The oldest and still very popular sealing devices are the braided packings also referred to as compression packings. Made from relatively soft, pliable materials, they are formed by either twisting or braiding individual strands of yarns together to obtain either a round or square finished product.
Braided Packings find their major use in the process industries such as petro-chemical, paper, and steel mills, and in the service industries like utilities, marine, water sewage, food and nuclear. They seal all types of fluids including water, steam, acids, caustics, solvents, gases, oil, gasoline and other chemicals over a broad range of temperature and pressure conditions.
The base materials of construction are usually flax, cotton, carbon or graphite and even copper or aluminum foils are formed into strands, then braided or twisted together. Used in conjunction with these yarns are binders, lubricants or compounds such as Teflon, graphite, mineral oils and various elastomers or graphite based lubricants. These materials are used as blocking agents to fill the voids left during the braiding process and to help reduce friction between the packing and the other sealing surface.
Twisted Packings are constructed of yarns of various materials, twisted together or around a core to obtain the desired size. One size of packing can be used for several stuffing box sizes, because of its twisted construction. Strands from a larger size can be untwisted and removed so that the remaining packing will fit a smaller size stuffing box. When metallic materials are used in the packing, they can be made to resist high temperatures and pressures, to resist the penetration of fluids, and to conform to the irregularities of worn equipment.
Braid-Over-Braid is also known as round braid or multiple braids. Tubular jackets of various materials are braided one over the other to obtain the desired size packing. Some styles have the tubular jacket braided over a center core, which may be extruded, twisted, wrapped or knotted from a wide range of materials. Finished packings can be supplied in round, square, or rectangular cross section. Braid-Over-Braid packings are relatively dense and are recommended for high-pressure, slow moving applications such as valve stems, expansion joints, groove gasketing, etc.
Square Braid is also known as plaited braid. Yarns of various materials are processed on equipment where strands pass over and under strands running in the same direction. Resulting packings are usually supplied in square cross section but rectangular sizes can be also made in this method. The packing is usually soft and can carry a large percentage of lubricant. Square braided packings are not as harmful to equipment and are generally used for high speed rotary services at relatively low pressures.
Interlock Braid yarns either alone or in combination, are processed on equipment where the strands criss-cross from the surface diagonally through the body of the packing. Each strand is strongly locked by other strands to form a solid integral structure that cannot easily ravel apart. Interlock packing has a more even distribution of yarn density and generally has an improved lubricant retention. The finished packing is relatively dense, but flexible. These packings are used for applications on both reciprocating and centrifugal pumps, agitators, valves, expansion joints and in grooves.