Maintenance Methods for Mechanical Seals Under Severe Operating Conditions

Mechanical seal failures account for approximately half of the total maintenance workload in pump malfunctions. This highlights the critical role of mechanical seals in pump maintenance. Pumps handle a wide range of media and operate under diverse process conditions, so understanding these factors is essential for the selection and maintenance of mechanical seals. Based on years of experience in mechanical seal selection and maintenance, this article outlines the operating characteristics and key considerations for mechanical seals under several special operating conditions.

1. Mechanical Seals Under High-Temperature Conditions

Typically, a seal is considered a high-temperature seal when the medium temperature exceeds 120°C. The main challenges for mechanical seals under high temperatures include:

As the temperature of the friction pair end faces rises, the friction coefficient increases, leading to intensified wear and higher temperature elevation. This can cause thermal deformation or thermal cracking of the seal rings, resulting in seal failure.

The auxiliary seal rings in mechanical seals are usually made of rubber or PTFE. Prolonged exposure to high temperatures easily causes aging and decomposition, leading to seal failure.

Elastic components of mechanical seals are prone to fatigue and creep under high temperatures, resulting in seal failure.

High temperatures accelerate corrosion of metal materials, shortening the service life of mechanical seals.

To ensure the normal operation of mechanical seals under high-temperature conditions, the following measures can be taken:

Install a cooling and flushing device for the mechanical seal.

Select high-temperature-resistant materials, and choose different materials based on specific operating temperatures.

Use metal bellows mechanical seals. In recent years, metal bellows seals have been widely used in high-temperature sealing applications and achieved excellent results.

2. Mechanical Seals Under Low-Temperature Conditions

Media transported under low-temperature conditions generally have low viscosity, poor lubricity, and high volatility. When the medium vaporizes, it absorbs a large amount of heat, causing a sharp drop in the surrounding temperature. This leads to condensation of water vapor in the air, resulting in icing on and around the end faces, which impairs the tight fit of the end faces and causes leakage. Meanwhile, poor lubricity leads to excessive heat generation from end face friction, which further intensifies vaporization at the end faces and accelerates early failure of the seal faces. In addition, the auxiliary seal rings of low-temperature seals become hard and lose elasticity under low temperatures, which also easily causes leakage.

For low-temperature seals, in addition to selecting appropriate materials for the auxiliary seal rings, the following points should be noted:

Improve the seal structure and select the correct friction pair materials.

Choose an appropriate end face specific pressure. Due to poor lubricity, high volatility, and icing risks of the end faces in low-temperature seals, the end face specific pressure can be appropriately reduced while ensuring reliable sealing. The recommended specific pressure for general mechanical seals is 0.29-0.59MPa, and the lower limit can be adopted for low-temperature seals. For metal bellows mechanical seals, since there is no axial friction on the rotating ring, the end face specific pressure can even be lower than the lower limit.

Implement corresponding process measures, such as minimizing the water content in the medium, maintaining relatively stable pressure in the seal chamber, equipping flushing and cooling devices as much as possible, and properly preheating the mechanical seal area before starting the equipment.

3. Mechanical Seals Under High-Speed Conditions

A seal is generally considered a high-speed seal when the rotational speed exceeds 3000r/min or the linear speed of the friction pair exceeds 25m/s. Under high-speed conditions, to reduce the impact of centrifugal force and vibration, the number of rotating components should be minimized, and a spring-stationary structure should be adopted. Additionally, cooling and lubrication of the friction pair end faces can be enhanced. Furthermore, friction pair materials with high [PV] values can be selected, the contact width of the end faces can be reduced, or a controlled-film mechanical seal can be used.

4. Mechanical Seals Under High-Pressure Conditions

Mechanical seals operating at pressures exceeding 4-5MPa fall into the high-pressure seal category. Under high-pressure conditions, excessive specific pressure on the seal end faces causes end face heating, which destroys the formation of the liquid film and leads to abnormal wear. High pressure may also cause deformation and cracking of the friction pair. High-pressure seals generallyadopt a balanced structure to keep the end face specific pressure within an appropriate range. However, considering seal reliability, the balance coefficient cannot be increased indefinitely. Therefore, in some cases, even with a balanced structure, the end face specific pressure may still be relatively high. A balanced structure alone may not meet the application requirements, so a multi-end-face mechanical seal or a controlled-film mechanical seal can be used in such cases.

5. Mechanical Seals Under Corrosive Environments

Some mechanical seals operate in corrosive environments. In such cases, attention should be paid not only to the corrosion resistance of each component of the mechanical seal itself but also to preventing galvanic coupling effects that may occur when these components are combined. For mechanical seals in corrosive environments, corrosion-resistant materials should be selected during selection. In addition, external-mounted mechanical seals can be used. For highly corrosive environments that may cause safety accidents, double-end-face seals can be used, with an isolation fluid introduced for protection.

6. Mechanical Seals Under Particulate Medium Conditions

Some seals operate in media containing solid particles. When solid particles enter the friction end faces, they easily cause severe wear, leading to rapid failure of the seal end faces. In addition, solid particles may prevent the rotating ring seal from moving axially, making the rotating ring unable to float and lose its compensating ability, resulting in failure of the mechanical seal. An effective solution to this problem is to install an external flushing device. Since the pressure of the flushing fluid introduced into the seal chamber is higher than the pressure of the medium in the seal chamber, it can effectively prevent particulate media from entering the seal chamber. Additionally, a double-end-face mechanical seal can be used, with a sealing fluid injected between the seal chambers. An external-mounted mechanical seal can also be adopted.

Currently, mechanical seals have become the most common seal type in rotating equipment of oil refining and chemical plants. With the continuous improvement of their design and manufacturing standards and the continuous development of new materials, their application range will be further expanded.

If you need, I can help you optimize the translation of professional terms in specific sections (such as "friction pair" "balanced structure") for different industry scenarios, or convert this document into a more readable English version in the form of a table.