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.
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.
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
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
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
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
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
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.
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.
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.
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
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
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