Dynamic seal and static seal of the hottest valve

  • Detail

The sealing problem of how to solve the dynamic and static sealing forms of valves cannot be ignored, because the phenomenon of running, emitting, dripping and leaking of valves mostly occurs here. Next, we will discuss the dynamic and static sealing of valves

1. Dynamic seal

dynamic seal of valve mainly refers to valve stem seal. Preventing the medium in the valve from leaking with the movement of the valve stem is the central topic of the dynamic sealing of the valve

1) stuffing box form

at present, the valve dynamic seal is mainly stuffing box. The basic form of stuffing box is:

(1) gland type

this is the most used form

the same form can be distinguished in many details. For example, from the perspective of compression bolts, they can be divided into T-shaped bolts (for low-pressure valves with a pressure ≤ 16 kg/cm2), stud bolts and union bolts. From the gland, it can be divided into integral type and combined type

(2) compression nut type

this form has small overall dimensions, but the compression force is limited, so it is only used for small valves

2) packing

in the stuffing box, the packing is in direct contact with the valve stem and fills the stuffing box to prevent medium leakage. There are the following requirements for packing:

(1) good sealing

(2) corrosion resistance

(3) small friction coefficient

(4) adapt to medium temperature and pressure

common fillers are:

(1) asbestos packing: asbestos packing has good temperature resistance and corrosion resistance, 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 twisting, the other is knitting. It can also be divided into circle and square

(2) PTFE braided packing: the PTFE ribbon is braided into packing, which has excellent corrosion resistance and can be used in cryogenic media

(3) rubber O-ring: under low pressure, the sealing effect is good. The use temperature is limited. For example, natural rubber can only be used at 60 ℃

(4) plastic molding filling can be seen from this CES Exhibition: generally made into three pieces, but also made into other shapes. Most of the plastics used are polytetrafluoroethylene, and nylon 66 and nylon 1010 are also used

in addition, users often explore various effective filler forms according to their own needs. For example, in the 250 ℃ steam valve, the asbestos packing and lead ring are alternately overlapped, and the steam leakage will be reduced; For some valves, the medium is often changed. If asbestos packing and PTFE raw material belt are used together, the sealing effect will be better. In order to reduce the friction of the valve rod, molybdenum disulfide (M0S2) or other lubricants can be added in some cases

at present, new fillers are being explored. For example, the molding filler with excellent sealing performance can be obtained by impregnating polyacrylonitrile fiber with polytetrafluoroethylene lotion and sintering and pressing in the mold after pre oxidation; Another example is the corrugated filler made of stainless steel sheet and asbestos, which 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, which has more stringent requirements for valve sealing. In some cases, packing seal can no longer be used, so a new sealing form - bellows seal has been produced. This kind of seal does not need packing, so it is also called packless seal

both ends of the bellows are welded with other parts. When the valve rod rises and falls, the bellows expands and contracts. As long as the bellows itself does not leak, the medium cannot be discharged. For the sake of safety, the double seal of bellows and packing is often used

2. Static seal

static seal usually refers to the seal between two static surfaces. The sealing method is mainly to use gaskets

1) gasket material

(1) non metallic materials: such as paper, hemp, cow leather, asbestos products, plastic, rubber, etc

paper, hemp, cow leather and the like have pores and are easy to penetrate. They must be impregnated with oil, wax or other impermeable materials when used. Stretching rubber does not require a lot of force, which is rarely used in general valves

asbestos products, including asbestos tape, rope, board and asbestos rubber board. Among them, asbestos rubber plate has 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 and are widely used. Varieties include polyethylene, polypropylene, soft PVC, polytetrafluoroethylene, nylon 66, nylon 1010, etc

rubber products are soft in texture, and various rubbers have certain acid, alkali, oil and sea water resistance. Varieties include natural rubber, styrene butadiene rubber, nitrile rubber, neoprene, isobutylene rubber, polyurethane rubber, fluororubber, etc

(2) metal materials: Generally speaking, metal materials have high strength and strong temperature resistance. But this is not the case with lead, only because of its resistance to dilute sulfuric acid. Commonly used varieties include brass, copper, aluminum, low carbon steel, stainless steel, Monel alloy, silver, nickel, etc

(3) composite materials: such as metal clad (internal asbestos) washers, combined corrugated washers, spiral wound washers, etc

2) performance of common gaskets

when using valves, the original gaskets are often replaced according to the specific situation. There are often washers: rubber flat washer, rubber O-ring, plastic flat washer, PTFE wrapped washer, asbestos rubber washer, metal flat washer, metal special-shaped washer, metal wrapped washer, wave washer, spiral wound washer, etc

(1) rubber flat washer: it is easy to deform and effortless to compress, but its pressure resistance and temperature resistance are poor. It is only used in places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, and the service temperature should not exceed 60 ℃; Neoprene can also resist some acid and alkali, and the service temperature is 80 ℃; Nitrile rubber is oil resistant and can be used up to 80 ℃; Fluororubber has good corrosion resistance and better temperature resistance than ordinary rubber. It can be used in 150 ℃ medium

(2) rubber O-shaped gasket: the cross-section shape is round, which has a certain self tightening effect. The sealing effect is better than that of flat gasket, and the compression force is smaller

(3) plastic flat washer: the biggest feature of plastic is good corrosion resistance, and most plastics have poor temperature resistance. Polytetrafluoroethylene is the crown of plastics. It not only has excellent corrosion resistance, but also has a wide range of temperature resistance. 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 gasket is made. In this way, it is not only corrosion-resistant as the PTFE flat washer, but also has good elasticity, which enhances the sealing effect and reduces the compression force. The section shape is shown in the figure

(5) asbestos rubber gasket: cut from asbestos rubber plate. Its components are 60 ~ 80% asbestos and 10 ~ 20% rubber, as well as fillers, vulcanizing agents, etc. It has good heat resistance, cold resistance, chemical stability, and abundant supply and low price. When in use, the pressing force does not need to be great. Because it can adhere to metal, it is best to coat a layer of graphite powder on the surface to avoid laborious 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 ℃); Purple red, used for high voltage (brand xb-450, pressure resistance of 100 kg/cm2, temperature resistance of 450 ℃); Green, used for oils, with good pressure resistance

(6) metal flat heating ring: lead, temperature resistance 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 anisotropic washer:

lens washer: it has self tightening effect and is used for high-pressure valves

oval washer: it 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., which are generally only used in high and medium pressure valves

(8) metal clad gasket: metal has both good temperature and pressure resistance and good elasticity. Wrapping materials include aluminum, copper, low carbon steel, stainless steel, Monel alloy, etc. The filling materials inside 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 non-metal is often used to make the nanocomposite laminate containing graphene have excellent conductive function

(10) spiral wound gasket: a thin metal belt and a non-metal belt are tightly wound together into a multi-layer circle, with a wavy section, which has good elasticity and tightness. 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 talking about the performance of sealing washers. It must be noted that these figures are closely related to flange form, medium condition and installation and repair technology. Sometimes they can be exceeded and sometimes they cannot be achieved. Moreover, the pressure resistance and temperature resistance are also mutually transformed. For example, when the temperature is high, the pressure resistance is often reduced. These subtle problems can only be realized in practice

3) new materials and new technologies

the sealing gasket introduced above is not comprehensive, and the sealing technology is in rapid development. 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 in recent years, which is used for static sealing; This new technology is usually called liquid sealing. The principle of liquid sealing is to make use of the adhesion, fluidity and monolayer effect of liquid sealant (6. Absolutely stop the experiment. The thinner the explosive 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 sealing: PTFE is also a high molecular organic compound. It is called raw material before burning into products. It has 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. As long as there is pressure on any joint, it will form an annular membrane that plays a uniform sealing role. As the gasket between the valve body and the valve cover in the valve, you can pry open a gap without taking out the valve disc or ram, plug in the raw material and take it away. The pressing force is small, and it does not stick to the hand or the flange surface, so it is very convenient to replace. It is most suitable for grooved flanges. PTFE raw material can also be made into tubular and rod shapes for sealing

(3) metal hollow O-ring: it has good elasticity, small compression force and self tightening effect. A variety of metal materials can be selected to adapt to low temperature, high temperature and strong corrosive media

(4) graphite plate sealing ring: in people's impression, graphite is a brittle material, lacking elasticity and toughness, but after special treatment, graphite is soft and elastic. In this way, the heat resistance and chemical stability of graphite can be shown in the gasket material; Moreover, this kind of gasket has small pressing force and excellent sealing effect. This kind of graphite can also be made into strips, which match with metal strips and have excellent composition and performance

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