Mechanical seal

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Mechanical seals offer excellent sealing performance, stable operation, low leakage rates, low frictional power consumption, and a long service life. They cause little wear to the shaft and can meet the requirements of various operating conditions; as such, they are widely used in industries such as chemicals and petrochemicals. However, they have a complex structure, require high manufacturing precision, are expensive, and are difficult to maintain.  Definition and working principle of mechanical seals. Mechanical seals, also known as face seals, have at least one pair of faces that are perpendicular to the axis of rotation. Under the action of fluid pressure and forces designed to counteract mechanical expansion forces, along with the assistance of additional sealing elements, these faces remain in close contact with each other while sliding relative to one another, thereby preventing fluid leakage. Due to the tight fit between the two sealed end surfaces, a tiny gap is formed at their junction. When a pressurized medium passes through this gap, a very thin film of liquid is created, which generates resistance and prevents the leakage of the medium. At the same time, this film lubricates the end surfaces, thereby ensuring long-term sealing performance.  Composition of mechanical seals and functions of their components. A mechanical seal is primarily composed of the following four main parts: 1) A pair of sealing surfaces made up of a stationary ring and a rotating ring; these two surfaces are usually ground, and they are sometimes referred to as the friction pair. 2) A compensation and buffering mechanism based primarily on elastic elements. 3) Auxiliary sealing ring. 4) A transmission mechanism that rotates the rotating surface together with the shaft.  Classification of mechanical seals 1. Classification by purpose 1) Classification by the industrial sector in which they are used: There are mechanical seals used in industries such as transportation machinery manufacturing, household appliance manufacturing, power machinery and pump manufacturing, the chemical and petroleum industries, the defense industry, and shipbuilding. 2) Classified by the host application: mechanical seals for pumps, kettles, centrifuges, fans, submersible motors, refrigerators, internal combustion engines, cooling water pumps, marine pumps, and other types of hosts. 2. Classification by principle of operation and structure: 1) Based on the number of sealing surfaces, there are single-surface mechanical seals, double-surface mechanical seals, and multi-surface mechanical seals. Among these, double-surface mechanical seals can be further divided into axial double-surface mechanical seals and radial double-surface mechanical seals. 2) Based on whether the fluid pressure acting on the sealing surface is unloaded or not, mechanical seals can be classified into unbalanced mechanical seals, partially balanced mechanical seals, and fully balanced mechanical seals. 3) Depending on whether the stationary ring is installed on the inside or outside of the sealed end face, they are divided into internal mechanical seals and external mechanical seals. 4) When the spring is located within the fluid, it is a spring-mounted mechanical seal; otherwise, it is a spring-externally mounted mechanical seal. 5) Based on the number of springs in the compensation mechanism, mechanical seals are divided into single-spring type mechanical seals and multi-spring type mechanical seals. 6) Depending on whether the elastic element rotates with the shaft, there are rotating mechanical seals and stationary mechanical seals. 7) If the leakage direction of the sealing fluid between the sealing surfaces is in line with the centrifugal force caused by the rotation of the shaft, it is an internal-flow mechanical seal; otherwise, it is an external-flow mechanical seal. 8) Based on whether the back side of the compensation ring, which is farthest from the sealing end face, is on the low-pressure side or the high-pressure side of the fluid, mechanical seals are classified into back-side low-pressure mechanical seals and back-side high-pressure mechanical seals. 9) When the sealed end faces are in direct contact, it is a contact mechanical seal; otherwise, it is a non-contact mechanical seal, which can be further divided into hydrostatic and hydrodynamic mechanical seals. 10) Based on the presence or absence of a bellows component, mechanical seals are divided into bellows-free types and bellows-type types; bellows can be of various types such as hydraulically formed metal bellows, welded metal bellows, polytetrafluoroethylene bellows, and rubber bellows. 3. Classification by operating conditions: 1) Based on the temperature of the sealing chamber, mechanical seals are divided into high-temperature mechanical seals, medium-temperature mechanical seals, normal-temperature mechanical seals, and low-temperature mechanical seals. Mechanical seals with a sealing chamber temperature > 200°C are considered high-temperature mechanical seals ； >80~200℃ is the medium-temperature mechanical seal ； -50~80°C is the normal temperature range for mechanical seals ； It is an ultra-high pressure mechanical seal at 15 Mpa ； Mechanical seals with a seal chamber pressure of >5~15 Mpa are considered high-pressure mechanical seals; those with a pressure of >0.8~5 Mpa are medium-pressure mechanical seals, while those operating at normal pressure up to 0.8 Mpa are low-pressure mechanical seals ； When the pressure in the sealing chamber is negative, it is a vacuum mechanical seal. 3) They can be classified based on the linear velocity of the sealed end face as follows: when the average linear velocity v of the end face is >100 m/s, it is considered an ultra-high-speed mechanical seal ； v≥30~100m/s corresponds to high-speed mechanical seals ； A medium-speed mechanical seal has a velocity of 2.5~30 m/s; for example, the average linear velocity at the end face is 120 mm/s ； For standard shaft-diameter mechanical seals, the shaft diameter ranges from 25 to 120 mm ； Small-bore mechanical seal, shaft diameter 120mm. (2) A mechanical seal is considered medium-sized if it meets one of the following three parameters or has a certain shaft diameter: Seal chamber pressure: 5 Mpa ≥ p ≥ 0.5 Mpa ； Sealed chamber temperature: -50℃