In industrial production, valves are indispensable equipment, and the valve stem, as one of the core components of a valve, directly affects the safety and stability of the entire system. The valve stem not only has to withstand the pressure of the medium but also deal with pressure changes caused by various uncertain factors. Therefore, understanding the principles of valve stem sealing, common problems, and their solutions is crucial for ensuring the safety and efficiency of industrial production. Failure of valve stem sealing can lead to a series of problems that affect the normal operation of the valve. These signs of failure not only impact production efficiency but can also lead to serious safety accidents. Thus, it is essential to detect and address these signs in a timely manner to maintain the normal operation of industrial production.
The valve stem is a key part of a pressure piping component. During operation, it has to bear the pressure of the medium and deal with pressure changes caused by various uncertain factors. The sealing performance of the valve stem directly affects the reliability and safety of the valve. If the valve stem sealing fails, it can lead to medium leakage and even safety accidents, especially in high-risk industries such as chemical and petroleum, where the consequences can be unimaginable.
During the opening and closing process of the valve stem, the inner hole of the packing gland will gradually wear out due to friction, creating gaps that lead to valve leakage. Therefore, the valve stem needs to have appropriate hardness and elasticity, be resistant to medium erosion and abrasion, and possess excellent machinability. In addition, to further enhance the wear and corrosion resistance of the valve stem, surface treatment is required to prevent medium leakage caused by factors such as valve stem bending, corrosion, and surface tension.
Valve stem sealing is a complex system composed of multiple components. The basic principle is to promote the axial pre-tightening of the gland through the application of bolt torque or gland pre-tightening, thereby balancing the relationship between the compression deformation of the packing and the torque of the valve stem movement. When an external force is applied to axially compress the packing in the packing gland, the packing undergoes significant radial deformation, which seals the valve stem.
There are differences in surface roughness at the microscopic level on the valve stem, which results in inconsistent contact between the packing and the valve stem surface. The contact parts reach a state of extreme lubrication, creating small oil grooves through non-contact grooves. When the packing is axially compressed, the high plasticity of the packing forms a radial residual compressive stress, tightly contacting the valve stem surface. At the same time, after lubricating oil is added, it is squeezed between the mating surfaces to form an oil film layer. This unbalanced contact state makes the contact parts boundary lubrication and the non-contact grooves thicker oil film small oil grooves. When the shaft and packing move, an irregular labyrinth is formed between the contact and non-contact parts, preventing liquid flow leakage.
The radial pressure distribution and average pressure distribution of the packing are such that the radial pressure increases exponentially from the inside out, while the average pressure decreases exponentially from the inside out. The sealing packing can partially seal the exterior and be close to the gland radial 2 to 3 packing rings of compressive force. This design allows the packing to maintain good sealing performance during the movement of the valve stem.
Valve stem sealing failure can lead to a series of problems that affect the normal operation of the valve. Understanding these signs of failure helps to detect and address problems in a timely manner to avoid greater losses.
The most intuitive method to judge is to observe whether there is medium leakage around the packing gland. If you see liquid dripping, seeping, or vapor (for volatile media) escaping at the valve stem (i.e., the packing gland position) of the valve, this is a clear sign of packing seal failure. For gaseous media, the soap water test can be used by applying soapy water around the packing gland. If bubbles appear, it indicates gas leakage.
Minor leakage may manifest as slight dampness or tiny droplets around the packing gland. This situation may temporarily not affect the normal use of the valve, but it requires close monitoring. If the leakage is severe, such as continuous liquid flow or large amounts of gas leakage, affecting the working environment or failing to meet process requirements (for example, in chemical production, leakage may cause safety accidents or quality problems), the packing needs to be replaced immediately.
When operating the valve (manually or electrically) to open and close, if you feel a significant increase in operating force, it may be due to increased friction between the packing and the valve stem. This could be because the packing has become compact after wear, or the packing has aged and dried out, losing its elasticity, resulting in tighter contact with the valve stem. For example, if a handwheel that was originally easy to turn now requires more effort to operate, this is a signal that the packing may need to be replaced.
During valve operation, pay attention to the movement of the valve stem. If the valve stem movement is not smooth, with sticking or shaking, in addition to considering problems with the valve stem itself, it may also be due to uneven friction between the packing and the valve stem. This could be caused by packing wear, damage, or improper installation. For example, partial damage to the packing may cause inconsistent resistance to the valve stem during movement, resulting in sticking.
When performing regular maintenance on the valve or suspecting a problem with the packing, you can open the packing gland cover (if possible) to inspect the appearance of the packing. If the packing shows signs of hardening, brittleness, discoloration (for example, if the originally black graphite packing turns grayish-white, it may be due to chemical erosion), it indicates that the packing has aged or undergone chemical changes. Aged packing cannot effectively fill the gap between the valve stem and the packing gland, leading to leakage. Check for wear on the packing. Normally, there will be a certain degree of friction between the packing and the valve stem, and the packing will gradually wear out with time and usage. If the packing is severely worn and its thickness is significantly reduced, it can no longer provide sufficient sealing force and needs to be replaced. For example, for braided packing, if the braided structure is loose or fibers are broken, it indicates severe wear.
Press the packing with your finger to check its elasticity. If the packing loses its elasticity and does not return to its original shape after pressing, it indicates that the sealing performance of the packing has decreased. Packing with good elasticity can adaptively fill the gaps during the movement of the valve stem, maintaining good sealing effect.
Review the maintenance records of the valve. If the packing has been repaired multiple times before, such as adding packing or adjusting the packing gland cover, but leakage or abnormal operating force still occurs, it is likely that the packing cannot restore its performance through simple repairs and needs to be replaced.
Based on the frequency of valve use and working environment, estimate the service life of the packing. Generally, in normal working conditions, the service life of packing may vary from one to several years. For example, in the chemical industry, valves that are frequently operated and in contact with corrosive media may need to replace the packing within about a year; while in ordinary water supply and drainage systems, the service life may be relatively longer. If the valve has been in use for a long time and the above-mentioned leakage or changes in operating force occur, it is a prudent practice to replace the packing.
Packing wear is one of the common causes of valve stem sealing failure. As the number of valve opening and closing operations increases, the packing will gradually wear out, the gap will gradually increase, and eventually lead to leakage. For rising stem valves, the packing is mostly PTFE V-shaped packing, asbestos packing, or graphite packing. These packings will wear out faster when the valve is frequently opened and closed. Statistics show that after more than 2,000 valve opening and closing operations, leakage may occur at the packing. At this time, the packing gland cover needs to be tightened again, but this solution is suitable for occasions with fewer valve opening and closing operations.
If the valve has a high frequency of opening and closing (for example, 100,000 to 300,000 times a year), traditional packing sealing solutions will significantly increase the use and maintenance costs of the valve. Therefore, it is necessary to find a design that can maintain sealing even after 1 to 2 million valve opening and closing operations.
One solution is to adopt hydraulic or pneumatic sealing technology, which changes the traditional valve design concept. A cylinder sealing slip ring is used at the packing position, and an O-ring is added. This method requires calculating the friction force and sealing specific pressure to determine the number of slip rings and O-rings. Through practical application, this design can achieve a non-leakage effect for up to 2 million opening and closing operations. However, the aging problem of the O-ring limits its service life to about 5 years, and the cost is relatively high.
Another solution is to add a washer below the packing and a spring below the washer. The pre-tightening force of the spring needs to be accurately calculated to ensure that it can effectively compensate for packing wear. When the packing wears out, the spring will re-tighten the packing to maintain sealing. This solution has been tested through practical application and can achieve a non-leakage effect for up to 500,000 to 1 million opening and closing operations. However, the user needs to re-tighten the packing gland cover during the annual major overhaul to reduce the maintenance workload during normal operations.
Choosing the right valve stem sealing solution requires a comprehensive consideration of factors such as the frequency of valve use, working environment, medium characteristics, and cost.
If the valve has fewer opening and closing operations, traditional packing sealing solutions (such as PTFE V-shaped packing, asbestos packing, or graphite packing) can meet the requirements. These packings are low in cost and relatively simple to replace and maintain. However, if the valve has frequent opening and closing operations, hydraulic, pneumatic sealing technology, or spring compensation solutions should be considered.
The working environment has a significant impact on the performance of the packing. In environments with corrosive media, the packing needs to have good corrosion resistance. For example, in the chemical industry, valves that are frequently operated and in contact with corrosive media may need to replace the packing within about a year. In contrast, in ordinary water supply and drainage systems, the service life of the packing may be relatively longer.
The characteristics of the medium also affect the choice of packing. For example, for high-temperature, high-pressure, or corrosive media, packing with corresponding performance should be selected. Graphite packing has good high-temperature and corrosion resistance and is suitable for these special environments. For general media, PTFE packing may be a better choice.
Cost is an important factor to consider when choosing a valve stem sealing solution. Traditional packing sealing solutions are low in cost but require frequent maintenance. Hydraulic, pneumatic sealing technology, and spring compensation solutions can significantly improve the service life and sealing performance of the valve, but they are more expensive. Therefore, a balance needs to be struck based on actual needs and budget.
Valve stem sealing is a crucial link in ensuring the safe operation of valves. By understanding the working principles of valve stem sealing, common problems, and their solutions, appropriate sealing solutions can be better selected to improve the service life and reliability of valves. Whether it is the traditional packing sealing solution or advanced hydraulic, pneumatic sealing technology and spring compensation solutions, a comprehensive consideration of factors such as the frequency of valve use, working environment, medium characteristics, and cost is necessary. Regular maintenance and upkeep are important guarantees for ensuring the performance of valve stem sealing. Through scientific management and maintenance, the risk of valve leakage can be effectively reduced, ensuring the smooth progress of industrial production. In practical applications, choosing the right valve stem sealing solution can not only improve production efficiency but also reduce maintenance costs and ensure production safety. It is hoped that this article can provide valuable reference for relevant technical personnel and managers, helping them better deal with various problems of valve stem sealing.
