February 2024, Vol. 251, No. 2
Tech Notes
Using Dry Gas Sealing Solutions to Drive Energy Efficiency
By Stefano Rossi, Portfolio Director for Gas Compression, Support and Control Systems, and Mike Weegenhausen, Senior Director of Turbomachinery & Service Delivery, John Crane Turbomachinery Products and Services
(P&GJ) — Natural gas is widely considered the bridge fuel in the transition from hydrocarbons to renewables like wind, solar and geothermal. And centrifugal compressors play a critical role in efficiently transporting the large volumes of natural gas used as a feedstock and fuel source in industries ranging from oil and gas to petrochemicals to LNG.
An effective sealing system is crucial to keeping a compressor running at peak efficiency. Wet seals, once the standard sealing solution, have well-known design limitations that impact compressor reliability and performance. Chief among these, wet seals make direct contact with the rotating shaft during operation, necessitating a circulating oil as a lubricant. The circulating oil tends to absorb natural gas at the high-pressure seal face and then release it from the compressor casing into the environment.
This pervasive problem made wet-seal compressors a leading source of methane emissions in the natural gas sector. The U.S. Environmental Protection Agency (EPA) estimates that leaks from centrifugal compressors account for 6% and 9% of methane emissions in gas processing and gas transmission, respectively.1
Leaks spell trouble for pipeline and plant operators trying to meet their environmental, social, and governance (ESG) commitments and progress toward their net-zero emission goals. Higher methane emissions also hurt an operator’s bottom line by turning a valuable product – the natural gas – into a waste stream.
Wet seals require elaborate oil circulation pumps and treatment facilities that draw significant power, making them both mechanically complex and expensive to operate and maintain. And because they can fail without warning, wet seals increase the risk of compressor downtime.
Boosting Efficiencies
For more than 40 years, dry gas seals have been a proven sealing alternative to wet seals. Dry gas seals are standard issue in most new compressors and a common retrofit solution in legacy wet-seal compressor systems thanks to the efficiency gains and environmental protections they provide.
As their name implies, dry gas seals run dry, with no need for oil lubrication associated with contacting seals. As a result, dry gas seals leak at far lower rates than wet seals, reducing methane emissions by 95% or more and eliminating oil seepage into the process stream to avoid product contamination, pipeline degradation, and loss of system efficiency.
Because dry gas seals don’t require ancillary oil-circulation equipment, they are mechanically simpler to operate and consume up to 80% less power. They are also non-contacting during operation for longer run life, greater reliability, and improved maintenance planning, all of which help lower operating costs.
Dry gas seals lower the risk of unplanned shutdowns that necessitate a blowdown to minimize emissions further, retain more valuable product, and maximize overall system efficiency.
Field Performance
John Crane dry gas seals are reducing gas emissions, lowering maintenance costs and improving energy efficiency for a growing number of projects worldwide.
For example, a pipeline station operator in the Middle East retrofitted two large compressor trains from wet seals to dry gas seal technology. The turnkey retrofit included installing and commissioning John Crane’s Type 28 dry gas seals, which fit into the compressor’s existing seal cavities with only minor modifications.
The operator achieved 10 years of continuous operation with this compressor upgrade. The new seals reduced emissions by 98% and lowered gas consumption in the turbine by 2%. They also eliminated oil contamination of the process gas to improve product quality while reducing maintenance costs and boosting reliability.
In another case, a natural gas compressor station in Louisiana required a similar retrofit solution to retain more product and curtail methane emissions, which reached rates of 200 standard cubic feet per minute.
Facing a tight turnaround schedule toward the end of its budget year, the operator worked with John Crane to replace the wet seal technology with Type 28XP dry gas seals and a corresponding dry gas seal support system.
Following successful startup and commissioning, the new dry gas seals increased compressor efficiency by reducing power consumption, maintenance costs, and unplanned downtime. The average mean time between refurbishments for the station’s compressors increased by 133%, from three to seven years.
Additionally, a natural gas liquid (NGL) extraction plant in the United Arab Emirates wanted to retrofit its wet seal-equipped compressors. A comprehensive dry gas seal solution comprising a new housing structure was developed to accommodate Type T28 dry gas seals.
Metal bridges were built between each train's low-pressure and high-pressure compressors to help install and afford easier access to the Dry Gas Seal Support system. A rotor-dynamics study uncovered a rotor instability issue that prompted John Crane to install new tilting pad journal bearings with a squeeze film damper.
The retrofit solution improved the compressor train efficiency by minimizing rotor vibration, lowering maintenance costs, improving reliability and operational safety, and reducing emissions by 97%.
Greater Gains
While dry gas seals emit methane at a fraction of the rates of wet seals, John Crane saw opportunities for further emission reductions and operational efficiency gains. This prompted the development of a Seal Gas Recovery (SGR) system, which recovers more vent gas from a dry gas seal and diverts it back to the process stream.
The SGR system mixes the low-pressure vent gas with a high-pressure gas stream. The resulting high-pressure gas mixture circulates back into the process to reduce product losses and minimize the need for flaring waste gas. The SGR system is easily incorporated to boost the efficiency of existing compressor systems. And with no rotating elements, the system improves reliability and minimizes maintenance-related downtime.
Easing Transition
The retrofit examples presented here demonstrate how a seemingly minor switch from wet to dry gas seals can deliver major efficiency gains in a compressor system.
As the energy transition continues, John Crane’s retrofittable dry gas seals, SGR systems, and installation expertise are helping operators minimize their emissions, improve compressor reliability and runtime, and capture the maximum value of natural gas as the bridge fuel to a low-carbon energy future.
Reference:
https://www.epa.gov/natural-gas-star-program/estimates-methane-emissions-segment-united-states
Authors: Stefano Rossi is the John Crane global portfolio director for Gas Compression, Support and Control Systems. In his 26 years with the company, he has covered leadership roles across Europe, MEA and North America.
Mike Weegenhausen is the senior director of Turbomachinery & Service Delivery, covering all John Crane Turbomachinery Products and Services for both Global OEM and end-user customers. In his role, he leads a global team of turbo engineers and SMEs located around the world.
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