Natural gas and water are transported to caustic and high-temperature steam applications, and when compressed packing is properly used, it is a cost-effective, high-performance seal. Unfortunately, compressive fillers create friction, which can be a major problem in some applications. Knowing how to reduce this friction is critical to reducing these problems.
Users of pneumatic and electric operated control valves (AOV and MOV) typically require low friction packing that allows for accurate, efficient and consistent drive while maintaining an effective seal on the media. The frictional encapsulation applied on the dynamic surface is primarily a function of material type, contact surface area and compressive load. While other system variables and inputs can also affect friction, they are more difficult to quantify or modify.
The anti-friction strategy includes modifying the sealing material, configuration and installation procedures to achieve a low friction load. Different applications have different allowable leak rates. Graphite may be required to effectively seal one application, while another may require a PTFE based seal to reduce friction. Users can also have cost-based development. There is no single solution to all sealing issues; the strategies discussed here are conceptually applicable to most applications, but they need to be validated before implementation. Each application will have an optimal solution, taking into account the currently available sealing technologies and strategies.
The compression seal acts as a barrier to the movement of the medium from the higher pressure system to the lower pressure surrounding where the valve is operating. The sealing mechanism of the compressed packing is based on a tight fit between the packing and the dynamic sealing surface. This cooperation is produced by the application of axial compression which causes radial movement of the filler against the sealing surface 1. Figure 1 shows the dynamics of axial compression and radial expansion.
The size of the leak is determined by system variables such as media, pressure, structure and installation, shaft yaw and temperature. An important point here is that the friction and sealing are separate, but with regard to compression filling. The usually optimized solution is a combination of these two factors. Hypothetically, low friction can be achieved by not installing any seals; but the result will be a leaking valve. Conversely, an excellent seal can be achieved by welding the valve stem to the valve cover; however, the valve cannot be actuated. In fact, the factors that the operator immediately controls the emissions include the type and number of packing rings used, the correct installation and the axial load.
There are three basic strategies for reducing friction while maintaining an effective sealing system. These include reducing the load on the stuffing box, reducing the number of rings and replacing the packing material.
1. Reducing the number of rings in the packing group limits the contact area with the shaft. This reduces the uncompressed fill height (H), which is proportional to the lower friction. In theory, most of the applied stress only affects the two closest rings. These rings provide most of the sealing effect; the remaining rings provide a small seal but increase the total friction exerted on the moving shaft. Figure 4 shows actual test data relating to the number of packing rings required for actuation. Adding a ring adds friction, but this is not a linear relationship and depends on the material and structure. Removing the ring can create potential problems with spacing and sealing. The spacing can be maintained by mounting a carbon or steel bushing that maintains the height of the packing set without contacting the shaft. The number of turns required for sealing depends on the application and should be determined by the person familiar with the system.
2. Replace the filler material with a material with a lower coefficient of friction (COF) to reduce friction. The coefficient of friction (μ) quantifies how the packaging material resists motion on the dynamic sealing surface. However, the coefficient of friction is not the same as COF. The friction factor is a lumped variable that describes the friction of a particular configuration or braid. This is different from COF which describes the properties of intrinsic materials. The friction factor varies for different types of compression packages. For example, a PTFE-based braid may have a coefficient of friction of 0.08; a graphite braid with lubrication may be about 0.09; and a graphite group formed by the mold may approach 0.1. These coefficients of friction differ from the actual values because of the manufacturer's safety factors, taking into account the worst-case conditions, and the average of the dice of different sizes and styles. Figure 5 shows the actuation force required for the four-ring set 3/8-in. PTFE coated carbon fiber, a pure graphite group formed by a mold, a lubricant-impregnated pure PTFE fiber and a PTFE fiber woven layer having a lattice structure.
Graphite and PTFE are the main low friction materials used to compress the filler. PTFE is a highly lubricious material, but is limited by its 500°F (260°C) temperature rating and high creep and flow characteristics. Graphite can withstand temperatures up to 850°F (454°C) in an oxidizing atmosphere and withstand temperatures of 1,200°F (649°C) in a vapor environment. Both materials can be used as the main material for the filler or can be added to reduce friction. Graphite, PTFE and other polymers and lubricants are typically added by impregnation or dispersion to reduce friction during operation. They can also be made into pure PTFE or graphite sealed products.
Typically, graphite is formed into a sealed product by molding a flexible graphite foil into a solid ring. PTFE can form fibers and weave, similar to other fiber braids. PTFE and graphite materials can also be processed with other fibers and fillers to optimize the desired properties, such as lower friction and extrusion resistance. For example, a thin coating of PTFE on a carbon or graphite braid can significantly reduce friction while the carbon core maintains the structural integrity and creep resistance of the braid.
3. A graphite set formed using a mold having an angular plane that promotes radial motion that minimizes the compressive load required for effective sealing. This reduction in compressive load combined with the soft material properties of graphite creates an effective seal and reduces the frictional load on the travel rod. The soft graphite ring does not apply high friction but is deformed to the point of equilibrium between shear friction and material strength. Moreover, the reduced compressive load required for sealing means that the end ring, typically a more sturdy woven material, sees a smaller compressive load and then exerts less friction on the moving rod. This generally means that the use of a mold forming set produces less friction than an equivalent woven material. The PTFE on carbon showed the lowest friction of the braid tested in Figure 5.
There is no standard test method for the friction generated by the compression packing, so manufacturers develop their own standardized tests to compare the friction properties of different products. Standard tests usually measure COF. ASTM G1115-103 provides guidance for measuring and reporting COF under specific controlled settings. However, the results of analyzing a particular portion of the braid for friction are not particularly useful because the rod friction in the valve is created by the dynamic interaction of constantly changing variables such as lubrication, finish, temperature, and number of cycles.
There are other factors that directly affect the friction generated by the compressed packing set, but they are more difficult to measure and control in the field than simply changing the packing material. Among them is the shaft end face, recommended on a 32 microinches (AARH) or better reciprocating valve stem. The bounce of the rod or travel shaft unevenly loads and unloads the packing, possibly exceeding the limits of compressibility and recovery performance of the material, which adversely affects the seal. In addition, the gland follower may interfere with the stroke stem.