A mechanical seal keeps gas or fluid contained within the compressor casing while allowing the shaft to rotate — so nothing leaks into the environment.
If you’re considering a gas compressor, you’ll likely notice two options for shaft sealing: single face mechanical seals or double bellows (double face) mechanical seals. Each has its place depending on your application. Here’s how they compare.
Single Face Mechanical Seals
While the concept of a seal is straightforward, in operation inside a rotary sliding vane compressor, the execution is more complex. Single face mechanical seals utilize a spring to press together two flat surfaces (faces), which slide against each other. Lubricating oil is injected into the seal housing to cool and lubricate the seal’s internals. The lubricating oil forms into a thin film between the sealing faces, creating a barrier between the process gas and the atmosphere.
When to Use a Single-Face Seal
- For clean, non-hazardous gases or fluids.
- When pressures and temperatures are moderate and there’s no aggressive or toxic media.
Pros
- Simple, robust design
- Easy to install and maintain
- Lower initial cost than double-seal systems
What You Must Do to Keep it Working
- Ensure proper lubricating oil flow and quality
- Keep debris away from sealing faces
- Have clean installation and undamaged O-rings or seal faces
Double Bellows Mechanical Seals
This type of seal has two (2) sets of contact faces:
- Two sets of faces arranged back-to-back (dual-opposed)
- Faces are separated by a coil spring
- Spring pushes each face set in opposite directions
The double bellows mechanical seal requires a lube oil support system. This is designed to:
- Keep the seal parts submerged in oil
- Maintain oil system pressure higher than the pressure inside the compressor
- Ensure that if leakage occurs at the outer seal, it moves toward the atmosphere (preventing air from being drawn into the compressor)
- Ensure that if leakage occurs at the inner seal, lube oil is pushed into the compressor
- Detect either type of leakage with a pressure sensor on the oil support system so the compressor can be shut down before any gas leaks into the atmosphere
Details of the required lube oil support system can be found in Ro-Flo’s Installation, Operation and Maintenance Manual.
Critical Operating Requirement
It is important to note:
- The lube oil support system must be pressurized before pressurizing the compressor or gas piping system
- This applies whether the system is being pressure-tested or operated
- If the gas side is pressurized before the support system, the seal is likely to fail
Common Applications
Double bellows mechanical seals are used in applications where:
- Additional protection is desired
- Safety and environmental regulations require enhanced containment
- Hazardous or sensitive gas management is essential
- Industries such as oil and gas or biogas need reliable sealing performance
Because many Ro-Flo users operate in these industries, this type of seal is quite common.
Key Differences at a Glance
| Feature / Consideration | Single-Face Seal | Double-Face Seal (Double Bellows) |
| Sealing faces | 1 set (2 faces) | 2 sets (4 faces) |
| Lubrication / barrier | Uses pump oil film | Uses dedicated barrier oil between seals |
| Complexity | Simple, fewer components | More complex; requires oil support system |
| Suitability | Clean fluids, non-hazardous, moderate conditions | Hazardous/toxic gases, high pressure, safety-critical cases |
| Maintenance & cost | Lower cost & easier maintenance | Higher cost and requires more care |
| Leak protection | Basic leak prevention | High containment / redundancy |
Key Takeaways
- Single-face seals offer simplicity and cost-efficiency and are ideal for non-hazardous, moderate-duty applications.
- Double-face (double-bellows) seals provide enhanced containment and safety which is essential for hazardous, toxic, or high-pressure gas applications.
- For demanding applications (e.g., oil & gas, biogas, sour gas), a double seal significantly reduces the risk of environmental release or safety incidents, but only if installed and maintained correctly.
- Seal selection should always be based on fluid type, pressure/temperature, safety/environmental requirements, and risk tolerance, not just initial cost.
Need Guidance?
If you’re ready to purchase a gas compressor, trust the experts at Ro-Flo to help you through the process. Our team has years of experience and is trusted by the top 25 largest oil and gas companies in the world. Contact us today to get started.
