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Dry Gas Seal

Tips for Extending Mechanical Seal Life

by:Lepu Seal     2022-09-18
Coup for prolonging the life of mechanical seals Mechanical seals working in liquid media generally rely on the liquid film formed between the friction surfaces of the dynamic and static rings for lubrication by the liquid medium. Therefore, maintaining the liquid film between the friction surfaces is essential to ensure the stable operation of the mechanical seal and prolong its service life. Several structures to enhance lubrication 1. Slotting on the end face: It is difficult for high-pressure and high-speed machinery to maintain the liquid film between the friction surfaces, and the liquid film is often destroyed by the friction heat generated by high pressure and high speed. In this case, it is very effective to take the form of grooves to intensify lubrication. Both the moving ring and the static ring can be grooved, generally on more wear-resistant materials. The moving and stationary rings should not be grooved at the same time as this will reduce the lubrication effect. In order to prevent dirt or wear debris from entering the friction surface as much as possible, the liquid is sealed in the direction of centrifugal force (outflow), and the groove should be opened on the static ring to avoid the centrifugal force from introducing dirt into the friction surface. On the contrary, when the liquid flows in the opposite direction of centrifugal force (internal flow), the groove should be opened on the moving ring, and the centrifugal force will help to throw the dirt out of the groove. The small grooves on the friction surface are rectangular, wedge-shaped, or other shapes. The grooves should not be too many or too deep, otherwise the leakage will increase. 2. Hydrostatic lubrication: The so-called hydrostatic lubrication is to directly introduce the pressure lubricating liquid into the friction surface to play the role of lubrication. The lubricating liquid introduced is supplied by a separate liquid source, such as a hydraulic pump, etc. This pressurized lubricant is used to oppose the fluid pressure within the machine, and this form is also commonly referred to as a hydrostatic seal. For the mechanical seal of gas medium, measures should be taken to establish gas film lubrication, such as the use of gas static pressure controlled film mechanical seal or solid lubrication, that is, self-lubricating material actuating ring or static ring. As long as the conditions allow, the gas medium conditions should be converted into liquid medium conditions as much as possible, which is convenient for both lubrication and sealing. 3. Eccentricity of the end face: In general mechanical seals, the center of the moving ring, the center of the static ring and the center line of the shaft are all on a straight line. The line is offset by a certain distance, so that the ring can continuously bring in lubricating fluid to the sliding surface to lubricate the sliding surface when it rotates. It should be pointed out that the eccentric size should not be too large, especially for high pressure, the eccentricity will make the pressure on the end face uneven, resulting in uneven wear. For high-speed seals, it is not advisable to use the moving ring as an eccentric ring, otherwise the machine will vibrate due to the balance of centrifugal force. The friction between the dynamic and static rings of the mechanical seal will appear as follows according to different conditions: (1) Semi-liquid lubrication: There is liquid in the pit on the sliding surface, and a thin liquid film is maintained between the contact surfaces, so The heat generation and wear conditions are good, because the liquid film between the dynamic and static rings has surface tension at its outlet, so the leakage of the liquid is limited. (2) Complete liquid lubrication: When the pressure between the dynamic and static rings is insufficient and the gap increases, the liquid film thickens, and there is no solid contact at this time, so there is no friction. However, due to the large gap between the dynamic and static rings in this case, the sealing effect cannot be achieved, and the leakage is serious. This situation is generally not allowed in practical applications (except for controlled membrane mechanical seals). Most of the working conditions between the dynamic and static rings of mechanical seals are in the state of boundary lubrication and semi-liquid lubrication, while semi-liquid lubrication can obtain the best results when the friction coefficient is the smallest, that is, the wear and heat generation are satisfactory. sealing effect. To make the mechanical seal work under good lubrication conditions, it is necessary to comprehensively consider factors such as medium characteristics, pressure, temperature, sliding speed, etc., but select the appropriate pressure between the dynamic and static rings, a reasonable lubrication structure and improve dynamic and The friction surface quality of the static ring is also an important factor to ensure the effective operation of the seal. (3) Dry friction: There is no liquid entering on the sliding friction surface, so there is no liquid film, only dust and oxide layers, and adsorbed gas molecules, etc. When the dynamic and static rings are running, the result must be that the friction surface is heated and the wear is aggravated, resulting in leakage. (4) Boundary lubrication: When the pressure between the dynamic and static rings increases or the ability of the liquid to form a liquid film on the friction surface is poor, the liquid will be squeezed out from the gap. Since the surface is not absolutely flat, but uneven, there will be contact wear in the convex, while the lubricating performance of the liquid is maintained in the concave, resulting in boundary lubrication. The degree of wear and heat generation for boundary lubrication is moderate.
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