Mechanical seal
1. Mechanical seal method and working principle
Mechanical seal is a filler-free end face sealing device that limits the leakage of working fluid along the rotating shaft. It is mainly composed of static ring, dynamic ring, elastic (or magnetic) element, transmission element and auxiliary sealing ring.
When the mechanical seal works, it relies on the dynamic ring fixed on the shaft and the static ring fixed on the pump housing, and uses the elastic force of the elastic element and the pressure of the sealing fluid to promote the close fit of the dynamic and static ring end faces to complete the sealing function. In the mechanical sealing device, the pressure shaft seal water, on the one hand, blocks the high-pressure leakage water, and on the other hand, squeezes into the dynamic and static rings to maintain a layer of moving smooth liquid film, so that the dynamic and static ring end faces do not touch. Because the moving film is very thin and is affected by high-pressure water, the leakage is very small. Auxiliary sealing rings are used between the static ring and the sealing cover (B), between the dynamic ring and the rotating shaft (C), and between the sealing cover and the housing (D) to solve the sealing problems of these leakage points.
2. Working process of mechanical seal
The static ring and the sealing cover are sealed with a static ring sealing rubber ring, and the elastic tightening force of the sealing rubber ring is used to fix the static ring on the sealing cover, and an anti-rotation pin is used to prevent the static ring from rolling; the dynamic ring uses the elastic force of the elastic element to fit tightly with the static ring, and the dynamic ring and the shaft are sealed with a dynamic ring sealing rubber ring, and are connected with the elastic element through a transmission pin, and rotate with the elastic element. The elastic element is fixed on the shaft through a fastening screw and rotates with the shaft. In this way, when the shaft rotates, the rotating shaft drives the elastic element to roll through the fastening screw, and the elastic element drives the dynamic ring to rotate together through the transmission pin, so that relative reverse motion and excellent fitting contact occur between the dynamic ring and the static ring, achieving the purpose of sealing.
3. Types of mechanical seals
1. According to the end face, it is divided into: single-end face and double-end face mechanical seals.
(1) Single-end face mechanical seal: a mechanical seal composed of a pair of sealing end faces. The structure is simple, and the manufacturing and installation are simple. It is generally used under the conditions where the medium itself has good lubricity and allows for trace leakage.
(2) Double-end mechanical seal: A mechanical seal composed of two pairs of sealing end faces. When the medium itself has poor lubricity, is toxic, flammable, explosive, easy to radiate, and has strict requirements on the leakage volume. A sealing cooling liquid with a pressure higher than the medium pressure is introduced between the two end faces for sealing and cooling. It is possible to achieve "zero leakage" of the medium. It is also divided into axial and radial double-end faces.
(3) According to the balancing method, it is divided into balanced and unbalanced mechanical seals.
a: Balanced mechanical seal: It can unload the pressure of the medium on the sealing end face. It is divided into partial balanced (partial unloading) and over-balanced (full unloading) according to the degree of unloading. It can reduce friction and wear on the end face, reduce friction heat, and has a large load-bearing capacity, but the structure is relatively complex. Generally, a step needs to be processed on the shaft or sleeve, which is costly.
b: Unbalanced mechanical seal: The medium cannot unload the pressure on the sealing end face. The structure is simple and is widely used when the medium pressure is less than 0.7Mpa.
3. According to the placement of the spring, it is divided into spring-inside mechanical seal and spring-outside mechanical seal.
(1) Spring-inside mechanical seal: The spring is in contact with the medium. It is susceptible to corrosion and is easily blocked by debris in the medium. If the spring rotates with the shaft, it is not suitable for use in high-viscosity media.
(2) Spring-outside mechanical seal: The spring is not in contact with the medium. It is used in equipment with highly corrosive, high-viscosity and easily crystallized media.
4. According to the number of springs, it is divided into single-spring mechanical seal and multi-spring mechanical seal.
(1) There is only one spring in the elastic element (sealing compensator). The spring wire is thicker, corrosion-resistant, and solid particles are not easy to accumulate at the spring, but the end face force is uneven.
(2) There is a group of springs in the elastic element (sealing compensator). The end face is subjected to a relatively uniform force, and it is easy to increase or decrease the number of springs to adjust the spring force. The axial length is short, but the spring wire is thin, the corrosion resistance life is short, and the installation size must be strictly followed.
5. According to the form of the elastic element (sealing compensator), it is divided into: rotary mechanical seal and stop mechanical seal.
(1) Rotary mechanical seal: The elastic element (sealing compensator) is installed with the shaft. It is widely used, because the centrifugal force on the spring during installation will affect the pressure on the sealing end face. It is not suitable for high speed conditions.
(2) Stop mechanical seal: The elastic element (sealing compensator) is not installed with the shaft. It is suitable for high speed conditions. It is suitable for high speed conditions.
6. According to the leakage direction of the sealing fluid (medium), it is divided into: internal flow mechanical seal and external flow mechanical seal.
(1) The direction of leakage of the sealing fluid (medium) between the end faces is opposite to the direction of the centrifugal force. Small leakage and reliable sealing.
(2) The direction of leakage of the sealing fluid (medium) between the end faces is the same as the direction of the centrifugal force. When the speed is extremely high, it is more suitable to use it to enhance the end face smoothness, but the medium pressure should not be too high, generally 1-2MPa.
7. According to the contact method of the sealing end face: contact type and non-contact type mechanical seals.
(1) Contact type mechanical seal: The sealing end face is in a boundary or semi-liquid smooth state. The structure is simple and the leakage is small, but the wear, power consumption and heat generation are large, and the use under high speed and high pressure is subject to certain restrictions.
(2) Non-contact type mechanical seal: The sealing end face is in a fully liquid smooth state. The heat generation and power consumption are small, there is no wear during normal operation, and it can operate under harsh conditions such as high pressure and high speed, but the leakage is large. It is also divided into: fluid static pressure non-contact type and fluid dynamic pressure non-contact type mechanical seal.
a. Fluid static pressure non-contact mechanical seal: uses external pressure fluid or the sealed medium itself to generate fluid static pressure effect through the pressure drop of the sealing end face.
b. Fluid dynamic pressure non-contact mechanical seal: uses the relative rotation of the end face to generate fluid dynamic pressure effect, such as spiral groove end face seal.
Others include bellows mechanical seals and single-pole, double-pole (multi-pole) mechanical seals.
