Dry gas seals and wet gas seals are two critical components in industrial compressors.
While both serve to prevent gas leakage, they operate differently.
In this blog post, we’ll explore the key distinctions between these two seal types, helping you make an informed decision for your specific application and optimize your compressor’s performance.
What is a Wet Gas Seal
A wet gas seal, also known as an oil seal or liquid seal, is a type of mechanical seal used in centrifugal compressors to prevent the leakage of process gas along the rotating shaft. It utilizes a pressurized liquid, usually oil, to create a barrier between the process gas and the atmosphere.
How Does a Wet Gas Seal Work
In a wet gas seal, high-pressure oil is circulated between a series of rings around the compressor shaft. The seal assembly typically consists of a rotating ring attached to the shaft and two stationary rings on each side in the seal housing. The oil flows between these rings, lubricating them and acting as a barrier to prevent gas leakage.
The center ring rotates with the shaft, while the outer rings remain stationary, pressed against the thin oil film. O-rings are used to prevent leakage around the stationary rings. A small amount of gas dissolves into the oil barrier, but the majority of the seal oil leakage and absorbed gas is collected and passed through a degassing process before recirculating.
Advantages of Wet Gas Seals
- Tolerance to Contaminants: Wet seals are generally more tolerant of small amounts of dirt, debris, or liquids in the process gas compared to dry gas seals.
- Proven Technology: They have been in use for many decades and are a well-proven technology for many compressor applications.
- Suitability for Older Designs: Wet seals may be a more suitable choice for certain older compressor designs or operating conditions where dry gas seals are not feasible.
- Lower Initial Capital Cost: The initial capital cost of a wet seal system is often lower than a dry gas seal system.
Disadvantages of Wet Gas Seals
- Higher gas leakage rates: Wet seals exhibit significantly higher gas leakage rates compared to dry gas seals, with most leakage occurring during the oil degassing process and often vented to the atmosphere.
- Additional equipment required: The seal oil system necessitates extra equipment like pumps, filters, degassing tanks, and treatment facilities, thereby increasing complexity and maintenance requirements.
- Increased power consumption: The circulating oil system consumes notably more power than dry gas seal systems, adding to operational costs.
- Risk of contamination: Oil contamination of the process gas or vice versa is possible, potentially affecting downstream equipment or gas quality.
- Reduced reliability and lifespan: Wet seals tend to be less reliable and have shorter operating lifespans compared to dry gas seals, with seal oil issues commonly causing unplanned compressor downtime.
Applications of Wet Gas Seals
Wet gas seals are used in many types of centrifugal compressors in oil & gas production, gas processing, petroleum refining, chemical manufacturing, and other industries.
Wet seals are still common in older compressors.However, they are now typically only specified where absolutely required by the compressor design and operating parameters.
What is Dry Gas Seal
A dry gas seal is a type of non-contacting mechanical seal used in centrifugal compressors and pumps to prevent process gas leakage along the rotating shaft. Unlike traditional wet (oil-lubricated) seals, dry gas seals rely on a thin film of gas to maintain a gap between the stationary and rotating seal faces, eliminating physical contact and wear.
How Dry Gas Seals Work
Dry gas seals consist of a stationary primary ring and a rotating mating ring. The rotating ring features shallow spiral grooves that, during operation, generate a fluid-dynamic lifting force.
This force separates the stationary ring from the rotating ring, creating a minute gap (typically 3-5 microns) between the seal faces. The gap is maintained by a small amount of filtered seal gas (usually nitrogen) that flows between the faces, preventing process gas from escaping to the atmosphere.
Advantages of Dry Gas Seals
- Higher reliability and longer life: Since the seal faces are separated by a thin film of gas and do not contact each other, there is virtually no wear. Dry gas seals can operate for over 10 years before needing to be refurbished, much longer than oil seals.
- Elimination of oil contamination: Using gas as the sealing medium prevents the process gas from being contaminated by seal oil.
- Reduced emissions: Gas leakage rates are 10 times lower with dry gas seals compared to oil seals.
- Lower power consumption: Dry gas seals do not require oil circulation systems and pumps, reducing energy usage by 90% or more compared to oil seal systems. Frictional losses are also much lower.
- Simpler design and lower maintenance: Dry gas seal systems have fewer components and no rotating machinery, making them more reliable and less costly to maintain than complex oil seal systems.
Disadvantages of Dry Gas Seals:
- Sensitivity to contamination: The small clearances between the seal faces can be clogged by particulates or liquids in the gas, causing the seal to fail.
- Requirement for clean, dry sealing gas: An external source of clean, dry nitrogen or process gas must be supplied at the right pressure and flow rate for the dry gas seal to function properly. This requires additional equipment.
- Potential for failures during transients: When the compressor is starting up, shutting down, or operating at low speed, the lifting force generated by the seal may be insufficient, allowing the faces to contact and potentially causing wear.
- Higher initial cost: While operating costs are lower, the upfront cost of dry gas seals and their support systems is typically higher than oil seals.
Applications of Dry Gas Seals:
- Natural gas pipelines and processing
- Hydrogen compression and transportation
- Hydrocarbon refrigeration compressors
- Synthesis gas compressors
- Carbon dioxide compression
- Air separation plants
The Key Difference between Dry Gas Seal and Wet Gas Seal
Sealing Medium
- Dry gas seals use pressurized gas (usually nitrogen or process gas) as the sealing medium between the rotating and stationary seal faces. No liquid is used.
- Wet gas seals use oil as the sealing medium. The oil acts as both a lubricant and sealant between the rotating and stationary components.
Emissions and Leakage
- Dry gas seals have significantly lower emissions and gas leakage rates, typically 1-6 scfm for a tandem seal arrangement. The sealing gas either leaks back into the compressor or is vented.
- Wet seals leak more gas, anywhere from 40-200 scfm, because the oil absorbs process gas at high pressures. This gas is then vented to atmosphere when the oil is degassed.
Auxiliary Systems
- Dry gas seal systems are simpler with fewer components – just filters, valves, and instrumentation to supply clean sealing gas.
- Wet seals require complex oil circulation systems with pumps, reservoirs, filters, coolers, degassing tanks, valves and controls to supply the sealing oil.
Contamination Risk
- Dry gas seals eliminate the risk of process gas contamination and catalyst poisoning from seal oil leaking across the seal faces.
- With wet seals, seal oil can leak into the process gas, causing quality and operational issues downstream.
Power Consumption
- Dry gas seal systems consume very little energy, about 5 kW, since they don’t have auxiliary pumps and systems.
- Wet seal oil systems require circulation pumps and heating, consuming 50-100 kW.
Reliability and Maintenance
- Dry gas seals are more reliable with lifetimes over 10 years since they are non-contacting and have fewer failure modes.
- Wet seals have more maintenance requirements and potential failure points due to the many components in the oil system. Lifetimes are typically 2-4 years.
Is a Wet Seal Better Than a Dry Seal
A wet gas seal is better for sealing high-pressure gases with liquid contaminants, as it uses a thin film of oil to lubricate and cool the seal faces.
In contrast, a dry gas seal is better for clean, dry gas applications, as it relies on a small amount of clean filtered gas to keep the seal faces apart.
Conclusion
In conclusion, both dry gas seals and wet gas seals have their unique advantages and limitations in sealing applications.
The choice between the two depends on factors such as operating conditions, maintenance requirements, and cost-effectiveness.
To make an informed decision for your specific sealing needs, consult with trusted seal manufacturers and industry experts.
