How to Reduce Friction in Valve Packing: A Practical Guide

In the industrial field, whether it is the transmission of natural gas or water, when it comes to caustic alkalis and high-temperature steam applications, compression packing plays a crucial role. It is a low-cost and high-performance sealing method that can effectively prevent media from leaking from a high-pressure system into a low-pressure environment. However, compression packing is not flawless; during operation, it generates friction, which in certain applications may trigger a series of problems. Therefore, mastering how to reduce this friction is particularly critical for optimizing equipment operation, reducing maintenance costs, and improving production efficiency.

For users of pneumatic and electric operated control valves (AOV and MOV), low-friction packing is a key factor for achieving precise, efficient, and stable actuation. The friction applied to dynamic surfaces mainly depends on the type of sealing material, the contact surface area, and the compression load. Although other system variables and inputs can also influence friction, these factors are often difficult to quantify or modify. Therefore, we need to start with sealing materials, configuration, and installation procedures, and achieve the goal of low-friction load through reasonable adjustments and optimization.

Different application scenarios have different tolerances for leakage rates. In some cases, graphite may be required as the sealing material to ensure sealing performance; in other cases, PTFE-based sealing materials may be a better choice to reduce friction. Users can also choose an appropriate sealing solution based on cost-effectiveness. But it must be made clear: there is no universal solution for all sealing problems. The strategies discussed in this article conceptually apply to most applications, but strict verification must be carried out before practical use. Each application scenario has its own unique optimal solution, which requires comprehensive consideration of currently available sealing technologies and strategies.

The sealing mechanism of compression packing is based on the close fit between the packing and the dynamic sealing surface. This fit is achieved by applying axial compression, which causes the packing to generate radial movement and press tightly against the sealing surface. The degree of leakage is determined by a variety of system variables, including the nature of the medium, pressure, structure and installation method, shaft runout, and temperature. It is important to recognize that although friction and sealing are two independent concepts, they are closely related in the application of compression packing. Usually, the optimized solution is to consider both factors together.

In theory, if no seal is installed at all, friction will be minimized; but the result will be massive valve leakage. Conversely, if the valve stem is welded to the bonnet, perfect sealing can be achieved, but the valve will not operate properly. In practice, operators can immediately adjust emissions by controlling the following factors: the type and number of packing rings used, the correct installation method, and the axial load. While maintaining an effective sealing system, there are three basic strategies to reduce friction: reducing the load on the stuffing box, reducing the number of rings, and changing the packing material.

Since friction is an inevitable issue in the application of compression packing, how can we effectively reduce it? Fortunately, through scientific methods and reasonable strategies, we can significantly reduce friction while maintaining sealing performance. The following introduces several proven strategies to reduce friction, which are not only based on theoretical research but also widely verified in practical applications.

Reducing the number of rings in the packing set can effectively limit the contact area with the shaft. This not only reduces the uncompressed packing height (H) but also proportionally lowers friction. In theory, most of the applied stress mainly affects the two nearest rings. These two rings provide the majority of the sealing effect, while the remaining rings contribute little to sealing but increase the total friction applied to the moving shaft. Actual test data show that adding rings increases friction, but the relationship is not linear and depends on material and structure. Removing rings may cause spacing and sealing performance issues. To solve this problem, carbon or steel bushings can be installed to maintain spacing without contacting the shaft, thereby maintaining the packing set height. The number of rings required for sealing depends on the specific application and should be determined by professionals familiar with the system.

Replacing the packing material with one that has a lower coefficient of friction (COF) can significantly reduce friction. The coefficient of friction (μ) quantitatively describes the ability of the packing material to resist motion on a dynamic sealing surface. However, the friction factor is not exactly the same as COF. The friction factor is a concentrated variable used to describe the friction characteristics of a particular configuration or braid, while COF describes the inherent properties of a material. Different types of compression packing have different friction factors. For example, PTFE-based braids can have a friction factor of 0.08; lubricated graphite braids around 0.09; and molded graphite sets close to 0.1. These friction factors differ from actual values because manufacturers adjust them for safety, worst-case scenarios, and averages across different braid sizes and styles.

Graphite and PTFE are two main low-friction materials used in compression packing. PTFE is a highly lubricated material, but its application is limited by its 500°F (260℃) temperature rating and its high creep and flow characteristics. By contrast, graphite can withstand up to 850°F (454℃) in oxidizing atmospheres and 1,200°F (649℃) in steam environments. These two materials can serve as the primary packing materials or be added to reduce friction. Graphite, PTFE, and other polymers and lubricants are often added through impregnation or dispersion to reduce friction during operation. They can also be made into pure PTFE or graphite sealing products.

Generally, graphite sealing products are made by molding flexible graphite foil into solid rings. PTFE can form fibers and be braided, similar to other fiber braids. PTFE and graphite materials can also be processed with other fibers and fillers to optimize desired properties such as lower friction and extrusion resistance. For example, coating a thin layer of PTFE on carbon or graphite braids can significantly reduce friction, while the carbon core maintains braid structural integrity and anti-creep properties. Using molded graphite sets with beveled planes that promote radial movement can minimize the compression load required for effective sealing. This reduced compression load, combined with the soft material properties of graphite, not only produces effective sealing but also reduces friction load on the traveling stem. Soft graphite rings do not apply high friction but deform to a balance point between shear friction and material strength. In addition, the reduced compression load required for sealing means that end rings (usually more robust braided materials) are subjected to lower compression load, thus applying less friction to the moving stem. This generally means molded sets generate less friction compared to equivalent braided materials. PTFE on carbon has shown the lowest friction among tested braids.

In addition to material selection and packing configuration, the installation process also has a significant impact on friction. Proper installation ensures a more uniform fit between packing and sealing surface, thereby reducing unnecessary friction. During installation, the manufacturer’s instructions should be strictly followed to ensure the packing is accurately positioned, avoiding displacement or distortion. At the same time, reasonably controlling the method and magnitude of axial load is also an important step in reducing friction. Excessive axial load not only increases friction but may also lead to over-compression of packing, thereby affecting sealing performance and service life.

Currently, there is no unified standard testing method for friction generated by compression packing. Therefore, manufacturers usually develop their own standardized tests to compare the friction performance of different products. These standard tests typically measure the coefficient of friction (COF), analyzing only the specific part of the braid used for friction, which cannot fully reflect actual conditions. In actual valve applications, stem friction results from dynamic interactions of constantly changing variables such as lubrication, finish, temperature, and number of cycles.

In addition to material selection and testing methods, there are other factors that directly affect friction in compression packing sets, but these are more difficult to measure and control in the field compared to simply changing the packing material. For example, the quality of the shaft end surface has a very significant impact on friction. A 32 micro-inch (AARH) or better finish on reciprocating valve stems is recommended to ensure smoothness and flatness. Stem or travel shaft runout can unevenly load and unload packing, potentially exceeding the limits of material compressibility and recovery, thus negatively affecting sealing. In addition, gland followers may interfere with the travel stem, which can also affect proper packing operation and further increase friction.

In industrial applications, compression packing, as a low-cost and high-performance sealing method, although it generates friction during operation, can effectively reduce this friction through reasonable strategies and methods, thereby optimizing equipment performance. From selecting suitable low-friction materials to optimizing packing configuration and installation processes, every step is crucial. At the same time, we must also recognize that each application scenario has its own unique requirements and challenges, so strict testing and verification of the selected sealing solution is essential before practical application.

Through continuous exploration and innovation, combined with currently available sealing technologies and strategies, we can find the best solutions for different application scenarios. This not only helps improve production efficiency and reduce maintenance costs but also ensures long-term stable operation of equipment. Finding the balance between sealing and low friction is an art that requires comprehensive consideration of multiple factors, and we are continuously working toward this goal.