Principle and characteristics of mechanical seals and material selection

[Principles and characteristics of mechanical seals and material selection]

1, the working principle of mechanical seal
The mechanical seal is a shaft seal device that resists leakage by a pair of or pairs of end faces that are relatively slid relative to the shaft under the action of the fluid pressure and the elastic force (or magnetic force) of the compensating mechanism to assist in sealing.

2, the selection of commonly used materials for mechanical seals

Clear water; normal temperature; (moving) 9Cr18, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) impregnating resin graphite, bronze, phenolic plastic.
River water (including sediment); normal temperature; (moving) tungsten carbide, (static) tungsten carbide seawater; normal temperature; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin graphite, tungsten carbide, metal ceramics;
Superheated water 100 degrees; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) impregnated resin graphite, tungsten carbide, cermet;
Gasoline, lubricating oil, liquid hydrocarbon; room temperature; (moving) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (static) impregnating resin or tin-bismuth alloy graphite, phenolic plastic.
Gasoline, lubricating oil, liquid hydrocarbon; 100 degrees; (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten; (static) dip bronze or resin graphite.
Gasoline, lubricating oil, liquid hydrocarbons; containing particles; (dynamic) tungsten carbide; (static) tungsten carbide.

3. Types and uses of sealing materials

The sealing material should meet the sealing function requirements. Due to the different media being sealed and the different working conditions of the device, 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 media;
2. Have proper mechanical strength and hardness:
3. Good compressibility and resilience, and small permanent deformation;
4, does not soften under high temperature, does not decompose, does not harden at low temperature, not brittle;
5, good corrosion resistance, long-term work in acid, alkali, oil and other media, its volume and hardness changes little, and does not adhere to the metal surface;
6. The friction coefficient is small and the wear resistance is good;
7, has a softness combined with the sealing surface;
8, good aging resistance, durable;
9, processing and manufacturing is convenient, the price is cheap, easy to take.

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

4, mechanical seal installation, use technical essentials

1. The radial runout of the equipment shaft should be ≤0.04 mm, and the axial yaw force should not be greater than 0.1 mm;
2, the sealing part of the equipment should be kept clean during installation, the sealing parts should be cleaned, the sealing end face is intact, and impurities and dust are prevented from being brought into the sealing part;
3. It is strictly forbidden to hit or knock during the installation process, so as to avoid the mechanical seal friction and damage and the seal failure;
4. A clean layer of mechanical oil should be applied to the surface in contact with the seal during installation so that it can be installed smoothly;
5. When installing the static ring gland, the screw must be evenly stressed to ensure the vertical requirement of the static ring end face and the axial line;
6. After the installation, push the moving ring by hand to make the moving ring move flexibly on the shaft and have certain elasticity;
7. After installation, the rotating shaft and the rotating shaft should be light and heavy.
8. The equipment must be filled with medium before operation to prevent dry friction and seal failure;
9. For easy crystallization and granular media, when the medium temperature is >80oC, corresponding flushing, filtering and cooling measures should be taken. For various auxiliary devices, please refer to the relevant standards of mechanical seals.
10. When installing, the surface in contact with the seal should be coated with a layer of clean mechanical oil. Pay special attention to the choice of mechanical oil. For different auxiliary sealing materials, avoid O-ring oil expansion or accelerated aging, resulting in premature seal failure. .

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

The seal between the moving ring and the stationary ring: the elastic pressure (spring, bellows, etc.) and the sealing liquid pressure generate a suitable pressing force on the contact surface (end surface) of the moving ring and the stationary ring (relatively moving) Pressing) makes the two smooth and flat end faces fit closely; maintaining a very thin liquid film between the end faces to achieve the sealing effect. This membrane has hydrodynamic and static pressures that act to balance pressure and lubricate the end faces. The reason why the two end faces must be highly smooth and straight is to create a perfect fit for the end faces and to make the specific pressure uniform, which is a relative rotary seal.

6, the type of mechanical sealing technology

New technologies for various mechanical seals using new materials and processes are progressing rapidly and have the following new mechanical seal technologies. Sealing surface groove sealing technology In recent years, various flow channels have been opened on the sealing end faces of mechanical seals to generate hydrostatic and dynamic pressure effects, which are now being continuously updated. Zero-leakage sealing technology used to mean that contact and non-contact mechanical seals are unlikely to achieve zero leakage (or no leakage). Israel uses slotted sealing technology to propose a new concept of zero-leakage non-contact mechanical end face seals and has been used in nuclear power plant lubrication pumps. Dry Run Gas Sealing Technology This type of seal uses slotted sealing technology for gas sealing. The upstream pumping sealing technique utilizes an open flow channel on the seal face to pump a small amount of downstream leakage fluid back upstream. The structural characteristics of the above types of seals are: shallow grooves are used, and the film thickness and the depth of the flow channels are all on the order of micrometers, and the lubrication grooves, the radial seal dams and the circumferential seals are used to form the seal and the load-bearing portion. It can also be said that the slotted seal is a combination of a flat seal and a slotted bearing. The advantages are small leakage (even no leakage), large film thickness, elimination of contact friction, low power consumption and low heat generation. Thermal fluid dynamic sealing technology utilizes various deeper sealing surface flow grooves to cause local thermal deformation to create a hydrodynamic wedge effect. This type of seal with hydrodynamic bearing capacity is called a thermohydrodynamic wedge seal.

The bellows sealing technology can be divided into a molded metal bellows and a welded metal bellows mechanical sealing technology.

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

7, mechanical seal flushing program and features

The purpose of the rinsing is to prevent the accumulation of impurities, to prevent the formation of the airbag, to maintain and improve the lubrication, etc., and to have a cooling effect when the temperature of the rinsing liquid is low. The main methods of washing are as follows:

First, internal flushing

1. 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: used for cleaning fluid, p1 is slightly larger than p. When the temperature is high or there are impurities, coolers, filters, etc. can be installed on the pipeline.

2. Backwashing (1) Features: Using the sealed medium of the working host, the outlet end of the pump is introduced into the sealed chamber, and after flushing, it flows back to the pump inlet through the pipeline.
(2) Application: used for cleaning fluids, and p

3. Full flushing (1) Features: Using the sealed medium of the working host, the outlet end of the pump is introduced into the sealed chamber through the pipeline, and then flushed back to the pump inlet through the pipeline.
(2) Application: The cooling effect is better than the first two, used for cleaning fluid, and p1 is close to p and p.

Second, external flushing

Features: Introduce the external system to the sealed cavity for cleaning with a cleaning fluid compatible with the sealed media.

Application: The external flushing fluid pressure should be 0.05--0.1 MPa larger than the sealed medium. It is suitable for occasions where the medium is high temperature or solid particles. The flow rate of the rinsing liquid should be such as to take away the heat, and also need to meet the needs of rinsing, and will not cause erosion of the seal. To do this, it is necessary to control the pressure of the sealed chamber and the flow rate of the flushing. The flow rate of the general cleaning rinse should be less than 5 m/s; the slurry containing the particles must be less than 3 m/s. To achieve the above flow rate, the rinse and the sealed chamber The difference in pressure should

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'The principle and characteristics of mechanical seal and material selection

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