Introduction
With the widespread adoption of Dry Gas Seal (DGS) technology, it has become the mainstream shaft sealing solution for compressors. In industries such as natural gas transmission, petrochemicals, and coal chemical processing, dry gas seals have gradually replaced traditional oil-film seals due to their advantages of low leakage, low power consumption, and long service life.
However, during project design and equipment selection, many users often ask the same question: Are dry gas seals used in screw compressors and centrifugal compressors the same?
Although both belong to the category of dry gas seals, significant differences exist in seal design priorities and auxiliary system configurations due to variations in compressor structure, operating parameters, and process conditions.
1. Compressor Structure Determines Seal Design Differences
Centrifugal compressors are high-speed turbomachinery that increase gas pressure through the high-speed rotation of impellers.
Their characteristics include:
- High rotational speed
- Small axial displacement
- Strict vibration control requirements
- Long continuous operating periods
In contrast, screw compressors are positive displacement machines that compress gas through the meshing of male and female rotors.
Their characteristics include:
- Relatively lower rotational speed
- Larger variations in axial force
- Frequent start-stop cycles
- Higher adaptability requirements for process gases
Due to these operational differences, the structural design of dry gas seals also varies accordingly.
2. Impact of Rotational Speed on Dry Gas Seals
The operating speed of centrifugal compressors is typically between 6,000 and over 30,000 rpm.
High rotational speed allows a stable gas film to form quickly between the seal faces, resulting in higher gas film stiffness and excellent sealing performance.
By comparison, screw compressors generally operate between 3,000 and 15,000 rpm.
Because of the lower surface velocity:
- Gas film lift-off capability is weaker
- Seal face clearance control becomes more critical
- Hydrodynamic groove design requires greater optimization
Therefore, dry gas seals for screw compressors often require specialized designs optimized for low-speed operating conditions.
3. Different Requirements for Axial Movement Compensation
Axial shaft movement is one of the key factors affecting dry gas seal service life.
Centrifugal Compressors
Centrifugal compressors are generally equipped with high-precision thrust bearings.
During normal operation:
- Axial displacement is relatively small
- Rotor stability is high
- Changes in seal face clearance are limited
As a result, dry gas seals operate under relatively stable conditions.
Screw Compressors
During operation, screw compressor rotors experience:
- Significant compression-induced forces
- Greater thermal expansion variations
- Noticeable axial movement during start-up and shutdown
Therefore, the seal must possess stronger compensation and tracking capabilities.
If the design is inadequate, it may lead to:
- O-ring hang-up
- Seal face contact
- Accelerated seal face wear
4. Differences in Process Gas Cleanliness
Based on practical engineering experience, process gas quality is one of the most important factors affecting dry gas seal reliability.
Centrifugal Compressor Applications
Typical process gases include:
- Natural gas
- Hydrogen
- Synthesis gas
- Cracked gas
These gases are usually subjected to strict filtration and treatment processes, resulting in relatively high cleanliness levels.
Screw Compressor Applications
Common process gases include:
- Flare gas
- VOC recovery gas
- Biogas
- Coke oven gas
- Oil-contaminated process gas
These media often contain:
- Liquid droplets
- Oil mist
- Dust particles
- Polymer contaminants
When contaminants enter the seal faces, they can block the hydrodynamic grooves, disrupt the gas film, and ultimately cause seal failure.
Therefore, in many projects, dry gas seal failures in screw compressors are not caused by the seal itself but by insufficient gas conditioning and filtration systems.
5. Differences in Seal Support System Configuration
Centrifugal compressors are typically equipped with complete seal gas systems in accordance with international standards.
A typical configuration includes:
- Primary filters
- Coalescing filters
- Pressure regulation systems
- Flow control systems
- Heaters
- Online monitoring instruments
For high-pressure natural gas compressors, a complete seal gas control panel is often provided.
In contrast, due to operating conditions and cost considerations, screw compressors usually employ simpler auxiliary systems, often consisting of:
- Filters
- Pressure regulation systems
However, for dirty gas applications, additional equipment is still recommended, including:
- Gas-liquid separators
- Coalescing filters
- Heaters
- Automatic drain devices
These components help ensure that the gas entering the dry gas seal remains clean and dry at all times, thereby improving seal reliability and service life.