Compression packing which is also called braided packing or rope packing. Braided packing is usually produced in a square or rectangular cross section and is braided from a range of different materials. Not only can compression packing vary by the material it is braided from, but the way in which it is braided can also vary.
Types of braid: One of the most common methods of braiding is in a square braid. A square braid is a relatively loose braid which produces a packing that when compressed will radially expand more than others braid types. This is particularly beneficial when equipment (usually a pump) has been in service for a long time resulting in a stuffing box and shaft sleeve that is well worn. When a tightly woven packing will not expand far enough to provide an effective seal, a square braided packing just might solve the problem.
Depending on who you buy your packing from, the second most common packing may be called a lattice packing, and interlock packing, Texlock packing, or Interbraid packing. All of which indicate the packing is tightly braided providing minimal leak paths between the braid. Because it is a tight braid which does seal better, it also will compress and radially expand less. This type of packing is excellent for equipment in like new tolerance condition. The sloppier the tolerance the more you need a square braid.
Packing can also be manufactured in a braid over core style. This is most common with valve packings. The core can be a tight braid, or an extruded core. The core is then braided over with another braid that often will contain wire for pressure containment (in the case of valve packing). For a mixer or agitator service, the core may be a rubber (Viton or Silicone generally). The beauty of this is it gives the packing recovery from compression which most styles of packing do not have. Because mixers and agitators often have high run outs in the shaft, the often densify the packing on two sides, creating a leak path. Adding a core of rubber allows the packing to bounce back from the compression, acting as a bearing and extending the life of the seal.
Materials of construction: The earliest of packings were all plant based. Jute, Hemp or cotton were the most common. Asbestos later became common because it doubled the temperature capabilities and we resistant to a wider range of chemicals. In the 1980's Asbestos was abandoned in the United States due to the health risks (and legal risks) associated with it. Asbestos is still used in other parts of the world. The most common materials used today include PTFE (if made of Dupont's product it is referred to as Teflon), Kevlar & other Aramid fibers, Fiberglass, Synthetic yarn (generally Acrylic) carbon, Graphite filament, flexible graphite, Grafoil, and various graphite/PTFE blends including GFO.
Choosing the right material for your application can be a science combined with the art of a good installer. For assistance and recommendations contact American Seal & Packing www.aspseal.com Phone: 714-361-1435 Fax: 714-593-9701