Basic knowledge of mechanical seals
Introduction to Mechanical Seals
Mechanical seals are one of the precision and structurally complex mechanical basic components, and are key components of various pumps, reaction synthesis kettles, turbo compressors, submersible motors, and other equipment. Its sealing performance and service life depend on many factors, such as selection, machine accuracy, correct installation and use, etc.
Basic knowledge of mechanical seals
1. The basic concept of mechanical seal:
Mechanical seal refers to a device that prevents fluid leakage by keeping at least one pair of end faces perpendicular to the axis of rotation in contact and sliding relative to each other under the action of fluid pressure and compensation mechanism elasticity (or magnetic force), as well as the cooperation of auxiliary seals. The auxiliary seal of the compensating ring, which is a metal bellows, is called a bellows mechanical seal.
2. Composition of mechanical seal:
There are mainly four types of components. a. Main sealing components: dynamic ring and static ring. b. Auxiliary seal: sealing ring. c. Pressing components: springs, push rings. d. Transmission components: spring basket seat and key or fixing screw.
Attention should be paid to issues
1. Precautions during installation
a、 Be very careful to avoid installation deviations that may occur during installation
(1) Tightening the gland should be done after aligning the coupling, and the bolts should be evenly supported to prevent the end face of the gland from tilting. Use a feeler gauge to check each point, and the error should not exceed 0.05 millimeters.
(2) Check the fit clearance (i.e. concentricity) between the gland and the outer diameter of the shaft or sleeve, ensuring uniformity around the edges. Use a feeler gauge to check that the allowable deviation at each point is not greater than 0.01 millimeters.
b、 The compression amount of the spring should be carried out according to the regulations, and there should be no excessive or insufficient phenomenon. The required error is 2.00 millimeters. Excessive pressure will increase the specific pressure on the end face and cause wear on the end face. Being too small can result in insufficient specific pressure and fail to provide a sealing effect.
c、 After the installation of the dynamic ring, the beard should be able to move flexibly on the shaft, and it should automatically bounce back when pressed against the spring.
2. Precautions for disassembly
a、 Be careful when dismantling mechanical seals, and it is strictly prohibited to use hammers and shovels to avoid damaging the sealing components. A pair of steel wire hooks can be used to insert into the gap of the transmission seat in the direction of self financing, and pull out the sealing device. If scaling cannot be removed, it should be cleaned thoroughly before disassembly.
b、 If mechanical seals are used at both ends of the pump, take care of each other during assembly and disassembly to prevent neglecting one aspect.
c、 For the mechanical seal that has been operated, if the cover is loose and causes the seal to move, the dynamic and static ring parts must be replaced and should not be tightened again to continue use. Because after such movement, the original trajectory of the friction pair will change, and the sealing of the contact surface is easily damaged.
Normal operation and maintenance of mechanical seals
1. Preparation and precautions before startup
a、 Conduct a comprehensive inspection of the mechanical seal, as well as the installation of auxiliary devices and pipelines, to ensure that they are complete and meet the technical requirements.
b、 Perform a static pressure test before starting the mechanical seal to check for any leakage. If there are many leaks, the cause should be investigated and efforts should be made to eliminate them. If it is still ineffective, it should be disassembled, inspected, and reinstalled. The general static pressure test pressure is 2-3 kilograms per square centimeter.
c、 Rotate the pump towards the turning wheel and check if it is light and even. If the turning gear is difficult or stationary, the assembly dimensions should be checked for errors and the installation should be reasonable.
2. Installation and shutdown
a、 Before starting, the sealed chamber should be filled with liquid. When transporting solidified media, steam should be used to heat the sealed chamber and melt the media. Before starting, it is necessary to turn the car to prevent sudden starting from causing the soft ring to break.
b、 For mechanical seals that utilize the pump's external sealing oil system, the sealing oil system should be started first. Stop the oil sealing system after parking.
c、 After the hot oil pump is shut down, the cooling water for the sealing chamber and end face seal cannot be stopped immediately. The cooling water should be stopped only when the oil temperature at the end face seal drops below 80 degrees to avoid damaging the sealing parts.
3. Operation
a、 If there is a slight leakage after the pump is started, it should be observed for a period of time. If the leakage does not decrease after running continuously for 4 hours, the pump should be stopped for inspection.
b、 The operating pressure of the pump should be stable, with pressure fluctuations not exceeding 1 kilogram per square centimeter.
c、 During the operation of the pump, the phenomenon of vacuum should be avoided to prevent dry friction and seal damage on the sealing surface.
d、 The sealing condition should be checked regularly. During operation, when the leakage exceeds the standard, the heavy oil should not exceed 5 drops/minute and the light oil should not exceed 10 drops/minute. If there is still no improvement trend within 2-3 days, the pump plug should be stopped to check the sealing device.
Sealing "has a long history of development in China. In ancient times, cotton, hemp and other fibers were used to make seals for water lifting machinery, while foreign countries did not use fillers until 1782. The importance of sealing is not emphasized here. Nowadays, there is a discipline abroad that studies sealing laws, sealing device design technology, and applied scientific principles - "sealing science". Research institutions also have specialized courses in sealing science. However, in China, as far as I know, there are courses in fluid mechanics and hydraulic transmission in universities, but there is still no "sealing department" that specifically studies sealing. Therefore, our research level still lags behind that of foreign countries.
There are many professional fields for seal design, including mechanics (including fluid mechanics, boundary layer theory, etc.), tribology, automatic control, and so on, in addition to materials and machinery. So for sealing, the research difficulty is relatively high. I personally believe that the gap between the domestic sealing industry and foreign countries should not be less than 50 years.
Regarding the sealing mechanism
If you want to learn sealing, you must first understand leakage. Once you understand the principle of leakage, you will have the corresponding sealing mechanism. There are basically three types of leaks——
One is leakage, which refers to the leakage between the sealing surface gaps
The second is leakage, which refers to the leakage of sealed fluid through the capillary tube of the sealing material
The third is diffusion, which refers to the transfer of substances through gaps or capillaries of materials in a sealed medium under the action of concentration differences.
Regarding the sealing method
There are roughly several sealing methods available——
1. Minimize the number of sealed areas as much as possible
2. Blockage and isolation
3. Introduction or injection
4. Increase leakage resistance
5. Add working elements in the channel
6. Multiple sealing methods combined
Common sealing forms
Gasket sealing, packing sealing, mechanical sealing, non-contact sealing, and injection type pressure sealing are common sealing forms. Among them, packing seal should be considered the most common, which also includes soft packing seal, hard packing seal, and molded packing seal. Shaped packing seals include commonly used O-rings, Y-rings, oil seals, and so on. Non contact seals include gap seals, labyrinth seals, floating seals, power seals, magnetic fluid seals, and fully enclosed seals.
