In industrial production, valve packing seals are crucial components that ensure the normal operation of equipment and prevent fluid leakage. However, packing seals often face wear issues during use, which not only affect the sealing effect but may also pose safety hazards. So, how can we effectively reduce the wear of packing seals and extend their service life? Lubrication is a key factor. This article will delve into the wear mechanisms of valve packing seals, the importance of lubrication, and how to select appropriate lubricants and methods, helping readers gain a comprehensive understanding of this important technical field.
Packing seals in valves serve to prevent fluid leakage by maintaining a tight contact between the packing and the valve stem (or rotating shaft). However, this contact generates friction during movement, leading to wear. Wear can occur not only on the packing but also on the valve stem. The extent of wear is related to various factors, including the wear resistance of the packing and valve stem materials, the speed of movement, lubrication and cooling conditions, and the quality of packing installation.
Normally installed packing experiences greater wear at the gland and gradually decreases inward. This is because the radial contact stress of the packing is unevenly distributed along the axial length, with the highest pressure at the gland. If the packing is installed improperly, such as being too tight or too loose, or if the layers of packing are uneven, high-stress areas will form, leading to abnormal wear. This abnormal wear can cause severe wear on the packing and valve stem in a short time, significantly affecting the sealing performance.
If the valve stem does not move along the same axis or has poor coaxiality with the packing box and packing rings during movement, it will cause circumferential non-uniform wear. This non-uniform wear makes the contact surface between the valve stem and packing uneven, increasing local pressure and thus accelerating wear. Therefore, ensuring the movement accuracy of the valve stem and the correct installation of the packing are important measures to reduce wear.
Lubrication is an effective means of addressing wear issues in packing seals. Lubricants can not only reduce the friction between the packing and the valve stem but also remove the heat generated by friction, preventing the packing from being damaged by high temperatures. Therefore, the selection and use of lubricants have a direct impact on the life and sealing capability of the packing.
There are various types of lubricants, with common ones including lubricating oils, greases, and solid lubricants. Most braided packings are impregnated with various self-lubricating and high-temperature-resistant lubricating materials during the manufacturing process. Additionally, lubricants can be supplied from the outside, or the leaking fluid can be used directly as a lubricant. For example, in high-temperature, high-pressure, and high-speed working conditions, forced lubrication and cooling measures are very important for packing seals.
The primary function of lubricants is to reduce friction and wear. They form a lubricating film between the packing and the valve stem, reducing direct contact between the two. Moreover, lubricants can remove the heat generated by friction, preventing the packing from being damaged by high temperatures. For example, in high-temperature sealing areas, if the impregnated lubricating oil is not heat-resistant, it will quickly evaporate, causing dry friction and a rapid increase in temperature, which can burn out the packing. Therefore, selecting the appropriate lubricant is crucial.
The selection of lubricants should be based on actual working conditions. The following are some factors to consider when choosing lubricants:
Speed of Movement: The higher the speed of movement, the stronger the penetration ability of the lubricant is required. Therefore, lubricating oils with not too high viscosity should be selected. High-viscosity lubricating oils or greases generate more heat, which is detrimental to both the packing and the valve stem. In low-speed conditions, where the load-bearing capacity is greater, the viscosity of the lubricating oil mainly plays a role, so lubricating oils with higher viscosity should be used.
Load or Pressure: The greater the load or pressure on the moving parts, the higher the viscosity of the lubricating oil should be. This is because different lubricating oils have the ability to bear certain loads, but their load-bearing capacity varies with viscosity. In low-speed, heavy-load moving parts, the load-bearing capacity of the lubricating oil should be considered first.
Impact and Vibration Loads: Impact and vibration loads can create extremely high pressures in an instant, and reciprocating and intermittent movements are not conducive to forming an oil film. Therefore, in these cases, lubricating greases or solid lubricants with higher viscosity should be used to ensure reliable lubrication.
Working Temperature: The working temperature refers to the temperature of the medium and the sealing area. For areas with lower working temperatures, lubricants with low viscosity should be used; for areas with higher working temperatures, lubricants with higher viscosity should be used. For conditions exceeding 200°C, solid lubricants should be considered.
Temperature Rise of Lubricants: The temperature rise of lubricants mainly depends on the speed of movement, pressure, medium, and the properties of the lubricating materials. Considering that various factors may change during actual operation, the temperature limit of the lubricant should be 20–30°C higher than the actual working temperature.
Environmental Conditions: In severely humid conditions, general lubricants are prone to deterioration or being washed away by water, so lubricating materials with anti-washout properties should be used. Greases have strong water resistance and are suitable for use in humid conditions. For equipment working in dusty environments, the packing box seal should be as tight as possible to prevent dust from entering and mixing with the lubricating grease to form an abrasive, causing wear. In places with severe corrosive atmospheres, lubricants with good anti-corrosion properties should be used because lubricating greases have a good covering ability on metals, which can prevent corrosive gases from directly contacting metals and causing corrosion.
In addition to selecting the appropriate lubricant, optimizing the lubrication system is also an important measure to reduce wear of packing seals. In the valve stem packing assembly, a follower ring is usually installed. The follower ring can not only be used for the injection of lubricants but also serves as a leakage monitoring channel. When leakage exceeds the standard, sealing compound can also be injected through the follower ring to further improve sealing performance.
The method of lubricant injection also affects the lubrication effect. In some complex working conditions, a forced lubrication system can be used. The forced lubrication system uses external power to deliver the lubricant between the packing and the valve stem, ensuring an adequate supply of lubricant. This system can effectively reduce dry friction caused by insufficient lubricant, thereby extending the service life of the packing.
Leakage monitoring is an important means to ensure the sealing performance of packing seals. By installing a follower ring in the middle of the packing, leakage can be monitored in real time. Once leakage exceeding the standard is detected, measures such as injecting sealing compound can be taken in time to prevent further leakage. This monitoring and remedial measure can effectively reduce equipment downtime caused by leakage and improve the operating efficiency of the equipment.
To better understand the relationship between packing seal wear and lubrication, let's look at a practical application case. In a high-temperature and high-pressure valve of a chemical plant, the packing seal often experienced rapid wear. After analysis, it was found that the main reason was that the lubricating oil impregnated in the packing was not heat-resistant and quickly evaporated, leading to dry friction. To solve this problem, the plant used a high-temperature-resistant solid lubricant and optimized the lubrication system by adding a forced lubrication device. After these improvements, the service life of the packing was significantly extended, and the leakage problem was effectively controlled.
Wear of valve packing seals is an important factor that affects their service life and sealing performance. By reasonably selecting lubricants and optimizing the lubrication system, wear can be effectively reduced, and the service life of the packing can be extended. In practical applications, it is necessary to comprehensively consider factors such as the type, viscosity, heat resistance, and water resistance of lubricants based on specific working conditions, and choose the appropriate lubricants and lubrication methods. At the same time, strengthening leakage monitoring and timely remedial measures are also important means to ensure the sealing performance of packing seals. It is hoped that the introduction of this article can help readers better understand and apply the technology of packing seal wear and lubrication, and improve the operating efficiency and safety of equipment.