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Valve Sealing
Mar 19, 2019

Valve Sealing

For the requirement of valve sealing performance, it is necessary to prevent leakage. According to the different parts and degrees of leakage, the leakage situation of the valve is different. Therefore, different leakage prevention measures need to be put forward.

 

1 Valve Sealing Principle

Sealing is to prevent leakage, so the principle of valve sealing is also studied from the prevention of leakage. There are two main factors causing leakage, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pairs, and the other is the pressure difference between the two sides of the sealing pairs. Valve sealing principle is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pairs.

 

Sealability of liquids

The sealing property of liquids is achieved by the viscosity and surface tension of liquids. When the capillary leaking from the valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. So the tangent angle is formed. When the tangent angle is less than 90 degrees, the liquid will be injected into the capillary tube, which will lead to leakage. Leakage occurs because of the different properties of the medium. Tests with different media will yield different results under the same conditions. You can use water, air or kerosene. Leakage also occurs when the tangent angle is greater than 90 degrees. Because it is related to grease or waxy film on metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface change. The previously excluded liquid will invade the wet surface and leak. In view of the above situation, according to Poisson's formula, the purpose of preventing leakage or reducing leakage can be achieved by reducing capillary diameter and medium viscosity.

 

Sealability of gases

According to Poisson's formula, gas sealing is related to gas molecule and gas viscosity. The leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and directly proportional to the diameter and driving force of the capillary. When the diameter of the capillary is the same as the average degree of freedom of the gas molecule, the gas molecule will flow into the capillary with free thermal motion. Therefore, when we do the valve sealing test, the medium must use water to play the role of sealing, using air or gas can not play the role of sealing. Even if we reduce the diameter of the capillary to below the gas molecule by plastic deformation, the flow of the gas can not be stopped. The reason is that gases can still diffuse through metal walls. So when we do the gas test, we must be more rigorous than the liquid test.

 

Sealing Principle of Leakage Channel

The valve seal consists of two parts: the irregularity scattered on the waveform surface and the waviness of the distance between peaks. Under the condition that the elastic strain force of most metal materials is low, if we want to achieve the sealed state, we need to put forward higher requirements for the compressive force of metal materials, that is, the compressive force of materials should exceed its elasticity. Therefore, when designing the valve, the sealing pair is matched with a certain hardness difference. Under the action of pressure, it will produce a certain degree of plastic deformation sealing effect. If all the sealing surfaces are metal materials, the uneven protrusions will appear earliest, and plastic deformation of these uneven protrusions can be produced initially with only a small load. When the contact surface increases, the surface roughness will become plastic-elastic deformation. At this time, the roughness of both sides in the concave will exist. It is necessary to apply a load which can cause serious plastic deformation of the underlying material and make the two surfaces contact closely. Only along the continuous line and circumferential direction can these remaining paths be closed.

 

Valve Sealing Pair

Valve seal pair is the part of the seat and closure that closes in contact with each other. The metal sealing surface is vulnerable to the damage of clamping medium, medium corrosion, wear particles, cavitation and erosion in the use process. For example, wear particles. If the unevenness of wear particles is smaller than that of the surface, the surface accuracy will be improved without deterioration when the sealing surface runs in. On the contrary, the surface accuracy will be deteriorated. Therefore, when choosing wear particles, the material, working condition, lubricity and corrosion of sealing surface should be considered comprehensively. Like wear particles, when we choose seals, we should consider all kinds of factors affecting their performance comprehensively, so as to play a leak-proof function. Therefore, it is necessary to select materials that are corrosion resistant, scratch resistant and erosion resistant. Otherwise, without any requirement, the sealing performance will be greatly reduced.

 

2 Main factors affecting valve seal

There are many factors affecting valve sealing, mainly the following:

1.Sealing pair structure

Under the change of temperature or sealing force, the structure of the sealing pair will change. And this change will affect and change the interaction between the sealing pairs, so that the performance of the valve seal is reduced. Therefore, when choosing seals, it is necessary to choose seals with elastic deformation. At the same time, attention should be paid to the width of the sealing surface. The reason is that the contact surface of the sealing pair can not match completely. When the width of the sealing surface increases, the force needed for sealing should be increased.

2.Specific pressure of sealing surface

The specific pressure of the sealing surface affects the sealing performance and service life of the valve. Therefore, the specific pressure of the sealing surface is also a very important factor. Under the same conditions, too high specific pressure will cause valve damage, but too low specific pressure wine will cause valve leakage. Therefore, we need to fully consider the appropriate specific pressure in the design.

3.Physical Properties of Medium

The physical properties of the medium also affect the sealing performance of the valve. These physical properties include temperature, viscosity and surface hydrophilicity. Temperature change not only affects the relaxation of the seal pair and the change of the size of the parts, but also has an inseparable relationship with the viscosity of the gas. The gas viscosity increases or decreases with the increase or decrease of temperature. Therefore, in order to reduce the influence of temperature on the sealing performance of the valve, we should design the sealing pair as an elastic valve seat with thermal compensation. Viscosity is related to the permeability of the fluid. Under the same conditions, the greater the viscosity, the smaller the permeability of the fluid. Surface hydrophilicity refers to the removal of a thin film on a metal surface. Because of the thin oil film, the hydrophilicity of the surface will be destroyed, resulting in blocking the passage of the fluid.

4.Quality of Sealing Pair

Seal pair quality mainly refers to the selection of materials, matching, manufacturing accuracy checks. For example, the disc and seat seals are in good agreement, which can improve the sealability. The characteristic of more circumferential waviness is its good labyrinth sealing performance.

Valve leakage is very common in life and production, which can cause waste or bring danger to life, such as tap water valve leakage, which can lead to serious consequences, such as toxic, harmful, flammable, explosive and corrosive intermediary leakage in chemical industry, and serious accidents that threaten personal safety, property safety and environmental pollution. A valve opened and closed by external rotating transmission has a sealing device which is used to install a certain number of packing rings in the packing culvert to achieve sealing effect, but how about the sealing condition? The leakage of the filler of the valve is one of the most vulnerable parts of the valve, but there are roughly two reasons.

 

3 Valve Sealing Form

Seals are also key components in valves. The sealing performance of the valve refers to the ability of the sealing parts of the valve to prevent medium leakage. It is the most important technical performance index of the valve.

There are three sealing parts of the valve:

The contact between the opening and closing parts and the two sealing surfaces of the valve seat; the fit between the filler and the stem and the packing box; and the connection between the valve body and the valve cover. The former leakage is called internal leakage, which is commonly referred to as lax closure. It will affect the ability of the valve to cut off the medium. Internal leakage is not allowed for truncation valves. Leakage at the last two places is called leakage, i.e. medium leaks from inside to outside of the valve.

Leakage can cause material loss, pollution of the environment, and serious accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.

How to solve the sealing problem is not negligent, valve running, rises, drips, leaks, as many as occur here. Next we will discuss the dynamic and static sealing of the valve.

 

1.Dynamic seal

Valve dynamic seal, mainly refers to stem seal. Keeping the medium inside the valve from leaking with the movement of the valve stem is the central subject of the dynamic seal of the valve.

1) Packing letter form

Valve dynamic seal, mainly packing box. The basic form of stuffing letter is:

gland type

This is in the most form.

The unified form can distinguish many details. For example, in terms of compression bolts, they can be divided into T-shaped bolts (for low-pressure valves with pressure less than 16 kg/square centimeter), double-ended bolts and live bolts, etc. As far as the cover is concerned, it can be divided into integral type and combined type.

Compression nut type

This type of valve has a small size, but its compression force is limited, so it can only be used in small valves.

2) packing

In the packing box, the packing is directly contacted with the valve stem and filled with the packing box to prevent the leakage of the medium. The following requirements are required for fillers:

(1) Good sealing;

(2) corrosion resistance;

(3) Small friction coefficient;

(4) Compliance with medium temperature and pressure.

Frequently used fillers are:

(1) Asbestos packing: Asbestos packing has good heat resistance and corrosion resistance, but when used in small bits, the sealing effect is unsafe and 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: The braided PTFE ribbon has excellent corrosion resistance and can be used in cryogenic media.

(3) Rubber O-ring: Under low pressure, the sealing effect is excellent. 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) is the most commonly used plastics.

In addition, for example, in 250 C steam valves, when asbestos pans and lead rings are alternately overlapped, the leakage will be reduced; with valves, the medium is often changed. If asbestos pans and PTFE raw material belt are used together, the sealing effect will be better. Molybdenum disulfide (M0S2) or other lubricants can be added in place to reduce friction on the stem.

New fillers are being sought. For example, polyacrylonitrile fibers are impregnated with polytetrafluoroethylene emulsion, and then pre oxidized, and sintered and pressed in the mold, so that the packing material can be formed with good sealing performance. If the corrugated packing is made of stainless steel sheet and asbestos, it can withstand high temperature, high pressure and erosion.

3) bellows seal

With the rapid growth of chemical industry and atomic energy industry and the increase of inflammable, explosive, highly toxic and radioactive materials, more stringent requirements have been placed on valve sealing. In some places, filler sealing has been unable to be used, and a new sealing form, bellows sealing, has taken place. This kind of seal does not need filler, also known as non-filler seal.

Both ends of bellows are welded to other parts. When the valve stem rises and falls, the bellows contract and the bellows do not leak, so the medium can not leak out. For the sake of safety, double seals of bellows and fillers are often used.

 

Static seal

What is static seal?

Static seal usually refers to the seal between two stationary surfaces. The main sealing method is to use gaskets.

1) gasket 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 the connection between valve itself and flange of 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 quality, all kinds of Rubber Division has a certain acid, alkali, oil, sea water resistance. Varieties include natural rubber, styrene-butadiene rubber, nitrile-butadiene rubber, chloroprene rubber, isobutadiene rubber, polyurethane rubber, fluorine rubber, etc.

(2) Metal materials: Generally speaking, metal materials have high strength and high temperature resistance. But lead is actually not like this, just because it is resistant to dilute sulfuric acid. 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 wrapped (internal asbestos) gaskets, combined waveform gaskets, wrapped gaskets, etc.

2) Frequently used gasket performance

When using valves, the original gasket is often replaced in specific circumstances. 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, wrapped gasket and so on.

(1) Rubber flat gasket: easy to deform, not hard 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, the use temperature is 80 C; butadiene nitrile rubber oil resistance, can be used to 80 C; fluorine rubber has good corrosion resistance, temperature resistance is better than ordinary rubber, can be used in 150 C medium.

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

(3) Plastic flat gasket: The most important characteristic of plastics is good corrosion resistance, and most plastics have poor temperature resistance. Polytetrafluoroethylene (PTFE) is the crown of plastics. It has excellent corrosion resistance and wide temperature range. It can be used for a long time in the range of - 180 ~200 ~C.

(4) Polytetrafluoroethylene (PTFE) wrapped gaskets: to enrich and elaborate the advantages of PTFE, while making up for its poor elasticity error, make PTFE wrapped rubber or asbestos rubber gaskets. In this way, it not only has the same corrosion resistance as the PTFE flat gasket, but also has excellent elasticity, enhances the sealing effect and reduces the compressive force.

(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60-80% asbestos and 10-20% rubber, as well as fillers and vulcanizers. It has good heat resistance, cold resistance, chemical stability, abundant supply, low price. When used, the compression force does not need to be very large. It can adhere to metal, preferably coated with graphite powder, so as to avoid the difficulty of disassembly.

(6) Metal flat-heat ring: lead, temperature resistance 100 C; aluminium 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 square, and other materials can withstand high pressure.

(7) Metal heterosexual gaskets:

Lens washer: self-tightening, used in high-pressure valves.

Elliptical washer: also belongs to high-pressure self-tightening washer.

Conical double gaskets: Used for self-cautious sealing in high pressure.

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

(8) Metal wrapping gasket: Metal has excellent temperature and pressure resistance, as well as excellent elasticity. The wrapping materials are aluminium, copper, low carbon steel, stainless steel, Monel alloy, etc. It is filled with asbestos, polytetrafluoroethylene, glass fiber and so on.

(9) Waveform gasket: It has the characteristics of small compaction force and good sealing effect. The combination of metal and non-metal is often used.

(10) Entangled gasket: It is a very thin metal belt and non-metal belt closely together, wrapped around a multi-layer circular, wavy cross-section, has good elasticity and sealing. The metal strips can be constructed with 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminium, titanium and Monel alloys. Non-metallic tape materials include asbestos, polytetrafluoroethylene and so on.

Above, some figures are listed when describing the performance of sealing gaskets. It must be pointed out that these figures are closely related to flange form, medium condition and installation technology. They can be leapfrogging, unreachable, pressure and temperature resistance, and also can be transformed into each other. For example, when the temperature is high, the pressure resistance often decreases. These subtle problems can only be appreciated in practice.

 

4 Valve Sealing Material

Valve sealing material is an important part of valve sealing, which acts as the direct contact surface of valve sealing. What are the sealing materials for valves? We know that there are two kinds of sealing ring materials for valves: metal and non-metal.

Valve sealing material is an important part of valve sealing, which acts as the direct contact surface of valve sealing. What are the sealing materials for valves? We know that there are two kinds of sealing ring materials for valves: metal and non-metal. Here is a brief introduction to the use of various sealing materials, as well as commonly used valve types.

1.synthetic rubber

The comprehensive properties of synthetic rubber, such as oil resistance, temperature resistance and corrosion resistance, are superior to those of natural rubber. Generally, the use temperature of synthetic rubber is less than 150 C, and that of natural rubber is less than 60 C. Rubber is used for sealing valves such as globe valves, gate valves, diaphragm valves, butterfly valves, check valves and pinch valves with nominal pressure PN less than 1 MPa.

2.nylon

Nylon has the characteristics of low friction coefficient and good corrosion resistance. Nylon is mostly used in ball valves and globe valves with temperature T < 90 C, nominal pressure PN < 32 MPa, etc.

3.Teflon

Polytetrafluoroethylene (PTFE) is mostly used in globe valves, gate valves and ball valves with temperature T < 232 C and nominal pressure PN < 6.4 MPa.

4.Cast iron

Cast iron is used for gate valves, globe valves, plug valves for gas and oil, etc. with temperature T < 100 C, nominal pressure PN < 1.6MPa.

5.Babbitt alloy

Babbitt alloy is used for ammonia shutoff valves with temperature t-70~150 C and nominal pressure PN < 2.5 MPa.

6.Copper alloy

The common materials of copper alloys are 6-6-3 tin bronze and 58-2-2 manganese brass. Copper alloy has good wear resistance. It is suitable for water and steam with temperature T < 200 C and nominal pressure PN < 1.6 MPa. It is often used in gate valves, globe valves, check valves, cock valves, etc.

7.Chromium stainless steel

Commonly used grades of chromium stainless steel are 2Cr13 and 3Cr13, which have good corrosion resistance after quenching and tempering. Usually used in water, steam and petroleum valves with temperature T < 450 C and nominal pressure PN < 32MPa.

8.Chromium-nickel-titanium stainless steel

Chromium-nickel-titanium stainless steel commonly used brand is 1Cr18Ni9ti, which has good corrosion resistance, erosion resistance and heat resistance. It is suitable for steam, nitric acid and other media with temperature T < 600 C and nominal pressure PN < 6.4 MPa. It is used for globe valves, ball valves, etc.

9.Nitriding steel

The common brand of nitriding steel is 38CrMoAlA, which has good corrosion resistance and abrasion resistance after carburizing treatment. It is commonly used in power station gate valves with temperature T < 540 C and nominal pressure PN < 10MPa.

10.Boronizing

Boronizing is directly processed from valve body or disc body material, and then boronized surface treatment. The sealing surface has good wear resistance. Used for sewage valve of power station.


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