What Are Dry Gas Seals

In the realm of rotating machinery, the effective sealing of process gases is of utmost importance for ensuring optimal performance and reliability. Dry gas seals have emerged as a widely adopted sealing solution, offering distinct advantages over traditional liquid-lubricated seals.

This blog post delves into the intricacies of dry gas seals, exploring their design, operating principles, and applications within various industrial sectors.

What Are Dry Gas Seals

Dry gas seals are a type of non-contacting mechanical seal commonly used in centrifugal compressors to prevent process gas leakage along the rotating shaft. These seals operate without the need for liquid lubrication, instead relying on a thin film of gas to maintain a gap between the sealing surfaces. This separation is achieved through a combination of grooves on the mating rings and the hydrodynamic forces generated by the rotation of the shaft.

Dry gas seals offer several advantages over traditional wet (oil-lubricated) seals, including lower energy consumption, reduced contamination risk, and improved compressor efficiency. They are widely used in various industries, such as oil and gas processing, petrochemical plants, and gas transport applications.

Components of Dry Gas Seals

• Rotating Ring: The rotating ring is attached to the compressor shaft and features a precision-machined face with a spiral groove pattern. This pattern helps to generate the hydrodynamic lift force necessary to maintain the gap between the sealing surfaces during operation.
• Stationary Ring: The stationary ring is mounted in the seal housing and has a smooth sealing face that mates with the rotating ring. It is typically made of a hard, wear-resistant material such as silicon carbide or tungsten carbide to withstand the harsh operating conditions.
• Seal Gas Supply System: The seal gas supply system provides a clean, dry gas (usually nitrogen) at a precisely controlled pressure to the seal cavity. This gas is essential for creating the thin film between the sealing faces and preventing contamination from the process gas. The seal gas supply system typically includes filters, pressure regulators, and flow control valves to maintain the optimal sealing environment.

In addition to these primary components, dry gas seals also incorporate secondary sealing elements, such as O-rings and gaskets, to prevent leakage between the stationary components.

How Dry Gas Seals Work

Dry gas seals operate on the principle of non-contacting, gap-type sealing technology. The sealing action is achieved through a thin film of pressurized gas that flows between the precisely manufactured seal faces, creating a barrier that prevents leakage.

The high-pressure seal gas is injected into the seal interface through ports in the stationary ring. As the gas enters the gap between the rotating and stationary rings, it expands and creates a pressure profile across the sealing faces. This pressure profile generates an opening force that separates the faces by a small gap, typically measuring a few microns.

The rotation of the shaft and the attached rotating ring generates a hydrodynamic effect within the gas film. This effect causes the gas pressure to increase radially outward, creating a stable, non-contacting seal between the rotating and stationary components.

To maintain the optimal sealing gap, dry gas seals rely on the balance between the opening force generated by the pressurized gas and the closing force exerted by the seal springs and the pressure differential across the seal faces. The seal gas supply system continuously provides clean, dry gas at a pressure slightly higher than the process gas to ensure positive sealing and prevent contamination.

The leaked gas from the primary seal is collected in the secondary seal chamber and safely vented or recycled back into the compressor.

Types of Dry Gas Seals

Dry gas seals come in various configurations to suit different application requirements. The three main types of dry gas seals are single seals, tandem seals, and double opposed seals.

Single Seals

Single dry gas seals consist of a single set of rotating and stationary rings. These seals are typically used in applications with lower pressure differentials and less demanding operating conditions. Single seals are more compact and cost-effective compared to other types of dry gas seals.

The seal gas is introduced between the rotating and stationary rings, creating a thin film that prevents direct contact between the surfaces. This film maintains a stable gap, allowing for efficient sealing and minimizing wear on the components.

Tandem Seals

Tandem dry gas seals feature two sets of rotating and stationary rings arranged in series. This configuration provides an additional layer of protection and redundancy, making tandem seals suitable for more critical applications or environments with higher pressure differentials.

In a tandem seal arrangement, the primary seal functions as the main sealing element, while the secondary seal serves as a backup. If the primary seal fails or experiences excessive leakage, the secondary seal takes over to maintain the sealing integrity of the system.

The seal gas is supplied to both the primary and secondary seals, ensuring a consistent and reliable sealing performance. Tandem seals offer enhanced safety and reliability, as they can continue operating even if one set of sealing components fails.

Double Opposed Seals

Double opposed dry gas seals consist of two sets of rotating and stationary rings arranged in an opposed configuration. In this arrangement, the sealing faces are positioned back-to-back, with the seal gas supplied between them.

The opposed design of double seals allows for pressure balancing, which helps to minimize the axial forces acting on the sealing components.

The seal gas flow in double opposed seals is managed through a series of ports and channels within the seal housing. This ensures an even distribution of the seal gas and helps to maintain a stable sealing interface between the rotating and stationary components.

Double opposed seals provide excellent sealing efficiency and are capable of handling higher pressure differentials compared to single and tandem seals. They are commonly used in demanding applications, such as high-pressure compressors and pumps, where reliable sealing performance is critical.

Advantages of Dry Gas Seals

Improved Reliability

Dry gas seals offer significantly higher reliability compared to traditional sealing methods. The non-contacting design of dry gas seals minimizes wear and tear, reducing the risk of failure and extending the seal’s operational life.

Enhanced Efficiency

Dry gas seals operate with a smaller clearance between the rotating and stationary components, resulting in reduced leakage and improved sealing efficiency.

Lower Maintenance Costs

The non-contacting nature of dry gas seals reduces friction and wear, minimizing the need for frequent maintenance and part replacements. This advantage leads to lower maintenance costs over the life of the equipment, as well as reduced downtime associated with seal maintenance activities.

Environmental Benefits

Dry gas seals provide a cleaner and more environmentally friendly sealing solution compared to traditional oil-based seals. By using inert gas as the sealing medium, dry gas seals eliminate the risk of oil leakage and contamination, helping to minimize the environmental impact of the equipment.

Disadvantages of Dry Gas Seals

Higher Initial Cost

The advanced technology and precision manufacturing required for dry gas seals often result in a higher initial cost compared to traditional sealing methods.

Sensitivity to Contaminants

Dry gas seals are more sensitive to contaminants in the sealing gas compared to traditional seals. Particles, liquids, or other impurities in the gas supply can damage the sealing faces, leading to premature failure.

Limited Temperature Range

Dry gas seals have a more limited operating temperature range compared to some traditional sealing methods. The materials used in dry gas seals may not be suitable for extremely high or low-temperature applications, restricting their use in certain industries or processes.

Applications of Dry Gas Seals

Centrifugal Compressors

Dry gas seals are widely used in centrifugal compressors, particularly in the oil and gas industry. They provide reliable and efficient sealing for high-pressure, high-speed applications, ensuring optimal compressor performance and minimizing gas leakage.

Turbomachinery

Dry gas seals find applications in various turbomachinery, such as steam turbines, gas turbines, and expanders. They offer superior sealing performance, reduced friction losses, and improved efficiency compared to traditional sealing technologies.

Petrochemical Processing

In petrochemical processing plants, dry gas seals are employed in pumps, mixers, and other rotating equipment. They help maintain process purity, prevent leakage of hazardous or valuable fluids, and contribute to the overall safety and efficiency of the plant.

Industrial Gases

Dry gas seals are used in the production, storage, and handling of industrial gases, such as hydrogen, nitrogen, and oxygen. Their non-contacting design and compatibility with various gas types make them suitable for maintaining the integrity and purity of the gases throughout the process.