Role of Packing Seals in Chemical Control Valves

In the chemical industry, control valves, as the final control elements in automatic control systems, are of unquestionable importance. Especially in complex chemical plants, high temperature, high pressure, strong corrosiveness, and flammable and explosive operating environments make the selection of control valves and the performance of their seals a key factor in ensuring production safety and efficiency. Once a valve leaks, it not only leads to waste of raw materials or products but also causes serious environmental pollution, and may even trigger fires, explosions, or poisoning, endangering human life. Therefore, the sealing issue of control valves must be taken seriously, and the reasonable selection of sealing materials and structures is a necessary step in design and selection.

Control valve seals are mainly divided into external seals and internal seals. External seals mainly refer to packing seals, while internal seals involve the sealing between the valve plug and the valve seat. This article focuses on external seals, that is, the structure and material selection of packing seals.

The valve part of a control valve consists of internal components and the valve body. The internal components include the valve plug, stem, packing box, and bonnet. The packing box is an inseparable part of the valve body, mainly used to seal the valve stem. It prevents leakage on the stem surface caused by the rotation or movement of the stem through elastic means. Almost all valve stem sealing is achieved using a packing box.

A commonly used packing box component with good sealing is the typical straight-through control valve packing box structure. This structure includes the packing chamber, gland, sleeve, and multiple V-shaped PTFE (polytetrafluoroethylene) packing rings of the same size. The packing is positioned and adjusted through a flange and double-headed bolts. The packing chamber and gland contact each other via a spherical interface. This design ensures that even if the packing chamber is misaligned due to wear, the valve stem will not be subjected to lateral forces. The sleeve provides a gap for the injection of lubricating oil. In most cases, before the stem moves through the packing box, there is a metal protective sleeve with a flange, and a felt sleeve is used on the flange to remove debris and wear marks from the stem.

The working principle of PTFE V-shaped packing rings is that under the pressure applied inside and outside the valve stem, the lips of the packing rings remain in close contact with the inner wall of the packing box when expanded, thus achieving a seal. Similarly, under pressure, the lips of the packing rings remain in tight contact with the valve stem, ensuring sealing. Even with vertical movement of the stem, this design ensures sealing performance.

The working principle of packing seals is that the packing is compressed inside the packing box, forming a seal between the valve stem and the packing box. When subjected to pressure, the packing expands to fill the gap between the stem and the packing box, preventing leakage. The sealing performance of the packing depends on the material, structure, and degree of compression. Suitable packing materials and structures can ensure sealing performance while reducing wear on the valve stem and extending the service life of the packing.

The selection of packing materials is crucial to ensuring the sealing performance, service life, and adaptability of valve packing. Ideal packing materials should have the following properties:

Strong chemical stability: They do not react with any chemicals and remain stable in various chemical media.

Low friction coefficient: Strong wear resistance, minimal valve stem wear, and reduced damage to the stem.

Good self-lubrication: Smooth sliding, no adhesion to the stem surface, and reduced friction.

Certain flexibility: Can adapt to stem movement while maintaining good sealing performance.

Good embedding and vibration resistance: Maintains sealing under high pressure and vibration.

Good temperature resistance: Does not coke, deteriorate, or decompose at high working temperatures and does not become brittle at low temperatures.

Currently, PTFE is widely used as valve packing because it nearly simultaneously possesses all the above properties. PTFE has excellent chemical inertness and lubrication, providing good sealing performance and low friction coefficient when operating between -40°C and 232°C. It is suitable for various chemical fluids, acids, bases, and oil-free environments.

PTFE can be used as a solid block or machined into V-shaped rings. The solid V-rings are often equipped with spring force to ensure sufficient preload on the valve stem, allow self-adjustment, require no lubrication, and can serve as a lubricant for graphite packing. However, PTFE packing also has drawbacks. Its expansion coefficient is high. When the medium temperature exceeds 300°C, PTFE expands sharply, causing the seal to seize the stem due to frictional heating, accelerating wear, and shortening the service life. PTFE cannot be used for molten alkali metals, high-temperature hydrofluoric acid, or media containing fluorine. It requires extremely smooth stem surfaces and correct sealing; if the stem or packing surface is damaged, leakage will occur. PTFE cannot be used alone for crystalline or solid-containing fluids. If necessary, graphite packing should be placed at the bottom of the packing box, with V-shaped PTFE packing above.

Flexible graphite packing is a new type of packing with excellent sealing, self-lubrication, corrosion resistance, high-temperature resistance, and minimal sensitivity to temperature changes. It is suitable for high-temperature or low-chloride conditions and is commonly used in high-pressure steam, water, and oil media, as well as media requiring high packing performance. Flexible graphite can withstand almost all media except fuming sulfuric acid and concentrated nitric acid. Its operating temperature ranges from -200°C to 600°C, suitable for nominal pressures not exceeding 32 MPa. Flexible graphite packing operates leak-free, has high thermal conductivity, and a long service life, but it produces high friction on the stem and usually requires a valve positioner to operate effectively. It is not suitable for strong oxidizers such as concentrated nitric acid and concentrated sulfuric acid. For small packing chambers in ball or plug valves, pure flexible graphite braided packing is sufficient.

In some special conditions, a single packing material may not meet requirements, in which case hybrid packing can be used. Hybrid packing mainly takes two forms: one is the combination of graphite packing and PTFE packing; the other is a combination of O-rings and V-rings. This hybrid packing combines the advantages of two materials, improving sealing performance and service life. For highly permeable media, graphite packing may be selected.

The use of packing is closely related to the packing box structure, operating conditions, and the material and form of the packing. When selecting packing, it should be based on specific conditions. Generally, the maximum value can only be satisfied between pressure and temperature. When selecting high-temperature valve packing, temperature should be prioritized. For corrosive media with relatively low temperature and pressure, corrosion-resistant materials should be preferred.

Packing selection varies under different operating conditions. For flammable substances such as hydrogen (H₂), due to its low density and tendency to leak, V-shaped PTFE combined with graphite packing is generally used. When the fluid temperature exceeds 200°C, packing boxes often adopt a dual-seal structure. Commonly used non-PTFE packing is generally silicone-based grease, but grease cannot be used for controlling hydrogen.

When selecting packing, temperature and pressure requirements must be balanced. Generally, high-temperature and high-pressure conditions require higher performance from packing. Flexible graphite packing is suitable for high-temperature conditions but produces higher stem friction, requiring a valve positioner for assistance. PTFE packing is suitable for lower-temperature conditions but may expand at high temperatures, reducing sealing performance.

For corrosive media, materials with high corrosion resistance should be preferred. PTFE packing has excellent chemical inertness and is suitable for a variety of corrosive media. However, for strong oxidizers such as concentrated nitric acid or concentrated sulfuric acid, flexible graphite packing may be a better choice.

In practical applications, in addition to selecting appropriate packing materials and structures, the following points should be noted:

Correct installation of packing: Packing must be installed correctly, ensuring even compression in the packing box and avoiding leakage points.

Regular inspection and maintenance: Regularly check for wear and replace damaged packing promptly to ensure sealing performance.

Proper adjustment of preload: The preload on the packing must be adjusted reasonably to ensure sealing while avoiding excessive compression that damages the stem.

Avoid improper lubricants: In some conditions, lubricants may react with the medium and reduce sealing performance; improper lubricants should be avoided.

The sealing performance of control valves plays a decisive role in the safe operation of petrochemical plants. When selecting valves, the stem packing should be chosen based on the specific medium, operating conditions, and valve type. Sealing must be reliable yet cost-effective to ensure long-term safe operation. Through proper selection of packing materials and structures, combined with correct installation and maintenance, the sealing performance and service life of control valves can be effectively improved, providing strong support for the safety and efficiency of chemical production.

In the chemical industry, control valve sealing cannot be ignored. By thoroughly understanding the structure, material selection, and usage conditions of packing seals, we can better cope with complex operating conditions and ensure smooth chemical production.