In the petrochemical industry, control valves, as one of the terminal control elements of automatic control systems, are of critical importance. This is especially true when dealing with complex conditions such as high temperatures, high pressures, toxic substances, highly corrosive media, and flammable or explosive environments. The selection and sealing performance of control valves are even more crucial in such cases, as they directly affect the safety operation, production efficiency, and environmental protection of the entire facility. Once a valve experiences leakage, it can lead to the waste of raw materials and products, severe environmental pollution, and even catastrophic accidents such as fires, explosions, or poisoning. Therefore, in-depth understanding of control valve sealing technology, along with the appropriate selection of sealing materials and structures, is essential for ensuring the long-term safe operation of petrochemical facilities.
Control valve sealing is mainly divided into external sealing and internal sealing. External sealing refers mainly to packing seals, which play a key role in preventing leakage caused by the valve stem's movement. Internal sealing involves the sealing between the valve plug and the valve seat, ensuring that the fluid inside the control valve is regulated according to the desired flow rate and pressure. In chemical installations, due to the complexity of the medium and the harsh operating conditions, external packing seals often face more significant challenges. If the packing seal fails, the risk of leakage greatly increases, which can then impact the normal operation of the entire facility.
The packing seal section of the control valve primarily consists of the packing box, packing gland, lantern ring, and packing rings. The packing box is the core component, providing installation space for the packing and applying pressure through the packing gland to ensure that the packing tightly fits the valve stem, thereby achieving sealing. A commonly used packing box structure consists of a bolted packing flange, packing gland, lantern ring, and multiple identical PTFE (Polytetrafluoroethylene) V-shaped packing rings. This structure is simple, reliable, and easy to install and maintain.
The working principle of packing seals relies on the elastic deformation of the packing. Under the pressure applied by the packing gland, the packing tightly fits the valve stem and the inner walls of the packing box, thus preventing leakage. As the valve stem moves up and down, the packing maintains constant contact with the stem through its elastic deformation and friction, thereby achieving dynamic sealing. For example, PTFE V-shaped packing rings work by having the outer lip of the packing expand under the pressure of the medium, sealing tightly against the inner wall of the packing box to achieve static sealing. Simultaneously, the inner lip expands and makes contact with the valve stem to ensure dynamic sealing. This structure ensures that the packing maintains excellent sealing performance throughout the movement of the valve stem.
After understanding the structure and working principle of control valve packing seals, the next important topic is the selection of packing materials and their performance requirements. No matter how sophisticated the sealing structure is, if the packing material cannot meet the rigorous operating conditions, the sealing performance will be compromised. Therefore, selecting the right packing material is a critical step to ensuring sealing reliability, and understanding its performance requirements is fundamental to making the correct choice.
To ensure good sealing performance, a long service life, and a wide application range, packing materials must have several key properties. First, the packing must have sufficient tensile strength to withstand the pressure and temperature changes of the medium, and must not react with any chemicals, ensuring its chemical stability. Second, the packing must have a low coefficient of friction, strong wear resistance, and minimal wear on the valve stem, which will help reduce stem wear and extend its service life. Additionally, the packing should have good self-lubricating properties, be resistant to stickiness with the valve stem, and possess a certain level of flexibility to adapt to the stem's movement. In high-temperature environments, the packing should not coke, degrade, or decompose; in low-temperature environments, the packing should not become brittle. These performance requirements make the selection of packing materials particularly critical.
PTFE packing is one of the most widely used valve packing materials, as it possesses almost all the desired characteristics. It has excellent chemical inertness and lubrication, maintaining good sealing performance over a temperature range of -40 to 232°C, and is suitable for various chemicals, acids, alkalis, and oil-free environments. PTFE can be molded or processed into V-shaped rings, with the overall V-ring being compressed by spring force to provide adequate contact pressure with the valve stem and can self-adjust without requiring additional lubrication. However, PTFE also has its drawbacks. It has a high expansion coefficient, and when the medium temperature exceeds 300°C, PTFE expands dramatically, which can lead to shaft seizure and increased wear, reducing the packing's lifespan. Furthermore, PTFE is unsuitable for molten alkali metals, high-temperature hydrogen fluoride, and fluorine-based media, as it requires smooth valve stem surfaces for proper sealing. For fluids containing crystalline substances or solid particles, PTFE packing should not be used alone, but instead, graphite packing should be placed at the lower part of the packing box, with PTFE V-shaped packing on top.
Flexible graphite packing is a new type of packing material that has excellent sealing properties, self-lubricating capabilities, corrosion resistance, and is less affected by temperature changes. It is suitable for high- and low-temperature environments and is commonly used for high-pressure steam, water, oil, and other demanding sealing applications. Its operating temperature range is from -200 to 600°C, and it is suitable for conditions with a nominal pressure up to 32 MPa. Flexible graphite packing offers leak-free operation, high thermal conductivity, and long service life. However, flexible graphite packing generates higher friction on the valve stem and typically requires a valve positioner to function effectively. It is not suitable for highly concentrated or high-temperature strong oxidizers such as concentrated nitric acid or sulfuric acid. For ball or plug valves requiring small packing dimensions, flexible graphite braided packing can meet the needs.
In certain special operating conditions, a single type of packing material may not meet the sealing requirements, so mixed packing can be used. Mixed packing typically combines two materials: graphite packing and PTFE packing or a mix of V-shaped rings. For permeable media, graphite packing is typically chosen. Mixed packing combines the advantages of different materials to improve sealing performance and service life.
After understanding the performance requirements for packing materials, we need to explore the principles for selecting packing seals. Even with knowledge of various packing materials, improper selection based on actual operating conditions will not yield the desired sealing results.
When selecting packing, comprehensive consideration must be given to specific medium conditions, operating requirements, and the valve type. Generally, only one of the pressure or temperature conditions can meet the maximum value. When selecting packing for high-temperature valves, temperature should be the primary consideration. For corrosive media with moderate temperature and pressure, materials with good corrosion resistance should be prioritized. For example, in cases where the fluid temperature exceeds 200°C, packing boxes are often designed with double sealing structures. For cases with negative pressure fluid, V-shaped PTFE packing should be installed in reverse (with the opening facing upwards). For alternating negative and positive pressure, dual-layer V-shaped PTFE packing should be used, with the lower layer in standard installation and the upper layer in reverse. For flammable substances like hydrogen gas, which are prone to leakage due to their low density, a combination of V-shaped PTFE and graphite packing is typically used.
While meeting sealing requirements, the selection of packing should also take into account economic reasonableness. Although some high-performance packing materials offer better sealing properties and longer service lives, they come at a higher cost. Therefore, when selecting packing, it is essential to strike a balance between sealing performance and cost to ensure the long-term safe operation of petrochemical facilities.
Regular Inspection and Maintenance: To ensure the reliability of packing seals, regular inspections and maintenance of control valves should be performed. Inspections should focus on the wear condition of the packing, the tightness of the packing gland, and the condition of the valve stem surface. If significant wear or damage to the valve stem is detected, packing should be replaced or the valve stem repaired to prevent leakage.
Proper Installation of Packing: When installing packing, attention must be paid to the correct installation direction and method. For V-shaped packing, the direction of the opening should be correct to ensure sealing performance. The packing should be installed evenly and tightly to avoid loosening or over-tightening. Over-tightened packing may cause excessive valve stem wear, while loose packing will fail to seal effectively.
Lubrication and Cooling of Packing: For certain packing materials requiring lubrication, regular lubrication should be added to reduce friction between the packing and the valve stem. Additionally, in high-temperature conditions, cooling measures should be implemented to prevent the packing from expanding or being damaged due to high heat.
The sealing performance of control valves is critical to ensuring the safe operation of petrochemical facilities. By thoroughly understanding the structure, working principle, material selection, and selection principles of packing seals, control valve sealing reliability can be improved, leakage risks reduced, and the long-term safe operation of petrochemical facilities ensured. In practical applications, it is necessary to select appropriate packing materials and structures based on specific operating conditions and requirements, and to perform regular inspections and maintenance to ensure that the packing seals remain in good condition. Only then can high-efficiency, safe, and environmentally friendly production goals be achieved in complex chemical production environments.