Vavle

- Oct 10, 2019-

Valve sealing principle

Sealing is to prevent leakage, so the principle of valve sealing is also studied

from leak prevention. There are two main factors that caused 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 sealing is also analyzed

from four aspects: liquid sealing, gas sealing, leakage channel sealing

principle and valve sealing pair.

The sealing of liquid

The sealing of the liquid is carried out by the viscosity and surface tension of

the liquid. When the leaking capillary of the valve is filled with gas, the

surface tension may repel the liquid or introduce the liquid into the capillary.

This creates a tangency angle. When the tangency angle is less than 90°, the

liquid is injected into the capillary, which causes leakage. The reason for the

leakage is the different nature of the medium. Experimenting with different

media will give different results under the same conditions.

It can be used with water, with air or with kerosene. When the tangency angle is

greater than 90°, leakage will also occur. Because it is related to grease or

waxy film on the metal surface.

Butterfly valve

Once the film on these surfaces is dissolved, the properties of the metal

surface changed, and the originally repelled liquid will invade the surface and

leak. In view of the above, according to the Poisson formula, it is possible to

achieve the purpose of preventing leakage or reducing the amount of leakage

while reducing the capillary diameter and the viscosity of the medium.

Gas tightness

According to the Poisson formula, the gas tightness is related to the viscosity

of gas molecules and gases. The leakage is inversely proportional to the length

of the capillary and the viscosity of the gas, and is 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 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 used for water to seal, and the air, that is, the gas, cannot function

as a seal.

Even if we reduce the capillary diameter below the gas molecules by plastic

deformation, it still cannot stop the flow of gas. The reason is that the gas

can still diffuse through the metal wall. Therefore, when we do gas test, we

must be stricter than liquid test.

Sealing principle of leakage channel

The valve seal consists of two parts: the unevenness spread on the wave surface

and the waviness between the peaks. In the case that most of the metal materials

in our country have low elastic strain, if the sealing state is to be achieved,

the compressive force of the metal material needs to be higher, that is, the

compressive force of the material exceeds its elasticity.

Therefore, when designing the valve, the sealing pair is combined with a certain

hardness difference to match, and under the action of pressure, a certain degree

of plastic deformation sealing effect is produced.

If the sealing surfaces are all metallic materials, the uneven spots on the

surface will appear at the earliest, and the uneven protruding points can be

plastically deformed at the beginning with a small load. When the contact

surface is increased, the unevenness of the surface becomes plastic-elastic

deformation. At this time, the roughness on both sides of the recess will exist.

When a load is required to cause severe plastic deformation of the underlying

material, and the two surfaces are in close contact, the remaining paths can be

brought into close contact along the continuous line and the circumferential

direction.

Valve seal

The valve seal pair is the portion of the valve seat and closure that closes

when in contact with each other. Metal sealing surfaces are susceptible to

damage from entrainment of media, media corrosion, wear particles, cavitation

and erosion during use. Such as wear particles.

If the wear particles are smaller than the unevenness of the surface, the

surface accuracy is improved when the sealing surface is worn-in, without

deterioration. On the contrary, the surface accuracy is deteriorated. Therefore,

when selecting wear particles, factors such as material, working condition,

lubricity and corrosion to the sealing surface should be considered.

Just like wear particles, when choosing a seal, we must consider various factors

that affect its performance to prevent leaks. Therefore, materials that are

resistant to corrosion, scratching and erosion must be selected. Otherwise, the

lack of any requirement will greatly reduce the sealing performance.

Main factors affecting valve sealing

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

Sealed secondary structure

The structure of the seal pair changes under the influence of temperature or

sealing force. Moreover, this change affects and changes the force between the

seal pairs, thereby reducing the performance of the valve seal.

Therefore, when selecting a seal, it is necessary to select a seal with elastic

deformation. Also, pay attention to the width of the sealing surface. The reason

is that the contact surface of the sealing pair cannot be completely matched.

When the width of the sealing surface is increased, the force required for

sealing is increased.

Sealing surface pressure

The specific pressure of the sealing surface affects the sealing performance of

the valve and the service life of the valve. Therefore, the sealing surface

specific pressure is also a very important factor. Under the same conditions,

the specific pressure will cause damage to the valve, but the pressure will be

too small to cause the valve to leak. Therefore, we need to fully consider the

appropriateness of the specific pressure in the design.

Physical properties of the medium

The physical properties of the media also affect valve sealing performance.

These physical properties include temperature, viscosity, and hydrophilicity of

the surface. Temperature changes not only affect the slack of the seal pair and

the dimensional change of the part, but also have an inseparable relationship

with the viscosity of the gas. The viscosity of the gas increases or decreases

following the temperature increases or decreases.

Therefore, in order to reduce the degree of influence of temperature on the

sealing performance of the valve, we design it as a thermally compensated valve

such as an elastic valve seat when designing the sealing pair. Viscosity is

related to the ability of the fluid to penetrate.

When under the same conditions, the viscosity greater, the ability of the fluid

to penetrate smaller. The hydrophilicity of the surface means that the film is

removed when there is a film on the metal surface. Because of this very thin oil

film, it will damage the hydrophilicity of the surface, leading to the passage

of fluids.

stainless steel Check valve

Quality of the seal pair

The quality of the seal pair mainly means that we have to check the selection,

matching and manufacturing precision of the materials. For example, the valve

flap fits well with the seat sealing surface to improve sealing. The

characteristic of the circumferential waviness is that the labyrinth sealing

performance is good.

Valve leakage is very common in life and production. It can cause waste or bring

danger to life. For example, the leakage of tap water valves can lead to serious

consequences, such as toxic, harmful, flammable and explosive in the chemical

industry. Corrosive dielectric leakage, etc., seriously threaten personal

safety, property safety and environmental pollution accidents.

A valve that relies on external force rotary drive to open and close is designed

with a sealing device that uses a certain number of packing seals in the packing

culvert to achieve the sealing effect, but what about the sealing situation?

Leakage at the packing of the valve is one of the most prone to leakage faults

in the valve, but there are two reasons for this.

Valve seal form

Seals are also a critical component in valves. The sealing performance of the

valve refers to the ability of the sealing parts of the valve to prevent the

leakage of the medium. It is the most important technical performance index of

the valve.

longzhu-casting-finshed-parts-storage

There are three sealing parts of the valve:

1.The contact between the sealing surface of the opening and closing member and

the valve seat; 2. the joint of the packing with the valve stem and the stuffing

box; 3.the connection between the valve body and the valve cover. The leak in

the former place is called endoleak, which is commonly referred to as the lack

of tightness, which will affect the ability of the valve to cut off the medium.

For shut-off valves, internal leakage is not allowed. The latter two leaks are

called external leaks, ie the medium leaks from the valve to the outside of the

valve.

External leakage can cause material loss, pollute the environment, and cause

accidents in severe cases. For flammable, explosive, toxic or radioactive media,

external leakage is not allowed, so the valve must have a reliable sealing

performance.

How to solve the sealing problem is not negligent, the valve runs, runs, drops,

leaks, the department occurs here. Below we will consider the problem of valve

dynamic sealing and static sealing.

Dynamic seal

The valve is dynamically sealed and the main finger is sealed. It does not allow

the medium in the valve to leak with the movement of the valve stem, which is

the center of the valve dynamic seal.

Stuffing box form: valve dynamic seal, mainly stuffing box. The basic form of

the stuffing box is:

1. Gland type: This is the most used form.

The unified form can be distinguished by many details. For example, from the

compression bolts, T-bolts (for low pressure valves with pressure ≤ 16 kg / cm

2 ), studs and joint bolts can be divided. From the gland, it can be divided

into integral and combined.

2, compression nut type: This type, small size, but the compression force is

limited, only used in small valves.

Packing: In the stuffing box, the packing is directly in contact with the valve

stem and filled with a stuffing box to prevent leakage of the medium. The

following requirements apply to the packing:

• Good sealing;

• resistant to erosion;

• The friction coefficient is small;

• Comply with media temperature and pressure.

Bellows seal:

The chemical industry and the atomic energy industry have grown rapidly,

flammable, explosive, highly toxic, and with radioactive materials. There are

more stringent requirements for valve sealing. Some places have no way to use

packing seals, and new sealing forms have emerged - bellows seals. This type of

seal does not require a packing, which 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, the bellows does not leak

by itself, and the medium cannot escape. For the sake of safety, a double seal

of bellows and packing is often used.

Static seal

What is a static seal? A static seal is usually a seal between two stationary

faces. The sealing method is mainly to use a gasket.

Always use gasket performance:

When using a valve, it is often the case that the original gasket is replaced.

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 sheathed washers, wave washers, wraparound entangled

washers, etc.

1. Rubber flat washer: It is easy to deform, not difficult when pressed, but the

pressure resistance and temperature resistance are poor. It is only used for low

pressure and low temperature. Natural rubber has certain acid and alkali

resistance, the use temperature should not exceed 60 °C; neoprene can also

resist certain acids and alkalis, use temperature 80 ° C; nitrile rubber

resistant to oil, can be used up to 80 ° C; fluororubber 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, there

is a certain self-tightening effect, the sealing effect is better than the flat

washer, and the pressing force is smaller.

3, plastic flat washer: the most important feature of plastic is the good

corrosion resistance; the plastic temperature resistance of the department is

not good. Teflon is the crown of plastic, excellent in corrosion resistance,

wide temperature range, and durable for use within -180 ° C ~ +200 ° C.

4, Teflon package gasket: enrich the advantages of PTFE, while making up for its

poor flexibility and error, made of PTFE wrapped rubber or asbestos rubber

gasket. In this way, it is as resistant to erosion as Teflon flat washers, and

has excellent elasticity, which enhances the sealing effect and reduces the

pressing force.

carbon steel Gate valve

5. Asbestos rubber gasket: cut from asbestos rubber sheet. Its composition is

60-80% asbestos and 10-20% rubber, and fillers, vulcanizing agents and the like.

It has good heat resistance, cold resistance, chemical stability, abundant

supply and low price. When used, the pressing force does not need to be very

large. It can adhere to the metal, and it is best to apply a layer of graphite

powder in order to avoid the trouble of disassembly.

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, metal isotropic gasket:

• Lens washer: Self-tightening and used for high pressure valves.

• Oval washer: Also belong to high pressure self-tightening washer.

• Cone double washer: Used for self-care sealing inside high pressure.

In addition, there are square, diamond, triangle, tooth, dovetail, B, C, etc.,

generally only used in high and medium pressure valves.

8. Metal foreskin gasket: Metal has excellent temperature and pressure

resistance and excellent 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. Metal and non-metal combinations are often used.

10. Surrounding entangled washer: It is a thin metal strip and a non-metallic

strip that are closely attached together, and are surrounded by entangled into a

multi-layered circular shape. The cross section is wave-like and has good

elasticity and sealing. The metal strip can be constructed 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 figures are closely related to the flange form, the

medium condition and the installation and repair technology. They can be

crossed, not reachable, withstand voltage and temperature resistance, and are

also mutually transformed. For example, the temperature is high and the pressure

resistance is often reduced. These subtle problems can only be perceived in

practice.

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