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How to solve the valve dynamic seal, static seal problem?


Release time:

2018-03-23

Dynamic seal

Valve dynamic seal, mainly refers to the stem seal. Do not let the valve with the valve stem movement and leakage of the medium, is the central subject of the valve dynamic seal.

 

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Dynamic sealing stuffing box form

At present, the valve dynamic seal, stuffing box based. the basic form of stuffing box is:

1. Cap type

This is the most used form. The same form can be distinguished by many details. For example, in terms of compression bolts, T-shaped bolts (for low-pressure valves with pressure ≤ 16kg/cm2), stud bolts and articulated bolts can be divided. From the gland, can be divided into integral and combined.

2. Compression nut type

This form, small size, but the pressing force is limited, only used in small valves. In the stuffing box, the packing is in direct contact with the valve stem and filled with the stuffing box to prevent the leakage of the medium.

The filler has the following requirements:

Good sealing; corrosion resistance; friction coefficient is small; adapt to medium temperature and pressure.

Common packing for dynamic seal

 

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1. Asbestos packing: Asbestos packing has good temperature resistance and corrosion resistance, but when used alone, the sealing effect is not good, so it is always impregnated or additional materials. Oil-impregnated asbestos packing: There are two basic structural forms, one is twisted and the other is braided. It can be divided into round and square.

2, PTFE braided packing: the PTFE ribbon woven packing, has excellent corrosion resistance, but also can be used for cryogenic medium.

3. Rubber O-ring: In the low pressure state, the sealing effect is good. The use temperature is limited, such as natural rubber can only be used at 60°C.

4, plastic molding filler: generally made into three-piece, can also be made into other shapes. The plastics used are polytetrafluoroethylene, and nylon 66 and nylon 1010 are also used.

In addition, the use of units according to their own needs, often explore a variety of effective filler forms. For example, in the 250 ℃ steam valve, with asbestos packing and lead ring alternately stacked, the leakage of steam will be reduced; some valves, the medium is often changed, such as asbestos packing and PTFE raw material with common use, the sealing effect will be better. In order to reduce the friction on the valve stem, in some cases, you can add molybdenum disulfide (M0S2) or other lubricants.

At present, the novel filler is being explored. For example, with polyacrylonitrile fiber impregnated by polytetrafluoroethylene emulsion, and after pre-oxidation, sintering in the mold pressing, can be excellent sealing performance of the molding filler; such as stainless steel sheet and asbestos made of corrugated packing, can be high temperature, high pressure and corrosion.

Dynamic seal Bellows seal

With the rapid development of the chemical industry and the atomic energy industry, flammable, explosive, toxic and radioactive substances increased, the valve seal has more stringent requirements, some occasions have been unable to use packing seal, resulting in a new form of sealing-bellows seal. This seal does not need packing, so it is also called no packing seal.

The two ends of the bellows are welded to other parts. When the valve stem is raised and lowered, the bellows retractable, as long as the bellows itself does not leak, the medium cannot be released. For the sake of insurance, the double seal of bellows and packing is often used.

Static seal

Static seal

A static seal generally refers to a seal between two stationary surfaces. Sealing method is mainly the use of gaskets.

 

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Static seal gasket material

1. Non-metallic materials: paper, hemp, cowhide, asbestos products, plastic, rubber, etc.

Paper, linen, cowhide and the like, with fine pores, easy to penetrate, when used to be impregnated with oil, wax or other impermeable materials. General valves are rarely used.

Asbestos products, asbestos belt, rope, board and asbestos rubber sheet, etc. Among them, the asbestos rubber sheet has a compact structure, good pressure resistance and good 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 include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc.

Rubber products, soft texture, all kinds of rubber have a certain acid, alkali, oil, sea water resistance. Varieties include natural rubber, styrene butadiene rubber, nitrile rubber, chloroprene rubber, isobutyl rubber, polyurethane rubber, fluorine rubber, etc.

Metal materials: Generally speaking, metal materials have high strength and strong temperature resistance. But lead is not so, only take its resistance to dilute sulfuric acid characteristics. 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 sheath (internal asbestos) washers, combined wave washers, wound washers, etc.

Performance of common gasket for static seal

When using the valve, often according to the specific situation, replace the original with gasket. There are often washers: rubber flat washers, rubber O-rings, plastic flat washers, PTFE-wrapped washers, asbestos rubber washers, metal flat washers, metal shaped washers, metal wrapping washers, wave washers, wound washers, etc.

 

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1. Rubber flat washer: easy to deform, effortless when pressed, but poor pressure and temperature resistance, only used for low pressure and low temperature. Natural rubber has a certain acid and alkali resistance, the use temperature should not exceed 60 ℃; chloroprene rubber can also withstand some acid and alkali, the use temperature of 80 ℃; nitrile rubber oil resistance, can be used to 80 ℃; fluorine rubber corrosion resistance is very good, temperature resistance is also stronger than ordinary rubber, can be used in 150 ℃ medium.

2. Rubber O-shaped gasket: The cross-sectional shape is a perfect circle, which has a certain self-tightening effect. The sealing effect is better than that of the flat gasket, and the pressing force is smaller.

3. Plastic flat washers: The biggest feature of plastics is good corrosion resistance, and most plastics have poor temperature resistance. PTFE is the crown of plastic, not only excellent corrosion resistance, but also a wide range of temperature resistance, can be used for a long time at -180 ℃ ~ 200 ℃.

4, PTFE coated gasket: in order to give full play to the advantages of PTFE, while making up for its poor elasticity of the shortcomings, made of PTFE coated rubber or asbestos rubber gasket. In this way, it is not only as corrosion-resistant as PTFE flat washers, but also has good elasticity, enhances the sealing effect and reduces the pressing force. Its cross-sectional shape is shown in Figure 4-20.

5. Asbestos rubber gasket: cut from asbestos rubber sheet. 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, low price. When used, the pressing force does not need to be very large. Because it can adhere to metal, it is best to coat the surface with a layer of graphite powder to avoid the difficulty of disassembly.

Asbestos rubber sheet has four colors:

Gray, for low pressure (brand XB-200, pressure resistance ≤ 16kg/cm2, temperature resistance 200 ℃);

Red, used for medium pressure (brand XB-350, pressure resistance up to 40kg/square centimeter, temperature resistance 350 ℃);

Purple red for high pressure (brand XB-450, pressure 100kg/cm2 temperature 450 ℃);

Green, for oil, pressure resistance is also very good.

6. Metal flat hot ring: lead, temperature resistant 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 64kg/cm2, and other materials can withstand high pressure.

7. Metal heterosexual washer:

Lens gasket: a self-tightening effect, used in high-pressure valves.

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, dovetail, B- shaped, C- shaped, etc., generally only used in high and medium pressure valves.

8, metal wrapping washer: metal has good temperature resistance and pressure resistance, and good elasticity. The wrapping materials are aluminum, copper, low carbon steel, stainless steel, monel alloy and so on. Inside the filling material is asbestos, PTFE, glass fiber and so on.

9. Wave washer: It has the characteristics of small pressing force and good sealing effect. Often in the form of a combination of metal and non-metal.

10, winding gasket: is a very thin metal belt and non-metal belt tightly together, wound into a multi-layer round, the cross section is wavy, has a good elasticity and sealing. The metal belt can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, Cr18Ni9Ti, copper, aluminum, titanium, monel, etc. Non-metallic belt materials are asbestos, PTFE and so on.

Above, when describing the performance of the sealing gasket, some figures are listed. It must be noted that these figures are closely related to the flange form, medium condition and installation and repair technology. Sometimes they can be exceeded and sometimes they cannot be reached. Moreover, the pressure resistance and temperature resistance are also transformed into each other. For example, when the temperature is high, the pressure resistance is often reduced. These subtle problems can only be realized in practice.