How to solve the problem for knowing the valve dynamic and static

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How to solve the problem for knowing the valve dynamic and static seal

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Preface: Sealing problem can not be ignored because the valve run, drop and leakage phenomenon, the vast majority occurred here. Here we will discuss the valve dynamic seal, static sealing problem.

1, Dynamic seal
Valve dynamic seal, mainly refers to the stem seal. Do not let the valve medium with the valve stem movement and leakage, which is the valve moving seal the central issue.
1) Stuffing box form
At present, the valve dynamic seal to fill the main letter. The basic form of the stuffing box is:
(1) Gland type
This is the most used form.
The same form can be many different details. For example, from the compression bolts, can be divided into T-bolts (for pressure ≤ 16 kg / cm low-pressure valve), studs and live bolts. From the gland, the integral and modular.
(2) Compression nut type
This form, small size, but the compression force is limited, only used in small valves.

2) Fillers
Fill the letter to the filler in direct contact with the stem and filled with stuffing box to prevent media leakage. The filler has the following requirements:
(1) Good sealing;
(2) Corrosion resistance;
(3) The friction coefficient is small;
(4) To adapt to 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 poor, so always impregnated or attached to other materials. Oil asbestos packing: its basic structure in two ways, one is twisted, the other is knot. And can be round and square.
(2) Teflon woven packing: Teflon tapes woven into packing, excellent corrosion resistance, but also for the cryogenic medium.
(3) Rubber O-ring: in the low pressure state, the sealing effect is good. The use of temperature is limited, such as natural rubber can only be used for 60 ℃.
(4) Plastic molding filler: generally made three-piece, can also be made into other shapes. Plastic used in polytetrafluoroethylene as much, but also the use of nylon 66 and nylon 1010.
In addition, the use of units according to their own needs, often explore a variety of effective forms of filler. For example, in the 250 ℃ steam valve, with asbestos packing and lead ring alternately overlap, leakage of the situation will be reduced; some valves, media often change, such as asbestos packing and polytetrafluoroethylene raw material with the common use , The sealing effect will be better. In order to reduce the friction on the stem, and some occasions, you can add molybdenum disulfide (M0S2) or other lubricants.
At present, we are exploring novel packaging. For example, polyacrylonitrile fiber impregnated with polytetrafluoroethylene emulsion, and after pre-oxidation, in the mold sintering press, you can get excellent sealing performance of the filler; and if made of stainless steel sheet and asbestos made of wave packing, high temperature , High pressure and corrosion.
3) Bellows seal
With the rapid development of the chemical industry and the atomic energy industry, flammable, explosive, highly toxic and radioactive material increases, the valve seal has more stringent requirements, some occasions can not use the packing seal, resulting in a new Sealed form - bellows seal. This seal does not require packing, so it is called no packing seal.
The ends of the bellows, welded with other parts. When the stem lift, the bellows telescopic, as long as the bellows itself does not leak, the media can not be vented. For insurance purposes, often using bellows and packing double seal.

2, Static seal
A Static seal usually refers to a seal between two stationary surfaces. The sealing method is mainly the use of gaskets.

1) Gasket material

(1) Non-metallic materials: such as paper, hemp, leather, asbestos products, plastics, rubber and so on.
Paper, hemp, leather and the like, with pores, easy to penetrate, the use of immersion oil, wax or other anti-infiltration materials. General valves are rarely used.
Asbestos products, asbestos, rope, plate and asbestos rubber sheet. Which asbestos rubber sheet structure is dense, good pressure performance, temperature performance is also very good, in the valve itself and the valve and pipe flange connection, the use of very wide.
Plastic products, have a good corrosion resistance, the use of more common. Varieties are polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010 and so on.
Rubber products, soft texture, a variety of rubber, respectively, a certain acid, alkali, oil, seawater resistance. Varieties of natural rubber, styrene butadiene rubber, nitrile rubber, chloroprene rubber, isobutyl rubber, polyurethane rubber, fluorine rubber and so on.
(2) Metal materials: Generally speaking, metal materials, high strength, strong temperature performance. But lead is not the case, only take it the characteristics of dilute sulfuric acid. Common varieties are brass, copper, aluminum, low carbon steel, stainless steel, Monel alloy, silver, nickel and so on.
(3) Composite materials: such as metal foreskin (internal asbestos) washers, combined wave washers, winding washers and so on.

2) Common gasket performance
When using the valve, often according to the specific circumstances, replace the original belt washers. Often gaskets are: rubber flat washers, rubber O-rings, plastic flat washers, polytetrafluoroethylene gaskets, asbestos rubber washers, metal flat washers, metal shaped washers, metal encased gaskets, wave washers, wound washers and so on.
(1) Rubber flat washers: easy deformation, pressure when the effort, but the pressure, temperature resistance are poor, only for low pressure, the temperature is not high place. Natural rubber has a certain acid and alkali properties, the use of temperature should not exceed 60 ℃; chloroprene rubber can also resistance to some acid and alkali, the use of temperature 80 ℃; nitrile rubber oil, available to 80 ℃; fluorine rubber corrosion resistance is very good Temperature performance is also stronger than the general rubber, can be used in the 150 ℃ medium.
(2) Rubber O-shaped gasket: section shape is round, there is a certain self-tight effect, the sealing effect is better than the flat washer, the clamping force is smaller.
(3) Plastic flat washers: plastic is the biggest characteristic of good corrosion resistance, most of the plastic temperature performance is not good. Teflon is the crown of plastic, not only excellent corrosion resistance, and the temperature range is relatively wide, can be -180 ℃ ~ +200 ℃ long-term use.
(4) Teflon gasket: In order to give full play to the advantages of PTFE, while making it less flexible shortcomings, made of PTFE wrapped rubber or asbestos rubber gasket. In this way, both with the same PTFE flat washers as corrosion-resistant, but also a good flexibility, enhance the sealing effect, reduce the clamping force. The shape of the cross-section shown in Figure 4-20.
(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its composition is 60 to 80% of asbestos and 10 to 20% of the rubber, as well as fillers, vulcanizing agents and so on. It has good heat resistance, cold resistance, chemical stability, and rich supply, cheap. When used, the pressing force is not very large. Because it can adhere to the metal, so the best surface coated with a layer of graphite powder, so as not to dismount when struggling.
Asbestos rubber sheet has four colors: gray, for low pressure (grade XB-200, pressure ≤ 16 kg / square centimeter, temperature 200 ℃); red, for medium pressure (grade XB-350, 40 kg / cm, temperature 350 ℃); purple, for high pressure (grade XB-450, pressure 100 kg / square meter temperature 450 ℃); green, for oil, pressure performance is also very good The
(9) Metal flat heat circle: lead, temperature 100 ℃; aluminum 430 ℃; copper 315 ℃; low carbon steel 550 ℃; silver 650 ℃; nickel 810 ℃; Monel (nickel copper) alloy 810 ℃, stainless steel 870 ℃ The Lead pressure resistance is poor, aluminum can be resistant to 64 kg / square centimeter, other materials can withstand high pressure.
(7) Metal anisotropic washers:
Lens washers: self-tightening, used in high-pressure valves.
Oval washers: also belong to high pressure self-tight washers.
Cone double washers: for self-tightening in high pressure.
In addition, there are square, diamond, triangle, tooth, dovetail, B-shaped, C-shaped, generally only used in high and medium pressure valves.
(8) metal coated gaskets: metal has both good temperature and pressure resistance, but also a good elasticity. Foreskin materials are aluminum, copper, low carbon steel, stainless steel, Monel and other alloys. Inside the filling material is asbestos, polytetrafluoroethylene, glass fiber and so on.
(9) Wave washer: with a small compression force, the sealing effect is good. Often used in the form of metal and non-metallic combination.
(10) winding washers: is a very thin metal band and non-metallic tape close together, wrapped into a multi-layer round, section was wavy, with good flexibility and sealing. Metal belt available 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel, and other production. Non-metallic materials are asbestos, polytetrafluoroethylene and so on.
Above, about the performance of sealing gaskets, listed some numbers. It must be stated that these numbers are closely related to the form of flanges, media conditions and installation and repair techniques, and sometimes can be exceeded, sometimes not reached, and pressure and temperature resistance are also converted to each other, such as high temperature Pressure capacity is often reduced, these subtle problems, only in practice experience.

3) New materials and new technologies

The sealing gasket described above is still very comprehensive, and sealing technology is in rapid development. Here are a few examples of new materials and new technologies.
(1) Liquid seal: With the rapid development of polymer organic synthesis industry, in recent years there have been liquid sealant, used in static seal; this new technology, usually called liquid seal. The principle of liquid seal is the use of liquid sealant adhesion, mobility and monomolecular membrane effect (the thinner the natural recovery of the membrane tend to), under appropriate pressure to make it like a gasket to play the same role. So the use of the sealant, also known as liquid washers.
(2) Polytetrafluoroethylene raw material seal: PTFE is also a polymer organic compounds, it sintered into products, called raw material, soft texture, but also a single molecular film effect. Made with raw materials made with a raw material belt, can be rolled into a long-term preservation. The use of free to shape, any joints, as long as a pressure, it will form a uniform role in the sealing ring membrane. As a valve in the valve body and the valve cover between the gasket, which can not remove the valve or the case of the gate, pry open a gap, stuffed into the raw material to go on the line. Compact force, non-stick hand, nor stick flange surface, the replacement is very convenient. For the tongue and groove flange the most appropriate. Teflon raw materials, but also made of tubular and rod-shaped, for sealing.
(3) Metal hollow O-ring: good elasticity, compact force, a self-tightening effect, the choice of a variety of metal materials, which in low temperature, high temperature and strong corrosive media can adapt.
(4) Graphite plate seal: in people's impression, graphite is brittle material, lack of flexibility and toughness, but after special treatment of graphite, but it is soft texture, good elasticity. In this way, the thermal resistance and chemical stability of the graphite can be displayed in the gasket material; and the gasket has a small pressing force and the sealing effect is excellent. This kind of graphite can also be made with the belt, with the metal band with excellent performance of the winding washers. Graphite plate seals and graphite - the emergence of metal spiral washers, high temperature corrosion resistance is a major breakthrough. Such gaskets, foreign countries have been mass production and use.