Common sense and working principle of mechanical seal


1. Common sense of mechanical seal The working principle of mechanical seal

Mechanical seal is a shaft seal device that achieves leakage resistance by relying on a pair or several pairs of relatively sliding end faces perpendicular to the shaft to maintain fit and match with auxiliary seals under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism.

2. Selection of common materials for mechanical seals

Clean water; Normal temperature; (dynamic) 9CR18,1CR13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin graphite, bronze, phenolic plastic.

River water (including sediment); normal temperature; (dynamic) tungsten carbide, (static) tungsten carbide

Seawater; Normal temperature; (dynamic) tungsten carbide, 1CR13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin graphite, tungsten carbide, cermet;

100 degrees of overheated water; (dynamic) tungsten carbide, 1CR13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin graphite, tungsten carbide, cermet;

Gasoline, lubricating oil, liquid hydrocarbon; Normal temperature; (dynamic) tungsten carbide, 1CR13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin or tin antimony alloy graphite, phenolic plastics.

Gasoline, lubricating oil, liquid hydrocarbon; 100 degrees; (dynamic) tungsten carbide, 1CR13 surfacing cobalt chromium tungsten; (static) impregnated bronze or resin graphite.

Gasoline, lubricating oil, liquid hydrocarbon; containing particles; (dynamic) tungsten carbide; (static) tungsten carbide.

3. Types and uses of sealing materials

Sealing materials should meet the requirements of the sealing function. Because the sealed medium is different, and the working premise of the equipment is different, the sealing material is required to have different adaptability. The requirements for sealing materials are generally:

1) The material has good compactness and is not easy to leak the medium;

2) have appropriate mechanical strength and hardness;

3) good compression and resilience, small permanent deformation;

4) No softening or decomposition at high temperature, no hardening or embrittlement at low temperature;

5) Good anti-erosion function, can work for a long time in acid, alkali, oil and other media, its volume and hardness change is small, and it does not adhere to the metal surface;

6) small friction coefficient, good wear resistance;

7) It has the flexibility to combine with the sealing surface;

8) Good aging resistance and durability;

9) processing and manufacturing convenience, cheap, easy to get.

Rubber is the most commonly used sealing material. In addition to rubber, it is suitable for sealing materials such as graphite, PTFE and various sealants.

4. Technical essentials of mechanical seal installation and use

1) The radial runout of the rotating shaft of the equipment shall be ≤ 0.04mm, and the axial movement shall not be greater than 0.1mm;

2) The sealing part of the equipment should be kept clean during installation, the sealing parts should be cleaned, and the sealing end face should be intact to prevent impurities and dust from being brought into the sealing part;

3), in the installation process is strictly prohibited, knock, so as to avoid the mechanical seal friction to pay damage and seal failure;

4) When installing, the surface in contact with the seal should be coated with a layer of clean mechanical oil so that it can be installed smoothly;

5) When installing the static ring gland, the force must be averaged when tightening the screw to ensure the vertical requirements of the static ring end face and the axis line;

6) After installation, the moving ring is pushed by hand, which can make the moving ring move flexibly on the shaft and have certain elasticity;

7) After installation, turn the rotating shaft by hand, and the rotating shaft should feel insignificant;

8) The equipment must be covered with media before operation to prevent dry friction and seal failure;

9), easy to crystallize, granular media, the medium temperature "80OC, should take the corresponding flushing, filtering, cooling measures, various auxiliary devices, please refer to the mechanical seal of the relevant scale.

10), installation in contact with the seal surface should be coated with a layer of clean mechanical oil, pay special attention to the choice of mechanical oil for different auxiliary sealing material, avoid causing O-ring oil invasion expansion or accelerated aging, resulting in early seal failure.

5. What are the three sealing points of the mechanical shaft seal and the sealing principle of these three sealing points

The seal between the moving ring and the static ring is to generate an appropriate pressing force (specific pressure) on the contact surface (end face) of the moving ring and the static ring by the elastic element (spring, bellows, etc.) and the sealing liquid pressure to make the two smooth and straight end faces fit closely. A very thin liquid film is maintained between the end faces to achieve the sealing effect. This film has hydrodynamic pressure and static pressure, which plays the role of balancing pressure and lubricating the end face. The reason why the two end faces must be highly smooth and straight is to create a perfect fit for the end faces and the premise of averaging the specific pressure. This is a relative rotating seal.

6. Common sense of mechanical seal Types of mechanical seal technology

At present, the new technology of various mechanical seals using new materials and processes has made rapid progress, and there are the following new mechanical seals. Sealing surface slotting sealing technology In recent years, a variety of flow slots have been opened on the sealing end face of the mechanical seal to produce hydrostatic and dynamic pressure effects, which are still being updated. Zero leakage sealing technology in the past always thought that contact and non-contact mechanical seals could not achieve zero leakage (or no leakage). A new concept of zero-leakage non-contact mechanical end-face seal is proposed in Israel using slotted seal technology and has been used in lubricating oil pumps for nuclear power plants. Dry running gas seal technology This type of seal is the slotted seal technology for gas sealing. Upstream pumping sealing technology is the use of open flow grooves on the sealing surface to pump a small amount of leakage fluid downstream back to the upstream. The structural characteristics of the above types of seals are: the use of shallow grooves, and the film thickness and the depth of the flow groove are all micron-level, and the use of lubrication grooves, radial sealing dams and circumferential sealing weirs to form sealing and bearing departments. It can also be said that the slotted seal is a combination of a flat seal and a slotted bearing. Its strengths are small leakage (even no leakage), large film thickness, elimination of contact friction, power consumption and fever. Hot fluid dynamic pressure sealing technology It is the use of a variety of shape deep sealing surface flow groove, resulting in local thermal deformation, in order to produce fluid dynamic wedge effect. This seal with hydrodynamic bearing capacity is called a thermohydrodynamic wedge seal.

Bellows sealing technology can be divided into forming metal bellows and welding metal bellows mechanical seal technology.

Multi-end sealing technology is divided into double sealing, intermediate ring sealing and multi-sealing technology. In addition, there are parallel surface sealing technology, monitoring sealing technology, combination sealing technology, etc.

7, mechanical seal common sense mechanical seal flushing scheme and characteristics

The purpose of flushing is to prevent the accumulation of impurities, prevent the formation of air bags, maintain and improve lubrication, etc., when the temperature of the flushing liquid is low, it has a cooling effect. The main ways of scouring are as follows:

scour in 1.

1. Positive scouring

(1) Features: The sealed medium of the working host is used to introduce the sealed cavity through the pipeline at the outlet end of the pump.

(2) Application: Used for cleaning fluid, P1 is slightly greater than P. When the temperature is high or there are impurities, coolers and filters can be set on the pipeline

2. Backwash

(1) Features: The sealed medium of the working host is used to introduce the sealed cavity from the outlet end of the pump, and flow back to the pump inlet through the pipeline after flushing.

(2) Application: For cleaning fluid, and P into 3. full scour

(1) Features: The sealed medium of the working host is used to introduce the sealed cavity through the pipeline at the outlet end of the pump, and then flow back to the pump inlet through the pipeline after flushing.

(2) Application: The cooling effect is better than the first two, when it is used to clean the fluid, and P1 is close to P in and P out.

2. external scour

Features: Introduce cleaning fluid compatible with the sealed medium from the external system to the sealed cavity for flushing.

Application: The external flushing hydraulic pressure should be 0.05 larger than the sealed medium -- 0.1MPA, which is used for occasions where the medium is high temperature or solid particles. The flow of the flushing liquid should be guaranteed to take away the heat, but also to meet the needs of the erosion, will not produce the erosion of the seal. For this reason, it is necessary to control the pressure of the sealing cavity and the flow rate of flushing. Generally, the flow rate of cleaning flushing liquid should be less than 5 M/S; Slurry liquid containing particles must be less than 3 M/S. In order to achieve the above flow rate value, the difference between flushing liquid and sealing cavity pressure should be "0.5MPA, generally 0.05-0.1MPA, and 0.1-0.2MP for double-end mechanical seal. The position of the orifice for the flushing liquid to enter and discharge the sealing cavity shall be set around the sealing end face and close to the moving ring side. In order to prevent the graphite ring from being eroded or deformed by temperature difference due to uneven cooling, as well as impurity accumulation and coking, tangential introduction or multi-point flushing can be adopted. If desired, the flushing fluid may be hot water or steam.

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Jiangsu Wanben specializes in the production of various door and window sealing strips for automobiles, construction machinery, construction, railways, etc. The products cover EPDM, PVC, CR, NBR, NR, TPV and other materials to design various seals like plastic and rubber. It can realize the full matching of automobiles and construction machinery vehicles and has independent export management rights. Its products are exported to more than ten countries and regions such as Asia, America and Europe.

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