Dynamic seal and static seal form of the valve

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How to solve the sealing problem can not be ignored, because the running, running, dripping and leaking of the valve, most of them happen here. Below we will discuss the dynamic and static sealing of the valve.

1. Dynamic seal

The dynamic seal of the valve is mainly referred to as the stem seal. It is the central issue of valve dynamic sealing that does not allow the medium in the valve to leak with the movement of the valve stem.

1) Stuffing box form

At present, the valve is dynamically sealed, mainly based on stuffing box. The basic form of the stuffing box is:

(1) Gland type

This is the most used form.

The same form can be distinguished by many details. For example, from the compression bolts, T-bolts (for pressure & Le; 16 kg / cm 2 low pressure valve), studs and joint bolts can be divided. From the gland, it can be divided into integral and combined.

(2) Compression nut type

This form has a small form factor but is limited in pressing force and is used only for small valves.

2) Filler

In the stuffing box, the filler is in direct contact with the valve stem and filled with a stuffing box to prevent leakage of the medium. The following requirements apply to the filler:

(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, good temperature and corrosion resistance, but when used alone, the sealing effect is not good, so always dipping or attaching other materials. Oil-impregnated asbestos packing: There are two basic structural forms, one is twisting and the other is knitting. It can be divided into circles and squares.

(2) Teflon braided packing: The PTFE strip is woven into a packing, which has excellent corrosion resistance and can be used for deep cooling media.

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

(4) Plastic molding filler: generally made of three pieces, can also be made into other shapes. The plastic used is mostly polytetrafluoroethylene, and nylon 66 and nylon 1010 are also used.

In addition, the use of units often explores a variety of effective filler forms depending on their needs. For example, in a 250 ° C steam valve, asbestos packing and lead ring are alternately stacked, the leakage of steam will be reduced; some valves, the medium often changes, such as the use of asbestos packing and Teflon tape The sealing effect will be better. In order to reduce the friction of the valve stem, in some cases, molybdenum disulfide (M0S2) or other lubricants may be added.

At present, new fillers are being explored. For example, the polyacrylonitrile fiber is impregnated with a polytetrafluoroethylene emulsion, and after pre-oxidation, it is sintered and pressed in a mold to obtain a molding filler having excellent sealing performance; and a stainless steel sheet and asbestos are used to form a wave filler, which can withstand 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 substances have increased, and there are stricter requirements for valve sealing. In some cases, packing seals have not been used, thus creating new ones. Sealed form - bellows seal. This seal does not require a packing, so it is also called a packing-free seal.

Both ends of the bellows are welded to other parts. When the valve stem is lifted and lowered, the bellows expands and contracts, and as long as the bellows itself does not leak, the medium cannot be discharged. For the sake of safety, double seals of bellows and packing are often used.

2, static seal

A static seal is usually a seal between two stationary faces. The sealing method is mainly to use a gasket.

1) Gasket material

(1) Non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc.

Paper, linen, cowhide and the like, with pores, easy to penetrate, must be immersed in oil, wax or other anti-penetration materials. Typical valves are rarely used.

Asbestos products, asbestos belts, ropes, plates and asbestos rubber sheets. Among them, the asbestos rubber sheet has a compact structure, good pressure resistance and good temperature resistance, and is widely used in the valve itself and the flange connection of the valve and the tube.

Plastic products have good corrosion resistance and are commonly used. The varieties are polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010 and the like.

Rubber products, soft texture, all kinds of rubber have certain acid, alkali, oil and sea water resistance. The varieties include natural rubber, styrene butadiene rubber, nitrile rubber, neoprene rubber, butyl rubber, urethane rubber, and fluoro rubber.

(2) Metal materials: Generally speaking, metal materials have high strength and high temperature resistance. But lead is not the case, only take its resistance to dilute sulfuric acid. Commonly used varieties are brass, copper, aluminum, low carbon steel, stainless steel, Monel, silver, nickel and so on.

(3) Composite materials: for example, metal foreskin (internal asbestos) washers, combined wave washers, wound washers, etc.

2) Common gasket performance

When using a valve, the original gasket is often replaced as the case may be. Commonly available gaskets include: rubber flat washers, rubber O-rings, plastic flat washers, Teflon gaskets, asbestos rubber washers, metal flat washers, metal profile washers, metal sheath washers, wave washers, wrap washers, etc.

(1) Rubber flat washer: easy to deform, no effort when pressing, but poor pressure and temperature resistance, only used in places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, the use temperature should not exceed 60 °C; neoprene can also withstand certain acids and alkalis, use temperature 80 ° C; nitrile rubber oil resistant, can be used up to 80 ° C; fluorine rubber corrosion resistance is very good, resistant The temperature performance is also stronger than normal rubber and can be used in 150 ° C medium.

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

(3) Plastic flat washers: The biggest characteristic of plastics is good corrosion resistance, and most plastics have poor temperature resistance. Teflon is the crown of plastic, which not only has excellent corrosion resistance, but also has a wide temperature range and can be used for a long time from -180 °C to +200 °C.

(4) Teflon-coated gasket: In order to give full play to the advantages of PTFE and to make up for its shortcomings of poor elasticity, it is made of a gasket of Teflon-coated rubber or asbestos rubber. In this way, it is corrosion-resistant and has good elasticity as the PTFE flat washer, which 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 composition is 60-80% asbestos and 10-20% rubber, as well as fillers, vulcanizing agents and the like. It has good heat resistance, cold resistance, chemical stability, and is rich in supply and low in price. When used, the pressing force does not have to be large. Since it can adhere to metal, it is best to apply a layer of graphite powder on the surface to avoid laborious disassembly.

Asbestos rubber sheet is available in four colors: grey, for low pressure (brand XB-200, pressure ≤ 16 kg / cm 2 , temperature resistance 200 ° C); red, for medium pressure (brand XB-350, pressure resistant Up to 40 kg / cm 2 , temperature resistance 350 ° C); purple red, for high pressure (brand XB-450, pressure 100 kg / cm 2 temperature resistance 450 ° C); green, for oil, pressure resistance is also very it is good.

(6) Metal flat hot ring: lead, temperature resistance 100 ° C; aluminum 430 ° C; copper 315 ° C; low carbon steel 550 ° C; silver 650 ° C; nickel 810 ° C; Monel (nickel copper) alloy 810 ° C, stainless steel 870 ° C . Among them, lead has poor pressure resistance, aluminum can withstand 64 kg/cm 2 , and other materials can withstand high pressure.

(7) Metallic gaskets:

Lens washer: Self-tightening for high pressure valves.

Oval washer: Also a high pressure self-tightening washer.

Cone double washer: for high pressure internal self-tightening seal.

In addition, there are square, diamond, triangle, tooth, dovetail, B, C, etc., generally only used in high and medium pressure valves.

(8) Metal sheathed gasket: Metal has good temperature and pressure resistance and good elasticity. The sheath material is aluminum, copper, low carbon steel, stainless steel, Monel and the like. The filling materials are asbestos, PTFE, fiberglass and the like.

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

(10) Wound washer: It is a thin metal strip and a non-metal strip that are closely attached together, and are wound into a multi-layered circular shape with a wavy cross section and good elasticity and sealing. The metal strip can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, monel or the like. Non-metallic strip materials are asbestos, polytetrafluoroethylene, and the like.

Above, when describing the performance of the gasket, some numbers are listed. It must be stated that these numbers are closely related to the flange form, the medium condition and the installation and repair technology. Sometimes they can be exceeded, sometimes not reached, and the pressure and temperature resistance are also mutually transformed, for example, the temperature is high, and the resistance is high. The pressure capacity is often reduced, and these subtle problems can only be realized in practice.

3) New materials and technologies

The gaskets described above are still not comprehensive, and sealing technology is rapidly developing. Here are some examples of new materials and technologies.

(1) Liquid sealing: along with polymer organic synthesizers