1. Principle of valve sealing

Sealing is to prevent leakage, so the principle of valve sealing is also studied from leakage prevention. There are two main factors that cause 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 pair. The principle of valve tightness is also analyzed from four aspects: liquid tightness, gas tightness, leakage channel sealing principle and valve sealing pair.

Liquid tightness

The tightness of the liquid is carried out by the viscosity and surface tension of the liquid. When the valve leaking capillary is filled with gas, surface tension may repel the liquid or introduce liquid into the capillary. This forms a tangent angle. When the tangent angle is less than 90°, the liquid will be injected into the capillary, which will cause leakage. Leaks occur because of the different properties of the media. Experiments with different media will yield different results under the same conditions. Water, air or kerosene can be used. When the tangent angle is greater than 90°, leakage will also occur. Because it is related to the grease or wax film on the metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface change, and the liquid that was originally repelled will wet the surface and leak. In view of the above situation, according to the Poisson formula, the purpose of preventing leakage or reducing leakage can be achieved under the condition of reducing the diameter of the capillary and the viscosity of the medium.

 gas tightness

According to Poisson's formula, the tightness of gas is related to the viscosity of gas molecules and gas. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and proportional to the diameter of the capillary and the driving force. When the diameter of the capillary is the same as the mean degree of freedom of the gas molecules, the gas molecules flow into the capillary with free thermal motion. Therefore, when we are doing the valve sealing test, the medium must be watered to play a sealing role, and air, that is, gas, cannot play a sealing role. Even if we reduce the diameter of the capillary below the gas molecules by plastic deformation, the flow of the gas still cannot be stopped. The reason is that the gas can still diffuse through the metal wall. So when we do gas test, we must be more strict than liquid test.

The principle of sealing the leakage channel

The valve seal consists of two parts, the roughness that is scattered on the wave surface and the waviness of the distance between the wave crests. Under the circumstance that most of the metal materials in our country have low elastic strain force, if the sealing state is to be achieved, it is necessary to raise higher requirements for the compressive force of the metal material, that is, the compressive force of the material must exceed its elasticity. Therefore, when designing the valve, the sealing pair is matched with a certain hardness difference, and under the action of pressure, a certain degree of plastic deformation sealing effect will be produced. If the sealing surfaces are all metal materials, the uneven protruding points on the surface will appear first, and only a small load can be used to plastically deform these uneven protruding points at the beginning. When the contact surface increases, the unevenness of the surface becomes plastic-elastic deformation. At this time, the roughness on both sides of the recess will exist. When it is necessary to apply a load that can cause severe plastic deformation of the underlying material, and make the two surfaces in close contact, these remaining paths can be made close along the continuous line and hoop direction.

Valve sealing pair

The valve sealing pair is the part that is closed when the valve seat and the closing member are in contact with each other. During use, the metal sealing surface is easily damaged by intercalated media, media corrosion, wear particles, cavitation and erosion. such as wear particles. If the wear particles are smaller than the unevenness of the surface, the surface accuracy will be improved without deterioration when the sealing surface is run-in. On the contrary, the surface accuracy will be deteriorated. Therefore, when selecting wear particles, factors such as their material, working conditions, lubricity and corrosion to the sealing surface should be comprehensively considered. Just like wear particles, when we choose seals, we must comprehensively consider various factors that affect their performance in order to play the function of preventing leakage. Therefore, materials that are resistant to corrosion, scratches and erosion must be selected. Otherwise, the lack of any one of the requirements will make its sealing performance significantly reduced.

Second, the main factors affecting valve sealing

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

1. Sealing auxiliary structure

Under the change of temperature or sealing force, the structure of the sealing pair will change. Moreover, this change will affect and change the force between the sealing pairs, thereby reducing the performance of the valve sealing. Therefore, when choosing a seal, be sure to choose a seal with elastic deformation. At the same time, pay attention to the width of the sealing surface. The reason is that the contact surfaces of the sealing pair cannot be completely matched. When the width of the sealing surface increases, the force required for the sealing should be increased.

2. Specific pressure of sealing surface

The specific pressure of the sealing surface affects the sealing performance of the valve and the 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 will cause valve leakage. Therefore, we need to fully consider the appropriateness of the specific pressure when designing.

3. Physical properties of the medium

The physical properties of the medium also affect the valve sealing performance. These physical properties include temperature, viscosity, and hydrophilicity of the surface, among others. The temperature change not only affects the slackness of the sealing pair and the change of the size of the parts, but also has an inseparable relationship with the viscosity of the gas. Gas viscosity increases or decreases with increasing or decreasing temperature. Therefore, in order to reduce the influence of temperature on the sealing performance of the valve, when designing the sealing pair, we should design it as a valve with thermal compensation such as an elastic valve seat. Viscosity is related to the permeability of the fluid. When under the same conditions, the greater the viscosity, the less the penetrating ability of the fluid. The hydrophilicity of the surface means that when there is a film on the metal surface, the film should be removed. Because of this very thin oil film, it will destroy the hydrophilicity of the surface, resulting in blockage of fluid channels.

4. The quality of the sealing pair

The quality of the sealing pair mainly refers to the selection, matching and manufacturing accuracy of materials that we need to check. For example, the valve disc fits well with the valve seat sealing surface, which can improve the sealing performance. The characteristic of many circumferential waviness is that its labyrinth sealing performance is good.

Valve leakage is very common in life and production, which can cause waste or bring danger to life, such as leakage of tap water valves, and serious consequences, such as toxic, harmful, flammable, explosive and The leakage of corrosive medium, etc., is an accident that seriously threatens personal safety, property safety and environmental pollution. A valve that relies on external force to rotate and drive to open and close is designed with a sealing device, which is used to install a certain number of packing sealing rings in the packing culvert, so as to achieve the sealing effect, but how is the sealing condition? The leakage at the packing of the valve is one of the most prone to leakage failures in the valve, but there are roughly two reasons for it.

3. Valve sealing form

The seal is also a very critical component in the valve. The sealing performance of the valve refers to the ability of each sealing part of the valve to prevent the leakage of the medium, which 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;

      Where the packing fits with the valve stem and stuffing box;

      The connection between the valve body and the valve cover.

       The leakage in the former part is called internal leakage, which is commonly referred to as lax closure, which will affect the ability of the valve to cut off the medium. For shut-off valves, internal leakage is not allowed. The leakage in the latter two places is called external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve.

External leakage will cause material loss, pollute the environment, and even cause accidents in severe cases. 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 cannot be ignored. The phenomenon of valve running, running, dripping and leaking occurs most of them here. Below we will discuss the dynamic sealing and static sealing of the valve.

Dynamic seal

The valve is dynamically sealed, and the main refers to the valve stem seal. Preventing the medium in the valve from leaking with the movement of the valve stem is the central issue of the dynamic sealing of the valve.

1. Stuffing box form. The dynamic sealing of the valve is mainly based on the stuffing box.

The basic form of stuffing box is:

(1) Gland type: This is the most used form. The unified form can be distinguished by many details. For example, in terms of compression bolts, it can be divided into T-shaped bolts (for low pressure valves with pressure ≤ 16 kg/cm²), stud bolts and joint bolts. As far as the gland is concerned, it can be divided into integral type and combined type.

(2) Compression nut: This type of form has small external dimensions, but the pressing force is limited, and it is only used for small valves.

2. Filling. In the stuffing box, the packing is in direct contact with the valve stem and is covered with the stuffing box to prevent the leakage of the medium. There are the following requirements for packing:

(1) Good sealing performance;

(2) Corrosion resistance;

(3) The friction coefficient is small;

(4) Comply with the medium temperature and pressure.

Commonly used fillers are:

(1) Asbestos packing: Asbestos packing has good temperature and erosion resistance, but when used alone, the sealing effect is not good, and it is always impregnated or added with other materials. Oil-immersed asbestos packing: It has two basic structural forms, one is twisting and the other is knitting. It can be divided into round and square.

(2) PTFE braided packing: PTFE tape is woven into packing, which has excellent corrosion resistance and can be used for cryogenic medium.

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

(4) Plastic molding filler: generally made into three pieces, but also can be made into other shapes. The plastics used are mostly polytetrafluoroethylene.

       In addition, for example, in a 250°C steam valve, asbestos packing and lead rings are alternately stacked to reduce steam leakage; if there is a valve, the medium is often changed, such as the use of asbestos packing and PTFE raw material tape. , the sealing effect is better. In order to reduce the friction on the valve stem, molybdenum disulfide (MOS2) or other lubricants can be added in some places.

Demand for novel fillers is ongoing. For example, polyacrylonitrile fiber is impregnated with polytetrafluoroethylene emulsion, and after pre-oxidation, it is sintered and pressed in a mold to form a filler with excellent sealing performance; another example is a corrugated filler made of stainless steel sheet and asbestos, which can withstand high temperature and high pressure. with erosion.

3. Bellows seal

The chemical industry and the atomic energy industry have grown rapidly, and the flammable, explosive, highly toxic and radioactive materials have increased, and stricter requirements have been placed on valve seals. In some places, packing seals can no longer be used, and a new sealing form - bellows seals has emerged. This type of seal does not require packing, also known as packingless seal.

Both ends of the bellows are welded to other parts. When the valve stem rises and falls, the bellows expands and contracts, the bellows does not leak by itself, and the medium cannot leak out. For the sake of insurance, double sealing of bellows and packing is often used.

static seal

What is a static seal called?

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

1. Gasket material

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

Paper, linen, cowhide, etc. have capillary pores and are easy to penetrate. When using, they must be impregnated with oil, wax or other impermeable materials. General valves are rarely used.

Asbestos products, there are asbestos belts, ropes, boards and asbestos rubber sheets. Among them, the asbestos rubber sheet has a compact structure, good pressure resistance and good temperature resistance. It is extremely widely used in the connection of the valve itself and the valve and the pipe flange.

Plastic products have good corrosion resistance and are widely used. Varieties are polyethylene, polypropylene, soft PVC, PTFE, nylon 66, nylon 1010 and so on.

Rubber products, soft in quality, all kinds of rubber are divided into certain acid resistance, alkali resistance, oil resistance and seawater resistance. Varieties are natural rubber, styrene-butadiene rubber, nitrile rubber, neoprene rubber, isobutylene rubber, polyurethane rubber, fluorine rubber and so on.

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

(3) Composite materials: such as metal sheathed (internal asbestos) gaskets, combined wave gaskets, entangled gaskets, etc.

2. Frequent use of gasket performance

When using the valve, it is often necessary to replace the original gasket according to the specific situation. Common gaskets are: rubber flat washer, rubber O-ring, plastic flat washer, teflon-coated washer, asbestos rubber washer, metal flat washer, metal special-shaped washer, metal sheathed washer, wave washer, entangled gasket, etc.

(1) Rubber flat washer: easy to deform, not difficult to press, but poor pressure and temperature resistance, only used for low pressure and low temperature. Natural rubber has a certain acid and alkali resistance, and the operating temperature should not exceed 60 °C; neoprene rubber can also withstand certain acids and alkalis, and the operating temperature is 80 °C; nitrile rubber is oil-resistant and can be used up to 80 °C; Fluorine rubber has good corrosion resistance and is resistant to The temperature performance is also stronger than that of general rubber, and it can be used in 150 ℃ medium.

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

(3) Plastic flat washer: The biggest feature of plastic is good corrosion resistance, and most plastics have poor temperature resistance. PTFE is the crown of plastics, with excellent corrosion resistance, wide temperature resistance range, and can be used for a long time within -180℃～+200℃.

(4) PTFE-wrapped gasket: fully display the advantages of PTFE, and at the same time make up for its poor elasticity, and make PTFE-wrapped rubber or asbestos rubber gaskets. In this way, it is not only as resistant to erosion as the PTFE flat gasket, but also has excellent elasticity, which enhances the sealing effect and reduces the pressing force.

(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60-80% asbestos and 10-20% rubber, and fillers, vulcanizing agents, etc. It has good heat resistance, cold resistance, chemical stability, abundant supply and low price. When using, the pressing force does not need to be very large. It can adhere to metal, and it is best to apply a layer of graphite powder on the surface to avoid strenuous disassembly.

(6) Metal flat heating ring: lead, temperature resistance 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 64 kg/square centimeter, and other materials can withstand high pressure.

(7) Metal heterosexual gasket:

Lens washer: It has self-tightening effect and is used for high pressure valves.

Oval washer: It is also a high-pressure self-tightening washer.

Conical double gasket: used for high pressure internal self-care sealing.

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

(8) Metal-clad gasket: Metal has both excellent temperature and pressure resistance and excellent elasticity. Foreskin materials include aluminum, copper, low carbon steel, stainless steel, Monel alloy, etc. The filling materials are 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 metals and non-metals.

(10) Surrounding entanglement washer: It is a thin metal belt and a non-metallic belt that are closely attached together, and are entangled into a multi-layer circle, with a wave-shaped section, which has good elasticity and tightness. Metal strips can be constructed of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel, etc. Non-metallic belt materials include asbestos, polytetrafluoroethylene, etc.

Above, when describing the performance of sealing gaskets, some numbers are listed. It must be noted that these numbers are closely related to the flange form, medium conditions and installation and repair technology, etc., which can be crossed, not reached, and the pressure and temperature resistance performance are also mutually transformed. For example, when the temperature is high, the pressure resistance capacity tends to decrease. , These subtle issues can only be experienced in practice.

3. Valve sealing material

The valve sealing material is an important part of the valve sealing, which acts as the direct contact surface of the valve sealing. What are the valve sealing materials? We know that the valve sealing ring materials are divided into two categories: metal and non-metal.

The valve sealing material is an important part of the valve sealing, which acts as the direct contact surface of the valve sealing. What are the valve sealing materials? We know that the valve sealing ring materials are divided into two categories: metal and non-metal. The following is a brief introduction to the use conditions 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 better than those of natural rubber. Generally, the use temperature of synthetic rubber is t≤150℃, and the temperature of natural rubber is t≤60℃.

2. Nylon

Nylon has the characteristics of small friction coefficient and good corrosion resistance. Nylon is mostly used for ball valves and globe valves with temperature t≤90℃ and nominal pressure PN≤32MPa.

3. PTFE

PTFE is mostly used for globe valves, gate valves, ball valves, etc. with temperature t≤232℃ and nominal pressure PN≤6.4MPa.

4. Cast iron

Cast iron is used for gate valve, globe valve, plug valve, etc. for temperature t≤100℃, nominal pressure PN≤1.6MPa, gas and oil.

5. Babbitt alloy

Babbitt alloy is used for ammonia globe valve with temperature t-70~150℃ and nominal pressure PN≤2.5MPa.

6. Copper alloy

Common materials for copper alloys are 6-6-3 tin bronze and 58-2-2 manganese brass. Copper alloy has good wear resistance and is suitable for water and steam with temperature t≤200℃ and nominal pressure PN≤1.6MPa. It is often used in gate valves, globe valves, check valves, plug valves, etc.

7. Chrome stainless steel

The commonly used grades of chromium stainless steel are 2Cr13 and 3Cr13, which have been quenched and tempered, and have good corrosion resistance. It is commonly used in valves for media such as water, steam and petroleum with temperature t≤450℃ and nominal pressure PN≤32MPa.

8. Chromium-nickel-titanium stainless steel

The commonly used grade of chromium-nickel-titanium stainless steel 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℃ and nominal pressure PN≤6.4MPa, used for globe valve, ball valve, etc.

9. Nitrided steel

The commonly used grade of nitrided steel is 38CrMoAlA. After carburizing, it has good corrosion resistance and scratch resistance. Commonly used in power station gate valve with temperature t≤540℃ and nominal pressure PN≤10MPa.

10. Boronizing

Boronizing directly processes the sealing surface from the material of the valve body or disc body, and then carries out boronizing surface treatment, the sealing surface has good wear resistance. For power station blowdown valve