In industrial production, the sealing performance of equipment such as pumps and fans is extremely important, as it not only affects the operational efficiency of the equipment but also directly impacts production safety and economic efficiency. Mechanical seals and packing seals, as two common sealing methods, each have their own advantages and disadvantages in practical applications. This article will provide a detailed comparison of the performance characteristics of mechanical seals and packing seals and explore their applicability in different application scenarios.
Mechanical seals are an advanced sealing technology, widely used in various industrial equipment. Their main principle relies on the end faces of the stationary ring and rotating ring fitting tightly together, and maintaining relative sliding under the action of fluid pressure and the elastic force of the compensation mechanism, thereby effectively preventing fluid leakage.
Mechanical seals generally have a long service life. They can typically be used for 1–2 years or even longer in oil and water media, and usually for more than six months in chemical media. The service life mainly depends on the usage time of the friction pair. Under normal operating conditions, a thin liquid film exists between the friction pairs, which not only serves as lubrication but also effectively cools the friction surfaces, reducing wear. In addition, many pump mechanical seals use hard alloys and graphite as the rotating and stationary rings. Graphite itself has good lubricating properties, further reducing friction wear, so the service life of mechanical seals can generally reach more than 8,000 hours, and during operation, they require no maintenance or repair, allowing long-term continuous operation.
The sealing performance of mechanical seals is excellent. They are not sensitive to shaft vibration, eccentricity, or misalignment of the shaft relative to the seal chamber and have good floating capability. During long-term operation, the sealing condition of mechanical seals is very stable, with extremely low leakage, usually only 3–5 milliliters per hour, and often almost invisible to the naked eye. In contrast, the leakage of packing seals is often tens of times higher than that of mechanical seals.
Mechanical seals have a very wide range of applications. They can adapt to low temperature, high temperature, vacuum, high pressure, different rotational speeds, and various corrosive media and abrasive media. Whether it is centrifugal pumps, pipeline pumps, chemical process pumps, circulation pumps in chemical plants, or large fans, mechanical seals can replace traditional packing seals.
The friction loss of mechanical seals is relatively small. Since the friction pair only contacts at the end face, with a small contact area and low pressing force, the friction power consumption is low. Tests have shown that the friction power of mechanical seals is generally only about 10%–50% of that of packing seals. This makes mechanical seals more energy-efficient during operation.
Packing seals are a traditional sealing method with a simple structure, low cost, and convenient maintenance. They work by compressing the packing in the gland, causing the packing to deform and tightly contact the outer surface of the shaft, thereby preventing fluid outflow and air ingress into the pump. The sealing of packing can be controlled by adjusting the tightness of the packing gland. A reasonable tightness should allow liquid to drip from the packing chamber, controlled at about 15–20 drops per minute.
Packing seals are suitable for transporting general media, such as water. Because they have higher leakage and power consumption, they are generally not suitable for petroleum and chemical media, especially for sealing valuable, explosive, or toxic media. When selecting packing, factors such as plasticity, chemical stability, self-lubricating performance, wear resistance, and floating elasticity need to be considered. The packing should have a certain plasticity to produce radial force under pressing and tightly contact the shaft; sufficient chemical stability to avoid contaminating the medium, swelling in the medium, or being dissolved by impregnating agents, and not corroding the sealing surface; good self-lubrication, wear resistance, and low friction coefficient; enough floating elasticity to accommodate minor shaft misalignment; simple manufacturing and easy packing installation.
Packing seals require frequent adjustment of the tightness during use. Over time, the lubricant in the packing gradually disappears, requiring compression of the packing to replenish the lubricant and compensate for the loosening caused by volume changes. However, frequent compression can deplete the impregnating agent, so regular replacement of packing is necessary. In addition, to maintain the liquid film and remove friction heat, a certain amount of leakage must be maintained at the packing, but this leakage is difficult to control and can reduce motor effective power, consuming energy, sometimes reaching a surprising 5%–10%.
For conveying media containing particles, packing seals have some advantages because, in such conditions, ordinary mechanical seals may experience accelerated wear of the sealing surfaces due to the particles, while packing seals are relatively unaffected. However, packing seals also have obvious disadvantages. The packing directly contacts and rotates relative to the shaft, causing wear on the shaft and sleeve, requiring regular or irregular replacement. To dissipate the friction heat generated between the packing and shaft or sleeve, a certain amount of leakage is required, which not only wastes the medium but may also cause environmental pollution.
Despite their limitations, packing seals still have application value in certain scenarios. To improve performance, optimization measures can be taken, such as selecting appropriate packing materials based on the medium's temperature, pressure, and corrosiveness; optimizing the packing structure to enhance sealing performance and service life; and using new packing technologies, such as flexible graphite or carbon fiber packing, which provide better sealing performance and wear resistance.
After a detailed understanding of the characteristics of mechanical seals and packing seals, it is evident that both sealing methods have advantages and limitations in industrial applications. To more intuitively present their differences and facilitate better decision-making in equipment selection and maintenance, the following performance comparison is provided:
The sealing effectiveness of mechanical seals is significantly better than that of packing seals. Under normal operation, mechanical seals have minimal leakage, often invisible to the naked eye. Packing seals, in contrast, leak tens of times more, making them unsuitable for scenarios requiring high sealing performance.
Mechanical seals have a long service life, generally exceeding 8,000 hours, requiring no maintenance or repair during operation, and can operate continuously for long periods. Packing seals have a relatively shorter service life, requiring frequent adjustment and replacement to maintain sealing performance.
Mechanical seals have low friction loss, generally only 10%–50% of packing seals, because only the end faces of the friction pairs contact, with a small area and low pressing force. Packing seals, by contrast, have larger contact surfaces and higher friction, increasing energy consumption and potentially reducing equipment efficiency.
Mechanical seals have a wide range of applications, suitable for low/high temperature, vacuum, high pressure, variable speeds, corrosive and abrasive media. Packing seals are mainly suitable for general media like water, and are not suitable for petroleum, chemical, valuable, explosive, or toxic media.
Packing seals have a simple structure, easy installation, and low maintenance cost. Mechanical seals require higher installation and maintenance skill levels and higher costs, but for scenarios demanding high sealing performance and reliability, the investment in mechanical seals is justified.
Choosing the appropriate sealing method in practice requires considering multiple factors, including equipment operating conditions, medium characteristics, operational requirements, and cost.
In chemical industries, equipment often handles corrosive, toxic, or explosive media, with high sealing performance requirements. Mechanical seals are the first choice in such cases, effectively preventing medium leakage, reducing environmental pollution and safety accidents. For centrifugal pumps and chemical process pumps, mechanical seals provide long-term stable operation, improving efficiency and safety.
For pumps conveying media with particles, packing seals are recommended over ordinary mechanical seals, as the latter would experience accelerated wear due to the particles. Packing seals are also easier to replace.
For equipment handling simple media like water, with no strict sealing performance requirements and cost considerations, packing seals offer an economical choice. They are simple, low-cost, and convenient to maintain while meeting basic sealing needs.
In high-temperature, high-pressure, or vacuum conditions, mechanical seals have clear advantages, maintaining reliable sealing performance and ensuring normal operation. For large fans or high-pressure pumps, mechanical seals are essential.
Both mechanical and packing seals are widely used in industrial production. Mechanical seals provide excellent sealing, long service life, low friction loss, and broad applicability, suitable for high-precision, high-reliability, and long-life sealing requirements. Packing seals have the advantages of simple structure, low cost, and easy maintenance, suitable for general media. In practice, the sealing method should be selected based on specific operating conditions and medium characteristics to improve equipment efficiency and safety. With continuous technological advancement, both mechanical and packing seals are evolving, promising even more efficient and reliable sealing technologies in the future.