Centrifugal compressors have been the workhorse of the oil and gas, petrochemicals and power generation markets for many years. Before the 1980s, wet seal technology was the primary shaft sealing solution for this equipment. In the late 1980s, dry gas seals were introduced to the market, which revolutionized compressor reliability.
Today, globally approximately 99% (John Crane Webinar, 2018) of centrifugal compressors are shipped from the OEMs using dry gas seal technology. This newer, simpler technology is broadly accepted to simultaneously meet the needs of end-users, government regulations, non-governmental organization (NGO) pressures and environmental initiatives. However, there are still thousands of wet seal equipped compressors still in operation with a substantial fleet of legacy equipment, which continues to age.
Dry gas seals have several advantages over conventional wet seals: higher reliability, safer operation, reduced emissions, lower operational and maintenance costs and improved process gas quality.
Typical Wet Seal Challenges
Outdated Technology and System Complexity
Outdated Technology and System Complexity
End-users using legacy equipment generally experience an increase in maintenance and operating costs caused by deteriorating compressor efficiency due to age, outdated technology, system complexity and lower reliability.
Maintenance Issues
While there is an increase in maintenance costs, there is also a lack of support for maintenance and repair from OEMs. The outdated technology creates a lack of spare parts availability, and documentation is often unavailable in modern digital formats.
Oil Leakage
Wet seal designs have inherent issues with contacting faces that require lubrication. To get lubrication to the seal faces there must be a system to support that. Hot oil from the seal faces can cause fouling in the internal processing parts of the compressor and the permanent consumption of seal oil due to the leakage into the process gas or loss to the sour oil drain.
Some extensive cases of oil losses have reached hundreds of gallons per day, which can be a severe safety hazard, while oil leakage can also compromise the quality of the final product.
Lower Reliability
Lower reliability can also plague oil systems, requiring maintenance as often as six months to a year. At this point, equipment becomes compromised and production is lost. Lube oil contaminated by the process can also be a problem, and this directly relates to the reliability. Older systems require consistent attention from both operators and maintenance personnel, which can be an added economic driver to upgrade to newer dry gas technology.
Environmental Drawbacks
Finally, there are the environmental challenges to consider. Centrifugal compressors equipped with wet seal technology are generally acknowledged as being the leading source of methane emissions in offshore applications and the fourth most significant onshore. (Frost and Sullivan, 2023)
Higher Reliability of Dry Gas Seal Technology
A Compelling Argument
Dry gas seals are non-contacting, dry-running mechanical face seals that consist of a mating (rotating) ring and a primary (stationary) ring. When operating, lifting geometry in the rotating ring generates a fluid-dynamic force, causing the stationary ring to separate and create a gap between the two rings.
Machined-in grooves outside the mating ring, direct gas inwards toward the sealing surfaces. The gas that is routed into the grooves generates a pressure that maintains a minute gap between the faces, optimizing fluid film stiffness and providing the highest possible degree of protection against face contact. The seal's film stiffness is maintained by a dynamic balance between the sealing pressure and the spring load of the primary ring.
By moving from a wet seal to a dry gas seal, end-users are moving from a contacting scenario to a non-contacting scenario, where there is no need for the injection of oil. This improves reliability and reduces the cost profile. This difference between contacting and non-contacting technology provides the most significant justification when considering retrofitting a compressor equipped with wet seals to dry gas seals.
Economic Benefits
Gas seal retrofitting is a considerable cost-effective solution, with an average pay-off period of 12-36 months. John Crane engineers have developed a Lifecycle Cost Calculator (LCC) with industry groups, such as the Fluid Sealing Association (FSA), which allows users to input their individual compressor specifications and processing parameters to quantify the financial benefits of dry gas seal upgrades.
Click here to calculate the payback on retrofitting your compressors.
Environmental Benefits
Enabling Efficient and Sustainable Operations
The amount of power required to drive a dry gas seal per system is up to 80% less per year, when compared to wet seals, primarily because the seal faces have less frictional energy losses due to the extreme precision mating between the stationary and rotating elements:
Wet Seal Systems
- Oil requires cooling (Fin Cooler & Fan 10kW)
- Trapped sour oil needs heating so it can degas (10-20kW)
- Pumping oil with high flow at a high discharge pressure requires high-capacity electrical motors (30-50kW)
- Hydraulic and mechanical losses in the seals and system (10-20kW)
Total Power Consumption: 60-100kW
Dry Gas Seal Systems
- Heater to prevent liquid condensation (10-20 kW, if applicable)
- Booster (2-3 kW, if applicable)
- Mechanical losses (<1 kW)
Total Power Consumption: 15-25 kW
Elimination of Fluid Consumption and Leakage
Wet seals used in abrasive pumping applications require significant volumes of liquid to be injected into the seal chamber. Dry gas seals require no flush liquid to be injected into the seal chamber, minimizing leakage to the atmosphere.
Dry gas seals require gas seal support systems to ensure the proper function of the seal. If applicable, John Crane also provides an innovative dry gas system upgrade with the Seal Gas Recovery (SGR) System, which is designed to work with tandem seal installations to recover seal leakage and divert the gas stream for more productive purposes.
Over the last ten years, John Crane’s wet-to-dry upgrades have helped customers reduce approximately 278,000 tons of CO2 equivalent emissions per year. By switching to wet-to-gas seals, end-users around the globe are minimizing methane leaks and emissions while boosting the reliability of their compressors. These retrofit solutions improve a plant's efficiency and reduce its environmental footprint while significantly cutting emissions.
Read our case study: Emissions Reduced at Natural Gas Liquid Extraction Plant in the Middle East by Using John Crane Dry Gas Seals
Next Steps Forward
The decision to retrofit wet-to-gas seals isn’t one that should be taken lightly and may be dictated by economic factors, HSE constraints or technical and resource constraints. That’s why the first stage of John Crane’s retrofitting projects starts with a detailed feasibility study and on-site discussion, to assess if a retrofit is the right solution.
To find out more about John Crane’s wet-to-gas retrofits contact us today.