Principle and characteristics of the hottest mecha

2022-07-31
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Principle and characteristics of mechanical seal and material selection

content tips: introduction to working principle of mechanical seal, selection of common materials of mechanical seal, technical essentials for installation and use of mechanical seal, flushing scheme and characteristics of mechanical seal

1. Working principle of mechanical seal

mechanical seal is a shaft seal device that relies on a pair or several pairs of end faces that slide relatively perpendicular to the shaft to keep sticking and matching with auxiliary seal 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 atmospheric temperature; (dynamic) 9Cr18, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) resin impregnated graphite, bronze, phenolic plastic

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

seawater; normal atmospheric temperature; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) resin impregnated graphite, tungsten carbide, cermet

superheated water 100 ℃; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) resin impregnated graphite, tungsten carbide, cermet

gasoline, lubricating oil, liquid hydrocarbon; normal atmospheric temperature; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt to improve the overall competitiveness of the industry chromium tungsten, cast iron; (static) resin impregnated or tin antimony alloy graphite, phenolic plastic

gasoline, lubricating oil, liquid hydrocarbon; 100 degrees; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten; (static) impregnated with 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 shall meet the requirements of sealing function. Due to different sealed media and different working conditions of equipment, sealing materials are required to have different adaptability. The general requirements for sealing materials are:

1. The materials have good compactness and are not easy to leak media

2. Appropriate mechanical strength and hardness

3, good compressibility and resilience, small permanent deformation

4. No softening and decomposition under high temperature, no hardening and embrittlement under low temperature

5. Good corrosion resistance. It can work in acid, alkali, oil and other media for a long time. Its volume and hardness change little, and it does not adhere to the metal surface

6, small friction coefficient and good wear resistance

7. It has softness combined with the sealing surface

8. Good aging resistance and durability

9. It is easy to manufacture, cheap and easy to obtain materials

rubber is the most commonly used sealing material. In addition to rubber, graphite, polytetrafluoroethylene and various sealants are also suitable for sealing materials

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 displacement shall not be greater than 0.1mm

2. The sealing parts of the equipment shall be kept clean during installation, and the sealing parts shall be cleaned. The lines with deep shadow and the edge sealing end face with clear outline are as good as the appearance of natural slate, so as to prevent impurities and dust from entering the sealing parts

3. It is strictly prohibited to bump or knock during installation to avoid damage of mechanical seal friction pair and seal failure

4. During installation, a layer of clean mechanical oil shall be coated on the surface in contact with the seal to facilitate smooth installation

5. When installing the static ring gland, the tightening screws must be stressed evenly to ensure that the end face of the static ring is perpendicular to the axis

6. Push the moving ring by hand after installation to make the moving ring move flexibly on the shaft and have certain elasticity

7. Turn the rotating shaft manually after installation, and the rotating shaft shall have no sense of weight

8. The equipment must be filled with media before operation to prevent seal failure due to dry friction

9. For crystallizable and granular media, when the media temperature is 80oC, corresponding washing, filtering and cooling measures should be taken. For various auxiliary devices, please refer to the relevant standards of mechanical seals to provide customers with convenience seals for picking up goods from other places and exchanging standard products in tandem for non-standard products

10. During installation, a layer of clean mechanical oil shall be coated on the surface in contact with the seal. Special attention shall be paid to the selection of mechanical oil. For different auxiliary seal materials, avoid oil invasion and expansion of O-ring or accelerated aging, resulting in premature seal failure

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

the sealing between the moving ring and the stationary ring: it relies on the elastic elements (springs, bellows, etc.) and the sealing liquid pressure to generate an appropriate pressing force (specific pressure) on the contact surface (end face) of the relatively moving moving moving moving ring and the stationary ring, so that the two smooth, straight end faces can closely fit; A very thin liquid film is maintained between the end faces to achieve the sealing effect. This layer of film has liquid dynamic pressure and static pressure, which plays the role of balancing pressure and lubricating the end face. The reason why the surfaces at both ends must be highly smooth and straight is to create conditions for perfect fitting and uniform specific pressure for the end faces, which is a relative rotating seal

6. Types of mechanical seal technologies

at present, various new mechanical seal technologies using new materials and processes have made rapid progress, including the following new mechanical seal technologies. In recent years, various flow grooves have been opened on the sealing face of mechanical seals to produce hydrostatic and dynamic pressure effects, which are still being updated. Zero leakage sealing technology used to think that contact and non-contact mechanical seals could not achieve zero leakage (or no leakage). A new concept of zero leakage non-contact mechanical face seal was proposed by Israel using slotted seal technology and has been used in lubricating oil pump of nuclear power plant. Dry running gas seal technology this kind of seal uses slotted seal technology for gas seal. The upstream pumping sealing technology is to pump a small amount of downstream leakage fluid back to the upstream by using the flow groove on the sealing surface. The structural characteristics of the above seals are as follows: shallow grooves are used, and the film thickness and the depth of the flow grooves are micron level. Lubricating grooves, radial sealing dams and circumferential sealing weirs are used to form the sealing and bearing parts. It can also be said that slotted seal is the combination of plane seal and slotted bearing. The utility model has the advantages of small leakage (even no leakage), large film thickness, elimination of contact friction, low power consumption and heat generation. Thermal fluid dynamic pressure sealing technology uses various deep grooves on the sealing surface to cause local thermal deformation and produce hydrodynamic wedge effect. This kind of seal with hydrodynamic bearing capacity is called thermal hydrodynamic wedge seal

bellows sealing technology can be divided into formed metal bellows and welded metal bellows mechanical sealing technology

multi face seal technology is divided into double seal, intermediate ring seal and multi seal technology. In addition, there are parallel sealing technology, monitoring sealing technology, combined sealing technology, etc

7. Flushing scheme and characteristics of mechanical seal

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 flushing liquid is low, it also has the cooling effect. The main flushing methods are as follows:

I. internal flushing

1 Forward flushing

(1) features: using the sealed medium of the working host, the outlet end of the pump is introduced into the sealing chamber through the pipeline

(2) application: for cleaning fluid, P1 is slightly greater than p inlet. When the temperature is high or there are impurities, cooler and filter can be set on the pipeline

2 Backwashing

(1) features: using the sealed medium of the working host, it is introduced into the sealing chamber from the outlet end of the pump, and then flows back to the pump inlet through the pipeline

(2) application: the competitiveness of the market will also be further improved for cleaning fluids, and P is

3 Full flushing

(1) features: using the sealed medium of the working host, it is introduced into the sealing chamber through the pipeline from the outlet end of the pump, and then flows back to the pump inlet through the pipeline after flushing

(2) application: the cooling effect is better than the first two. It is used to clean the fluid, and P1 is close to p inlet and P outlet

II. External flushing

features: clean fluid compatible with the sealed medium in the external system is introduced to the sealing cavity for flushing

application: the pressure of external washing liquid shall be 0.05--0.1mpa greater than that of the sealed medium, which is applicable to the occasions where the medium is high temperature or solid particles. The flow of flushing fluid shall ensure to take away heat and meet the needs of flushing without eroding the seals. Therefore, it is necessary to control the pressure of the sealing chamber and the flow rate of flushing. Generally, the flow rate of cleaning flushing fluid should be less than 5m/s; The slurry liquid containing particles must be less than 3m/s. In order to reach the above flow rate value, the difference between the flushing liquid and the pressure in the seal chamber should be 0.5MPa, generally 0.05--0.1mpa, and 0.1--0.2mp for the double face mechanical seal The orifice where the flushing liquid enters and exits the sealing cavity shall be set near the sealing end face and near the moving ring. In order to prevent the graphite ring from being eroded or temperature difference deformation caused by uneven cooling, as well as impurity accumulation and coking, tangential introduction or multi-point flushing can be used If necessary, the flushing fluid can be hot water or steam

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