Mechanical seals belong to one of the precision and more complex structure of mechanical basic components, which are key parts of various pumps, reaction synthesis kettles, turbocompressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as selection, precision of the machine, correct installation and use, etc.
1、The basic concept of mechanical seal
2、Composition of mechanical seal
There are mainly the following four types of parts. a. Main seal: moving ring and static ring. b. Auxiliary seal: seal ring. c. Compression parts: spring, push ring. d. Transmission parts: bullet skip seat and key or set screw.
Attention should be paid to issues
1、Installation precautions
(1) to pay great attention to avoid installation deviations arising from the installation
a. Tighten the gland should be carried out after the coupling is corrected, the bolts should be evenly supported to prevent the gland end deflection, check each point with a stopper, the error is not more than 0.05 mm.
b. Check the fit clearance (i.e. concentricity) between the gland and the outer diameter of the shaft or sleeve, evenly all around, and check the tolerance of each point with a stopper no more than 0.01 mm.
(2) the spring compression should be carried out in accordance with the provisions, do not allow too large or too small phenomenon, the required error of 2.00 mm. Too large will increase the end surface than the pressure, another speed end wear. Too small will cause insufficient pressure and can not play a sealing role.
(3) After the installation of the dynamic ring beard to ensure flexible movement on the shaft, the dynamic ring should be able to automatically spring back after pressing the spring.
2、Caution when disassembling
(1) in the disassembly of mechanical seals to be careful, forbidden to use hand hammer and flat shovel, so as not to damage the sealing components. A pair of steel wire hooks can be made in the direction of the negative gain and loss to reach into the transmission seat gap, the seal device will be pulled out. If the scale disassembly can not be, should be cleaned and then disassembled.
(2) If the mechanical seal is used at both ends of the pump, in the assembly, disassembly process to take care of each other to prevent loss of care.
(3) for the operation of the mechanical seal, where there is a loose gland to seal the case of movement, the static and dynamic ring parts must be replaced, should not be tightened again to continue to use. Because in the kind of pillar after the movement, the original trajectory of the friction vice will change, the contact surface of the seal will be easily destroyed.
Mechanical seals normal operation and maintenance
1、Preparation work and precautions before starting
(1) A comprehensive check of the mechanical seal, as well as ancillary equipment and piping installation is complete, whether in line with technical requirements.
(2) Mechanical seal start before the hydrostatic test, check whether the mechanical seal leakage phenomenon. If there is more leakage, the cause should be investigated to eliminate. If still invalid, it should be disassembled and reinstalled. General hydrostatic test pressure with 2 ~ 3 kg / cm2.
(3) according to the pump rotation of the car, check whether the light and fast even. If the coiled car strains or does not move, it should be checked whether the assembly size is wrong and whether the installation is reasonable. Public number “Mechanical Design Union”, the engineer’s refueling station!
2、Installation and shutdown
(1) The sealing chamber should be kept full of liquid before starting. For conveying solidified media, the seal chamber should be heated with steam to melt the media. Before starting, you must pan the car to prevent the sudden start and cause the soft ring to break.
(2) For the mechanical seal using the oil sealing system outside the pump, the oil sealing system should be started first. Stop the oil sealing system after the last stop.
(3) The cooling water of the oil sealing cavity and end seal cannot be stopped immediately after stopping the hot oil pump, and the cooling water should be stopped only when the oil temperature at the end seal drops below 80 degrees to avoid damaging the seal parts.
3、Running
(1) If there is slight leakage after the pump starts, it should be observed for a period of time. If the leakage is not reduced after 4 hours of continuous operation, the pump should be stopped for inspection.
(2) The operating pressure of the pump should be smooth and the pressure fluctuation should not be more than 1 kg/cm2.
(3) Pump in operation, should avoid the phenomenon of pumping, so as not to cause dry friction and seal damage to the sealing surface.
(4) The sealing condition should be checked frequently. In operation, when its leakage exceeds the standard, heavy oil is not more than 5 drops / min, light oil is not more than 10 / min, such as 2-3 days there is still no trend of improvement, then the pump should be stopped bolts to check the sealing device.
Seal in our country has a long history of development, the ancients used to use cotton, hemp and other fibers to do water lifting machinery seals, while foreign countries to 1782 before the use of packing. The importance of sealing, not to mention here, nowadays, foreign countries have emerged to study the laws of sealing, sealing device design technology and the application of scientific principles of the discipline – “sealing”, scientific research institutions also have specialized in the study of sealing professional courses, while in China. So far, as far as I know, there are courses of fluid mechanics and hydraulic transmission in universities, but there is not yet a “sealing department” specializing in seals, so our research level is still a gap compared to foreign countries.
Seals are designed for many fields of expertise, including mechanics (including fluid mechanics, boundary layer theory, etc.), tribology, automatic control, etc., in addition to materials and mechanical aspects. Therefore, in terms of sealing, the research is relatively more difficult. The level of the domestic sealing industry, I think the gap with foreign countries should be no less than 50 years.
About the sealing principle
Want to learn to seal, you must first understand the leakage, the principle of leakage to understand, that corresponds to the mechanism of the seal will also have. There are no more than three kinds of leaks-One is leakage through the gap between the sealing surface, that is, the leakage.Second is leakage, that is, the seal of the fluid through the capillary leakage of the seal material.The third is diffusion, which is the material transfer of the sealed medium through the gap or capillary of the material under the effect of concentration difference.
About sealing methods
Sealing methods are roughly so many –1、Minimize the setting of sealing parts2、Blocking and isolation3、Lead out or inject4、Increase the leakage resistance5、Adding work elements in the channel6、A combination of sealing methods
Common forms of seals
Gasket seal, packing seal, mechanical seal, non-contact type seal and injection with pressure plugging are the common forms of seal. Among them, packing seal should be considered the most common, which also includes soft packing seal, hard packing seal and molding packing seal. Formed packing seal includes our common O-ring, Y-ring, oil seal and so on. Non-contact type seals include gap seals, labyrinth seals, floating seals, power seals, magnetic fluid seals and fully enclosed seals.
Commonly used gasket performance and new materials and new technologies
1.Commonly used gasket performance
When using the valve, often according to the specific circumstances, replace the original with the gasket. There are often gaskets: rubber flat gaskets, rubber O-rings, plastic flat gaskets, polytetrafluoroethylene package gaskets, asbestos rubber gaskets, metal flat gaskets, metal shaped gaskets, metal-clad gaskets, waveform gaskets, winding gaskets, etc.
(1) rubber flat gasket: deformation is easy, no effort when pressed, but pressure and temperature resistance are poor, only for low pressure, temperature is not high. Natural rubber has a certain acid and alkali resistance, the use of temperature should not exceed 60 ℃; neoprene rubber can also be resistant to certain acids and bases, the use of temperature 80 ℃; nitrile rubber oil resistance, available to 80 ℃; fluorine rubber corrosion resistance is very good, temperature resistance is also stronger than the general rubber, can be used in 150 ℃ mediu
(2) Rubber O-shaped gasket: section shape is positive round, there is a certain self-tightening effect, sealing effect is better than the flat gasket, compression force is smaller.
(3) Plastic flat gasket: plastic’s most important feature is good corrosion resistance, most plastic temperature resistance is not good. Polytetrafluoroethylene for the crown of plastic, not only excellent corrosion resistance, and temperature range is relatively wide, can be used within -180 ℃ to +200 ℃ for a long time.
(4) polytetrafluoroethylene package gasket: In order to give full play to the advantages of polytetrafluoroethylene, while making up for its poor elasticity shortcomings, made of polytetrafluoroethylene wrapped rubber or asbestos rubber gasket. In this way, both with PTFE flat gasket as corrosion-resistant, but also has good elasticity, enhance the sealing effect, reduce the compression force.
(5) asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60 to 80% of asbestos and 10 to 20% of rubber, as well as fillers, vulcanizing agents, etc.. It has very good heat resistance, cold resistance, chemical stability, and abundant supply, cheap. When used, the compression force need not be large. Since it can adhere to metal, it is best to apply a layer of graphite powder on the surface to avoid laborious disassembly.
Asbestos rubber sheet is available in four colors: gray, for low pressure (brand XB-200, pressure resistance ≤ 16 kg/cm2, temperature resistance 200 ℃); red, for medium pressure (brand XB-350, pressure resistance up to 40 kg/cm2, temperature resistance 350 ℃); fuchsia, for high pressure (brand XB-450, pressure resistance 100 kg/cm2 temperature resistance 450 ℃); green. used for oil, pressure resistance is also very good.
(6) metal flat heat ring: lead, temperature resistance 100 ℃; aluminum 430 ℃; copper 315 ℃; low carbon steel 550 ℃; silver 650 ℃; nickel 810 ℃; Monel (nickel-copper) alloy 810 ℃, stainless steel 870 ℃. Among them, lead is less resistant to pressure, aluminum can be resistant to 64 kg / cm2, other materials can be resistant to high pressure.
(7) metal heterogeneous gasket:
Lens gasket: has a self-tightening effect, used in high-pressure valves.
Oval gasket: also belongs to the high-pressure self-tightening gasket.
Tapered double gasket: used for high-pressure internal self-tightening seal.
In addition, there are square, diamond-shaped, triangular, tooth-shaped, dovetail, B-shaped, C-shaped, etc., generally used only in high and medium pressure valves.
(8) metal clad gasket: metal both good temperature and pressure resistance, but also good elasticity. Skin materials are aluminum, copper, mild steel, stainless steel, Monel alloy, etc.. Filled with materials such as asbestos, polytetrafluoroethylene, glass fiber.
(9) waveform gasket: has a small compression force, good sealing effect. Often used in the form of metal and non-metal combination.
(10) winding gasket: is a very thin metal belt and non-metallic belt close together, winding into a multi-layer round, wavy cross-section, with good elasticity and sealing. Metal belt available 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy and other production. Non-metallic belt materials are asbestos, polytetrafluoroethylene, etc.
Above, when telling about the performance of sealing gasket, some figures are listed. It must be noted that these figures are closely related to the flange form, media situation and installation and repair technology, sometimes can exceed, sometimes not up to, and pressure and temperature performance, but also the conversion of each other, such as high temperature, pressure resistance is often reduced, these subtle issues, only in practice experience.
2. New materials and new technologies
The sealing gasket introduced above is still very incomplete, and sealing technology is in rapid development. Here are a few examples of new materials and new technologies.
(1) liquid seal: With the rapid development of polymer organic synthesis industry, the emergence of liquid sealant, used in static sealing; this new technology, usually called liquid seal. The principle of liquid sealing, is the use of liquid sealant adhesion, mobility and single-molecule film effect (the thinner the film natural tendency to return), under the appropriate pressure, so that it acts like a gasket. So the sealant in use, also called liquid gasket.
(2) polytetrafluoroethylene raw material seal: polytetrafluoroethylene is also a polymer organic compounds, it is sintered into products before, called raw material, soft texture, also has a single molecular film effect. The tape made of raw material is called raw material tape, which can be rolled into discs for long-term storage. When used, it can be shaped freely, and any joint can be made, and as soon as pressure is applied, a ring-shaped membrane is formed which acts as a seal evenly. As a gasket between the valve body and the valve cover, you can pry open a gap without taking out the valve flap or the gate, and stuff it into the raw material belt. Small compression force, not sticky hands, and not sticky flange surface, very easy to replace. For tongue and groove flange is most suitable. Teflon raw material, can also be made into tubular and rod-shaped, for sealing.
(3) metal hollow O-ring: good elasticity, small compression force, self-tightening effect, can choose a variety of metal materials, so that in low temperature, high temperature and strong corrosive media can be adapted.
(4) Graphite plate seal ring: in the impression that graphite is brittle material, lack of elasticity and toughness, but after special treatment of graphite, but the texture is soft and good elasticity. Thus, the heat-resistant properties of graphite and chemical stability, can be shown in the gasket material; and this gasket compression force is small, the sealing effect is exceptionally superior. This graphite can also be made into a belt, with metal belt, to form an excellent performance winding gasket. Graphite sheet seals and graphite —- metal wound gaskets, is a major breakthrough in high-temperature corrosion-resistant seal. This type of gasket, foreign countries have been mass production and use.