During operation, control valve packing leakage can occur due to various reasons, including changes in operating conditions and differences in the valve body's structure. This is the most common control valve malfunction, a significant factor affecting continuous production, and a common source of pollution in many chemical plants and refineries. It not only wastes materials but also pollutes the environment. Therefore, preventing control valve packing leakage is a crucial aspect of routine instrument maintenance, a challenging problem, and a topic worthy of our research.
Functions and classifications of control valve packing
Control valve packing is typically installed in the stuffing box of the upper valve cover. Its function is to prevent leakage of the controlled medium caused by the movement of the valve stem.
Commonly used fillers are mainly divided into two categories according to their materials: polytetrafluoroethylene and flexible graphite (asbestos products have been banned in many countries, so they will not be discussed in this article). Many kinds of fillers are derived from these two basic materials to meet the needs of different occasions.
1. Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a synthetic resin material with a carbon atom backbone and fluorine atoms symmetrically and uniformly distributed around it, forming a tight barrier that gives it excellent resistance to chemical corrosion. It has stable physical and chemical properties, and its corrosion resistance even surpasses that of glass and ceramics. It also exhibits good resistance to strong acids, strong alkalis, and strong oxidants, making it an excellent sealing material. Furthermore, it possesses good anti-aging properties, a low coefficient of friction, and excellent sealing performance, making it an ideal replacement for asbestos. However, its temperature resistance is poor; PTFE begins to decompose in trace amounts above 200°C, and it is prone to creep under pressure and heat, affecting its sealing performance. It is also unsuitable for use in molten alkali solutions or with fluorides.
Commonly used polytetrafluoroethylene (PTFE) fillers:
Polytetrafluoroethylene molded woven filler
It is made of PTFE bulk woven and pressed, and is an open packing material. It has good flexibility, durable wear resistance, good sealing effect, and is easy to replace. It is one of the most widely used packing materials.
V-type polytetrafluoroethylene packing
It is generally machined from polytetrafluoroethylene (PTFE) rods. The packing structure is V-shaped. When both ends are compressed, the low coefficient of friction of PTFE provides lubrication and good sealing performance. There are two main structures: 60° V-type for ordinary valves and 90° V-type for high-pressure valves.
PTFE-graphite filler
Adding glass fiber, graphite, and molybdenum disulfide can improve the creep resistance and thermal conductivity of tetrafluoroethylene, but it also increases hardness, reduces corrosion resistance, and decreases sealing performance.
2. Flexible graphite
Also known as expanded graphite, it is made from natural graphite flakes and is an ideal sealing material. Due to its porous, loose, and curled structure, it has good resilience and flexibility; the bonding force between its crystals is very small, and when subjected to external forces and friction, slippage easily occurs between the layers, and carbon molecules transfer to the abrasive surface, generating self-lubrication and good sealing performance. Expanded graphite has a large specific surface area and surface energy, and its surface always adsorbs a large amount of gas or water molecules, forming an extremely thin gas or liquid film that prevents the medium from penetrating. Its permeability decreases with increasing density. Generally, it has better impermeability than other sealing materials. Flexible graphite's main component is carbon, so its chemical properties are inert, and it has good corrosion resistance and resistance to high and low temperatures. However, it has high friction, causing creeping in control valves and significant hysteresis. It cannot be used with strong oxidizing agents such as concentrated sulfuric acid and concentrated nitric acid, and it is brittle and easily broken.
There are three main types of graphite fillers:
filamentous or woven
This type of packing has good elasticity, better surrounds the valve stem, and cleans the valve stem surface. Braided graphite packing can overcome the leakage caused by wear of other types of graphite packing, but the fibrous structure of braided graphite packing creates voids, making it prone to penetration.
flake graphite
The packing ring, made by pressing several graphite plates together, has a texture perpendicular to the valve stem surface, forming a square cross-section. This makes it difficult for fluid to penetrate, but it also reduces the ratio of axial pressure to radial pressure, requiring a greater clamping force to seal.
Curved flake graphite
Its texture is parallel to the valve stem axis, and it is made from several graphite sheets. The pressure applied during the pressing of the curved graphite sheets is much higher than the pressure applied during operation, therefore it does not shrink further within the stuffing box. As its principle dictates, it easily permeates at high temperatures. However, due to its ease of processing and low cost, it is currently widely used.
The main causes of packing leakage
Packing leakage in control valves can be caused by a variety of reasons. Therefore, each control valve should be analyzed specifically during maintenance. Only in this way can the packing leakage be fundamentally resolved and potential hazards eliminated.
1. Inappropriate selection of packing material and type. For example, if PTFE packing is used in 150℃-200℃ operating conditions, the packing will undergo slight creep under critical conditions for a long time, and the sealing performance of the packing will decrease under the action of high pressure medium. On the other hand, if woven packing is used for highly permeable media such as liquid ammonia, tar, and fuel oil, leakage is likely to occur under the influence of high temperature and high pressure medium.
2. Improper packing installation. After the packing is installed in the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it generates radial force and makes close contact with the valve stem. If the packing is tight at the top and loose at the bottom during installation, the force is uneven, making this contact very uneven. Some parts may have loose contact or even no contact, causing packing leakage.
3. The packing material ages and loses its elasticity; frequent operation of the control valve causes wear on the packing material.
4. The valve stem is bent, worn, corroded, and its surface finish is reduced.
5. Changes in the operating conditions of the control valve. For example, during the start-up and heating process of the unit, the control valve changes from a cold state to a hot state, or the medium in the confluence three-way valve changes greatly between cold and hot states. The valve stem is affected by thermal expansion and contraction, which increases the packing clearance and causes serious packing leakage.
6. Loose or misaligned packing glands, or horizontal installation of the control valve, can cause poor contact between the valve stem and packing, resulting in excessive or insufficient clearance.
Countermeasures to prevent packing leakage
1. Improve the surface precision and smoothness of the valve stem and stuffing box. If the sum of the friction coefficients between all moving parts were zero, the force acting on the stuffing gland would be evenly transmitted to the entire packing without any attenuation. However, actual friction always exists and cannot be zero. The radial force acting on the packing decreases with increasing distance from the gland, and the greater the friction, the greater the pressure attenuation. Once the sealed pressure exceeds the force acting on the packing, leakage begins. Therefore, during maintenance, the valve stem and stuffing box must be free of scratches, pitting wear, and have a good surface finish.
2. Select suitable materials. These materials must be able to resist temperature changes and have resistance to creep, relaxation, and oxidation. Generally, PTFE is preferred if these conditions are met; otherwise, graphite is chosen. Mixed packings can also be used: one is a mixture of graphite and PTFE packings; the other is a mixture of O-rings and V-rings. For highly permeable media, graphite packings can be selected.
3. When loading the packing, add it one ring at a time, and use a pressure tool to tighten and secure it one ring at a time to ensure even stress on the packing. Open packing should be staggered vertically by 90° or 120°. The number of packing rings added should be such that none are exposed. If too few rings are added, the packing gland will enter the stuffing box, easily causing leakage.
4. For PTFE V-type packing with spring action, the gland screws should be tightened as much as possible. Other types of packing do not need to be tightened too much; just tighten them until there is no leakage. The gland flange should be tightened evenly and symmetrically, without any deviation.
5. After a new valve or a control valve that has just been repaired is put into use, the packing should be checked for leaks. If any leaks are found, they should be dealt with promptly to prevent the leakage from getting worse.
summary
Over the years of production practice, maintenance personnel have successfully solved the problem of packing leakage in many control valves. However, it should also be noted that there are still a few control valve packing leakage problems that have not been completely solved, and continuous exploration is needed in future work.
