What Is The Difference Between Dry Gas Seal And Wet Gas Seal?

Gas seals are an essential component in various industries, including oil and gas, petrochemicals, and power generation. They are used to prevent gas leakage and ensure the efficient operation of rotating equipment such as compressors, pumps, and turbines. When it comes to gas seals, two common types are dry gas seals and wet gas seals. While both serve the same purpose, they have distinct differences in terms of design, operation, and application. In this article, we will explore the dissimilarities between dry gas seals and wet gas seals, providing a comprehensive comparison to help you understand their unique characteristics and determine the most suitable option for your specific needs.

The Basics of Dry Gas Seals

Dry gas seals are designed to separate the process gas from the atmosphere without using any external liquid. They are primarily used in applications where process gas leakage is not permissible, such as in hydrocarbon processing, natural gas compression, and refinery processes. The main components of dry gas seals include seal faces, a rotor, a stator, and a barrier gas system. The seal faces are constructed of hard material, such as silicon carbide or carbon, to withstand the high pressures and temperatures commonly encountered in industrial processes. The barrier gas system supplies a clean and dry gas to the seal faces, creating a barrier that prevents the process gas from escaping into the atmosphere. Dry gas seals are known for their high reliability and long mean time between repairs (MTBR), making them a popular choice for critical rotating equipment.

One of the key advantages of dry gas seals is their ability to operate without the need for an external liquid supply. This eliminates the risk of environmental contamination and simplifies the seal system design. Additionally, dry gas seals are capable of handling a wide range of process gas compositions and operating conditions, making them versatile for various industrial applications. However, it is important to note that dry gas seals require a reliable source of barrier gas to maintain an effective seal, and any deviation from the specified operating conditions can lead to seal failure.

The Fundamentals of Wet Gas Seals

Contrary to dry gas seals, wet gas seals rely on an external liquid, typically oil or water, to create a seal between the process gas and the atmosphere. They are commonly used in applications where the process gas contains a significant amount of liquid or where the operating conditions do not allow for a reliable supply of clean and dry barrier gas. Wet gas seals consist of seal faces, a seal chamber, a liquid supply system, and a drainage system. The seal faces are designed to operate in a lubricated environment, which helps to reduce friction and wear between the sealing surfaces. The liquid supply system provides a continuous flow of clean liquid to the seal faces, creating a barrier that prevents the process gas from leaking out. The drainage system ensures that any liquid that bypasses the seal faces is safely collected and removed from the system.

One of the primary advantages of wet gas seals is their ability to effectively seal process gas streams that contain liquid contaminants. This makes them suitable for applications such as gas compression, refrigeration, and vapor recovery, where the presence of liquid components in the process gas cannot be avoided. Additionally, wet gas seals require less stringent requirements for the quality of the liquid supply compared to dry gas seals, making them more forgiving in terms of operating conditions. However, the reliance on an external liquid introduces potential risks associated with liquid contamination, seal face clogging, and the need for careful monitoring and maintenance of the liquid supply and drainage systems.

Design and Operation Differences

The design and operation of dry gas seals and wet gas seals differ significantly due to their reliance on different sealing mechanisms. Dry gas seals utilize a non-contacting seal face arrangement, where the seal faces do not touch during normal operation. Instead, they are separated by a small film of barrier gas, typically nitrogen, which creates a gas barrier that prevents the process gas from leaking out. This non-contacting design reduces friction and wear on the seal faces, leading to longer service life and reduced maintenance requirements. In contrast, wet gas seals use a contacting seal face arrangement, where the seal faces come into direct contact with the liquid supplied to the seal chamber. This contacting design provides effective sealing in the presence of liquid contaminants but requires proper lubrication to prevent excessive friction and wear on the seal faces.

Another design difference between dry gas seals and wet gas seals is the presence of a secondary sealing system. Dry gas seals incorporate a secondary seal, such as a labyrinth seal or a magnetic non-contacting seal, to provide an additional barrier against gas leakage in the event of a primary seal failure. This dual-seal configuration enhances the reliability and safety of dry gas seal systems, especially in critical applications where gas leakage can have serious consequences. On the other hand, wet gas seals typically do not include a secondary sealing system, as the continuous supply of liquid to the seal faces serves as the primary sealing mechanism. However, some wet gas seal designs may incorporate a dual liquid supply system or a separate liquid containment system to address potential issues with liquid leakage.

Application Considerations

When selecting between dry gas seals and wet gas seals for a specific application, it is essential to consider various factors, including the process gas composition, operating conditions, reliability requirements, and environmental considerations. Dry gas seals are well-suited for applications where the process gas is clean and dry, and the risk of environmental contamination must be minimized. They are commonly used in gas compressors, turboexpanders, and high-speed rotating equipment operating in hydrocarbon processing and petrochemical plants. Dry gas seals are also favored in applications with stringent safety and emission control requirements, as they offer a high level of reliability and environmental compliance.

In contrast, wet gas seals are preferred for applications where the process gas contains liquid contaminants, such as oil, water, or hydrocarbons, and a reliable source of clean and dry barrier gas is not available. They are commonly employed in gas compressors, refrigeration systems, and vapor recovery units, where the presence of liquid components in the process gas necessitates a seal design that can effectively handle liquid contamination. Wet gas seals are also used in applications with fluctuating operating conditions or limited access to utilities, as they are less sensitive to variations in the quality of the sealing fluid and require less stringent maintenance of the barrier liquid supply.

When evaluating the suitability of dry gas seals and wet gas seals for a specific application, it is essential to consider the trade-offs between reliability, maintenance requirements, and environmental impact. Dry gas seals offer high reliability and minimal environmental impact but require a reliable source of clean and dry barrier gas. Wet gas seals provide effective sealing in the presence of liquid contaminants but require proper maintenance of the liquid supply and drainage systems to prevent potential issues with liquid contamination and seal face wear. Ultimately, the selection between dry gas seals and wet gas seals should be based on a thorough assessment of the application requirements and a comprehensive evaluation of the benefits and limitations of each sealing solution.

Conclusion

In conclusion, the choice between dry gas seals and wet gas seals depends on a range of factors, including the process gas composition, operating conditions, reliability requirements, and environmental considerations. Dry gas seals offer high reliability and minimal environmental impact but require a reliable source of clean and dry barrier gas. Wet gas seals provide effective sealing in the presence of liquid contaminants but require proper maintenance of the liquid supply and drainage systems. Both sealing solutions have their unique advantages and limitations, and it is crucial to carefully evaluate the specific needs of the application to determine the most suitable option. By understanding the differences between dry gas seals and wet gas seals, as well as their respective design, operation, and application considerations, you can make an informed decision when selecting a gas sealing solution for your rotating equipment.