The Difference Between Dry and Wet Seals
For centrifugal compressors that produce natural gas, also known as methane, seals on the machine’s rotating shafts prevent pressurized gas from escaping from the compressor’s casing. A conventional method for sealing the casing utilizes oil under high pressure to act as a barrier to stop gas from escaping. These are known as wet gas seals. Yet, for many situations, companies involved in producing and transmitting natural gas are moving towards using dry seals. Gas seals like these instead use pressurized nitrogen or another similar substance in gaseous form, which reduces both methane emissions and operational expenses.
While wet gas seals allow anywhere from 40 to 200 cubic feet (1.13 to 5.66 cubic meters) of methane to escape per minute, pressurized dry gas seals leak much less methane, only up to about 6 cubic feet (under .17 cubic meter) per minute in a two-seal system. Additionally, dry gas seals require less power, improve a compressor’s reliability and have much lower maintenance requirements while also enhancing the performance and efficiency of both pipelines and compressors. Unfortunately, not all wet seals can be replaced by dry gas seals due to functional requirements or housing layout. Yet replacing wet with dry gas seals on centrifugal compressors wherever possible can provide businesses involved in natural gas production considerable savings, as they greatly enhance the efficiency of its production.
Main Differences Between Wet & Dry Gas Seals
Both wet and dry gas seals are mechanical seals used to prevent gaseous substances from escaping a pressurized system, each with advantages and disadvantages. Typically, a dry gas seal consists of two metal rings that fit together closely and are held together by a spring. The rings within dry gas seals are made up of graphite, tungsten carbide, or another substance with low friction properties. Conversely, wet gas seals use a liquid film to help prevent leakage, typically delivered by a pressurized system that helps keep the space between the two sealing surfaces lubricated. This reduces wear from friction and acts as a sealant barrier to prevent leaking.
Centrifugal compressors that utilize wet gas seals use oil as a sealant around the compressor’s rotating shaft, where it protrudes from its casing. While “beam” compressors use a seal at each of the two compressor’s ends, “over-hung” varieties feature a single seal on the side where the motor is located to prevent methane from escaping. Pressurized oil circulates around the compressor’s shaft to create a barrier against gas leakage, with the center ring attached to the rotating shaft and two other rings on the seal’s housing that remains stationary. A thin oily film flows between the rings, both providing lubrication and acting as a barrier to leakage. Though very little gas can escape through this oily barrier, the high pressures within the compressor cause the oil to absorb the gas, which contaminates the seal’s oil. This absorbed methane is then purged via heaters, flash tanks, and other degassing methods, after which it’s then generally vented into the atmosphere.
Advantages & Trade-Offs of Dry Gas Seals
Dry gas seals can often save organizations hundreds of thousands of dollars annually, often paying for themselves in under a year. In certain cases, they drastically reduce emissions as well, by as much as 97 percent. Besides these economic and environmental benefits, they also make compressors more efficient, so also decrease operating expenses.
Key economic and environmental advantages of using dry gas seals include:
- Decreased energy consumption: Since dry gas seals don’t require pumps and systems to circulate oil, they require only about 5 kW of electricity hourly, about 5-10 percent of what wet seal systems need.
- Dependability: With the highest proportion of a compressor’s downtime resulting from issues within their seal systems when utilizing wet seals, the fact that dry gas seals have fewer parts to support makes them more reliable and means the compressor is less likely to experience downtime.
- Durability: When correctly fitted and maintained, dry gas seals last over double that of wet ones.
- Eliminates seal oil leakage: Wet seals leak oil into gas pipelines, causing them to degrade; dry gas seals prevent this contamination.
- Gas leak rates: Though wet seals leak at a similar rate to dry gas seals at the seal face, the additional venting of gas into the atmosphere means they leak far more methane over time, making dry gas seals more environmentally friendly.
- Less maintenance: Dry gas seals require less maintenance than wet ones because they have fewer moving components, such as control valves, pumps, and relief valves.
- Mechanical simplicity: Dry gas seals don’t need a complex system for circulating oil to components.
- Safety: Dry gas seals are safer, as high-pressure oil systems aren’t needed.
Most of the savings natural gas producers receive from replacing wet with dry gas seals come from decreased methane leakage. These savings can be assessed by measuring most of the lost methane using a high-flow sampler or by bagging at the seal oil degassing unit at the vent. Estimating the gas that escapes at the seal face is harder to measure, though it’s generally less than 10 percent of the emissions from the degassing unit.
Where Wet & Dry Gas Seals Are Used
Despite the clear economic benefits that the use of dry gas seals provides for most applications, there are situations in which wet seals are preferred. If hazardous chemicals are used in a process, it makes sense to use wet seals instead. While wet seals trap dangerous fumes in the oil that acts as a barrier to keep them from leaking into the atmosphere, dry gas seals can’t contain these gasses as effectively. In such cases, more pollution is released into the environment.
There are other aspects that make dry gas seals unsuitable in some situations. While dry seals are routinely used for applications of 1500 pounds per square inch (psi) (10342.14 kilopascal) and are often safely used with compressors that utilize 3000 psi (20684.27 kilopascal), at higher pressures, they may not be safe to use. Dry gas seals also shouldn’t be used at temperatures over 300°F to 400°F (about 149°C to 204°C), depending on the application and type of seal. In certain cases, the design of the compressor also makes retrofitting with dry gas seals impractical.
Hayes Pump: Wet & Dry Gas Seals
Hayes Pump, Inc. offers the broadest range of mechanical seals on the market, including a comprehensive selection from John Crane. With a reputation for innovation going back to 1917 and as the maker of the first automotive mechanical seal, their mechanical seals helped transform sealing technology. As the most widely used seals in the industry, our warehouse is always well-stocked with John Crane mechanical seals, and our capabilities allow for their rapid delivery.
As the largest and oldest distributor of pumps and pump components in the Northeastern United States, we offer a variety of sealing solutions for pumps involved in various applications, including wastewater treatment, oil and gas, energy generation, chemical, and other industries. To learn more about wet and dry gas seals and which seal is right for your application, we invite you to contact the experts at Hayes Pump today.