Thread Content
Common faults and preventive measures for general valves, as well as packing box leakage – these are the main causes of leaks, which are frequently encountered in factories. The reasons for packing box leakage include the following: ① The packing is not compatible with the corrosiveness, temperature, and pressure of the working medium ； ②The filling method is incorrect; in particular, inserting the entire packing disc coiled together is the most likely cause of leaks ； ③The valve stem lacks sufficient machining precision or surface finish; it may be elliptical or have scratches ； ④The valve stem has suffered pitting, or has rusted due to lack of protection in an outdoor environment ； ⑤Valve stem bending ； ⑥The filler has been in use for too long and has aged ； ⑦The operation is too forceful. The methods to eliminate packing leakage are: ① Selecting the right packing ； ②Load it in the correct manner ； ③If the valve stem is not properly processed, it must be repaired or replaced. The surface finish should be at least at level ▽5; for more critical applications, it should reach level ▽8 or higher, with no other defects present ； ④Take protective measures to prevent rust; those that are already rusted should be replaced ； ⑤The bent valve stem needs to be straightened or replaced ； ⑥The filler needs to be replaced after being used for a certain period of time ； ⑦Careful and steady operation is required, with slow opening and closing to prevent sudden temperature changes or medium shock. 2. Leakage from the closing element: Leaks in the packing gland are usually referred to as external leaks, while leaks from the closing element itself are called internal leaks. Leakage at the closing element, inside the valve, is difficult to detect. Leakages at the closing elements can be divided into two categories: one is leakage at the sealing surface, and the other is leakage at the root of the sealing ring. The reasons for the leakage are: ① The sealing surface was not ground properly ； ②The sealing ring does not fit tightly with the valve seat and valve disc ； ③The valve disc is not securely connected to the valve stem ； ④The valve stem is bent and twisted, causing the upper and lower closing elements to be out of alignment ； ⑤Shut down too quickly, poor contact between the sealing surfaces, or they are already damaged ； ⑥The material was not selected properly and cannot withstand corrosion by the medium ； ⑦Use globe valves and gate valves as control valves. The sealing surface cannot withstand the erosion caused by high-speed flowing media ； ⑧In some media, gradual cooling after the valve is closed can cause tiny cracks to appear in the sealing surfaces, which can also lead to erosion ； ⑨Certain sealing surfaces are connected to the valve seat and valve disc by threads, which can easily lead to the formation of oxygen concentration cell effects, resulting in corrosion and loosening ； ⑩Impurities such as slag, rust, and dust getting embedded, or mechanical parts falling off within the production system and blocking the valve core, prevent the valve from closing properly. Preventive measures include: ① It is necessary to conduct a thorough pressure test and leak check before use; if leakage is detected at the sealing surface or at the root of the seal ring, it must be fixed before use ； ②It is necessary to check in advance whether all components of the valve are in good condition; valves with a bent valve stem or an unreliable connection between the valve disc and the valve stem should not be used ； ③When closing the valve, do so gently and steadily, without force. If it is found that the sealing surfaces are not in good contact or there are obstructions, open the valve slightly immediately to allow debris to flow out, and then close it carefully again ； ④When selecting a valve, it is necessary to consider not only the corrosion resistance of the valve body but also that of the sealing element ； ⑤It is necessary to use the valve correctly in accordance with its structural characteristics; components that are required to regulate flow should employ control valves ； ⑥In cases where the medium cools down after the valve is closed and there is a significant temperature difference, the valve should be tightened again after cooling ； ⑦When the valve seat, valve disc, and seal ring are connected by threads, a polytetrafluoroethylene strip can be used as packing between the threads to eliminate any gaps ； ⑧For valves that are at risk of being clogged by impurities, a filter should be installed in front of the valve. 3. Failure in the lifting of the valve stem. The reasons for this failure include: ① Excessive force applied during operation, resulting in damage to the threads ； ②Lack of lubrication or failed lubricant ； ③Valve stem bending and torsion ； ④The surface finish is not good enough ； ⑤The fit tolerance is inaccurate, resulting in excessive tightness ； ⑥Valve stem nut is tilted ； ⑦Improper material selection, such as using the same material for the valve stem and the valve stem nut, can lead to seizing ； ⑧The threads are corroded by the medium (referring to stem valves or valves with the valve stem nut located at the lower part) ； ⑨Outdoor valves lack protection; the valve stem threads are covered with dust and sand, or become corroded by rain, dew, frost, and snow. Prevention methods: ① Operate with care; avoid using force when closing, and do not reach the upper dead center when opening. After opening enough, turn the handwheel back one or two turns to ensure a tight fit on the upper side of the threads, thereby preventing the medium from pushing the valve stem upward ； ②Regularly check the lubrication level to maintain an optimal state of lubrication ； ③Do not use a long lever to open or close valves; workers who are accustomed to using short levers must strictly control the amount of force applied to prevent the valve stem from bending (this applies to valves where the handwheel is directly connected to the valve stem) ； ④Improve the quality of processing or repair to meet regulatory requirements ； ⑤The material must be corrosion-resistant and suitable for the operating temperature and other working conditions ； ⑥The valve stem nut should not be made of the same material as the valve stem ； ⑦When using plastic for valve stem nuts, it is necessary to verify their strength; one should not consider only their good corrosion resistance and low friction coefficient – strength also needs to be taken into account. If the strength is insufficient, such materials should not be used ； ⑧Outdoor valves should be equipped with stem protectors ； ⑨For normally open valves, the handwheel should be turned regularly to prevent the valve stem from rusting. 4. Others: Gasket leakage: The main reasons are lack of corrosion resistance and inability to withstand the working temperature and pressure ； There is also the temperature change of high-temperature valves. Prevention method: Use washers that are suitable for the working conditions; for new valves, check whether the material of the washers is appropriate, and replace them if it is not. For high-temperature valves, the bolts should be tightened again before use. Valve body cracking: usually caused by freezing. In cold weather, the valves must be equipped with insulation and heating measures; otherwise, after shutdown, the water in the valves and connected pipelines should be drained completely (if there are plug valves at the bottom, these can be opened to drain the water). Damaged handwheel: caused by impact or forceful operation with a long lever. It can be avoided as long as the operators and other relevant personnel are careful. Filler gland fracture: Uneven force is applied when compressing the filler, or the gland (usually made of cast iron) is defective. To compress the filler, turn the screw symmetrically without any skew. During manufacturing, attention should be paid not only to large and critical components but also to minor parts such as gland seals, otherwise it will affect performance. Failure in the connection between the valve stem and the valve disc: Gate valves often use a rectangular-shaped end on the valve stem to connect with the T-shaped groove in the valve disc; sometimes this T-shaped groove is not machined, which leads to faster wear of the rectangular end of the valve stem. It is mainly addressed from the manufacturing perspective. However, the user can also perform further machining on the T-slot to give it a certain level of smoothness. The shutters of a double-shutter valve cannot be pressed tightly against the sealing surface: The tension on the dual shutters is generated by the wedges; in some valve types, these wedges are made of poor-quality material (low-grade cast iron), and they wear out or break after short use. The top wedge is a small component that requires little material; the using unit can manufacture it itself from carbon steel to replace the existing cast-iron parts.