Analyzing common faults of boiler safety valves
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Abstract: This paper analyzes common fault causes of safety valves in boilers, such as valve leakage, leakage at the joint surfaces of the valve body, failure of the main safety valve to operate after the impulse safety valve activates, excessive delay in the return to its normal position of the main safety valve after the impulse safety valve retracts, low return pressure of the safety valves, as well as frequent tripping and fluttering. Solutions are proposed for these fault causes. Keywords: safety valve, impulse, main safety valve 1. Introduction A safety valve is a very important type of protective valve that is widely used in various pressure vessels and piping systems. When the pressure in a pressurized system exceeds a specified value, it opens automatically to release the excess fluid into the atmosphere, thereby ensuring the safe operation of the pressure vessels and piping systems and preventing accidents. It then closes automatically when the pressure in the system drops back to the operating pressure or slightly below it. The reliability of the safety valve’s operation is directly related to the safety of equipment and personnel, so it must be given due attention. ?お? 2. Analysis of common failure causes and solutions for safety valves?お? 2.1 Valve leakage?? At the normal operating pressure of the equipment, leakage occurs at the sealing surface between the valve disc and the valve seat; leakage in safety valves not only results in a loss of fluid. Furthermore, continuous leakage of the medium can also damage the hard sealing materials. However, in common safety valves, the sealing surfaces are made of metal against metal; although efforts are made to keep them smooth and flat, it is very difficult to ensure complete leaklessness under pressure. Therefore, for safety valves whose working medium is steam, at the specified pressure value, if no leakage can be seen with the naked eye or heard at the outlet, the sealing performance is considered satisfactory. Generally, there are mainly three reasons for valve leakage: One reason is that dirt and impurities settle on the sealing surface, covering it and creating a gap between the valve stem and the valve seat, which leads to leakage. The method to eliminate this fault is to remove the dirt and impurities that have accumulated on the sealing surface. Generally, when the boiler is prepared for major maintenance or shutdown, a safety valve test is carried out first; if leakage is detected, the boiler is shut down for detailed inspection and repair. If leakage is found in the safety valve during such a test after the boiler has been started up, it is likely that this is the cause of the problem. In that case, wait 20 minutes after the test before conducting another test to flush the sealing surface. Another scenario is damage to the sealing surface. The main reasons for damage to the sealing surface are as follows: first, the material of the sealing surface is of poor quality. For example, in Reactors 3 to 9, due to years of maintenance work, the sealing surfaces between the main safety valve stems and their seats have become significantly worn down, which reduces the hardness of these sealing surfaces. This leads to a decline in their sealing performance. The best way to address this issue is to machine away the existing sealing surfaces and then remanufacture them according to the specifications provided, thereby increasing the surface hardness of the sealing surfaces. Be sure to maintain high quality during the processing; if defects such as cracks or pitting appear on the sealing surface, they must be removed by turning before further processing is carried out. The newly processed valve spools and seats must meet the requirements of the drawings. At present, the sealing surfaces of valve cores fabricated by surfacing with YST103 general-purpose steel electrodes yield good results. Secondly, the quality of maintenance is poor; the grinding of valve cores and seats does not meet the required quality standards. To resolve this issue, the sealing surfaces are repaired by grinding or by grinding after turning, depending on the extent of the damage. ?? Another cause of safety valve leakage is improper assembly or inappropriate dimensions of the related parts. During the assembly process, the valve core and seat may not be fully aligned, there may be light leakage at the joint surface, or the sealing surfaces of the valve core and seat may be too wide, which hinders proper sealing. The solution is to check the size and uniformity of the clearances around the valve spool, ensure that the hole at the tip of the valve spool is aligned properly with the sealing surface, and verify that there are no gaps in any part that could cause the valve spool to lift ； Reduce the width of the sealing surface appropriately as required by the drawings to achieve effective sealing. 2.2 Leakage at the joint surface of the valve body: This refers to leakage that occurs at the joint between the upper and lower valve bodies. The main reasons for such leakage are as follows: firstly, the tightening force of the bolts at the joint surface is insufficient or uneven, resulting in poor sealing at that joint. The solution is to adjust the bolt tension; when tightening the bolts, it is necessary to do so in a diagonal manner. It is best to measure the gaps at various points while tightening, and keep tightening until the bolts can no longer be tightened any further, ensuring that the gaps across the joint surfaces are uniform. Second, the toothed seal gasket at the valve body joint surface does not meet the standards. For example, minor radial grooves on the toothed gasket, poor parallelism, or defects such as overly sharp or excessively sloped teeth can all lead to sealing failure. Thus causing leakage at the valve body joint surface. During maintenance, it is important to ensure the quality of spare parts; using standard-compliant toothed gaskets can help prevent this phenomenon from occurring. Third, the flatness of the joint surface of the valve body is too poor, or it is blocked by hard impurities, resulting in seal failure. For leaks at the valve body joints caused by poor flatness of these joints, the solution is to disassemble the valve and re-grind the joints until they meet the quality standards. In cases where seal failure is caused by impurities blocking the area, thorough cleaning of the mating surfaces should be carried out during valve assembly to prevent impurities from getting there. 2.3 The main safety valve does not activate after the impulse safety valve operates. This phenomenon is commonly referred to as failure of the main safety valve to function. The failure of the main safety valve to operate poses a severe threat to boilers that are in use; it represents a serious hazard to the equipment and significantly affects its safe operation. If the pressure of the medium inside the pressure vessels and pipelines while they are in use exceeds the specified value, and the main safety valve fails to function, it can lead to the equipment operating under excessive pressure, thereby causing damage to the equipment and resulting in serious accidents. Before analyzing the reasons for the main safety door failing to operate, let’s first examine the operating principle of the main safety door. As shown in Figure 1, when the pressure inside the pressure vessel rises to the set pressure of the impulse safety valve, this valve operates, and the fluid flows from within the vessel through the piping into the piston chamber of the main safety valve. This results in a slight increase in volume and decrease in pressure within the piston chamber. If the pressure in this chamber at that time is P1 and the throttling area of the piston is Shs, then the force acting on the piston, denoted as f1, is given by:f1 = P1 × Shs……………………(1)
If the area of the valve core is Sfx, then the upward force exerted by the fluid on the valve core is given by:
f2 = P2 × Shx……………….(2)
Generally, the diameter of the piston in a safety valve is larger than that of the valve core; therefore, in equations (1) and (2), Shs > Sfx. Moreover, P1 ≈ P2.
If we denote the upward pull exerted by the spring on the valve core through the valve stem as f3, and the frictional forces between the moving parts and the fixed parts (mainly the friction between the piston and the piston chamber) as fm, then for the main safety valve to operate, it is necessary that the force acting on the piston, f1, be slightly greater than the sum of the upward force acting on the valve core, f2, the upward pull from the spring, f3, and the frictional forces, fm. In other words, f1 > f2 + f3 + fm for the main safety valve to activate. Through practice, it has been found that the failure of the main safety door to operate is mainly related to the following three factors: First, there is sticking in the moving parts of the valve. This may be due to improper assembly, the presence of dirt and impurities, or component corrosion ； The poor surface finish of the piston chamber is caused by surface defects such as damage, grooves, and hard spots. This increases the friction force fm between the moving part and the fixed part; with all other conditions unchanged, f1 < f2 + f3 + fm, so the main safety door refuses to open. For example, during the drift test of the superheater main safety valve prior to the refitting of Reactor No. 3 in 2001, the main safety valve failed to operate. During maintenance and disassembly inspection, it was found that there was a large amount of rust and debris inside the piston chamber; the piston could not move within this chamber, which led to the main safety door failing to operate. During maintenance, the piston, expansion ring, and piston chamber were descaled, and defects such as grooves in the piston chamber were ground smooth. Before assembly, lead powder was evenly applied to the inner wall of the piston chamber, and the valve was assembled strictly in sequence. During the boiler hydrostatic test, the pulse tube was flushed, and then the main safety valve was connected to the impulse safety valve; a safety valve drift test was conducted again when the boiler was started up after the overhaul, and everything worked normally. Second, the main safety door piston chamber has a high air leakage rate. When the air leakage in the valve piston chamber is large, the force exerted on the piston by the f1 term in equation (1) is relatively small; with all other conditions remaining unchanged, f1 < f2 + f3 + fm, and as a result the main safety valve fails to operate. The main reasons for high air leakage in the piston chamber are related to the airtightness of the valve itself, the fact that the piston rings do not meet the required dimensions, or excessive wear of the piston rings that prevents them from providing proper sealing. For example, the quality requirements for piston rings in the main safety valves of reactors 3 to 9 are that the edges of the piston rings should be smooth, the opening clearance in their free state should not exceed 14, the opening clearance after assembly should be △=1–1.25, the gap between the piston and the piston chamber should be B=0.12–0.18, and the gap between the piston ring and the piston chamber should be S=0.08–0.12. The piston rings must make good contact with the piston chamber, and the amount of light that passes through should not exceed 1/6 of the perimeter. The requirements for the piston chamber are that the depth of the grooves within it shall not exceed 0.08–0.1 mm, its ellipticity shall not exceed 0.1 mm, and its conicity shall not exceed 0.1 mm; it must also be smooth with no scratches. However, inspections during maintenance revealed that the piston rings, pistons, and piston chambers of the main safety valves in each furnace did not meet the specifications set out in the maintenance procedures. Currently, the clearance between the piston rings and the piston chambers is generally S ≥ 0.20, and the defects on the surface of the piston chambers are even more severe, which significantly affects the airtightness of these chambers and results in higher steam leakage rates. The method to eliminate this defect is to treat the inner surface of the piston chamber, replace it with qualified pistons and piston rings, reduce the opening degree of the throttle valve in the impulse safety device system equipped with a throttle valve, increase the amount of steam entering the piston chamber of the main safety valve; where conditions permit, the stroke of the impulse safety valve can also be increased to boost the amount of steam entering the piston chamber of the main safety valve, thereby triggering the operation of the main safety valve. Third, there is an improper matching between the main safety valve and the impulse safety valve, with the steam flow rate of the impulse safety valve being too low. The nominal diameter of the impulse safety valve is too small, resulting in an insufficient amount of steam flowing into the piston chamber of the main safety valve. As a result, the force f1 acting to push the piston downward is not sufficient; that is, f1 < f2 + f3 + fm, which prevents the valve element of the main safety valve from moving. This phenomenon often occurs when a main safety valve or an impulse safety valve needs to be replaced, as a result of inadequate consideration. For example, during the major overhaul of Furnace No. 5 in 2002, two weight-type impulse safety valves were replaced with two A49H-P54100VDg20 pulse safety valves produced by Harbin Valve Factory. These safety valves are generally used in conjunction with A42H-P54100VDg125 type spring-loaded main safety valves; when used together with the old-style Dg150×90×250 type main safety valves produced in Jiangsu, such main safety valves not only have a larger nominal diameter than the A29H-P54100VDg125 type spring-loaded main safety valves but also exhibit poorer airtightness. This led to the main safety valves failing to operate during the weight-testing procedure after the saturation safety valves of Furnace No. 5 were adjusted. Later, we disassembled the impulse safety valve and increased the gap between its guide sleeve and the part that comes into contact with the valve core, in order to increase its flow area; another weight-drop test was then conducted successfully. Therefore, an improper match between the impulse safety valve and the main safety valve, as well as a smaller nominal diameter, can also cause the main safety valve to fail to operate. ?
2.4 Excessively long delay in the reseating of the main safety valve after the impulse safety valve has reseated? The main reasons for this fault are as follows: On one hand, it is related to the amount of steam leaking from the piston chamber of the main safety valve; although the impulse safety valve has reseated, the pressure of steam in the pipelines and within the piston chamber remains high, resulting in a strong force that pushes the piston downward. This leads to a delayed reseating of the main safety valve. This type of fault occurs frequently in A42Y-P5413.7VDg100-type safety valves, as these valves have good steam-sealing properties in their piston chambers. The method to eliminate this fault is mainly to increase the opening degree of the throttle valve as well as the diameter of the throttle orifice. An increased opening degree of the throttle valve and a larger diameter of the throttle orifice both help to expel the steam remaining in the pulse tube more quickly, thereby reducing the pressure inside the piston and swiftly decreasing the downward force exerted on it. As a result, the valve core returns to its original position under the upward force of the steam in the steam collection tank and the upward pull exerted by the spring of the main safety valve. Another reason is that excessive friction between the moving parts and the fixed parts of the main safety valve can also lead to delayed reseating of the valve. The solution to this problem is to keep the clearance between the moving parts and the fixed parts of the main safety valve within the standard range. 2.5 Low reset pressure of the safety valve: A low reset pressure of the safety valve poses a serious threat to the efficient operation of the boiler; an excessively low reset pressure leads to excessive discharge of fluid over time, resulting in unnecessary energy losses. This type of failure occurs frequently in the A49H type spring pulse safety valves used in 200MW units. The main reasons for this are as follows: First, a large amount of steam is discharged from the spring pulse safety valve; when such an impulse safety valve opens, fluid continues to be discharged, which in turn triggers the operation of the main safety valve. On the one hand, the pressure in front of the impulse safety valve continues to rise due to insufficient discharge of fluid from the main safety valve; as a result, the steam inside the pulse tube flows further toward the drum or gas collection box in order to keep the impulse safety valve functioning. On the other hand, in this type of impulse safety valve, the flow of the medium takes place through the gap between the valve core and the guide sleeve, toward the piston chamber of the main safety valve. The medium bursts out of the sealing surface of the impulse safety valve, creating a region of kinetic pressure around it; this pressure lifts the valve core, allowing the impulse safety valve to continue discharging steam. The greater the amount of steam discharged, the higher the pressure in the kinetic pressure region around the valve core, and thus the greater the upward force acting on the valve core. As a result, it becomes more difficult for the impulse safety valve to return to its original position. To resolve this issue, the throttle valve can be reduced in size, thereby decreasing the flow rate of the medium exiting the impulse safety valve and lowering the pressure in the kinetic pressure region, which allows the impulse safety valve to return to its normal position. The second factor contributing to low reseating pressure is an inappropriate clearance between the valve core and the guide sleeve; when this clearance is too small, after the impulse safety valve opens, instantaneous throttling occurs at this location, creating a high kinetic energy pressure zone that lifts the valve core and delays its reseating. When the pressure inside the container drops, the pressure in the kinetic energy pressure zone decreases, and the impulse valve reseats. The method to eliminate this fault is to carefully check the dimensions of all parts of the valve core and guide sleeve. When the clearance between them is too small, reducing the diameter of the steam-blocking cap on the sealing surface of the valve disc, or increasing the radial clearance between the valve disc and the guide sleeve, can increase the flow area in that area. This prevents excessive throttling as steam passes through, thereby avoiding an increase in local pressure that could create a high kinetic energy pressure zone. Another reason for the low reset pressure is high friction between the moving parts, with some areas experiencing sticking. The solution is to carefully inspect all moving components, carry out maintenance on them in strict accordance with the relevant standards, adjust the clearance between these components to the standard range, and eliminate the possibility of sticking. 2.6, Frequent opening and closing of the safety valve Frequency opening and closing refers to the phenomenon where, after the safety valve returns to its closed position, it opens again as soon as the pressure rises slightly; this happens several times in a row, and such behavior is known as the \"frequent opening and closing\" of the safety valve. The mechanical characteristics of safety valves require that, during the entire operation process, when they reach the specified opening height, there should be no occurrence of sticking, vibration, or frequent pulsations. Frequent pulsation is extremely detrimental to the sealing of safety valves, as it can easily lead to leakage at the sealing surfaces. The analysis indicates that the cause is mainly related to the high reset pressure of the safety valve. When this reset pressure is high, less of the excess medium in the container is discharged, and the safety valve returns to its normal position. However, if the operators make improper adjustments, the pressure inside the container rises rapidly again, which triggers the safety valve to activate once more. This situation can be resolved by increasing the opening degree of the throttle valve. When the throttle valve is opened wider, the amount of steam flowing into the piston chamber of the main safety valve decreases, resulting in a weaker force pushing the piston downward. This reduces the likelihood of the main safety valve activating, thereby preventing it from starting repeatedly. 2.7 Vibration of safety valves The shaking phenomenon that occurs during the discharge process of a safety valve is known as vibration of the safety valve. This vibration can easily lead to metal fatigue, reducing the mechanical properties of the safety valve and creating serious risks for the equipment. The main reasons for such vibration include the following: One reason is improper use of the valve – choosing a valve with a discharge capacity that is too high (compared to the actual amount that needs to be discharged). The solution is to select a valve whose rated discharge capacity is as close as possible to the actual discharge requirements of the equipment. On the other hand, it is due to the fact that the diameter of the inlet pipe is too small, smaller than the inlet diameter of the valve, or the resistance in the inlet pipe is too high. The solution is to ensure that the inner diameter of the inlet pipe is at least as large as the valve’s inlet diameter when installing the valve, or to reduce the resistance in the inlet pipe. Excessive resistance in the discharge pipeline, which results in high back pressure during discharge, is also a factor that causes valve flutter; this issue can be resolved by reducing the resistance of the discharge pipeline. ?Oh? 3. Conclusions?Oh? An analysis was conducted on the common causes of failures in boiler safety valves, and specific solutions were proposed. Although today’s power plant boilers are equipped with both primary and auxiliary valves, along with dual protection mechanisms based on mechanical and thermal controls, some failures still can occur. However, only by fully understanding the common causes of failures in safety valves and the methods to address them can one handle such issues effectively when they arise, which is of great importance for ensuring the safe operation of the equipment.
I wonder if everyone knows that some economizers also need to be equipped with safety valves? What should the specific choice be
Back to Floor 3: Article 131 of the \"Regulations on Safety Supervision of Steam Boilers\" stipulates that a safety valve must be installed at the outlet of the fractional economizer. The safety valve can be of the spring-type full-opening type or the slightly-opening type; in the boiler drawings I have seen, slightly-opening type safety valves are used more frequently for economizers.