How to solve the problem of sealing can not be ignored, because the valve run, run, drip, leakage, most of the occurrence here. Next we will discuss the valve dynamic seal, static seal problem.
1. Dynamic seal
Valve dynamic seal, mainly refers to the stem seal. Do not let the valve medium with stem movement and leakage, is the central issue of the valve dynamic seal.
1) stuffing box form
At present, the valve dynamic seal, mainly in the packing box. The basic form of stuffing box is:
(1) gland type
This is the most commonly used form.
The same form can be different in many details. For example, in the case of compression bolts, t-bolts (for low pressure valves with pressures less than 16 kg/cm2), studs and union bolts are available. From the gland, can be divided into integral and composite.
(2) compression nut type
In this form, the dimensions are small but the compressive force is limited and used on small valves only.
Packing box with packing in direct contact with stem and fill the packing box to prevent media leakage. Packing requirements:
(1) good sealing;
(2) corrosion resistance;
(3) small friction coefficient;
(4) suitable for medium temperature and pressure.
Common fillers are:
(1) asbestos packing: asbestos packing, temperature and corrosion resistance are very good, but when used alone, the sealing effect is not good, so always dip or additional materials. Oil-impregnated asbestos packing: it has two basic structural forms, one is twisted, the other is braiding. It can also be divided into a circle and a square.
(2) ptfe braided packing: the ptfe thin belt is braided as packing, which has excellent corrosion resistance and can be used for cryogenic medium.
(3) rubber o-ring: good sealing effect under low pressure. Use temperature is restricted, such as natural rubber can only be used for 60 ℃.
(4) plastic molding filler: generally made into three pieces, also can be made into other shapes. The plastics used are mostly polytetrafluoroethylene, but also nylon 66 and nylon 1010.
In addition, the use of units according to their own needs, often explore a variety of effective filler forms. For example, in 250 ℃ steam valves, the use of asbestos packing and lead lap alternating lamination, steam leakage situation will ease; Some valves, the medium often changes, such as asbestos packing and ptfe raw materials with common use, sealing effect is better. Molybdenum disulfide (M0S2) or other lubricants may be added to reduce stem friction.
At present, novel fillers are being explored. For example, polyacrylonitrile fiber impregnated by polytetrafluoroethylene emulsion, and after pre-oxidation, sintered in the mold pressed, can get excellent sealing performance molding filler; It can withstand high temperature, high pressure and corrosion.
3) bellows seal
With the rapid development of chemical industry and atomic energy industry, flammable, explosive, highly toxic and radioactive substances increase, the valve seal has more stringent requirements, some occasions have been unable to use packing seal, so a new form of sealing - bellows seal. This kind of seal does not need the packing, therefore also calls the packing not to seal.
The ends of the bellows are welded to death with other parts. When the valve stem lifting, bellows expansion, as long as the bellows itself does not leak, the medium can not be released. A double seal of bellows and packing is often used for safety.
2. Static seal
A static seal is usually a seal between two stationary surfaces. The main method of sealing is to use gaskets.
1) gasket material
(1) non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc.
Paper, hemp, cowhide and so on, has the capillary hole, is easy to permeate, when using must impregnate oil, wax or other impermeable material. Valves are rarely used.
Asbestos products, and asbestos belt, rope, board and asbestos rubber board, etc. Among them, the asbestos rubber sheet is of compact structure, good pressure resistance and temperature resistance. It is widely used in the valve itself and the flange connection between the valve and the pipe.
Plastic products, have good corrosion resistance, the use of more common. Varieties of polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010 and so on.
Rubber products, soft texture, all kinds of rubber are acid resistance, alkali resistance, oil resistance, resistance to sea water. Varieties of natural rubber, styrene butadiene rubber, nitrile butadiene rubber, neoprene, isobutyl rubber, polyurethane rubber, fluorine rubber and so on.
(2) metal materials: generally speaking, metal materials have high strength and high temperature resistance. But this is not the case with lead, which is only resistant to dilute sulfuric acid. Commonly used varieties are brass, copper, aluminum, low carbon steel, stainless steel, monel alloy, silver, nickel and so on.
(3) composite materials: such as metal wrapping (internal asbestos) washer, combined waveform washer, winding washer, etc.
2) common washer performance
When using a valve, always replace the original gasket as the case may be. Often have a washer: rubber plain washer, rubber o-ring, plastic plain washer, polytetrafluoroethylene bag washer, asbestos rubber washer, metal plain washer, metal shaped washer, metal foreskin washer, waveform washer, winding washer and so on.
(1) rubber plain washer: easy deformation, easy to press, but poor pressure, temperature resistance, only for low pressure, low temperature places. Natural rubber is, to some extent, acid-proof alkaline can use temperature should not be more than 60 ℃; Neoprene also ability some acid and alkali, temperature 80 ℃; Nitrile rubber oil resistant, available to 80 ℃; Fluorine rubber corrosion resistant performance is good, heat resistance performance is better than general rubber, can be used in the 150 ℃ medium.
(2) rubber o-washer: section shape is round, has a certain self-tightening effect, sealing effect is better than flat washer, compression force is smaller.
(3) plastic plain washer: the biggest feature of plastic is good corrosion resistance, most of the plastic temperature performance is not good. Ptfe as the crown of plastic, not only the excellent corrosion resistance, and heat resistance range is wide, can be in within 180 ℃ ~ + 200 ℃, use for a long time.
(4) ptfe gasket: in order to give full play to the advantages of ptfe, at the same time to make up for its poor elasticity, it is made of ptfe wrapped rubber or asbestos rubber gasket. In this way, both with ptfe flat gasket as corrosion resistance, and good elasticity, enhance the sealing effect, reduce the compression force. Its section shape is shown in figure 4-20.
(5) asbestos rubber gasket: cut from asbestos rubber sheet. Its composition is 60 ~ 80% asbestos and 10 ~ 20% rubber, as well as filler, vulcanization agent. It has good heat resistance, cold resistance, chemical stability, and abundant supply, cheap price. When in use, the pressure force need not be very big. As it can adhere to metal, it is best to surface coated with a layer of graphite powder, so as not to remove the effort.
Asbestos rubber sheet have four color: grey, used for low pressure (brand XB - 200, pressure or less 16 kg/cm2, heat-resistant 200 ℃). Red, used in medium voltage (brand XB - 350, pressure can reach 40 kg/cm2, heat-resistant 350 ℃). Amaranth, used in high pressure (brand XB - 450, pressure of 100 kg/cm2 and heat-resistant 450 ℃). Green, for oil, also good pressure resistance.
(6) metal flat hot laps: lead, 100 ℃ heat resistance; Aluminum 430 ℃; Copper 315 ℃; Low carbon steel 550 ℃; Silver 650 ℃; Nickel 810 ℃; Monel nickel copper alloy is 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 metamaterials:
Lens washer: self - tightening for high pressure valves.
Oval washer: also belongs to the high pressure self - tightening washer.
Cone face double washer: used for high pressure internal self-sealing.
In addition, there are square, diamond, triangle, tooth, dovetail, B, C, etc., generally only in high pressure valves.
(8) metal cover gasket: metal has good temperature and pressure resistance, and good elasticity. Wrapping material has aluminium, copper, low carbon steel, stainless steel, monel alloy. It is filled with asbestos, polytetrafluoroethylene, glass fiber, etc.
(9) corrugated washer: it has the characteristics of small compression force and good sealing effect. A combination of metal and nonmetal is often used.
(10) winding gasket: it is a very thin metal belt and non-metal belt tightly together, wound into a multi-layer round, 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 and so on. Non - metallic materials are asbestos, polytetrafluoroethylene, etc.
Above, some figures are enumerated in relation to the sealing gasket performance. Must explain, these Numbers follow flange form, medium circumstance and installation repair technology to have close concern, can exceed sometimes, cannot achieve sometimes, and bear pressure and bear temperature performance, also be mutual translate, for example temperature was high, bear pressure ability often reduces, these subtle problems, can experience in practice only.
3) new materials and technologies
The gasket described above is not comprehensive, and 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 emerged in recent years, which is used for static seal; This new technology is often called a liquid seal. The principle of liquid seal, is the use of liquid sealant adhesion, fluidity and monomolecular membrane effect (the thinner the film natural recovery tendency is greater), at the appropriate pressure, so that it works like a gasket. So for the sealant used, also known as the liquid washer.
(2) polytetrafluoroethylene raw seal: polytetrafluoroethylene is also a macromolecular organic compound, it is called raw material before sintering products, soft texture, also has monomolecular membrane effect. Raw material made of tape called raw material tape, can be rolled into a long - term preservation disk. It can be freely formed when used, any joint, as long as there is pressure, will form a ring film which ACTS as a seal evenly. As a gasket between the valve body and valve cover in the valve, can be in the absence of disc or gate, pry open a gap, into the raw material belt to go. Small pressure, non - stick hand, non - stick flange surface, easy to replace. Most suitable for mortise flange. Polytetrafluoroethylene raw material, also can be made into tubes and bars, as a seal.
(3) metal hollow o-ring: good elasticity, small pressure, self-tightening effect, can choose a variety of metal materials, thus in low temperature, high temperature and strong corrosive medium can adapt.
(4) graphite plate seal ring: in people's impression, graphite is brittle material, lack of elasticity and toughness, but after special treatment of graphite, but the texture is soft, 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. The graphite can also be made into ribbons, which can be combined with metal bands to make excellent winding gaskets. The appearance of graphite plate seal ring and graphite - metal wound gasket is a great breakthrough in high temperature corrosion resistance seal. Such gaskets have been mass produced and used abroad