Drive energy efficiency and reduce emissions with dry gas sealing solutions
S. ROSSI, John Crane, Muggio, Italy; and M. WEEGENHAUSEN, John Crane, Dubai, UAE
Natural gas is widely considered the bridge fuel in the transition from hydrocarbons to renewables like wind, solar and geothermal. 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 liquefied natural gas (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 limitations, 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 has caused wet-seal compressors to be a leading source of methane (CH4) 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 CH4 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 emissions goals. Higher CH4 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. Because they can fail without warning, wet seals also increase the risk of compressor downtime.
Boost efficiencies by retrofitting to dry gas seals. For more than 40 yr, dry gas seals have been a proven sealing alternative to wet seals. Dry gas seals are a 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 and do not need the oil lubrication associated with contacting seals. As a result, dry gas seals leak at far lower rates than wet seals, reducing CH4 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 do not 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 products and maximize overall system efficiency.
Proven performance in the field. The authors’ company’s 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 the installation and commissioning of one of the company’s dry gas sealsa, which fit into the compressor’s existing seal cavities with only minor modifications. The operator achieved 10 yr 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.
- A natural gas compressor station in Louisiana, U.S. required a similar retrofit solution to retain more product and curtail CH4 emissions, which reached rates of 200 sft3/min. Facing a tight turnaround schedule toward the end of its budget year, the operator worked with the authors’ company to replace the wet seal technology with proprietary dry gas sealsb (like that shown in FIG. 1) 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 3 yr to 7 yr.
FIG. 1. The authors’ company’s dry gas sealb is a non-contacting sealing element that exhibits high durability and longer run life over a wide temperature range.
- A natural gas liquid (NGL) extraction plant in the United Arab Emirates approached the company to retrofit its wet seal-equipped compressors. A comprehensive dry gas seal solution comprising a new housing structure was developed to accommodate one of the company’s dry gas sealsc. 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 the authors’ company 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%.
Recover more gas for greater gains. While dry gas seals emit CH4 at a fraction of the rates of wet seals, the authors’ company saw opportunities for further emissions 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. With no rotating elements, the system improves reliability and minimizes maintenance-related downtime.
Easing into the energy 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, the authors’ company’s retrofittable dry gas seals, SGR system 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.
NOTES
a John Crane Type 28 dry gas seals
b John Crane Type 28XP dry gas seals
c John Crane Type T28 dry gas seals
LITERATURE CITED
1 U.S. Environmental Protection Agency (EPA), “Estimates of methane emissions by segment in the United States,” online: https://www.epa.gov/natural-gas-star-program/estimates-methane-emissions-segment-united-states
ABOUT THE AUTHORS
Stefano Rossi works as the John Crane Global Portfolio Director for gas compression, support and control systems. In his 26 yr with the company, he has served in leadership roles across Europe, the Middle East and Africa (MEA), and North America.
Mike Weegenhausen is the Senior Director of Turbomachinery, covering John Crane turbomachinery products and services for both global original equipment manufacturers (OEMs) and end user (EU) customers. In his role, Weegenhausen leads a global team of turbo engineers and subject matter experts (SMEs) strategically located around the world, providing increased customer interaction (OEM and EU) with a common global approach and focus.
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