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Describe the boiler safety valve failure analysis
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. 2, safety valve common causes of failure analysis and solution ?? 2.1, 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, the safety valve leakage will not only cause the media loss. 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 padding, 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 cause of leakage 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; . 2.2, valve body surface leakage refers to leakage between the upper and lower valve body junction, resulting in leakage of the main reasons for the following aspects: First, the binding surface of the bolt tension is not enough or tight bias, resulting in Poor sealing surface. 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 avoid this phenomenon. 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. 2.3, Impulse safety valve does not move after the main safety valve action 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, at this time on the piston f1: f1 = P1 × Shs ........................ (1)? If this time The pressure of the medium in the pressure-bearing 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) In general, the piston diameter of the safety valve is larger than the diameter of the valve plug. Therefore, Shs> Sfx? P1≈P2 in the formula (1) and (2) f3 and the friction between the moving part and the fixed part (mainly the friction between the piston and the piston chamber) is set to fm, the main safety gate action preconditions: only the force acting on the piston f1 slightly larger than the role of the valve The force on the core to make it upward f2 and spring through the valve stem upward pull force f3 and moving parts and fixed parts When the friction between (mainly friction between the piston and the piston chamber) fm sum, namely: f1> f2 f3 when fm the main security gate to start. 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 components fm increases, under 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 gas leakage, f1 (1) in a force acting on the piston is too small, other conditions remain unchanged f1 <f2 f3 fm so the main security 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, conicity of not more than 0.1mm, should be smooth and no scratches, but disintegration inspection found that each owner Safety door 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, severely affected the piston chamber seal Sex, 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 Steam volume may also be increased by increasing the stroke of the relief valve to increase the amount of intake air entering the piston chamber of the main safety gate, as conditions permitting, to facilitate actuation of the main relief valve. Third, the main safety valve and impulse safety valve mismatch, impulse safety valve steam flow is too small. The nominal diameter of the impulse safety valve is too small, resulting in insufficient amount of steam flowing into the piston chamber of the main relief valve and insufficient force f1 to push the piston downward. That is, f1 <f2 f3 fm causes the main safety valve spool not to move. This phenomenon occurs mostly in the main safety valve impulse relief valve has a replacement, due to poor consideration caused. For example, in 2002 overhaul of furnace 5, the two heavy hammer type safety relief valve replaced by two Harbin valve factory production A49H-P54100VDg20 pulse safety valve, the safety valve and A42H-P54100VDg125 type spring main safety match, It will be used with the Soviet-made Dg150 × 90 × 250-type old main safety valve, the main safety valve and the A29H-P54100VDg125 spring-type main safety valve than the nominal diameter not only larger and less air tightness, at 5 No. Furnace safety valve is set to be saturated, running test caused by the main safety valve refused to move. Later, we disassembled the impulse safety valve and expanded the gap between the guide sleeve and the valve plug to increase its flow area, and ran the test once more successfully. So impulse safety valve and the main valve matching properly, the smaller nominal diameter will cause the main valve to refuse to move. 2.4, after the impulse safety valve recoil after the main safety valve delay back seat too long? The main reason for this failure occurs in the following two aspects: On the one hand, the main relief valve piston chamber leakage size, although the impulse safety However, the pressure of the steam in the pipeline and the piston chamber is still high, and the force of pushing down the piston is still very large, so the main safety valve is sluggish due to the recoil. This kind of failure occurs in A42Y-P5413 .7 VDg100 type safety valve, because this type of safety valve piston chamber Gland is good. The way to eliminate this kind of failure is mainly solved by opening the large throttle opening and increasing the throttle aperture. The large opening of the throttle valve and the increase of the throttle aperture allow the steam remaining in the pulse tube to be drained rapidly , Thereby reducing the pressure inside the piston, so that the downward thrust on the piston quickly reduce the thrust in the steam turbine steam accumulator up thrust and the main safety valve spring pull up rapidly under its own back seat. Another reason is that the main safety valve friction between the moving parts and fixed parts can also cause the main safety valve back seat slowly, to solve this problem is the main safety valve moving parts and fixed parts with the gap Console standard range. 2.5, the safety valve seat back pressure is low? Safety valve seat back pressure is low on the economic operation of the boiler have a great hazard, back seat pressure is too low will cause a large number of media out of time, resulting in unnecessary energy loss. This failure occurred in 200MW units used in the A49H-type spring pulse safety valve, the analysis of the reasons are mainly caused by the following factors: First, spring pulse relief valve on the steam discharge, this form of impulse After the safety valve is opened, the medium is continuously discharged to promote the action of the main safety valve. On the one hand, the pressure before the relief valve due to the main relief valve medium displacement is not enough and continue to rise, so the pulse tube steam along the steam drum or gas header continue to flow to the impulse safety valve to maintain impulse safety valve action. On the other hand, due to this type of impulse safety valve medium flow through the gap between the valve core and the guide sleeve flows to the main safety valve piston chamber, medium out of the sealing surface of the impulse safety valve to form a dynamic pressure zone around it, The valve lift, so to achieve the momentum of the relief valve to continue emissions, the greater the steam emissions, the spool part of the kinetic energy pressure zone pressure greater role in the upward thrust on the spool greater impulse safety valve, the less Easy to back to seat, then eliminate this failure is to throttle the small, so that flow out of the flow of the relief valve to reduce the flow of media, reducing pressure in the kinetic energy area, so that the impulse safety valve back seat. The second factor that causes the low seat back pressure is that the fitting clearance between the valve core and the guide sleeve is not suitable and the gap between the valve seat and the guide sleeve is too small. After the impulse safety valve opens, a high dynamic pressure zone is throttled instantaneously in this position. Spool elevation, delayed back seat time, when the container down to a lower pressure in the kinetic energy area decreases, impulse valve back seat. Way to eliminate this failure is to carefully check the size of each part of the valve plug and the guide sleeve, with the gap is too small, reducing the diameter of the valve disc sealing surface straight valve cap or increase the radial clearance between the valve disc and the guide sleeve , To increase the flow area of ​​the site, so that steam will not flow too much over the throttling, leaving the local pressure rise to form a high kinetic energy pressure zone. Another reason for the low seat back pressure is the friction of various moving parts, some parts have jam, the solution is to carefully check the various moving parts, maintenance standards in strict accordance with the maintenance of the various components, the components of the gap to adjust to Standard range, eliminating the possibility of jam. 2.6, safety valve frequency jump Frequency jump refers to the safety valve back seat, to be slightly elevated pressure, the safety valve will open again and again several times, this phenomenon is called the safety valve "frequency hopping" . Safety valve mechanical requirements Safety valve in the entire process reaches the required opening height, not allowed to appear jam, tremor and frequency hopping phenomenon. Occurrence of frequent jumps on the safety valve seal is extremely unfavorable, can easily lead to the sealing surface of the leak. Analysis of the main reasons for the safety valve seat back pressure up high, back seat pressure is high, the container less excess media emissions, safety valve has been back to the seat, and when the improper adjustment of the operator, the pressure in the container will be very fast Rise, it also caused the safety valve action, like this situation can be opened by opening a large throttle method to be eliminated. When the throttling valve is opened, the steam source leading to the main safety valve piston chamber is reduced, the force pushing the piston downward is smaller, and the main safety valve is less likely to operate, thereby avoiding the continuous starting of the main safety valve. 2.7, safety valve flutter? Safety valve in the discharge process of jitter phenomenon, called the safety valve flutter, flutter phenomenon can easily lead to metal fatigue, the safety valve to reduce the mechanical properties, resulting in Serious equipment problems, chatter occurs mainly in the following aspects: On the one hand the improper use of the valve, the use of the valve discharge capacity is too large (relative to the amount of emissions must be), the elimination of the method should be selected valve The rated displacement is as close as possible to the required discharge of the equipment. On the other hand is due to the inlet pipe diameter is too small, smaller than the inlet diameter of the valve, or inlet pipe resistance is too large, the elimination of the valve is installed, the inlet pipe diameter not less than the inlet diameter of the valve or reduce the import pipe resistance. Excessive discharge pipe resistance, resulting in excessive discharge when the North pressure is also a factor that causes the valve flutter can be reduced by reducing the resistance of the discharge pipe to be resolved. ?? 3, concluding remarks on the boiler safety valve common causes of failure were analyzed and put forward specific solutions, although the power station boiler safety valve is composed of the main and auxiliary valves, and the use of mechanical and thermal control Double protection, some faults are not easy to occur, but only fully grasp the safety valve common causes of failure and elimination methods, when the fault occurs to handle it handy, to ensure the safe operation of equipment is of great significance.