If you want to learn sealing, you must first understand leakage, the principle of leakage is understood, and the corresponding mechanism of sealing is also available. There are three kinds of leaks

One is the leakage, that is, the leakage between the sealing surface gap

The second is leakage, that is, the leakage of the sealed fluid through the capillary tube of the sealing material

The third is diffusion, which refers to the material transfer of the sealing medium through the gap or the capillary tube of the material under the action of the concentration difference.

About sealing method

There are several types of sealing methods

1, minimize the sealing parts

2. Plugging and isolation

3. Extraction or injection

4. Increase leakage resistance

5. Add work elements in the channel

6. Combination of various sealing methods

Common seal form

Gasket seal, packing seal, mechanical seal, non-contact seal and injection type with pressure plugging are common sealing forms. Among them, the packing seal should be the most common, which also includes soft packing seal, hard packing seal and formed packing seal. The formed packing seal includes our common O-ring, Y-ring, oil seal and so on. Non-contact seals include clearance seals, labyrinth seals, floating seals, dynamic seals, magnetic fluid seals and fully enclosed seals.

Common gasket properties and new materials and new technology

1, commonly used gasket performance

When the valve is used, the original belt gasket is often replaced according to the specific situation. There are often gaskets: rubber flat gaskets, rubber O-rings, plastic flat gaskets, polytetrafluoroethylene gaskets, asbestos rubber gaskets, metal flat gaskets, metal shaped gaskets, metal coated gaskets, wave gaskets, wound gaskets and so on.

(1) Rubber flat washer: easy deformation, no effort when pressing, but the pressure and temperature resistance are poor, only used for low pressure and low temperature places. Natural rubber has a certain acid-alkali resistance, the use of temperature should not exceed 60℃; Neoprene rubber can also withstand some acids and bases, and the use temperature is 80℃; Nitrile rubber oil resistant, can be used up to 80℃; Fluorine rubber corrosion resistance is very good, temperature resistance is also stronger than general rubber, can be used in 150℃ medium.

(2) Rubber O-shaped washer: the section shape is round, has a certain self-tightening effect, the sealing effect is better than the flat washer, and the compression force is smaller.

(3) Plastic flat washer: The biggest feature of plastics is good corrosion resistance, and most plastics have poor temperature resistance. Polytetrafluoroethylene is the crown of plastics, not only excellent corrosion resistance, but also a relatively wide temperature range, can be used for a long time within -180 ° C ~ +200 ° C.

(4) PTFE gasket: In order to give full play to the advantages of PTFE and make up for its poor elasticity, polytetrafluoroethylene wrapped rubber or asbestos rubber gasket. In this way, it is as corrosion-resistant as the flat polytetrafluoroethylene gasket, and has good elasticity, enhancing the sealing effect and reducing the compression force.

(5) Asbestos rubber gasket: cut by asbestos rubber sheet. Its components are 60 ~ 80% asbestos and 10 ~ 20% rubber, as well as fillers, vulcanizing agents and so on. It has good heat resistance, cold resistance, chemical stability, and abundant supply, cheap price. When used, the compression force need not be very large. Because it can adhere to metal, it is best to coat the surface with a layer of graphite powder, so as to avoid difficulty when disassembling. (We recommend you to pay attention to the "Mechanical Engineer" public number, the first time to grasp the knowledge of dry goods, industry information)

Asbestos rubber sheet has four colors: gray, for low pressure (grade XB-200, pressure ≤16 kg/cm2, temperature 200℃); Red, for medium pressure (grade XB-350, pressure up to 40 kg/cm2, temperature 350℃); Purplish red, for high pressure (grade XB-450, pressure 100 kg/cm2, temperature 450℃); Green, for oil, pressure resistance is also very good.

(6) Metal flat hot ring: lead, temperature 100℃; Aluminum 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 64 kg/cm2, and other materials can withstand high pressure.

(7) Metal heterosexual gaskets:

Lens washer: self-tightening, used in high pressure valves.

Oval washer: also belongs to high pressure self-tightening washer.

Conical double washer: for high pressure internal self-sealing.

In addition, there are square, diamond, triangle, tooth shape, dovetail shape, B shape, C shape, etc., generally only used in high pressure valves.

(8) Metal clad gasket: metal has both good temperature and pressure resistance and good elasticity. The coating materials are aluminum, copper, low carbon steel, stainless steel, Monel alloy and so on. It is filled with asbestos, polytetrafluoroethylene, glass fiber and so on.

(9) Waveform washer: has the characteristics of small compression force and good sealing effect. It is often used in the form of a combination of metal and non-metal.

(10) Winding washer: It is a very thin metal belt and non-metal belt closely together, wrapped into a multi-layer circle, the section is wavy, has good elasticity and sealing. The metal strip can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non-metallic belt materials include asbestos, polytetrafluoroethylene and so on.

Above, when describing the performance of the sealing gasket, some figures are listed. It must be explained that these figures are closely related to the flange form, the medium condition and the installation and repair technology, which can sometimes be exceeded, sometimes cannot be reached, and the pressure and temperature resistance are also converted to each other, such as the temperature is high, the pressure resistance is often reduced, and these subtle problems can only be experienced in practice.

2, new materials and new technology

The sealing gasket described above is still not comprehensive, and the sealing technology is in rapid development. Here are some examples of new materials and technologies.

(1) liquid seal: With the rapid development of polymer organic synthesis industry, liquid sealant has appeared, used in static seal; This new technology is often called liquid sealing. The principle of liquid sealing is to use the adhesion, fluidity and single-molecule film effect of liquid sealant (the thinner the film naturally tends to return to the greater), under appropriate pressure, so that it acts like a washer. So the sealant used is also called a liquid gasket.

(2) polytetrafluoroethylene raw material seal: polytetrafluoroethylene is also a polymer organic compound, it is called raw material before burning products, soft texture, but also a single molecular film effect. The tape made of raw material is called raw material tape and can be rolled into a tray for long-term preservation. When used, it can be freely formed, any joint, as long as there is pressure, it forms a uniform ring film that plays a sealing role. As a gasket between the valve body and the valve cover in the valve, it can be pry open a gap and stuffed into the raw material without taking out the valve disc or gate. The compression force is small, does not stick to the hand, does not stick to the flange surface, the replacement is very convenient. Best for mortise and groove flanges. Polytetrafluoroethylene raw material can also be made into tubes and rods for sealing.

(3) Metal hollow O-ring: good elasticity, small compression force, self-tightening effect, can choose a variety of metal materials, so as to adapt to low temperature, high temperature and strong corrosive media.

(4) Graphite plate seal ring: in people's impression, graphite is a brittle substance, lack of elasticity and toughness, but after special treatment of graphite, it is soft texture, good elasticity. In this way, the heat resistance and chemical stability of graphite can be shown in the gasket material; And the gasket compression force is small, sealing effect is exceptionally superior. This graphite can also be made into a belt, with a metal belt, to form an excellent wound washer. The emergence of graphite plate seals and graphite ---- metal wound gaskets is a major breakthrough in high-temperature corrosion resistance seals. This kind of gasket has been mass-produced and used abroad.