To be honest, the first time I heard the word “packing” in this small edition, I didn’t know what it was. After online inquiry and relevant information search, I learned that packing is also known as “braided packing”. It is a kind of sealing packing made of expanded graphite wire. Next, this small edition will introduce in detail what is packing and how it works? If you want to know, please join us to learn!
Prepare packing
Packing, also known as sealing packing, is usually woven from softer linear materials. Usually, strips with square or rectangular cross-sectional area and circular shape are filled in the sealing cavity to achieve sealing. The packing seal was first used as the shaft seal of the water lifting machine to prevent the leakage of the liquid flow by plugging the fiber such as cotton and hemp in the leakage channel. Because of the wide source of packing, easy processing, low price, reliable sealing and simple operation, it has been used up to now. Nowadays, packing is widely used for shaft seals of centrifugal pumps, compressors, vacuum pumps, mixers and ship propellers, reciprocating shaft seals of piston pumps, reciprocating compressors and refrigerators, and rotary seals of various valve stems.
Woven packing is made of expanded graphite thread, which is commonly used in most occasions except for strong oxidant. It can be used in boiling water, high temperature, high pressure steam, heat exchange medium, oil, acid, alkali, hydrogen, ammonia, organic solvent, hydrocarbon, low temperature liquid and other media. It is a unique new type of sealing packing.
Application: it is widely used for sealing of centrifugal and reciprocating pumps, valves, pipes, etc. in petroleum, chemical, electric power, metallurgy, machinery, paper and other industries.
working principle:
Working principle of packing: the surface of the shaft is very uneven under microscopic conditions, and it can only partially fit with the packing. So there is a small gap between the packing and the shaft, just like a labyrinth. The medium is intercepted many times in the labyrinth to achieve the sealing effect.
The packing seal is installed in the packing box by packing, and the packing is pressed on the surface of the shaft by the packing gland. Since the surface of the shaft is always rough, it can only partially fit with the packing, and some are not in contact, thus forming numerous labyrinths. When the pressurized medium passes through the shaft surface, the medium is throttled for many times, and the sealing is achieved by virtue of the “labyrinth effect”. The fitting and friction between the packing and the shaft surface is similar to that of the sliding bearing, and there should be enough liquid for lubrication to ensure a certain life of the seal, which is the so-called “bearing effect”. It can be seen that a good packing seal is the combination of the labyrinth effect and the bearing effect.
The pressing force of the packing on the shaft is generated by tightening the gland bolts. As the packing is an elastic-plastic body, when it is compressed in the axial direction, the friction force will be generated to gradually reduce the compression force in the axial direction. Meanwhile, the radial compression force will make the packing close to the shaft surface to prevent the medium from leaking. The distribution of radial compression force decreases sharply from the outer end (gland) to the inner end, and then becomes gentle. The distribution of medium pressure decreases gradually from the inner end to the outer end. When the medium pressure at the outer end is zero, there is little leakage, and when it is greater than zero, there is leakage.
With the continuous appearance of new materials, the structure of packing has also changed greatly, which will undoubtedly promote the wider application of packing seals. The materials used as packing should have the following characteristics: a certain degree of elastoplasticity. When the packing is pressed axially, it can generate a large radial pressing force to obtain a seal; When there is vibration between the machine and the shaft, or the shaft has runout and eccentricity, it can have certain compensation ability (following ability); Chemical stability. It will not be corroded and dissolved by the medium, nor will it pollute the medium; Impermeability. The medium has some penetration to most of the fibers, so the packing structure is required to be dense. Therefore, it is often necessary to impregnate and fill various lubricants and fillers when making the packing; Good self lubrication, low friction coefficient and wear resistance; Temperature resistance. It can withstand a certain temperature after friction heating; Easy to disassemble; Simple manufacture and low price.
Sealing packing
Packing, also known as sealing packing, is usually woven from softer threads, and is filled in the sealing cavity by strips with square cross-sectional area to achieve sealing.
Nowadays, the packing is mainly made of graphite and various fibers. According to different requirements, carbon fiber, copper wire, 304, 316L, inko nickel alloy wire and other materials are used for reinforcement.