Dynamic seal and static seal of valve
The sealing problem of the valve can not be ignored, because the valve running, emitting, dripping, leakage phenomenon, the vast majority occurs here. This paper will discuss the valve dynamic seal, static seal related issues.
The dynamic seal of valve mainly refers to the stem seal. It is the key to solve the dynamic sealing problem of valve to prevent the medium in the valve from leaking with the movement of valve stem.
(1) Stuffing box form at present, valve dynamic seal, mainly to stuffing box. The basic form of stuffing box is:
- (a) Gland type: This is the most used form. The same form can distinguish many details. For example, from the perspective of hold down bolt, it can be divided into T-shaped bolt (used for low-pressure valve with pressure ≤ 16 kg / cm2), stud bolt and looper bolt, etc. From the gland, it can be divided into integral type and combined type.
- (b) The compression nut type has small size, but the compression force is limited, so it is limited to use in small valves.
(2) In the packing box, the packing directly contacts with the valve stem and fills the packing box to prevent medium leakage. The requirements for packing are as follows:
- (a) Good sealing performance;
- (b) Corrosion resistance;
- (c) The friction coefficient is small;
- (d) Adapt to medium temperature and pressure.
Common fillers are:
Flexible graphite: a new sealing material.
Asbestos packing: asbestos packing, temperature resistance and corrosion resistance are very good, but when used alone, the sealing effect is poor, so it is always impregnated or added with other materials.
Oil impregnated asbestos packing: it has two basic structural forms, one is twisted, the other is braided. It can be divided into circle and square.
Polytetrafluoroethylene braided packing: the polytetrafluoroethylene ribbon braided packing has excellent corrosion resistance and can be used in cryogenic medium.
Rubber O-ring: good sealing effect under low pressure. The use temperature is limited, such as natural rubber can only be used at 60 ℃.
Plastic molding filler: generally made into three pieces, can also be made into other shapes. Most of the plastics used are polytetrafluoroethylene, some are nylon 66 and nylon 1010.
Other packing forms: in 250 ℃ steam valve, asbestos packing and lead ring are used alternately to reduce steam leakage; Some valves, medium often change, such as asbestos packing and polytetrafluoroethylene raw material with common use, sealing effect will be better. In order to reduce the friction on the valve stem, molybdenum disulfide (M0S2) or other lubricants can be added on some occasions.
(3) With the rapid development of chemical industry and atomic energy industry, the number of inflammable, explosive, highly toxic and radioactive substances in bellows seal is increasing, which has more stringent requirements for valve seal. In some cases, the packing seal can not be used, so a new sealing form, bellows seal, is produced. This kind of seal does not need packing, so it is also called packless seal. Both ends of bellows are welded with other parts. When the valve rod rises and falls, the bellows will expand and shrink. As long as the bellows itself does not leak, the medium cannot be released. For the sake of safety, the double seal of bellows and packing is often used.
Static seal usually refers to the seal between two stationary surfaces.
- (a) Non metallic gasket materials: such as paper, hemp, cowhide, asbestos products, plastic, rubber, etc. Paper, hemp, cowhide and so on, with pores, easy to penetrate, use must be impregnated with oil, wax or other impermeable materials. General valves are rarely used.
- Asbestos products include asbestos belt, rope, board and asbestos rubber board. Among them, asbestos rubber sheet has compact structure, good pressure resistance and temperature resistance. It is widely used in valve itself and flange connection between valve and pipe.
- Plastic products have good corrosion resistance and are widely used. Varieties of polyethylene, polypropylene, soft PVC, PTFE, nylon 66, nylon 1010, etc.
- Rubber products, soft texture, all kinds of rubber have a certain acid resistance, alkali resistance, oil resistance, sea water resistance. Varieties of natural rubber, styrene butadiene rubber, nitrile rubber, chloroprene rubber, isoprene rubber, polyurethane rubber, fluororubber, etc.
- (b) Metal gasket material: Generally speaking, metal material has high strength and high temperature resistance. However, 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, etc.
- (c) Composite gasket materials: such as metal sheath (internal asbestos) gasket, combined wave gasket, spiral wound gasket, etc.
Properties of common gaskets
According to the specific situation, the gasket will be replaced frequently when using the valve. Commonly used washers are: rubber flat washer, rubber O-ring, plastic flat washer, PTFE coated washer, asbestos rubber washer, metal flat washer, metal special-shaped washer, metal clad washer, corrugated washer, wound washer, etc.
- (a) Rubber flat washer: easy to deform, easy to compress, but poor pressure and temperature resistance, only used in places with low pressure and low temperature. Natural rubber has a certain acid-base resistance, and its service temperature should not exceed 60 ℃; Chloroprene rubber is also resistant to some acids and bases, and its service temperature is 80 ℃; NBR 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.
- (b) 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 pressing force is smaller.
- (c) Plastic flat washer: the biggest characteristic of plastic is good corrosion resistance, most of the plastic temperature resistance is not good. Polytetrafluoroethylene is the top of plastics. It not only has excellent corrosion resistance, but also has a wide temperature range. It can be used for a long time from – 180 ℃ to + 200 ℃.
- (d) Polytetrafluoroethylene wrapped gasket: in order to give full play to the advantages of polytetrafluoroethylene and make up for its poor elasticity, the gasket is made of polytetrafluoroethylene wrapped 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, enhances the sealing effect and reduces the pressing force.
- (e) Asbestos rubber gasket: cut from asbestos rubber sheet, with good heat resistance, cold resistance, chemical stability, abundant supply and low price. Its combined components are 60-80% asbestos and 10-20% rubber, as well as fillers and vulcanizers. When asbestos rubber gasket is used, the pressing force need not be great. Because it can adhere to metal, it is best to coat a layer of graphite powder on the surface to avoid difficulty in disassembly.
- Asbestos rubber sheet has four colors: gray, used for low pressure (brand xb-200, pressure resistance ≤ 16 kg / cm2, temperature resistance 200 ℃); Red, for medium voltage (brand xb-350, pressure resistance up to 40 kg / cm2, temperature resistance 350 ℃); Purplish red, used for high pressure (brand xb-450, pressure resistance 100 kg / cm2, temperature resistance 450 ℃); Green, for oil, pressure resistance is also very good.
- (f) Metal flat washer: made of lead, temperature resistant 100 ℃; Aluminum 430 ℃; Copper 315 ℃; Low carbon steel 550 ℃; Silver 650 ℃; Nickel 810 ℃; Monel nickel copper 810 ℃, stainless steel 870 ℃. Among them, the pressure resistance of lead is poor, aluminum can withstand 64 kg / cm2, and other materials can withstand high pressure.
- (g) Metal seal gasket: lens gasket: self tightening, used in high pressure valve. 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 shape, dovetail shape, B shape, C shape, etc., generally only used in high and medium pressure valves.
- (h) Metal sheath gasket: the sheath materials are aluminum, copper, low carbon steel, stainless steel, Monel alloy, etc. It is filled with asbestos, polytetrafluoroethylene, glass fiber, etc. Metal not only has good temperature and pressure resistance, but also has good elasticity.
- (i) Wave washer: it has the characteristics of small pressing force and good sealing effect. The combination of metal and nonmetal is often used.
- (j) Spiral wound gasket: it is a thin metal belt and non-metal belt close together, winding into a multi-layer circle, the section is wavy, with good elasticity and sealing. The metal strip can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium and Monel alloy. Non metallic belt materials include asbestos, polytetrafluoroethylene, etc.
New sealing materials and new sealing technology
- (a) Liquid sealing: with the rapid development of polymer organic synthesis industry, liquid sealant has appeared in recent years, which is used for static sealing; This new technology is often called liquid seal. The principle of liquid sealing is to make use of the adhesiveness, fluidity and monolayer effect of liquid sealant (the thinner the film, the greater the natural recovery tendency), and make it work like a gasket under appropriate pressure. So the sealant used is also called liquid gasket（ b) Polytetrafluoroethylene raw material seal: polytetrafluoroethylene is also a high polymer organic compound, which is called raw material before firing products, with soft texture and monolayer effect. The belt made of raw material is called raw material belt, which can be rolled into a disk for long-term storage. When in use, it can be formed freely, any joint, as long as there is pressure, it will form an annular membrane which plays a uniform sealing role. As the gasket between the valve body and the valve cover, you can pry open a gap without taking out the valve disc or gate, and then plug in the raw material. Small pressing force, not stick to the hand, also not stick to the flange surface, replacement is very convenient. It is most suitable for mortise flange. Polytetrafluoroethylene raw material can also be made into tube shape and rod shape for sealing.
- (c) Hollow metal O-ring: good elasticity, small compression force, self tightening, can choose a variety of metal materials, which can adapt to low temperature, high temperature and strong corrosive media.
- (d) Graphite plate seal ring: in people’s impression, graphite is a brittle material, lack of elasticity and toughness, but after special treatment of graphite, it is soft, good elasticity. In this way, the heat resistance and chemical stability of graphite can be shown in gasket materials; Moreover, this kind of gasket has small pressing force and excellent sealing effect. This kind of graphite can also be made into tape, which can cooperate with metal tape to form spiral wound gasket with excellent performance. The appearance of graphite plate seal ring and graphite metal wound gasket is a great breakthrough in high temperature corrosion resistant seal.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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