Introduction to mechanical seals
Mechanical seal is one of the precision and complex mechanical basic elements, and it is the key component of various pumps, reaction synthesis kettles, turbine compressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as model selection, machine accuracy, correct installation and use, etc.
Basic knowledge of mechanical seals
1. Basic concept of mechanical seal:
Mechanical seal refers to a device to prevent fluid leakage, which is composed of at least one pair of end faces perpendicular to the rotation axis under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism, and with the cooperation of the auxiliary seal. The auxiliary seal of compensation ring is metal bellows, which is called bellows mechanical seal.
2. Composition of mechanical seal:
There are four main types of components. a. Main seals: dynamic ring and static ring. b. Auxiliary seal: seal ring. c. Compression parts: spring, push ring. d. Transmission part: skip seat and key or fixing screw
Attention
1. Precautions during installation
a. Pay great attention to avoid installation deviation during installation
(1) The gland shall be tightened after the alignment of the coupling. The bolts shall be evenly supported to prevent the end face of the gland from deflecting. Check each point with a feeler gauge, and the error shall not be greater than 0.05 mm.
(2) Check the fit clearance (i.e. concentricity) between the gland and the outer diameter of the shaft or shaft sleeve. The circumference should be even. Use a feeler gauge to check that the tolerance of each point is not more than 0.01mm.
b. The spring compression shall be carried out according to the regulations, and it is not allowed to be too large or too small. The required error is 2.00mm. If it is too large, the end face pressure will be increased, and the end face will be worn at another speed. If it is too small, the specific pressure will be insufficient and the sealing effect will not be achieved.
c. The movable ring shall be able to move flexibly on the shaft after installation, and it shall spring back automatically after pressing the movable ring against the spring.
2. Precautions during disassembly
a. When disassembling the mechanical seal, be careful. Never use a hammer or flat shovel to avoid damaging the sealing elements. It can be used as a pair of steel wire hooks to extend into the notch of the transmission seat in the direction of self financing and pull out the sealing device. If scaling cannot be removed, it shall be cleaned before removal.
b. If mechanical seals are used at both ends of the pump, take care of each other during assembly and disassembly to prevent neglect of one thing and loss of another.
c. For the operated mechanical seal, if the gland is loose and the seal moves, the moving and stationary ring parts must be replaced and should not be tightened again for further use. Because the original running track of the friction pair will change after the sample movement, the sealing of the contact surface will be easily damaged.
Normal operation and maintenance of mechanical seals
1. Preparations and precautions before startup
a. Comprehensively check whether the mechanical seal, accessory devices and pipeline installation are complete and meet the technical requirements.
b. Static pressure test shall be carried out before starting the mechanical seal to check whether there is leakage of the mechanical seal. If there are many leaks, find out the causes and try to eliminate them. If it is still invalid, it shall be disassembled, checked and reinstalled. The general hydrostatic test pressure is 2~3 kg/cm2.
c. Turn the gear according to the rotation direction of the pump and check whether it is light and even. If the turning gear is hard or does not move, check whether the assembly size is wrong and the installation is reasonable.
2. Installation and shutdown
a. The sealing chamber shall be kept full of liquid before starting. When transporting solidified medium, the sealing chamber shall be heated by steam to melt the medium. The turning gear must be turned before starting to prevent the soft ring from breaking due to sudden starting.
b. For the mechanical seal using the external sealing oil system of the pump, the sealing oil system shall be started first. Finally stop the oil sealing system after shutdown.
c. After the hot oil pump is shut down, the cooling water of the seal oil chamber and the end face seal cannot be stopped immediately. The cooling water can only be stopped when the oil temperature at the end face seal drops below 80 degrees to avoid damaging the sealing parts.
3. Operation
a. If there is slight leakage after the pump is started, observe for a period of time. If the leakage does not decrease after 4 hours of continuous operation, stop the pump for inspection.
b. The operating pressure of the pump shall be stable, and the pressure fluctuation shall not exceed 1kg/cm2.
c. During the operation of the pump, evacuation shall be avoided to avoid dry friction on the sealing surface and seal damage.
d. The sealing condition shall be checked frequently. During operation, when the leakage exceeds the standard, the heavy oil shall not be more than 5 drops/min and the light oil shall not be more than 10/min. If there is no improvement trend within 2-3 days, the pump shall be stopped and the sealing device shall be checked.
The development of "seal" in China has a long history. The ancients used cotton, hemp and other fibers as the seals of water lifting machinery, while foreign countries only used filler in 1782. The importance of sealing will not be mentioned here. Now there has been a discipline of studying sealing laws, sealing device design technology and applied scientific principles abroad - "sealing science". Scientific research institutions also have professional courses specialized in sealing science. In China, so far, as far as I know, there are courses of fluid mechanics and hydraulic transmission in universities, However, there is no "sealing system" specialized in sealing, so our research level is still lagging behind that of foreign countries.
There are many professional fields of seal design, including mechanics (including fluid mechanics, boundary layer theory, etc.), tribology, automatic control, etc. in addition to materials and machinery. Therefore, for sealing, the research is relatively difficult. Personally, I think the gap between the domestic sealing industry and foreign sealing industry should not be less than 50 years.
About sealing principle
If you want to learn sealing, you must first understand the leakage. If you understand the principle of leakage, then the corresponding sealing mechanism will also be available. There are three kinds of leakage——
One is leakage, that is, leakage between sealing surface gaps
The other is leakage, that is, leakage of sealed fluid through capillary tube of seal material
The third is diffusion, which refers to the material transfer of sealing medium through gaps or capillary tubes of materials under the effect of concentration difference.
About sealing method
There are several sealing methods——
1. Minimize the parts with seals
2. Blocking and isolation
3. Lead out or injection
4. Increase leakage resistance
5. Add power element in the channel
6. Combination of multiple sealing methods
Common sealing forms
Gasket seal, packing seal, mechanical seal, non-contact seal and injection type leak stoppage under pressure are common sealing forms. Among them, packing seal is the most common, which also includes soft packing seal, hard packing seal and shaped packing seal. Formed packing seal includes our common O-ring, Y-ring, oil seal, etc. The non-contact seal includes clearance seal, labyrinth seal, floating seal, dynamic seal, magnetic fluid seal and totally enclosed seal.
Common gasket performance --- new materials and new technology
（1） Common gasket performance
When using the valve, the original gasket is often replaced according to the specific situation. Common gaskets include: rubber flat washer, rubber O-ring, plastic flat washer, PTFE wrapped washer, asbestos rubber washer, metal flat washer, metal profiled washer, metal sheath washer, corrugated washer, spiral wound washer, etc.
(1) Rubber flat washer: easy to deform, easy to compress, but poor pressure and temperature resistance. It is only used in places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, and the use temperature should not exceed 60 ℃; Neoprene can also resist certain acids and alkalis, and the operating temperature is 80 ℃; Buna-N rubber is oil resistant and can be used up to 80 ℃; Fluorine rubber has good corrosion resistance and temperature resistance, and can be used in 150 ℃ medium.
(2) Rubber O-ring washer: the section shape is round, which has a certain self tightening effect. The sealing effect is better than that of flat washer, and the compression force is smaller.
(3) Plastic flat washer: The biggest feature of plastic is its good corrosion resistance, while most plastics have poor temperature resistance. Polytetrafluoroethylene (PTFE) is the best plastic. It not only has excellent corrosion resistance, but also has a wide temperature resistance range. It can be used for a long time within - 180 ℃~+200 ℃.
(4) PTFE wrapped gasket: in order to give full play to the advantages of PTFE and make up for its poor elasticity, PTFE wrapped rubber or asbestos rubber gaskets are made. In this way, it not only has the same corrosion resistance as the PTFE flat washer, but also has good elasticity, enhancing the sealing effect and reducing the compression force. The section shape is shown in Figure 4-20.
(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60-80% asbestos and 10-20% rubber, as well as fillers and vulcanizing agents. It has good heat resistance, cold resistance, chemical stability, and is rich in supply and cheap. When in use, the pressing force need not be great. Since it can adhere to metal, it is better to coat the surface with a layer of graphite powder to avoid the difficulty of disassembly.
Asbestos rubber sheet has four colors: gray, used for low pressure (grade XB-200, pressure resistance ≤ 16kg/cm2, temperature resistance 200 ℃); Red, for medium voltage (XB-350, pressure resistance up to 40 kg/cm2, temperature resistance 350 ℃); Purplish red, used for high pressure (grade XB-450, pressure resistance 100 kg/cm2, temperature resistance 450 ℃); Green, used for oil, with good pressure resistance.
(6) Metal flat heating ring: lead, temperature resistance 100 ℃; Aluminium 430 ℃; Copper 315 ℃; Low carbon steel 550 ℃; Silver 650 ℃; Nickel 810 ℃; Monel (nickel copper) alloy 810 ℃, stainless steel 870 ℃. Among them, lead has poor pressure resistance, aluminum can withstand 64kg/cm2, and other materials can withstand high pressure.
(7) Metal anisotropic washer:
Lens washer: self tightening, used for high-pressure valves.
Oval washer: also belongs to high-pressure self tightening washer.
Conical double washer: used for high-pressure internal self tightening seal.
In addition, there are square, diamond, triangle, tooth, swallow tail, B, C, etc., which are generally only used in high and intermediate pressure valves.
(8) Metal sheath washer: metal has good temperature resistance and pressure resistance, and good elasticity. The foreskin materials include aluminum, copper, low carbon steel, stainless steel, Monel alloy, etc. The filling materials include asbestos, polytetrafluoroethylene, glass fiber, etc.
(9) Corrugated washer: It has the characteristics of small compression force and good sealing effect. The combination of metal and nonmetal is often used.
(10) Spiral wound gasket: it is a kind of thin metal belt and non-metallic belt that are tightly tied together and wound into a multi-layer circle with wavy cross section, which has good elasticity and sealing performance. The metal belt can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non metallic belt materials include asbestos, polytetrafluoroethylene, etc.
Above, some figures are listed when describing the performance of sealing washer. It must be noted that these figures are closely related to the flange form, medium condition, installation and repair technology, and sometimes can be exceeded, sometimes cannot be reached. Moreover, the pressure resistance and temperature resistance are also mutually transformative. For example, when the temperature is high, the pressure resistance is often reduced. These subtle problems can only be understood in practice.
（2） New materials and new technologies
The sealing gasket described above is not comprehensive, and the sealing technology is developing rapidly. Here are some examples of new materials and technologies.
(1) Liquid sealing: With the rapid development of polymer organic synthesis industry, liquid sealant has appeared, which is used for static sealing; This new technology is usually called liquid sealing. The principle of liquid sealing is to use the adhesiveness, fluidity and monolayer effect of liquid sealant (the thinner the film, the greater the natural recovery tendency) to make it work like a gasket under appropriate pressure. Therefore, the sealant used is also called liquid gasket.
(2) Teflon raw material seal: Teflon is also a macromolecular organic compound. It is called raw material before being sintered into products. It is soft and has a monolayer effect. The belt made of raw material is called raw material belt, which can be rolled into a reel for long-term storage. When in use, it can be formed freely. Any joint can form an annular membrane with uniform sealing function as long as there is pressure. As the gasket between the valve body and the valve cover in the valve, it can be used to pry open a gap without taking out the valve disc or gate, and then put the raw material into it. The pressing force is small, and it does not stick to the hand or the flange surface, so it is very convenient to replace. Best for tongue and groove flanges. Teflon raw material can also be made into tube and rod for sealing.
(3) Metal hollow O-ring: It has good elasticity, small compression force and self tightening effect. It can be made of a variety of metal materials, so it can adapt to low temperature, high temperature and strong corrosive media.
(4) Graphite plate seal ring: people have the impression that graphite is a brittle material, lacking elasticity and toughness, but the specially treated graphite is soft and elastic. In this way, the heat resistance and chemical stability of graphite can be shown in the gasket material; Moreover, the gasket has small compression force and excellent sealing effect. This graphite can also be made into a belt, which can be matched with the metal belt to form a spiral washer with excellent performance. The appearance of graphite plate seal ring and graphite metal spiral wound gasket is a major breakthrough in high-temperature anti-corrosion sealing. This kind of gasket has been produced and used in large quantities abroad.
