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Leakage is one of the common faults of mechanical equipment. There are two main reasons for leakage: first, due to the results of mechanical processing, there must be various defects and shape and size deviations on the surface of mechanical products, therefore, there will inevitably be a gap at the joint of mechanical parts; second, there is a pressure difference on both sides of the seal, and the working medium will leak through the gap.

Reducing or eliminating gap is the main way to prevent leakage. The function of sealing is to seal the gap between the joints, isolate or cut off the leakage passage, increase the resistance in the leakage passage, or add small power elements in the passage, which will cause pressure on the leakage, offset or completely balance with the pressure difference causing the leakage, so as to prevent leakage.

Seals can be divided into static seals between relatively static joints and dynamic seals between relatively moving joints.

Static seal usually refers to the seal between two stationary surfaces. Static seals are mainly gasket seals, sealant seals and direct contact seals.

Dynamic seal refers to the seal between relative moving parts in machine (or equipment).

1 Packing seal

Packing seals are mainly used for dynamic seals of moving parts of process machines and equipment in the mechanical industry, such as centrifugal pumps, compressors, vacuum pumps, mixers, rotating shaft seals of reaction kettles and reciprocating pumps, plunger or piston rods of reciprocating compressors, and seals between valve rods and fixed bodies for linear and spiral motion valves. The basic form of stuffing letter is:

(1) Asbestos disk root:

Temperature and corrosion resistance are very good, but when used alone, the sealing effect is not good, so it is always impregnated or additional materials. Oil-immersed asbestos packing: There are two basic structural forms, one is twisting, the other is knitting. It can also be divided into circles and squares.

(2) Polytetrafluoroethylene braided packing:

Polytetrafluoroethylene (PTFE) ribbons are woven into pans, which have excellent corrosion resistance and can be used in cryogenic media.

(3) Rubber O-ring:

At low pressure, the sealing effect is good. Use temperature is limited, such as natural rubber can only be used at 60 C.

(4) Plastic moulding fillers:

Generally made into three-piece type, can also be made into other shapes. Polytetrafluoroethylene (PTFE), nylon 66 and nylon 1010 are the most commonly used plastics.

(5) Soft packing seal:

Soft filler, also known as muddy filler, is a new generation of high-performance synthetic fibers with special barriers and lubricants refined from the high-level "packing root". Soft packing of arbitrary size, arbitrary shape, easy to install, through the special high-pressure gun can be directly injected, unlike the traditional packing root, need pre-cutting, can be applied to any size of packing cavity seal. Soft packing seals have a wide range of applications, such as high-pressure reciprocating pumps or low-speed pumps, high-temperature stem, valves or packing box seals. It has the advantages of simple structure, reliable operation, low price and so on. It is still widely used in occasions where the requirements are not too high.

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2 Expansion ring seal

Expansion ring is a notched metal elastic ring, commonly known as piston ring. When the expanding ring is in free state, the incision opens and closes after being loaded into the cylinder block. The outer circle of the expanding ring is tightly attached to the inner surface of the cylinder block by elastic force. When working, the expansion ring is tightened to the side of the expansion ring groove under the pressure of working medium, and the sealing effect is jointly played by the cylindrical surface outside the expansion ring and the tightening end face. Expansion rings are usually made of cast iron or tin bronze. They are mainly used for piston seals of reciprocating compressors and internal combustion engines with higher operating temperatures, and for rotating shaft seals of steam turbines.

3 Contact dynamic seal

Gasket seal

Sealing gasket is a kind of sealing spare parts used in machinery, equipment and pipeline as long as there is fluid. It uses internal and external materials to seal. Sealing gasket is made of metal or non-metal plate material by cutting, stamping or cutting. It is used for sealing connection between pipelines and machine parts. According to the material, it can be divided into metal sealing gasket and non-metal sealing gasket. Metals are copper gaskets, stainless steel gaskets, iron gaskets, aluminum gaskets, etc. Non-metallic asbestos gaskets, non-asbestos gaskets, paper gaskets, rubber gaskets, etc. The latest is graphite gaskets.

Thread seal

Thread seals generally have two forms, one is threaded connection gasket seal, the other is threaded filler seal, both of which are used in small diameter threaded connection sealing occasions. The sealing element of the threaded connection gasket seal is the gasket, while the threads only provide the compressive force. The sealing effect is in addition to the gasket's own performance. The roughness of the sealing surface and the relative geometric position accuracy of the threaded hole also have a great influence on the sealing effect. When tightening threads by sealing gaskets, they not only bear compressive force, but also bear torsion, which makes gaskets deformed or even damaged. Therefore, when gaskets are non-metallic, they are generally only suitable for low pressure situations, and if gaskets are metal, the applicable pressure can reach more than 30 Mpa.

Felt seal

The felt ring is installed in the trapezoidal groove of the bearing cover, and the felt is pressed on the shaft by the side pressure of the trapezoidal groove, thus playing a sealing role. It has good flexibility, good warmth retention, moisture retention and elasticity. In the first and second gear reducers, felt ring oil seals are commonly used. In the bearing end cap of the input and output shafts, the center position of contact with the shaft is trapezoidal ring groove with cross-section, in which the felt ring is placed to prevent dust and oil leakage.

Oil seal

Oil seal is also the seal of lubricating oil. It isolates the parts requiring lubrication from the output parts in the transmission parts so as not to let the lubricating oil leak. Oil seals are needed in all parts of the body box which have liquid lubricants and are connected with the outside. Some are rubber, some are metal, most are steel-reinforced rubber, such as crankshaft back oil seal, transmission front and rear oil seal, left and right half axle oil seal, main reducer front oil seal, air compressor crankshaft oil seal, etc.

Fully enclosed seal

Fully enclosed seal is to completely separate the leakage passage inside and outside the system, or to place the working machine (pump) and the prime mover in the same enclosed system, which can completely prevent the leakage of media. Therefore, in the production of highly toxic, radioactive and rare precious substances, as well as in precision experiments, fully enclosed seal is of great significance. Common forms are diaphragm seals. Non-contact dynamic seal

4 Non-contact dynamic seal

Floating Ring Seal

Floating ring seal is a kind of high-speed seal, which can be used to seal gases and liquids. When sealing gas, the floating ring device should be filled with lubricating oil to facilitate sealing and lubrication. Floating ring seal is also called oil film seal. Non-contact dynamic seal which restricts fluid leakage by minimal clearance between floating metal ring and shaft or shell. The floating ring can float freely in the shell, so the influence of high-speed rotation and vibration of the shaft on the seal is small, and the friction and wear are also small.

Spiral Seal

Spiral seal is a form of dynamic seal. It is a spiral groove machined on the rotating shaft or on the bearing sleeve of the shaft. There is a sealing medium between the shaft and the sleeve. The rotation of the shaft causes the spiral groove to act as a pump conveyor, thus preventing the leakage of sealing fluid. Its sealing capacity is related to the helical angle, pitch, tooth width, tooth height, the acting length of teeth and the clearance between shaft and sleeve. Because there is no friction between the seals, the service life of the seals is long, but because of the restriction of the structure space, the helix length is generally short, so its sealing ability is also limited. When the pump is used at a reduced speed, its sealing effect will be greatly reduced.

Centrifugal seal

Centrifugal seal is a device that prevents leakage by using the centrifugal force generated by the rotating of the added working element. Its sealing ability comes from the work done by the sealing element driven by the rotation of the machine shaft. There are two main types of centrifugal seal: back blade seal and auxiliary impeller seal. Back blade seal and auxiliary impeller seal are commonly used in centrifugal pumps. These two types of centrifugal seals can be used with large sealing clearance, small wear, long service life and near zero leakage. They are often used in impurity pumps and slurry pumps for transporting solid media, but they consume a lot of sealing power and need to be equipped with parking seals.

Dry gas seal

Dry gas seal is a new type of shaft end seal which uses grooved seal technology for gas seal. When the moving ring with hydrodynamic groove is rotated outside the end face, the hydrodynamic groove pumps the high-pressure isolated gas from the outer diameter side (called upstream side) into the sealing end face. The film pressure increases gradually from the outer diameter to the groove diameter, while the film pressure from the groove diameter to the inner diameter decreases gradually. The opening force of the film pressure is greater than the closing force acting on the sealing ring, and a thin layer of air film is formed between the friction pairs to make the seal work in a non-contact state. The gas film formed completely blocked the leakage passage of the relatively low pressure sealing medium, and realized zero leakage or zero escape of the sealing medium. The gas film seal has less leakage and can realize zero escape of medium, so it is an environmental protection seal. The gas film seal has small wear and tear, long service life and stable and reliable operation. The sealing power consumption is small, only about 5% of the contact mechanical seal, and the sealing fluid is not polluted by oil. The auxiliary system of the seal is simple and reliable, and does not need maintenance in use.

Labyrinth seal

Labyrinth seal is a series of annular sealing teeth arranged in sequence around the rotating shaft. A series of interception clearances and expansion cavities are formed between the teeth and teeth. The sealing medium produces throttling effect when it passes through the gap of the tortuous labyrinth to prevent leakage. It has the advantages of good sealing performance at high speed, no lubrication, no friction, simple maintenance, long service life and no need to use other sealing materials, but it is difficult to assemble because of its high processing accuracy. Magnetohydrodynamic seal Magnetohydrodynamic seal technology is developed on the basis of magnetic fluid. When magnetic fluid is injected into the gap of magnetic field, it can fill the gap and form a "liquid O-ring". The function of the magnetic fluid sealing device is to transfer the rotational motion to the sealed container, which is often used for vacuum sealing.

End face seal (mechanical seal)

"End seal" is a seal in the end, by another plane extrusion seal. Mechanical seal is a device to prevent fluid leakage, which consists of at least one pair of end faces perpendicular to the rotating axis, under the action of fluid pressure and elastic force (or magnetic force) of compensating mechanism, and with the cooperation of auxiliary seal, which keeps sticking and merging relative sliding. Elastic loading mechanism and auxiliary seal are mechanical seals of metal bellows, which we call metal bellows seals. In light seals, rubber bellows are also used as auxiliary seals. The elastic force of rubber bellows is limited. Generally, spring is needed to meet the loading elastic force. "Mechanical Seal" is usually referred to as "Mechanical Seal"