Talking about the Causes and Countermeasures of Fire Safety Failure of Regulating Valves in Chemical Enterprises

1. Introduction In the chemical control system with high fire risk, the regulating valve is used as the terminal actuator of the regulating system. It is widely used in various pressure vessels and pipeline systems to receive control signals to realize the adjustment of the chemical process when under pressure. When the pressure in the system exceeds the specified value, it can be automatically opened to discharge excess flammable or flammable media into the atmosphere to ensure the safe operation of pressure vessels and piping systems to prevent the occurrence of fire accidents, and when the pressure in the system returns When it is down to work pressure or slightly lower than working pressure, it can automatically shut down. The reliable operation of the regulating valve is directly related to the fire protection and production safety of production equipment and personnel.

2. Analysis of common fire safety failures of regulating valves and their solutions 2.1. Leakage of valves Under the normal working pressure of the equipment, leakage occurs beyond the permissible level at the sealing surfaces of the valve disc and valve seat. Leakage of the regulating valve not only causes media loss. In addition, the continuous leakage of the medium will also cause the hard seal material to be destroyed. However, the sealing surface of the commonly used control valve is a metal material to a metal material. Although it is designed to be smooth and smooth, it should be done under the pressure of the medium. It is very difficult to absolutely not leak. Therefore, for a regulating valve that is a flammable or flammable medium, if a professional testing tool is used to test under a specified pressure value, the sealing performance is considered to be acceptable if it meets the scope of the specification. There are three main causes for the leakage of the valve:

One is that the debris falls on the sealing surface and the sealing surface is cushioned, causing a gap between the valve plug and the valve seat, so that the valve leaks. The method to eliminate this kind of failure is to remove the debris that has fallen onto the sealing surface. Generally, when the pressure vessel is ready for maintenance, the pressure vessel safety door is first tested. When the leakage is found, the pressure vessel is stopped and the valve is inspected. Wash the sealing surface.

The second is the sealing surface damage. The main causes of damage to the sealing surface are the following: First, the material of the sealing surface is poor. Due to many years of use, the valve core and valve seat sealing surfaces have generally been studied very low, so that the hardness of the sealing surface is also greatly reduced, resulting in reduced sealing performance, the best way to eliminate this phenomenon is to turn the original sealing surface Go down, then re-weld according to the requirements of the drawings, to improve the surface hardness of the sealing surface. Pay attention to ensure the quality of processing in the processing process, such as cracks in the sealing surface, trachoma and other defects must be re-processing after turning. The newly processed valve seat must meet the requirements of the drawings. The second problem is that the quality of the repair is poor, and the valve seat grinding cannot meet the quality standards. The method to eliminate this kind of failure is to repair the sealing surface or reinstall the new regulating valve according to the degree of damage by grinding or grinding after turning.

Thirdly, another reason for the leakage of the regulating valve is due to improper assembly or improper size of the relevant parts. During the assembly process, the valve seat of the valve body is not completely aligned or light-transmitting at the joint surface, or the valve seat sealing surface is too wide to seal. The elimination method is to check the size and the uniformity of the clearance around the valve plug to ensure that the top hole of the valve plug is the same as the sealing surface. It is not allowed to check the gap of each part to lift the valve core, and the width of the sealing surface is appropriately reduced according to the requirements of the drawing to achieve effective sealing. .

Fourth, the valve core, seat deformation leakage. The main reason for the leakage of the valve plug and valve seat is due to the strengthening of corrosion caused by casting or forging defects in the production process of the regulating valve. The passage of corrosive medium, the flushing of the fluid medium can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, erosion and impact on the valve core and the seat material are caused, so that the valve core and the valve seat become elliptical or other shapes. As time goes by, the valve core and valve seat are not matched and there is a gap. Leakage occurs when the valve is closed. The key to solving the problem is to select the quality of the valve core, valve seat and other materials. Choose corrosion-resistant materials and strongly reject defective products such as pits and trachoma. If the valve core and the valve seat are not deformed seriously, they can be ground with fine sandpaper to eliminate the marks and improve the sealing finish to improve the sealing performance. If the damage is serious, the new valve should be replaced.

2.2. Body surface leakage

Leakage at the joint surface of the valve body refers to the leakage phenomenon at the joint surface between the upper and lower valve bodies. The main reasons for this leakage are the following: First, the tightness of the bolts on the joint surface is not enough or is tight, resulting in sealing of the joint surface. it is good. The elimination method is to adjust the tightness of the bolts. When tightly tightening the bolts, it must be carried out diagonally. It is best to measure the gaps at the tight edges and tighten the bolts until they are tight, and make the gaps at the joint surfaces. Consistent. The second is that the toothed seals on the valve body joint surface do not meet the standards. For example, toothed seals have slight groove marks in the radial direction, poor parallelism, and defects such as sharp teeth or over-slopes can cause seal failure. So that the valve body surface leakage. The quality of a good spare part can be shut off during inspection and the use of a standard toothed seal can avoid this phenomenon. Third, the flatness of the joint surface of the valve body is too bad or it is sealed by hard impurities, causing the seal to fail. For the leakage of the joint surface of the valve body due to the poor flatness of the joint surface of the valve body, the method of elimination is to disassemble the valve and regrind the joint surface until the quality standard is met. As the impurities are trapped and cause the seal to fail, carefully clean the joint surface during valve assembly to prevent impurities from falling.

2.3. After the action of the impulse control valve, the main control valve does not act. The action of the main control valve after the impulse control valve is actuated is usually referred to as the rejection of the main control valve. The main control valve refused to move to the equipment is a very serious hazard, is a major equipment fire hazard, seriously affecting the safe operation of the equipment, once the pressure in the pressure vessel and the pipeline in the operating medium exceeds the rated value, the main regulating valve Without action, overpressure operation of the equipment can easily cause equipment damage and major fire accidents. The main control valve refused to move mainly related to the following three factors:

One is that 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, defects such as grooved hard spots.

Second, the main regulator valve piston chamber leakage. The main reason for the large amount of air leakage in the piston chamber is related to the airtightness of the valve itself and the piston ring does not meet the dimensional requirements or the wear of the piston ring is too large to meet the sealing requirements. The method to eliminate this defect is to process the piston inner surface, replace the qualified piston and piston ring, and close the throttle opening in the impulse safety device with throttle valve to increase the access to the piston chamber of the main regulator valve. The amount of steam entering, if the conditions permit, can also increase the amount of steam entering the main control valve piston chamber by increasing the stroke of the impulse control valve to push the action of the main control valve.

Third, the main control valve and the impulse control valve are not properly matched, and the steam flow of the impulse control valve is too small. The nominal diameter of the impulse control valve is too small, so that the amount of steam flowing into the piston chamber of the main control valve is insufficient, and the force that pushes the piston to move downward is insufficient, resulting in the main control valve spool not moving. This phenomenon occurs mostly when one main valve type impulse control valve is replaced due to poor consideration.

2.4. The delay of the main valve after the impulse control valve is returned to the seat is too long. The main reasons for this fault are the following two aspects: On the one hand, the amount of steam leakage in the main control valve piston chamber, although the impulse control valve Seat, but there is still the pressure in the pipeline and the steam in the piston chamber is still very high, pushing the downward force of the piston is still very large, so the main control valve back seat slow. The method to eliminate this kind of failure is mainly solved by opening the large throttle valve opening and increasing the throttle aperture. The opening of the throttle valve and the increase of the throttle aperture make the medium remaining in the pulse tube be quickly discharged. Therefore, the pressure in the piston is reduced, and the thrust that acts on the piston to move downwards is rapidly reduced. The spool is quickly returned to seat by the upward thrust of the medium and the upward force of the main regulator valve itself.

The other reason is that the friction force between the moving part and the fixed part of the main regulating valve will cause the main control valve to return slowly. The method to solve this problem is to match the gap between the moving part of the main regulating valve and the fixed part. Within the control standard.

2.5, control valve back seat pressure is low

If the pressure of the control valve is too low, it will cause a large amount of medium to be discharged over time, posing a fire hazard to the production and equipment. The reasons for the analysis are mainly caused by the following factors:

One is the large amount of medium expulsion on the spring pulse control valve. After this type of impulse control valve is opened, the medium is continuously discharged to push the main regulating valve. On the one hand, the pressure before the impulse control valve continues to rise due to the lack of the medium discharge of the main control valve, and continues to flow toward the impulse control valve to maintain the action of the impulse control valve. On the other hand, the medium flow of the impulse control valve flows to the piston chamber of the main control valve through the gap between the valve core and the guide sleeve. The medium rushes out of the sealing surface of the impulse control valve, forming a dynamic pressure zone around it, lifting the valve core. Therefore, the impulse of the impulse control valve continues to be discharged, and the greater the pressure in the kinetic pressure region of the spool, the greater the upward thrust acting on the spool, and the less likely the impulse control valve to return to seat, eliminate this at this time. The fault method is to close the throttle valve to reduce the flow of media out of the impulse control valve and reduce the pressure in the kinetic pressure zone so that the impulse control valve can be returned to seat.

The second is that the clearance between the valve plug and the guide sleeve is not proper, and the clearance is small. After the impulse control valve is seated, a high kinetic pressure zone is formed by instantaneous throttle at this part, raising the spool and delaying the return time. When the container falls to a lower level, the pressure in the kinetic pressure zone decreases and the impulse valve returns to its seat. The method to eliminate this kind of failure is to inspect the dimensions of the valve plug and the guide sleeve carefully. If the clearance is too small, reduce the valve sealing surface to the diameter of the valve flap or increase the radial clearance between the valve flap and the guide sleeve. , To increase the area of ​​the flow through the area, so that the local pressure rises to form a high kinetic energy pressure area.

Third, each moving part has a large frictional force, and some parts are stuck. The solution is to carefully inspect the moving parts, strictly inspect and repair the parts according to the inspection standards, and adjust the gap of each part to the standard range to eliminate sticking. possibility.

2.6, adjust the valve frequency jumper

Frequency hopping means that after the control valve is returned to the seat, the pressure will rise slightly, and the regulating valve will be opened again and repeated several times. This phenomenon is called “frequency jump” of the regulating valve. The mechanical characteristics of the regulating valve require that the blocking valve, tremor, and frequency skipping are not allowed when the regulating valve reaches the specified opening height during the entire operation. The occurrence of frequency jump phenomenon is extremely unfavorable to the sealing of the regulating valve and can easily cause leakage of the sealing surface. The reason for the analysis is mainly related to the high pressure of the repositioning valve of the regulating valve. When the re-seating pressure is high, the excess medium discharge in the container is small, and the regulating valve has returned to the seat. When the operating personnel adjust it improperly, the pressure in the container will be very fast. When it rises, it will cause the regulating valve to operate again. The frequency jump of the regulating valve can be eliminated by opening the opening of the large throttle valve. After the throttle valve is opened, the steam source to the main regulator valve piston chamber is reduced, the force that pushes the piston to move downward is smaller, and the probability of the main regulator valve is smaller, thereby avoiding the continuous startup of the main regulator valve.

2.7, flutter control valve flutter adjustment valve in the discharge of the phenomenon of jitter, called the flutter of the regulating valve, chatter vibration phenomenon can easily lead to metal fatigue, so that the mechanical properties of the valve to reduce, causing serious Fire hazards, the reasons for the occurrence of chatter mainly in the following two aspects:

On the one hand, the improper use of the valve, the use of the valve's discharge capacity is too large (as opposed to the necessary emissions), the elimination method is to make the rated displacement of the selected valve as close as possible to the necessary emissions of the equipment.

On the other hand, because the diameter of the inlet pipe is too small, smaller than the inlet diameter of the valve, or the resistance of the inlet pipe is too large, the elimination method is that when the valve is installed, the inner diameter of the inlet pipe is not smaller than the inlet diameter of the valve or the inlet pipe is reduced. resistance. Excessive resistance of the discharge pipeline causes excessive north pressure at the time of discharge. It is also a factor that causes valve chatter and can be solved by reducing the resistance of the discharge pipeline.

3. Concluding remarks Through the analysis of the causes of failure of the regulating valve, proper treatment and improvement measures will greatly increase the utilization rate of the regulating valve, reduce the failure rate of the instrument, and increase the production efficiency and economic efficiency of the process technology and reduce the energy consumption. Has an important role, can effectively improve the quality of the adjustment system, only to fully grasp the control valve common fire failure causes and countermeasures, in the event of failure can be handled handy, to ensure the equipment fire safety operation has an important significance.