main safety gate is that only the force f1 acting on the piston is slightly larger than the force acting on the valve plug On the upward force f2 and the spring through the valve stem upward pull force f3 and moving parts and solid Friction between components (primarily the friction between the piston and the piston chamber) FM sum, that is: f1> f2 + f3 + fm to start the main security gate. Through practice, the main security door refused to move mainly with the following three factors: First, the 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. So that the friction between the moving parts and fixed parts fm increases, the other conditions remain unchanged f1 <f2 + f3 + fm Therefore, the main security door refused to move. For example, a major security door reflex movement occurred during the overheating main safety gate run-in test before the overhaul of furnace No. 3 in 2001. Inspection disintegration inspection found that the piston chamber has a lot of rust and impurities, the piston can not move in the piston chamber, resulting in the main security door refused to move. Maintenance of the piston, expansion ring and the piston chamber was rust treatment, the piston chamber groove marks and other defects were ground, the piston chamber before assembly evenly coated with lead powder inside the wall, and in strict accordance with the order of the valve assembly. In the boiler hydrostatic test, the pulse tube to be washed, and then the main safety valve and the impulse safety valve connection, after overhauling the furnace again when the safety valve running test is all normal. Second, the main security door piston chamber leak large. When the valve piston chamber a large amount of air leakage, the formula (1) in a force acting on the piston f1 is too small, other conditions remain unchanged f1 <f2 + f3 + fm Therefore, the main safety door refused to move. Piston chamber caused by a large amount of air leakage and the valve itself is not air tightness and piston ring does not meet the size requirements or excessive wear and tear of the piston ring seal does not meet the requirements. For example, 3 ~ 9 furnace main valve safety valve ring quality requirements of the piston ring should be smooth edges and corners, the free state of the opening gap is not greater than 14, the opening gap after assembly △ = 1 ~ 1.25, piston and piston chamber clearance B = 0.12 ~ 0.18, the gap between the piston ring and the piston chamber is S = 0.08 ~ 0.12, the piston ring is in good contact with the piston chamber, and the light transmission should not exceed 1/6 of the circumference. The requirements of the piston chamber is that the groove depth of the piston chamber shall not exceed 0.08 ~ 0.1mm, the ovality of not more than 0.1mm, the conicity of not more than 0.1mm, should be smooth and no scratches, but disintegration inspection found that each furnace The main safety gate piston ring, piston and piston chamber does not meet the requirements of maintenance procedures, the current general piston ring and piston chamber gap are S ≥ 0.20, and the piston chamber surface defects more serious, seriously affecting the piston chamber of the steam Density, resulting in excessive leakage of piston chamber steam. The way to eliminate this kind of defect is to deal with the inner surface of the piston chamber, replace the qualified piston and piston ring, and close the small throttle opening in the throttling safety device system with throttle valve to increase the inlet into the piston chamber of the main safety gate The amount of steam, if conditions permit, can also be increased by increasing the stroke of the relief valve

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