Mechanical seals, also known as end seals, are essential components used in pumps, compressors, and hydraulic systems to prevent leakage from rotating shafts. These seals consist of a pair of friction surfaces—moving and stationary rings—that work together under the influence of fluid pressure and elastic forces to maintain a tight seal.
A mechanical seal prevents fluid leakage by at least one set of end faces that are perpendicular to the axis of rotation. The sealing is maintained through the action of fluid pressure, an elastic force (or magnetic force), and auxiliary seals. One type of auxiliary seal is a metal bellows, which is commonly used in bellows mechanical seals.
The main components of a mechanical seal include: a. Primary seals – moving ring and static ring. b. Auxiliary seals – such as O-rings or other sealing elements. c. Pressing parts – typically a spring and push ring. d. Transmission parts – like the gland seat, key, or fixing screw.
There are several ways to classify mechanical seals: - Based on the position of the spring, they can be either rotating or stationary. - Depending on where the static ring is located, they can be internal or external. - By the direction of leakage, they can be either internal or external. - According to the balance of the seal, they can be balanced or unbalanced. - By the number of sealing faces, they can be single or double. - Based on the number of springs, they may be single-spring or multi-spring. - By the type of elastic element, they can be spring-loaded or bellows-type. - By the design, they can be hydrostatic, hydrodynamic, or dry gas seals. - By temperature, they can be high, medium, normal, or low-temperature seals. - By pressure, they can be ultra-high, high, medium, or low-pressure seals.
When installing a mechanical seal, it's important to avoid misalignment. The gland should be tightened only after the coupling is properly aligned. Bolts must be evenly distributed to prevent any uneven pressure. A feeler gauge should be used to check for deviations, ensuring the error does not exceed 0.05 mm.
It’s also crucial to check the concentricity between the gland and the shaft. The clearance should be uniform, with a tolerance of no more than 0.01 mm. The spring compression must be adjusted according to specifications, usually within a 2.00 mm range. Too much or too little compression can lead to wear or failure.
When disassembling a mechanical seal, care should be taken to avoid damaging the components. Hammers or flat chisels should not be used. Instead, a pair of wire hooks can be made to gently pull the seal out. If the seal has been in operation, the static and dynamic rings should be replaced rather than re-tightened, as the original friction path may have changed.
During operation, the mechanical seal should be monitored regularly. If there is a small amount of leakage, it should be observed for a few hours. If it doesn’t improve, the pump should be stopped. The operating pressure should remain stable, with fluctuations not exceeding 1 kg/cm². Avoiding dry running is critical to prevent damage to the seal faces.
Before starting the pump, ensure the sealed chamber is filled with liquid. For solidified media, heating may be necessary. Also, the cooling system should not be turned off immediately after stopping a hot oil pump. Wait until the temperature drops below 80°C before shutting down the cooling water to prevent damage to the seal components.
Proper installation, maintenance, and operation of mechanical seals are vital to their longevity and performance. Following these guidelines ensures safe and efficient operation of equipment using mechanical seals.
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