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Braided Sealing Packing Knowledge

Packing, also known as the static sealing packing, is usually braided from relatively soft fiber yarn. Usually, square、rectangular or rounded sectional area is filled into the sealed cavity.

1. Concept

Packing, is usually braided from relatively soft fiber yarn. Usually the cross-sectional area is square or rectangular, and the circular strips are filled in the sealed cavity to achieve sealing. . The earliest packing seals were cotton, hemp, and other fibers plugged into the leakage passages to prevent leakage of liquids. They were mainly used as shaft seals for water lifting machines. Due to the wide range of filler sources, easy processing, low cost, reliable sealing and simple operation, it is still used today. Today packing is widely used for shaft seals of centrifugal pumps, compressors, vacuum pumps, mixers and marine propellers, piston pumps, reciprocating compressors, reciprocating shaft seals for refrigerators, and rotary seals for various valve stems, etc. .

Braided packing is made of expanded graphite yarn and is often used in most occasions except for strong oxidants. It can be used in boiling water, high temperature, high pressure steam, heat transfer medium, oil, acid, alkali, hydrogen, ammonia, organic solvents, etc. Hydrocarbons, cryogenic liquids and other media. It is a unique new type of sealing packing.

2. Principle

Working principle of packing The surface of the shaft is very uneven in microscopic conditions and can only fit partially with the packing. So there is a slight gap between the packing and the shaft, just like a maze. The medium is blocked several times in the labyrinth to achieve sealing.

The packing seal is packed in the stuffing box by the packing, and the packing presses the packing on the shaft surface. Because the shaft surface is always rough, it can only partially fit with the filler, and some parts are not in contact, thus forming numerous mazes. When the pressure medium passes through the shaft surface, the medium is throttled several times, and the seal is achieved by virtue of this “maze effect”. The fitting and friction between the packing and the shaft surface are also similar to those of the sliding bearing. The solid should have enough liquid to be lubricated to ensure that the seal has a certain lifespan, the so-called "bearing effect". This shows that a good packing seal, that is, a 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. Because the filler is an elasto-plastic body, when the axial compression is applied, friction force is generated, so that the pressing force gradually decreases along the axial direction, and the generated radial pressing force causes the filler to closely contact the surface of the shaft and prevent the leakage of the medium. The distribution of radial compression force from the outer end (gland) to the inner end, followed by a gradual decline in the first flattened gradually, the distribution of the medium pressure gradually decreases from the inner end to the outer end, when the external media pressure is zero, then the leak is very Less, leakage when greater than zero.

With the continuous appearance of new materials, the structure of fillers has also changed greatly. This will undoubtedly promote the application of filler seals more widely. The materials used as fillers should have the following characteristics: They have certain elasticity and plasticity. When the packing is axially compressed, it can generate a large radial pressing force to obtain the seal; when the machine and shaft have vibration or the shaft has runout and eccentricity, it can have certain compensation ability (following property); chemical stability Sex. It is not corroded by medium, swelling, or contaminating medium; impermeability. The medium has some permeation to most of the fibers, so dense packing materials are required. For this reason, fillers are often required to be impregnated and filled with various lubricants and fillers; the self-lubricating properties are good, the friction coefficient is small, and the wear resistance is high; . When the friction and heat can withstand a certain temperature; easy removal; simple manufacturing, low prices.

3. Commonly used materials

Packing, sealing packing, is usually braided from relatively soft fiber yarn. It is sealed by filling a sealed cavity with a square cross-sectional area.

Nowadays, the packing is mainly made of graphite and various fibers. According to different requirements, carbon fiber, copper wire, 304, 316L, and nickel alloy wire are used to reinforce the material.

3.1 Aramid packing series

Gold Aramid Fiber Packing, Huang aramid fiber packing, aramid fiber intertwined with white PTFE Packing, aramid fiber intertwined with black PTFE Packing This series of packing is made of aramid fiber as the main material, repeatedly impregnated with lubricant, PTFE emulsion Such as precision compilation. It has good high rebound, chemical resistance, low cold flow and high line speed. Compared with other types of packing, it can resist grain crystallizing medium and higher temperature. It can be used alone or in combination with other packings. Mainly used for medium particles, easy to wear conditions.

3.2 PTFE Packing series

White PTFE Packing, black PTFE Packing, white PTFE Aramid Fiber Packing, black PTFE aramid packing, PTFE Packing, white PTFE impregnated packing, white four Fluoride-impregnated heart packing This series of packing is made of PTFE as the main material. Has excellent chemical stability, corrosion resistance, sealability, high lubricating non-sticky and good anti-aging endurance. It can work at +250°C to -180°C for a long period of time. Except for molten metal sodium and liquid fluorine, it can resist all other chemicals and boil in aqua regia. It is mainly used in conditions that do not allow pollution, such as sanitary conditions, strong corrosiveness, high line speed, and easy wear.

3.3 Graphite Packing series

Flexible Graphite Packing, metal-reinforced Graphite Packing, carbon fiber-reinforced Graphite Packing, carbon fiber metal-reinforced Graphite Packing This series of packing characteristics is based on graphite as the main material. Has good self-lubrication and thermal conductivity, low friction coefficient, versatility, good flexibility, high strength, protection of the shaft and other advantages. According to different requirements, the use of carbon fiber, copper wire, 304, 316L, Yancai nickel alloy wire and other materials to strengthen. Mainly used under high temperature and high pressure conditions.

3.4 Carbon fiber packing series

Carbon fiber-reinforced graphite packing and carbon fiber metal-reinforced graphite packing Carbon fiber is the main material of this series. Carbon fiber is a new material with excellent mechanical properties. Carbon fiber has no creep, good fatigue resistance, small coefficient of thermal expansion, corrosion resistance Good, good thermal conductivity. Mainly used in high temperature and high pressure wear conditions.

3.5 Reinforced rubber core packing series

Super-elastic silicone core is coated with high-quality synthetic fibers and woven with PTFE emulsion, lubricant and barrier agent. It can effectively absorb mechanical vibrations and control leaks caused by vibrations. Has excellent resilience, self-lubrication, flexibility, wear resistance, high tensile strength, good chemical resistance and so on.

3.6 High temperature and high pressure packing

The packing is precisely woven from high-quality synthetic fibers and has a fine structure. It has excellent high temperature and pressure resistance, low thermal conductivity, and non-flammability. Widely used in ultra-high temperature, high pressure and harsh environment of water, steam, acid, alkali and other media. Used for insulation and fire prevention in pipes, etc., static sealing at containers, heaters, manholes, furnace door heaters, high temperature flanges, etc. The use of high temperature in steel mills is used exclusively for blast furnace combustion valves.

In addition to the above materials, there are acrylic packing, high water-based packing, and ramie packing to meet various operating conditions.

4. Technical requirements

4.1 When cutting the packing, it is best to cut the packing around a round bar with the same diameter as the stem to ensure that the size is correct and the cuts are parallel. The incision should be neat and free of loose asbestos threads. The incision is preferably 45°.

4.2 Each packing ring should be pressed into the packing box separately. When pressing the valve packing, a small amount of scaly dry graphite powder or lubricating oil should be placed between each ring.

4.3 For ease of installation, press-fit aluminum foil wrapped with asbestos packing should be coated with a thin layer of scaly graphite powder blended with oil on the inner edge of the packing.

4.4 The width of the packing should be the same as the size of the packing box, or 1 to 2 mm.

4.5 When press-packing oil-impregnated asbestos packing, it is best to press dry asbestos packing on the first and last lap to prevent oil from oozing out.

4.6 When pressing the packing, the interface of the packing ring must be staggered. Generally, the interface is staggered by 120° or 180°; the packing should not be pressed too tightly and it must be properly adjusted according to the operating pressure.

4.7 The depth of the gland pressing into the packing box is generally a circle of packing height, but not less than 5, and should leave room for further tightening.

4.8 When installing the gland, the gap between the packing gland and the valve stem should be the same to prevent the bias, so that the packing root is not uniform and wear quickly. Generally should maintain the gap is not more than 0.1 ~ 0.15.

4.9 The key points for guaranteeing the installation quality of packing are: selection is correct; inspection is fine (check the stem, packing box, packing, etc.); accurate size; good pressure (pressing the first lap is the key to ensure that after pressing The packing of each circle is the same as the thickness and thickness.)

5. Choose

How to choose the right packing? There are two things that must be taken into account when choosing a packing.

5.1 Working conditions and packing performance should be consistent. According to the relevant technical data to select the best meet the requirements of the packing.

5.2 The packing size must be correctly selected according to the sealing requirements. To determine the cross section, use the following formula:

Cross section = (packing box diameter - stem diameter)/2

A damaged device may require a slightly larger cross-sectional packing to compensate for the damage.

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