Causes and Strategies for Control Valve Packing Leakage

Control valves play a crucial role in various industrial applications, with their sealing performance directly impacting the equipment's proper functioning and operational safety. However, packing leakage in control valves is a common issue that not only affects equipment efficiency but also poses potential safety risks. Understanding and identifying the main causes of packing leakage is a key step in ensuring stable equipment operation and enhancing operational safety. This article will delve into the common causes of control valve packing leakage and provide corresponding preventive measures.

Main Causes of Packing Leakage in Control Valves

 
Packing leakage is a common fault in the practical application of control valves, which not only directly affects the performance of the equipment but can also lead to safety hazards. Therefore, a deep understanding of the main reasons for packing leakage is crucial for maintaining the stable operation of the equipment and ensuring safe operation. The following are the main reasons that cause packing leakage.

1. Improper Selection of Packing Material and Type

 
Material Incompatibility with Working Conditions: The material of the packing must be compatible with the working environment. For instance, in conditions ranging from 150°C to 200°C, PTFE (Teflon) packing may undergo slight creep under long-term critical conditions, leading to a decline in sealing performance. For media with strong permeability such as liquid ammonia, tar, and fuel, if woven packing is used, it can also be affected by high temperature and high pressure, causing leakage.
Inappropriate Type of Packing: Different types of packing (such as graphite, PTFE) are suitable for different working conditions. If the selected packing has poor tolerance to certain media, it increases the risk of leakage.

2.Improper Packing Installation Method

 
Uneven Axial Pressure Application: During the installation of packing, if the packing is tightened unevenly or the pressure is not applied evenly, it may lead to uneven contact between the packing and the valve stem, causing leakage.
Non-standard Assembly: Incorrect assembly methods of packing, such as misaligned cut packing or uneven tightening of the gland, can affect the sealing effect of the packing.

3. Axial Movement in Control Valve Use

 
Axial Movement Leading to Leakage: The axial movement between the valve stem and the packing during the use of the control valve can lead to packing leakage. High temperature, high pressure, and media with strong permeability can cause leakage in the packing chamber, mainly due to interface leakage between the packing and the valve stem and seepage through tiny gaps in the packing fibers.
Aging and Pressure Decay of Packing: The aging of the packing, loss of elasticity, and gradual decay of contact pressure can lead to leakage of the pressure medium along the contact gap between the packing and the valve stem.

4. Frequent Movement and Mechanical Wear of Control Valves

 
Frequent Operation Leading to Wear: Frequent operation of the control valve, bending, wear, and corrosion of the valve stem can lead to wear of the packing, thereby causing leakage.
Decrease in Surface Finish: A decrease in the surface finish of the valve stem can also increase the risk of wear and leakage of the packing.

5. Changes in Operating Conditions

 
Temperature Changes: Changes in the operating conditions of the control valve during the heating or cooling process can affect the sealing effect of the packing. For example, the transition from a cold state to a hot state may cause thermal expansion of the valve stem, increasing the gap between the packing and leading to leakage.
Changes in Medium: Significant changes in the hot medium can severely reduce the sealing performance of the packing, leading to increased leakage.

6. Loose or Skewed Packing Gland

 
Improper Installation of Packing Gland: If the packing gland is not tightened in place or is skewed, it may lead to poor contact between the valve stem and the packing, causing improper gaps and leading to leakage.
Installation Posture Issues: The horizontal installation of the control valve may affect the sealing effect of the packing, leading to leakage problems.

Strategies for Control Valve Packing Leakage Prevention

 
To effectively prevent packing leakage in control valves, a systematic approach is needed, optimizing from design, material selection, installation to maintenance. The following are the main preventive measures for packing leakage issues.

1. Optimize Packing Chamber Design

 
Chamfer Design: Set a chamfer at the top of the packing chamber to facilitate the smooth installation of the packing and reduce resistance during assembly.
Metal Protective Ring: Place an erosion-resistant metal protective ring at the bottom of the packing chamber, ensuring that the contact surface of the protective ring is flat and avoiding beveled designs to prevent the packing from being pushed out under medium pressure.

2. Improve Surface Precision and Finish

 
Surface Machining: Improve the surface precision and finish of the valve stem and packing chamber. A smooth surface can reduce friction and ensure even pressure on the packing.
Avoid Defects: The surfaces of the valve stem and packing chamber should avoid scratches, pitting, and wear, which can lead to poor sealing and leakage.

3. Choose the Right Packing Material

 
Material Matching: Select the appropriate packing material based on the working conditions. Generally, PTFE packing is preferred for its excellent high-temperature resistance and chemical corrosion resistance; graphite packing can be used when conditions do not meet the requirements.
Mixed Packing: For media with strong permeability, mixed packing can be used, such as a combination of graphite and PTFE packing, or a combination of O-rings and V-rings, to improve sealing performance.

4. Standardize Packing Assembly

 
Add Packing in Turns: When installing packing, add it turn by turn and use a press to tighten each turn, ensuring even pressure on the packing.
Cut Packing Arrangement: Cut packing should be placed at 90° or 120° intervals to avoid leakage.
Number of Packing Rings: The number of packing rings should be sufficient to prevent the gland from entering the packing chamber, thereby reducing the risk of leakage.

5. Properly Tighten Gland Screws

 
For spring-actuated PTFE packing, gland screws should be tightened evenly and symmetrically to avoid skewing. Other types of packing do not need to be overly tight, just ensure there is no leakage.

6. Regular Inspection and Maintenance

 
Initial Inspection: New valves or valves that have just been repaired should be inspected initially after being put into use to ensure there is no leakage in the packing. If leakage is found, it should be dealt with promptly.
Regular Maintenance: Regularly check the condition of the packing and adjust and replace it according to the actual situation to maintain good sealing performance and extend the service life.
 
By deeply analyzing and understanding the reasons for packing leakage, effective measures can be taken to prevent such failures. Optimizing the design of the packing chamber, improving surface precision and finish, choosing the right packing material, standardizing packing assembly, properly tightening gland screws, and regular inspection and maintenance are all key steps to ensure the normal operation of control valves. Implementing these measures not only improves the efficiency of equipment operation but also greatly reduces the risk of leakage, thereby ensuring the performance and operational safety of the equipment.