Introduction to Mechanical Seals
Mechanical seals are one of the basic components of machinery with precision and complex structures, and are key components of various pumps, reaction synthesis kettles, turbo compressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as type selection, machine precision, correct installation and use, etc.
Basic knowledge of mechanical seals
1. Basic concept of mechanical seal:
Mechanical seal refers to a device that prevents fluid leakage by at least one pair of end faces perpendicular to the axis of rotation, under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism, and the cooperation of auxiliary seals to keep fit and slide relative to each other. The auxiliary seal of the compensation ring is a metal bellows called a bellows mechanical seal.
2. Composition of mechanical seal:
There are mainly the following four types of components. a. Main seals: moving ring and static ring. b. Auxiliary seal: sealing ring. c. Compression parts: spring, push ring. d. Transmission parts: spring skip seat and key or fixing screw
should pay attention to the problem
1. Matters needing attention during installation
a. Pay great attention to avoid installation deviation during installation
(1) The gland should be tightened after the coupling is aligned. The bolts should be evenly supported to prevent the end face of the gland from deflecting. Check each point with a feeler gauge, and the error should not exceed 0.05 mm.
(2) Check the fit clearance (i.e. concentricity) between the gland and the outer diameter of the shaft or bushing, and make sure that it is even around. Use a feeler gauge to check that the tolerance of each point is not greater than 0.01mm.
b. The amount of spring compression should be carried out according to the regulations. It is not allowed to be too large or too small, and the error is required to be 2.00 mm. If it is too large, the specific pressure of the end face will be increased, and the end face will be worn at a faster rate. Too small will result in insufficient specific pressure and can not play a sealing role.
c. After the moving ring is installed, ensure that it can move flexibly on the shaft, and it should spring back automatically after pressing the moving ring against the spring.
2. Precautions when disassembling
a. Be careful when disassembling the mechanical seal. It is strictly forbidden to use hand hammers and flat shovels, so as not to damage the sealing components. A pair of steel wire hooks can be used to extend into the gap of the drive seat in the self-financing direction to pull out the sealing device. If the scale cannot be disassembled, it should be cleaned before disassembly.
b. If mechanical seals are used at both ends of the pump, they should take care of each other during assembly and disassembly to prevent one from losing the other.
c. For the mechanical seal that has been in operation, if the gland is loose and the seal moves, the dynamic and static ring parts must be replaced, and it should not be re-tightened to continue to use. Because after such a movement, the original running track of the friction pair will change, and the sealing of the contact surface will be easily damaged.
Proper Operation and Maintenance of Mechanical Seals
1. Preparatory work and matters needing attention before starting
a. Comprehensively inspect the mechanical seal, and whether the installation of auxiliary devices and pipelines is complete and whether it meets the technical requirements.
b. Perform a static pressure test before starting the mechanical seal to check whether there is any leakage of the mechanical seal. If there is a lot of leakage, find out the cause and try to eliminate it. If it is still invalid, it should be disassembled for inspection and reinstalled. Generally, the static pressure test pressure is 2~3 kg/cm2.
c. Press the pump steering wheel to check whether it is brisk and even. If the cranking is difficult or does not move, you should check whether the assembly size is wrong and whether the installation is reasonable.
2. Installation and shutdown
a. Before starting, keep the sealed cavity filled with liquid. When transporting solidified medium, steam should be used to heat the sealed cavity to melt the medium. The car must be cranked before starting to prevent the soft ring from breaking due to sudden start.
b. For mechanical seals that use the external oil sealing system of the pump, the oil sealing system should be activated first. Stop the oil sealing system last after parking.
c. After the hot oil pump is out of service, the cooling water in the oil seal cavity and the end face seal cannot be stopped immediately. The cooling water should be stopped only when the oil temperature at the end face seal drops below 80 degrees, so as not to damage the sealing parts.
3. Operation
a. If there is a slight leakage after the pump is started, it should be observed for a period of time. If the leakage does not decrease after 4 hours of continuous operation, the pump should be stopped for inspection.
b. The operating pressure of the pump should be stable, and the pressure fluctuation should not exceed 1 kg/cm2.
c. When the pump is in operation, avoid pumping out to avoid dry friction on the sealing surface and damage to the seal.
d. The sealing condition should be checked frequently. During operation, when the leakage exceeds the standard, the heavy oil is not more than 5 drops/min, and the light oil is not more than 10/min. If there is no improvement within 2-3 days, stop the pump and check the sealing device.
"Sealing" has a long history of development in our country. The ancients used cotton, hemp and other fibers to make seals for water-lifting machinery, while foreign countries did not use packing until 1782. The importance of sealing is not mentioned here. There is a discipline called "Sealing Science" that studies the laws of sealing, sealing device design technology and applied scientific principles. Research institutions also have professional courses dedicated to the study of sealing science. In our country, so far, as far as I know, there are fluid Mechanics and hydraulic transmission and other courses, but there is no "seal system" that specializes in sealing, so our research level is still far behind compared with foreign countries.
There are many professional fields for seal design, in addition to materials and mechanics, there are also mechanics (including fluid mechanics, boundary layer theory, etc.), tribology, automatic control, etc. Therefore, for sealing, the difficulty of research is relatively large. The level of the domestic sealing industry, I personally think that compared with foreign countries, the gap should be no less than 50 years.
About the sealing principle
If you want to learn sealing, you must first understand leakage. Once you understand the principle of leakage, you will have the corresponding sealing mechanism. There are three kinds of leaks——
One is leakage, that is, the leakage between the gaps of the sealing surfaces
The second is leakage, that is, the leakage of the sealed fluid through the capillary of the sealing material
The third is diffusion, which refers to the material transfer that occurs when the sealing medium passes through the gap or the capillary of the material under the action of the concentration difference.
About the sealing method
There are roughly several sealing methods——
1. Minimize the number of sealed parts
2. Blockage and isolation
3. Extract or inject
4. Increase leakage resistance
5. Add active elements in the channel
6. Combination of multiple sealing methods
Common Sealing Forms
Gasket seals, packing seals, mechanical seals, non-contact seals, and injection pressure plugging are common sealing forms. Among them, the packing seal should be considered the most common, and it also includes soft packing seal, hard packing seal and formed packing seal. Formed packing seals include our common O-rings, Y-rings, oil seals and more. Non-contact seals include gap seals, labyrinth seals, floating seals, dynamic seals, magnetic fluid seals and hermetic seals.
Properties of common gaskets---and new materials and technologies
1) Commonly used gasket performance
When using the valve, the original gasket is often replaced according to the specific situation. Common gaskets are: rubber flat gasket, rubber O-ring, plastic flat gasket, PTFE bag gasket, asbestos rubber gasket, metal flat gasket, metal special-shaped gasket, metal foreskin gasket, wave gasket, wound gasket, etc.
(1) Rubber flat washer: easy to deform, easy to compress, but poor in pressure and temperature resistance, only used in places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, and the operating temperature should not exceed 60°C; neoprene rubber can also withstand certain acids and alkalis, and the operating temperature is 80°C; nitrile rubber is oil-resistant and can be used up to 80°C; The temperature performance is also stronger than ordinary rubber, and it can be used in 150°C medium.
(2) Rubber O-shaped washer: the section shape is a perfect circle, and it has a certain self-tightening effect. The sealing effect is better than that of the flat washer, and the pressing force is smaller.
(3) Plastic flat gasket: The biggest feature of plastic is its good corrosion resistance, and most plastics have poor temperature resistance. PTFE is the crown of plastics. It not only has excellent corrosion resistance, but also has a relatively wide temperature 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, it is made into a gasket wrapped with PTFE rubber or asbestos rubber. In this way, it not only has the same corrosion resistance as the PTFE flat washer, but also has good elasticity, which enhances the sealing effect and reduces the pressing force. Its cross-sectional 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, and chemical stability, and it is abundant in supply and cheap in price. When in use, the pressing force does not need to be very large. Because it can adhere to metal, it is best to coat the surface with a layer of graphite powder to avoid laborious removal.
There are four colors of asbestos rubber sheet: gray, used for low pressure (brand XB-200, pressure resistance ≤ 16 kg/cm2, temperature resistance 200 ℃); red, used for medium pressure (brand XB-350, pressure resistance up to 40kg/cm2, temperature resistance 350℃); purple, used for high pressure (grade XB-450, pressure resistance 100kg/cm2 temperature resistance 450℃); green, used for oil, pressure resistance is also very good .
(6) Metal flat heating ring: lead, temperature resistance 100°C; aluminum 430°C; copper 315°C; low carbon steel 550°C; silver 650°C; nickel 810°C; Monel (nickel-copper) alloy 810°C, stainless steel 870°C . Among them, lead has poor pressure resistance, aluminum can withstand 64 kg/cm2, and other materials can withstand high pressure.
(7) Metal anisotropic washers:
Lens gasket: It has self-tightening effect and is used for high-pressure valves.
Oval washers: also belong to high-pressure self-tightening washers.
Double tapered gasket: used for high-pressure internal self-tightening seal.
In addition, there are square, rhombus, triangle, tooth shape, dovetail shape, B shape, C shape, etc., which are generally only used in high and medium pressure valves.
(8) Metal sheathed gasket: Metal has good temperature and pressure resistance and good elasticity. Foreskin materials include aluminum, copper, low carbon steel, stainless steel, Monel alloy, etc. The filling materials inside include asbestos, polytetrafluoroethylene, glass fiber, etc.
(9) Wave washer: It has the characteristics of small pressing force and good sealing effect. It is often used in the form of a combination of metal and non-metal.
(10) Wound gasket: It is a thin metal strip and a non-metallic strip that are tightly bonded together and wound into a multi-layer circle. The section is wavy and has good elasticity and sealing. The metal belt can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non-metal strip materials include asbestos, polytetrafluoroethylene, etc.
Above, when describing the performance of sealing gaskets, some figures are listed. It must be noted that these numbers are closely related to the flange form, medium conditions, and installation and repair techniques. Sometimes they can be exceeded, and sometimes they cannot be reached. Moreover, the pressure and temperature resistance properties are also mutually transformed. For example, the higher the temperature, the higher the resistance. The pressure ability is often reduced, and these subtle problems can only be realized in practice.
2) New materials and technologies
The sealing gaskets introduced above are not comprehensive, and the sealing technology is developing rapidly. The following examples introduce several new materials and new technologies.
(1) Liquid sealing: With the rapid development of the polymer organic synthesis industry, liquid sealants have appeared for static sealing; this new technology is usually called liquid sealing. The principle of liquid sealing is to use the adhesion, fluidity and monomolecular film effect of liquid sealant (the thinner the film, the greater the natural recovery tendency), and make it work like a gasket under appropriate pressure. Therefore, the sealant used is also called liquid gasket.
(2) PTFE raw material seal: PTFE is also a high-molecular organic compound. Before it is sintered into a product, it is called raw material. It is soft in texture and has a monomolecular film effect. The tape made of raw material is called raw material tape, which can be rolled into a disk for long-term storage. It can be formed freely during use, and any joint, as long as there is pressure, will form a ring-shaped membrane that evenly acts as a seal. As the gasket between the valve body and the valve cover in the valve, it can be pried open a gap and stuffed into the raw material belt without taking out the disc or gate. The pressing force is small, not sticky to hands, not sticky to the flange surface, and it is very convenient to replace. Best suited for tongue and groove flanges. PTFE raw materials can also be made into tubes and rods for sealing.
(3) Metal hollow O-ring: good elasticity, small pressing force, and self-tightening effect. Various metal materials can be used, so that it can adapt to low temperature, high temperature and strong corrosive medium.
(4) Graphite plate sealing ring: In people's minds, graphite is a brittle substance that lacks elasticity and toughness, but graphite that has been specially treated is soft in texture and has good elasticity. In this way, the heat resistance and chemical stability of graphite can be displayed in the gasket material; moreover, the gasket has a small compression force and excellent sealing effect. This graphite can also be made into a belt, which can be combined with a metal belt to form a wound gasket with excellent performance. The emergence of graphite plate sealing rings and graphite----metal wound gaskets is a major breakthrough in high-temperature corrosion-resistant sealing. This type of gasket has been produced and used in large quantities abroad.
