In the industrial field, the sealing performance of valves is crucial for ensuring production safety and efficiency. The packing ring, as a key component of valve sealing, is of undeniable importance. This article will focus on the packing ring, detailing its manufacturing methods, installation steps, precautions, and how proper installation and maintenance can extend its service life and ensure reliable valve sealing.
The packing ring is a critical element used for sealing within the stuffing box and is typically manufactured using either the braiding method or the molding method. Packing materials made by the braiding method can be cut into segments according to the circumference of the stem/valve stem, with cuts usually at 45º (beveled) or 90º (perpendicular). The beveled method is widely recommended because it provides better end-face mating under axial load. Each packing ring must fully encircle the entire circumference of the stem/valve stem, and the tightness between the outer side and the stuffing box wall should be higher than that between the inner side and the valve stem. If the packing cord ends at the cut are too long, causing overlap, or too short, causing gaps, the packing ring density will be uneven, creating potential leak paths.
Packing rings manufactured by the molding method are more uniform and precise and are usually used in situations requiring higher sealing performance. These packing rings are preformed during the manufacturing process, eliminating the need for on-site cutting, which greatly simplifies the installation steps and improves installation efficiency.
Installing packing rings is a key step in ensuring valve sealing performance. Proper installation not only effectively prevents leakage but also extends the service life of the packing rings and reduces maintenance costs. Below are the detailed installation steps and precautions:
Before installing packing rings, a comprehensive hazard and risk assessment must be conducted. First, the valve must be properly isolated to ensure construction safety. Operators should wear protective clothing and goggles as required. Use packing cleaning tools to remove all old packing from the stuffing box and carefully inspect the valve stem for pitting and scratches using a borescope to ensure that the old packing has been completely removed. If necessary, refurbish the valve stem. Then, measure the outer diameter of the valve stem, the inner diameter of the stuffing box, and the depth of the stuffing box cavity to accurately calculate the packing size.
The formula for calculating the packing ring radius is (OD−ID)/2, where OD is the inner diameter of the stuffing box and ID is the outer diameter of the valve stem. Next, calculate how many layers of packing rings are needed to match the depth of the stuffing box cavity. The packing material can be wrapped around a mandrel with the same diameter as the valve stem and cut vertically (radially); or it can be cut at an angle using a packing-specific cutting tool. It is important to avoid stretching the packing material along its length or compressing it to prevent deformation of its cross-section. Also, the cutting surface must be neat and free of impurities or debris.
Never fill the stuffing box with packing material by winding it in a coil. Instead, use a specialized cutting tool to cut the packing braided material into individual packing rings with a 45º (beveled) cut, which can provide better end-face mating under load. Check that each packing ring fits snugly into the stuffing box; each ring's joint should be properly aligned, and the ring should be as closely in contact with the stuffing box wall as possible without stretching the braided material to match the inner diameter. All subsequent packing rings installed must have the same size and cutting method as the first one.
Preformed packing rings are very common and do not require on-site cutting, making them easy to install. First, inspect the valve bonnet bolts, nuts, and gaskets, and replace any that are damaged. Both bolts and nuts should be coated with lubricant. To effectively control fugitive emissions, a torque wrench must be used, and the operating procedures provided by the packing supplier must be followed. The stuffing box must also be ensured to be clean and free of impurities and grease.
Cleaning and Inspection: Place the first packing ring into the stuffing box, paying attention to the position of the joint, which should be inserted into the stuffing box first, followed by the rest of the packing ring. If it is a flexible graphite molded packing ring, each ring should be stretched into a spiral shape before installation to avoid damaging the packing ring. After each packing ring is installed, ensure that it is properly compressed and seated in the stuffing box before installing the next one. If the length of the bonnet follower is insufficient to reach the bottom of the stuffing box, temporary tools or extension rings may be needed to assist in the installation of the packing rings.
Avoiding Lubrication: Unless specifically stipulated, sealing elements should not be lubricated. Follow the manufacturer's operating specifications to apply load to the first installed packing ring. Repeat the same steps for the next packing ring, installing only one at a time. The joints of any two adjacent packing rings must be positioned at 90º and 180º angles to each other.
Operating the Valve Stem: After installation, the valve stem must be operated to cycle the valve open and closed three to ten times to ensure that the packing set is properly seated. During this process, the packing set will experience a certain degree of relaxation, so after the cycling operation is completed, it should be retightened according to the manufacturer's parameters to provide sufficient clearance to offset future relaxation. Similarly, after the valve is put into normal operation, the packing set should be tightened one last time to reserve enough clearance for future stress relaxation.
The selection of packing ring materials is crucial, as different materials are suitable for different working environments. Polytetrafluoroethylene (PTFE) is a commonly used packing material with a temperature range of approximately -101°C to 232°C. If used in environments with higher temperatures, its molecular structure may change, leading to the leakage of harmful gases, which can cause corrosion of the valve stem and may even release toxic PTFE fumes. These fumes may be carcinogenic, and as the fumes are released, the packing material will soften and lose weight, resulting in the leakage of harmful process media or product.
Flexible graphite is another commonly used packing material, with a temperature range of generally -196°C to 450°C in an oxidizing atmosphere. Considering the material properties, such as the content of ash, chlorides, sulfur, fluorides, and other halogen compounds, as well as the oxidation mass loss information provided by the manufacturer, it can be determined that graphite packing can reliably prevent fugitive emissions in environments up to 450°C. In steam conditions where no oxidation reaction occurs, it can withstand up to 650°C. Both flexible graphite and PTFE are widely applicable to media with a pH value of 0-14, and graphite material is also suitable for fire-safe sealing in some cases.
Maintenance and inspection of packing rings are key to ensuring valve sealing performance. Regular maintenance and inspection can promptly identify potential problems and prevent leaks and equipment damage. Below are the detailed maintenance and inspection steps and precautions.
Regular Inspection: Regularly checking the condition of packing rings is essential for ensuring valve sealing performance. During inspection, pay attention to signs of wear, deformation, or damage to the packing rings. If any abnormalities are found, the packing rings should be replaced in a timely manner to avoid the risk of leakage.
Adjustment and Tightening: During valve operation, packing rings may experience wear or relaxation, which can affect sealing performance. Regular adjustment and tightening of packing rings are necessary maintenance measures. Use a torque wrench to tighten according to the manufacturer's parameters to ensure the sealing performance of the packing rings.
Replacement and Renewal: When the sealing performance of packing rings decreases or leakage occurs, they should be replaced promptly. During replacement, strictly follow the installation steps to ensure the new packing rings are installed correctly and provide reliable sealing performance.
When installing packing rings, pay attention to some key details to ensure sealing performance and extend service life. Below are some important installation precautions.
Avoid Over-Compression: When installing packing rings, each ring must be installed in sequence until the entire packing set is completed. This ensures uniform distribution of axial stress, reducing friction and leakage rates. If a set of multiple packing rings is installed all at once, the top ring closest to the bonnet follower will bear the greatest load, while the bottom ring will bear very little, if any. This can lead to poor performance of the bottom ring in preventing medium leakage.
Avoid Over-Loading: Excessive load applied to the top of the packing rings can cause excessive friction during valve stem rotation and may even damage the packing set. Therefore, it is essential to apply load reasonably according to the manufacturer's operating specifications to ensure the sealing performance of the packing rings.
Ensure Cleanliness: The stuffing box and valve stem must be kept clean, free of impurities and grease. Before installing packing rings, use packing cleaning tools to remove all old packing from the stuffing box and carefully inspect the valve stem for pitting and scratches using a borescope.
If packing rings are installed correctly, the attenuation of axial stress converted to radial stress will be reduced. This helps the packing rings maintain reliable sealing performance for a longer period and ensures a reasonable distribution of frictional load. On this basis, the fugitive emission sealing performance of the valve, as well as production efficiency, will be significantly improved.
As a key component of valve sealing, the proper installation and maintenance of packing rings are crucial for ensuring valve sealing performance. By selecting the appropriate materials, using the correct installation methods, and strictly following operating procedures, the service life of packing rings can be effectively extended, leakage risks can be reduced, and production efficiency and safety can be improved. In actual operations, it is essential to strictly follow the manufacturer's guidance to ensure that every step is accurate and error-free. Only in this way can the sealing performance of packing rings be fully utilized to ensure the smooth progress of industrial production.
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