Abstract: The leakage of the valve of the boiler safety valve, the leakage of the valve body joint surface, the failure of the main safety valve after the impulse safety valve is actuated, the delay of the main safety valve after the impulse safety valve is returned, and the return pressure of the safety valve are analyzed. Common causes of failures such as low frequency, frequency hopping, and flutter, and solutions are proposed for the cause of the failure.
1. Preface 
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 and discharge excess medium to the atmosphere. Go to ensure the safe operation of the pressure vessel and piping system to prevent accidents, and automatically shut down when the pressure in the system drops back to the working pressure or slightly lower than the working pressure. The reliability of the safety valve work is directly related to the safety of the equipment and the human body, so it must be taken seriously. 
2. Analysis of common causes of safety valves and their solutions
2.1, valve leakage 
Under the normal working pressure of the equipment, leakage exceeds the allowable degree at the sealing surface of the valve disc and the valve seat, and leakage of the safety valve not only causes media loss. In addition, the continuous leakage of the medium will also damage the hard sealing material. However, the sealing surface of the commonly used safety valve is made of metal material to metal material. Although it is intended to be smooth and smooth, it should be done under the condition of medium pressure. It is also very difficult to never miss. Therefore, for a safety valve whose working medium is steam, if the leak is not visible to the naked eye at the specified pressure value, the sealing performance is considered to be acceptable. There are three main reasons for the leakage of the valve:
In one case, the dirt impurities fall on the sealing surface, and the sealing surface is padded, causing a gap between the valve core and the valve seat, so that the valve leaks. The way to eliminate this kind of failure is to remove the dirt and impurities that fall to the sealing surface. Generally, when the boiler is ready to be shut down, the safety door running test is first carried out. If it is found that the leak is stopped, the disassembly and repair are carried out. After the furnace was tested, the safety door leak was found. It is estimated that this situation is caused by the fact that it can be cooled after 20 minutes of running and then ruddered again to flush the sealing surface.
Another situation is damage to the sealing surface. The main reasons for the damage of the sealing surface are as follows: First, the sealing surface material is poor. For example, in the main safety door of No. 3-9 furnace, due to years of maintenance, the sealing surface of the main safety valve spool and the valve seat has been generally studied very low, so that the hardness of the sealing surface is also greatly reduced, thereby causing a decrease in sealing performance, eliminating this The best way to do this is to turn the original sealing surface down and then re-surge the welding according to the drawings to improve the surface hardness of the sealing surface. Note that the processing quality must be guaranteed during the processing. For example, cracks such as cracks and trachoma on the sealing surface must be turned and reworked. Newly processed spool seats must meet the drawing requirements. At present, the effect of the valve sealing surface of YST103 universal steel electrode surfacing welding is better. Second, the quality of the inspection is poor, and the valve seat is not up to the quality standard. The method to eliminate the failure is to repair the sealing surface by grinding or turning after grinding according to the degree of damage. 
Another cause of leakage of the safety valve is due to improper assembly or improper size of the part. During the assembly process, the valve seat is not completely aligned or the joint surface is transparent, or the sealing surface of the valve seat is too wide to seal. The elimination method is to check the size and uniformity of the matching clearance around the valve core, to ensure that the top hole of the valve core and the sealing surface are the same degree, and it is not allowed to lift the valve core to check the gap of each part; the width of the sealing surface is appropriately reduced according to the drawings to achieve effective sealing. .
2.2, valve body joint surface leakage
Refers to the leakage phenomenon at the joint surface between the upper and lower valve bodies. The main causes of such leakage are as follows: First, the bolt tightening force of the joint surface is insufficient or tight, resulting in poor sealing of the joint surface. The elimination method is to adjust the tightening force of the bolt. When tightening the bolt, it must be carried out in a diagonal manner. It is better to measure the gaps at the tight side, tighten the bolt until it is tight, and make the joint surface Consistent. Second, the toothed gasket of the valve body joint surface does not meet the standard. For example, the toothed seal has a slight groove in the radial direction, and the parallelism is poor, and the tooth shape is too sharp or over-slope to cause the seal to fail. Thereby, the valve body joint surface leaks. The quality of the spare parts is checked during maintenance, and this phenomenon can be avoided by using a standard toothed gasket. Third, the flatness of the valve body joint surface is too poor or the hard impurities are padded to cause the seal to fail. The leakage of the valve body joint surface due to the poor flatness of the valve body joint surface is eliminated by disassembling the valve and re-grinding the joint surface until the quality standard is met. If the seal is ineffective due to the impurity being trapped, carefully clean the joint surface during the assembly of the valve to avoid impurities falling in.
2.3. The main safety valve does not operate after the impulse safety valve is acted. This phenomenon is often referred to as the rejection of the main safety door. The main safety door refuses to damage the boiler in operation. It is a major equipment hazard, which seriously affects the safe operation of the equipment. Once the pressure of the pressure vessel and pipeline in operation exceeds the rated value, the main safety gate Do not operate, so that equipment overpressure operation can easily cause equipment damage and major accidents.
Before analyzing the cause of the main security door refusal, first analyze the action principle of the main security door. As shown in Fig. 1, when the pressure in the pressure vessel rises to the full pressure of the impulse safety valve, the impulse safety valve acts, and the medium is rushed from the container through the pipeline to the piston chamber of the main safety valve, and there will be a slight expansion in the piston chamber. Buck, if the pressure in the piston chamber is P1, the throttle area is Shs, and the f1 acting on the piston is:
F1=P1×Shs........................(1)
If the pressure of the medium in the pressure vessel is P2 and the area of ​​the valve core is Sfx, then the upward force f2 of the medium to the spool is:
F2=P2×Shx..............(2)
Usually the diameter of the piston of the safety valve is larger than the diameter of the valve core, so Shs>SfxP1≈P2 in equations (1) and (2)
If the spring force of the spring through the valve stem is set to f3 and the friction between the moving part and the fixed part (mainly the friction between the piston and the piston chamber) is fm, the preconditions for the action of the main safety door are: Only the force f1 acting on the piston is slightly larger than the force f2 acting on the valve core and the upward force f3 of the spring through the valve stem against the valve core and the friction between the moving part and the fixed part (mainly piston and piston) When the friction between the chambers is equal to fm, that is, when f1>f2+f3+fm, the main safety door can be started. 
Through practice, the main security door refusal is mainly related to the following three factors:
First, the moving parts of the valve are stuck. This may be due to improper assembly, contamination of dirt and impurities, or corrosion of parts; poor surface finish of the piston chamber, surface damage, and hard spots such as grooves. Thus, the frictional force fm between the moving member and the fixed member is increased, and f1 < f2 + f3 + fm when the other conditions are not changed, so that the main safety door is rejected.
For example, when the overheating main safety door running test was carried out before the overhaul of the No. 3 furnace in 2001, the main safety door refused. During the disassembly inspection, it was found that there was a large amount of rust and impurities in the piston chamber, and the piston could not move in the piston chamber, which caused the main safety door to refuse. During the overhaul, the piston, the expansion ring and the piston chamber were derusted, and the piston chamber grooves and other defects were ground. The inner wall of the piston was evenly coated with lead powder before assembly, and the valve was assembled in strict order. In the boiler water pressure test, the pulse tube is flushed, and then the main safety door is connected with the impulse safety valve. When the furnace is repaired after the overhaul, the safety valve running test is performed again.
Second, the main safety door piston chamber has a large air leakage. When the valve piston chamber leaks a large amount, the force acting on the piston in the equation (1) is small, and f1 < f2 + f3 + fm under other conditions, so the main safety door is rejected. The main cause of the leakage of the piston chamber is related to the airtightness of the valve itself and the fact that the piston ring does not meet the dimensional requirements or the piston ring wears too much to meet the sealing requirements.
For example, the quality requirement of the main safety valve of the No. 3 to No. 9 furnace for the piston ring is that the angular angle of the piston ring should be smooth, the free state opening gap is not more than 14, the opening gap after assembly is △ = 1 to 1.25, and the clearance between the piston and the piston chamber is 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 be no more than 1/6 of the circumference. The requirement for the piston chamber is that the groove depth in the piston chamber must not exceed 0.08~0.1mm, the ellipticity should not exceed 0.1mm, the conicality should not exceed 0.1mm, it should be clean and free of scratches, but every furnace should be inspected during disassembly inspection. The piston ring, piston and piston chamber of the main safety door do not meet the requirements of the maintenance regulations. At present, the gap between the piston ring and the piston chamber is generally S≥0.20, and the defects on the surface of the piston chamber are more serious, which seriously affects the steam of the piston chamber. The tightness causes the piston chamber to leak excessively.
The method for eliminating such defects is to treat the inner surface of the piston, replace the qualified piston and the piston ring, and close the opening of the small throttle valve in the impulse safety device system with the throttle valve to increase the entry into the piston chamber of the main safety door. The steam volume, if conditions permit, can also increase the amount of steam entering the main safety door piston chamber by increasing the stroke of the impulse safety valve to push the main safety valve.
Third, the main safety valve and the impulse safety valve are not properly matched, and the steam flow rate of the impulse safety valve is too small. The nominal diameter of the impulse safety valve is too small, so that the amount of steam flowing into the piston chamber of the main safety valve is insufficient, and the force f1 for pushing the piston downward is insufficient, that is, f1 < f2 + f3 + fm causes the main safety valve spool to not move. This phenomenon occurs mostly when the main safety valve type impulse safety valve has a replacement, which is caused by poor consideration.
For example, in the No. 5 furnace overhaul in 2002, two heavy hammer impulse safety valves were replaced by two Harbin valve factories to produce A49H-P54100VDg20 pulse type safety valve. This safety valve is generally matched with A42H-P54100VDg125 spring type main safety. It is used with the old Dg150×90×250 old main safety valve. The main safety valve and the A29H-P54100VDg125 spring type main safety valve are not only the nominal diameter but also the airtightness. The furnace safety valve is fixed, and the main safety valve is rejected when the running test is performed. Later, we disintegrated the impulse safety valve and expanded the gap between the guide sleeve and the valve core to increase the flow area. Therefore, the impulse safety valve is not properly matched with the main safety valve, and the nominal diameter is small, which may cause the main safety valve to refuse.
2.4. After the impulse safety valve is returned to the seat, the main safety valve is delayed for a long time.
The main reasons for this failure are as follows:
On the one hand, the amount of steam leakage in the piston chamber of the main safety valve, although the impulse safety valve is returned to the seat, the pressure of the steam in the pipeline and the piston chamber is still high, and the force pushing the piston downward is still large. Therefore, the main safety valve is slow to return to the seat, and this kind of failure mostly occurs on the A42Y-P5413.7VDg100 type safety valve, because the piston valve of this type of safety valve has good steam sealability. The method of eliminating such a failure is mainly solved by opening the opening of the large throttle valve and increasing the throttle orifice. The opening of the throttle valve and the increase of the orifice diameter cause the steam remaining in the pulse tube to be quickly discharged. , thereby reducing the pressure inside the piston, so that the thrust acting downward on the piston is rapidly reduced, and the spool is quickly returned by the upward thrust of the steam medium in the collector header and the upward pulling force of the main safety valve itself. seat.
On the other hand, the reason is that the excessive friction between the moving parts and the fixed parts of the main safety valve will also cause the main safety valve to return to the seat. The solution to this problem is to match the moving parts of the main safety valve with the fixed parts. Within the scope of the console standard.
2.5, the relief pressure of the safety valve is low
The low return pressure of the safety valve is very harmful to the economic operation of the boiler. If the return pressure is too low, a large amount of medium will be discharged over time, resulting in unnecessary energy loss. This kind of failure occurs mostly on the A49H spring pulse safety valve used in the 200MW unit. The reason for the analysis is mainly caused by the following factors:
First, the amount of steam discharged from the spring pulse safety valve is large. After this type of impulse safety valve is opened, the medium is continuously discharged to push the main safety valve.
On the one hand, the pressure before the impulse safety valve continues to rise due to insufficient medium discharge of the main safety valve, so the steam in the pulse tube continues to flow toward the impulse safety valve along the steam drum or the gas collection header to maintain the impulse safety valve action.
On the other hand, since the medium circulation of the impulse safety valve of this type flows through the gap between the valve core and the guide sleeve to the piston chamber of the main safety valve, the medium rushes out of the sealing surface of the impulse safety valve, and a kinetic energy pressure zone is formed around it. Raise the valve core, so that the impulse safety valve continues to discharge. The greater the steam discharge, the greater the pressure in the kinetic energy pressure zone of the spool, the greater the upward thrust acting on the spool, and the less the impulse safety valve It is easy to return to the seat. At this time, the method of eliminating the fault is to close the throttle valve, reduce the flow rate of the medium flowing out of the impulse safety valve, and reduce the pressure in the kinetic energy pressure zone, so that the impulse safety valve can be returned to the seat.
The second factor causing the return pressure is low: the clearance between the valve core and the guide sleeve is not appropriate, and the clearance is small. After the impulse safety valve is opened, a high kinetic energy pressure zone is formed in this part. The valve core is raised and the return time is delayed. When the container is lowered to a lower level, the pressure in the kinetic energy pressure zone is reduced, and the impulse valve is returned to the seat.
The way to eliminate this kind of failure is to carefully check the size of each part of the valve core and the guide sleeve. When the clearance is too small, reduce the diameter of the valve flap sealing surface to the valve flap or increase the radial clearance between the valve disc and the guide sleeve. In order to increase the flow area of ​​the part, the steam does not flow excessively, and the partial pressure rises to form a high kinetic energy pressure zone.
Another reason for the low back pressure is that the moving parts have large frictional force and some parts have jams. The solution is to carefully check the moving parts, and strictly check the parts according to the inspection standard, and adjust the matching clearance of each part to Eliminate the possibility of clicks within the standard range.
2.6, the frequency jump of the safety valve
The frequency jump refers to the safety valve returning to the seat. When the pressure rises slightly, the safety valve will open again and appear several times. This phenomenon is called the “frequency jump” of the safety valve. The mechanical characteristics of the safety valve require that the safety valve does not exhibit jamming, tremor and frequency jump when it reaches the specified opening height during the whole operation. The phenomenon of frequency jump is extremely unfavorable for the sealing of the safety valve, and it is easy to cause leakage of the sealing surface. The reason for the analysis is mainly related to the high pressure of the safety valve. When the return pressure is high, the excess medium in the container is discharged less, and the safety valve has returned to the seat. When the operator is not properly adjusted, the pressure inside the container will be very fast. It rises, so it causes the safety valve to act. This situation can be eliminated by opening the throttle valve. After the throttle valve is opened, the steam source to the piston chamber of the main safety valve is reduced, the force for pushing the piston downward is small, and the probability of the main safety valve being operated is small, thereby avoiding the continuous starting of the main safety valve.
2.7, the flutter of the safety valve
The vibration phenomenon of the safety valve during the discharge process is called the flutter of the safety valve. The occurrence of the flutter phenomenon is likely to cause metal fatigue, which causes the mechanical performance of the safety valve to drop, causing serious equipment hazards and flutter. The reasons are mainly as follows:
On the one hand, the improper use of the valve, the discharge capacity of the selected valve is too large (relative to the amount of emissions required), the elimination method should be such that the rated displacement of the selected valve should be as close as possible to the necessary discharge of the equipment.
On the other hand, because the diameter of the inlet pipe is too small, less than the inlet diameter of the valve, or the resistance of the inlet pipe is too large, the elimination method is to make the inner diameter of the inlet pipe not less than the inlet diameter of the valve or reduce the inlet pipe when the valve is installed. resistance. The resistance of the discharge pipe is too large, and the excessive north pressure caused by the discharge is also a factor causing the valve flutter, which can be solved by reducing the resistance of the discharge pipe. 
3. Conclusion 
The common causes of boiler safety valves are analyzed and specific solutions are proposed. Although the power supply boiler safety valves are composed of main and auxiliary valves, they are protected by mechanical and thermal control. Some faults are not easy to occur. However, only by fully grasping the common causes and elimination methods of the safety valve can be handled easily when the fault occurs, which is of great significance for ensuring the safe operation of the equipment.

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