Mechanical seal

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Overview of the working principle of mechanical seals for pumps. Mechanical seals are a common type of shaft seal used in rotating machinery. Mechanical seals are primarily used for sealing various media such as gases and liquids, in devices like pumps and reaction vessels; they are also used for gas sealing, such as sealing the shafts of compressors. Mechanical seals have widely replaced gland packing seals due to their low leakage rate and the fact that they require no routine maintenance. A mechanical seal typically has two sealing surfaces that are perpendicular to the axis of the rotating shaft, and therefore can also be referred to as “radial face seals”. One of the sealing surface components is mounted on the pump cover, end cover, or pump body and remains stationary; the other is fixed to the shaft and rotates along with it. The stationary one is called the static ring assembly (referred to as the floating ring assembly in stationary mechanical seals), while the rotating one is called the dynamic ring (rotating ring) assembly. To keep the leakage at an extremely low level, the tolerance for the flatness of the grinding surfaces on the rotating and stationary rings of conventional mechanical seals is generally 0.0006. To keep the heat generated by friction and the amount of wear on the two surfaces within acceptable limits, there must be a layer of fluid lubrication film between the sealing surfaces. For single-face seals, the sealing medium is usually used as a lubricant. However, for double-face or tandem mechanical seals used with toxic, harmful, flammable, explosive, or highly corrosive media, a sealing fluid that is compatible with such media is used as both a lubricant and a cooling fluid. To compensate for wear and manufacturing errors, there should be a sealing surface that can move a certain amount axially under the combined action of the elastic element and the medium pressure; meanwhile, to accommodate the oscillations caused by angular deviations of rotating parts, there should be a sealing end face that can move slightly radially. Usually, a single sealing surface serves both functions. For the sealing of rotating shafts, mechanical seals are the most common type of seal. Mechanical seals are commonly used for liquid sealing, such as in centrifugal pumps; they are also used for gas sealing, such as in compressors and mixing devices. In this textbook, we mainly introduce mechanical seals for pumps used with liquid media. Mechanical seals have widely replaced packing seals due to their low leakage rate and the fact that they require no routine maintenance. Mechanical seals generally have two sealing surfaces that are perpendicular to the axis of rotation, and therefore can also be referred to as “radial face seals”. One of the sealing surfaces is mounted on a fixed pump bracket or pump body and is called the \"static ring\", while the other is mounted on the rotating shaft and rotates together with it in a coaxial manner; it is called the \"dynamic ring\". To keep the leakage rate of the medium within acceptable limits, these two contacting surfaces that move relative to each other must maintain a very small gap, usually less than 0.001 mm; moreover, an auxiliary system is required to cool and lubricate the end faces of the mechanical seal. Figure 1–1 Sealing clearance: To keep the heat generated by friction between the two end faces as well as the amount of wear within acceptable limits, there must be a fluid lubrication film formed by the medium being sealed between the sealing surfaces. Theoretically, all mechanical seals have a certain degree of leakage, only less than that of other types of seals. In some cases, we cannot observe any leakage, as the liquid leaking out of the sealing surface has already vaporized before it can be seen. Generally speaking, for the same mechanical seal, leakage rate and lifespan are two parameters that are inversely related to each other: a higher leakage rate results in a thicker liquid film on the sealing surfaces, thereby leading to a longer lifespan. Therefore, when installing mechanical seals, one should not focus solely on achieving a low leakage rate; comprehensive factors need to be taken into account. If only a low leakage rate is pursued by increasing the compression of the spring, it will lead to severe wear of the sealing surfaces of the mechanical seal. At the microscopic level, the two sealing surfaces are not entirely smooth; rather, there are peaks and valleys due to surface roughness (see Figures 1-1 and 2). A liquid film exists between these peaks and valleys, but they are not connected to each other. Therefore, in a static state, leakage generally does not occur. Figures 1–2 show microscopic images of the moving ring sealing surface of the feed water pump mechanical seal. To compensate for wear and manufacturing errors, there should be a sealing surface that can move axially. At the same time, to accommodate the shaking caused by angular deviation of the rotating components, one of the sealing surfaces should be able to move radially. Usually, the same sealing surface serves both functions. The floating end can be either the stationary ring or the moving ring. However, when the linear speed on the surface of the rotating shaft exceeds 15 m/s, the floating end is set as a stationary ring to avoid the effects of centrifugal force. In fact, the operating condition of the sealing surface in a mechanical seal is the same as that in a sliding thrust bearing. However, due to the characteristics of mechanical seals such as the requirement for a small clearance, poor lubrication performance, and low lubrication flow rate, their design is difficult and their operational reliability is low. Next, we will compare the operating parameters of mechanical seals and thrust bearings: Table 1-1 shows a comparison of the parameters for mechanical seals and thrust bearings. Figures 1–3 illustrate the mechanical seal configurations that are commonly used in our power plants. Its sealing surface is the contact surface between the planes of the moving and stationary rings during relative rotation. The sealing surface is ground to achieve a very high level of flatness, which can be measured in terms of the wavelength of light. According to China’s JB4127-85 standard on technical requirements for mechanical seals, the flatness of the sealing surface should not exceed 0.9 um; however, the typical requirement at the time of manufacturing is that it be no more than 0.6 um. In rotary mechanical seals, the moving ring can move along the axis; in stationary mechanical seals, the stationary ring (also known as the floating ring) can move axially. In this way, during operation, the pressure of the sealing fluid and the force of the spring keep the sealing surfaces in contact, and the spring can still compensate for any lack of fluid pressure. Figures 1–3 Mechanical seal structure. The magnitude of the sealing compression force should be determined by taking into account both leakage and wear factors. In situations with significant mechanical vibration or pressure fluctuations, sufficient clamping force is necessary to ensure the bonding of the sealing surfaces. Auxiliary static seals usually use O-ring rubber gaskets. Friction resulting from the mutual sliding between the sealed end faces generates a large amount of heat. Seals of medium size and speed have a heat generation rate ranging from 100 to 1000 W. This heat is transferred through the sealing element and eventually to the surrounding liquid, where it is carried away by the sealing flushing or cooling system. The liquid that leaks out also carries away a small amount of heat. The temperature between the sealed end faces must be safely kept below the boiling point of the sealing medium; it is extremely important to note that vaporization of this medium can cause severe wear on the seal. Typically, liquid is drawn from the high-pressure side of the pump to flush the mechanical seal (as will be explained in more detail later). If the temperature of the medium is too high, then an additional cooling system is necessary. The sealing medium circulates within the cooling system due to the pressure difference at the inlet and outlet of the pump, or thanks to the pumping mechanism built into the design of the seal itself; the heat is then removed by the secondary cooling water passing through the cooler. To prevent particles and impurities in the medium from wearing out the sealing surfaces, a filter or hydrocyclone is usually also installed in this system. The material selection for the sealed end face takes into account the following key factors: good sliding properties (adhesion compatibility); resistance to chemical corrosion by the medium (chemical compatibility); and the ability to withstand temperature and other operational factors (such as wear resistance and thermal conductivity). Typically, one of the sealing rings is made of carbon-graphite material, while the other ring paired with it is made of metal, alloy, ceramic, or cemented carbide material. Carbon-11 graphite combines sliding properties with chemical compatibility (corrosion resistance). When used as a sealing ring, carbon-graphite is usually treated with resin or metal impregnation. The lubrication of the sealed interface is self-regulating. Depending on the structure and workload, its lubrication can be provided by a liquid film that separates the two end surfaces, or through point contact caused by the irregularities such as peaks and valleys on the end surfaces, in combination with a liquid film. Only when the sliding interface is properly lubricated can a satisfactory sealing effect be achieved; otherwise, the end surfaces of the mechanical seal will wear out quickly, resulting in a reduced lifespan.