From the perspective of mechanical packing manufacturers, there have been applications where braided mechanical packing fails. Whether the application is in pumps, mixers, valves or other special equipment. It is always a quick decision to consider braided packing as the main cause of failure or problem. There seem to be several reasons why people make this decision, because they may see packaging as low-level or outdated technology—perhaps this is just the last step in the process of getting their equipment back into service. When talking to people about their equipment, there will never be any problems with the equipment. Braided filler may be incorrectly identified as an application. Whether the problem is caused by a single factor or a combination of factors, each problem will affect performance and shorten the life expectancy of the filler.
Many new mechanical packaging solutions will start from a single user request for challenging applications or the further development and advancement of new raw materials. Manufacturers of mechanical packing may spend months or even years evaluating materials and material combinations in an effort to improve sealing performance.
The driving force for the creation of new mechanical packing solutions is to eliminate painful applications and expand sealing performance, which means thorough consideration of all options. Is the sealing medium liquid or gas? Does the medium range from abrasive substances to relatively clean liquids? Is the medium on the corrosive or acidic side of the pH range? What if the medium is gas? Gaseous media can be inert, explosive, and even harmful to health. Are you worried about emissions or steam? Does the medium have a constant stable temperature or is there temperature fluctuation? The filler can be used in high temperature and severe cold applications.
The Fourth Edition Compression Packing Manual of the Fluid Sealing Association (FSA) mentions the use of the "STAMPS" acronym. This is an easy-to-remember concise acronym that can guide users to logically and methodically understand the relevant details of the application every time.
Please pay attention to any special requirements that need to be met, and make sure to understand the context and background of the specific circumstances of the problem. Having these details, no matter how small or trivial, can provide direction for the design of new packing solutions. What constitutes a high-quality package usually has obvious characteristics. The first is strength. A good filler needs to be made of strong materials to withstand the physical requirements of pressure, temperature and surface speed. The next characteristic of a good filler is chemical resistance. The wider the pH range, the greater the number of chemicals and the concentration level that the packaging can handle. The last characteristic of good packaging is flexibility. This allows the packaging material to deform and recover under load. High-quality packing needs to have a certain degree of flexibility in its structure to provide energy and maintain a continuous seal.
Once the application details are determined, the focus will be on determining the best material choice for the application. Should the filler be carbon yarn or graphite yarn (Figure 1)? Maybe it should be a synthetic blend of fiber, para-aramid, polytetrafluoroethylene (PTFE) or expanded polytetrafluoroethylene (ePTFE) yarns (Figures 2 and 3)? Viewing the potential advantages and disadvantages of each material type requires an in-depth understanding of the material's fibers and their characteristics.
As important as the type of fiber in the filler is the type of coating. Coatings and lubricants are also part of the process to be considered for mechanical packing. How will the lubricant be used? Is the lubricant a sacrificial component of the filler or a sealant? Are there any restrictions on the contamination of lubricants or fillers in the process medium?
It is very important to be able to identify differences in the structure of braided fillers, such as cross braided (lattice) and square braided fillers (Figures 4 to 6). It is also important to understand why the characteristics and advantages of braided fillers are specified in the application—perhaps because of the material, material combination, or even the type of structure. Some examples of packing types used for equipment with specific needs include braided packing with a rubber core (Figure 4), mainly used to seal shaft/sleeve runout conditions and multi-fiber packing (Figure 6) to seal corrosive materials with graphite The heavy medium of the coated PTFE yarn and the extra strength of the Kevlar corner reinforcement.
From the advancement of material fibers and coatings, combined with the combination of material types and the improvement of braided packing structure, users have more choices for braiding mechanical packing than ever before.
Warren Montgomery is the global product line manager for mechanical packing and gaskets of AW Chesterton. He graduated from Central New England College with a degree in mechanical engineering technology. For more information, please visit chesterton.com.