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Dynamic and Static Sealing Forms of Valves

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How to solve the sealing problem can not be neglected, because the valve running, leaking, dripping, most of the phenomenon occurs here. Next we will discuss the dynamic and static sealing of valves.
 
1. Dynamic seal
 
The dynamic seal of the valve mainly refers to the stem seal. Keeping the medium inside the valve from leaking with the movement of the valve stem is the central issue of the dynamic seal of the valve.
 
1) stuffing box form
 
At present, filler box is the main type of valve dynamic seal. The basic form of stuffing letter is:
 
(1) gland type
 
This is the most used form.
 
The same form can differ in many details. For example, in terms of compression bolts, they can be divided into T-bolts (low-pressure valves with pressure less than 16 kg/cm square), double-ended bolts and live bolts, etc. As far as the cover is concerned, it can be divided into integral type and combined type.
 
(2) press nut type.
 
In this form, the shape size is small, but the compression force is limited, only for small valves.
 
2) packing
 
In the packing box, the packing contacts the valve stem directly and fills the packing box to prevent the leakage of the medium. The following requirements are required for fillers:
 
(1) good sealing performance;
(2) corrosion resistance;
(3) Small friction coefficient;
(4) Adapt to medium temperature and pressure.
 
Commonly used fillers are:
 
(1) Asbestos packing: Asbestos packing has good heat resistance and corrosion resistance, but when used alone, the sealing effect is not good, so it is always impregnated or added with other materials. Oil-immersed asbestos packing: There are two basic structural forms, one is twisting, the other is knitting. It can also be divided into circles and squares.
(2) Polytetrafluoroethylene braided packing: braided PTFE ribbon 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. Use temperature is limited, such as natural rubber can only be used at 60 C.
(4) Plastic moulding fillers: generally made into three-piece type, but also can be made into other shapes. Polytetrafluoroethylene (PTFE), nylon 66 and nylon 1010 are the most commonly used plastics.
 
In addition, users often explore various effective filling forms according to their own needs. For example, in steam valves at 250 C, when asbestos pans and lead rings are alternately overlapped, the leakage will be reduced; in some valves, the medium is often changed, such as using asbestos pans and PTFE raw material belt together, the sealing effect will be better. Molybdenum disulfide (M0S2) or other lubricants can be added in some cases to reduce friction on the stem.
 
Nowadays, new fillers are being explored. For example, polyacrylonitrile fibers are impregnated with polytetrafluoroethylene emulsion, and then pre oxidized, and sintered and pressed in the mold, so that the molding filler with excellent sealing performance can be obtained. For example, corrugated packing made of stainless steel sheet and asbestos can withstand high temperature, high pressure and corrosion.
 
3) bellows seal
 
With the rapid development of chemical industry and atomic energy industry, more and more flammable, explosive, highly toxic and radioactive materials have been added, which requires more stringent sealing requirements for valves. In some cases, filler sealing can no longer be used. Therefore, a new sealing form, bellows sealing, has emerged. This kind of seal does not need filler, so it is also called non-filler seal.
 
Both ends of bellows are welded to other parts. When the valve stem rises and falls, the bellows expand and contract, so long as the bellows themselves do not leak, the medium can not leak out. For the sake of safety, double seals of bellows and fillers are often used.
 
2. Static seal.
 
Static seal usually refers to the seal between two stationary surfaces. The main sealing method is to use gaskets.
 
1) washer material
 
(1) Non-metallic materials: such as paper, linen, cowhide, asbestos products, plastics, rubber, etc.
 
Paper, linen, Kraft and so on, with capillary holes, easy to penetrate, use must be impregnated with oil, wax or other impermeable materials. Valves are seldom used in general.
 
Asbestos products, asbestos belt, rope, board and asbestos rubber board. Among them, asbestos rubber sheet has compact structure, good pressure resistance and good temperature resistance. It is widely used in flange connection of valve itself and valve and pipe.
 
Plastic products have good corrosion resistance and are widely used. Varieties include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc.
 
Rubber products, soft texture, a variety of rubber respectively have a certain acid, alkali, oil, sea water resistance. Varieties of natural rubber, styrene-butadiene rubber, nitrile-butadiene rubber, chloroprene rubber, isobutadiene rubber, polyurethane rubber, fluorine rubber and so on.
 
(2) Metal materials: Generally speaking, metal materials have high strength and high temperature resistance. But lead is not like this, only because of its resistance to dilute sulfuric acid. Commonly used species 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) gaskets, combined waveform gaskets, winding gaskets, etc.
 
2) Performance of common gaskets
 
When using the valve, the original washer is often replaced according to the specific situation. Often there are gaskets: rubber flat gasket, rubber O-ring, plastic flat gasket, PTFE wrapped gasket, asbestos rubber gasket, metal flat gasket, metal special-shaped gasket, metal wrapped gasket, wavy gasket, winding gasket and so on.
 
(1) Rubber flat gasket: easy to deform, easy to press, but poor pressure and temperature resistance, only for low pressure, low temperature. Natural rubber has a certain acid and alkali resistance, the use temperature should not exceed 60 C; chloroprene rubber can also withstand certain acid and alkali, use temperature 80 C; butadiene nitrile rubber oil resistance, can be used to 80 C; fluorine rubber corrosion resistance is good, temperature resistance is also better than ordinary rubber.