Five Ways Non-Contacting Dry Gas Seals Have Changed the Face of the Industry

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Which Sealing Arrangement Is Right for You?
Sealing arrangement is an important part of any seal installation. Operating conditions, such as the type of gas to be sealed, pressure, temperature and speed, as well as abrasive contaminants, all are major factors in the selection of a sealing arrangement.

• Single Seal. Process fluid that is inert or non-toxic typically requires only a single seal. In this arrangement, process gas at the outside diameter of the seal face will flow to the atmospheric side of the seal.

• Double Seal. When leakage of the process gas to the atmosphere cannot be tolerated, or when the gas contains dirt and abrasive, or when a vacuum can occur in the seal chamber, a double seal is the best choice. An inert buffer gas such as nitrogen, typically at a pressure 10 percent higher than the maximum pressure being sealed, is used to pressurize the seal chamber. A small amount of gas will enter the compressor at the inboard seal. Double seals are typically used at lower pressure at or around plant nitrogen pressures, but contact John Crane for application specific inquiries.

• Tandem Seal. The most common used seal in the industry today is the tandem seal, with or without an intermediate labyrinth. The inboard seal is used to handle the full pressure differential of the application, while the outboard seals acts as a backup incase of inboard seal failure. The space between the two seals is vented to a vapor recovery system or flare system. When an intermediate labyrinth is used, the outboard seal is buffered with an inert gas, typically nitrogen, and the outboard seal leakage which is nitrogen instead of process gas, can be safely vented to atmosphere. John Crane has the highest pressure tandem seal running in the world, and we are pushing our capabilities to achieve 800 bar g / 11,600 psi.

Over the years, sealing technology has produced a host of different sealing concepts, each of which has met with varying degrees of success. Designs such as labyrinth seals, carbon ring seals, bushing seals, and circumferential seals have all been considered viable options for certain applications; however, their use often comes with limits on speed, pressure and operating conditions, and, with changes in condition, they are prone to problems such as leakage and relatively short periods of operation.

All of this changed in the 1960s when John Crane Inc. introduced the world to a revolutionary new piece of sealing technology-the non-contacting spiral groove gas seal. The seal uses a spiral groove seal face with a sealing dam on the inside diameter to provide resistance to gas flow and allow pressure to build up at the bottom of each groove, separating the seal faces. This separation allows a small amount of gas to flow across the seal faces and keep them cool and non-contacting, thereby minimizing the detrimental effects friction can have on a seal face.

Since John Crane Inc. received the patent on this technology 30 years ago, non-contacting dry gas seals have become standard for a wide range of sealing applications and have helped bring about several notable changes in the realm of sealing technology, as well as in the industry as a whole.

One: The need to have different types of seals for different applications has become obsolete.

Seal faces that move relative to each other create the heart of the contacting seal, and, therefore, much depends on the type of fluid sealed and its properties, for it is this fluid that will lubricate the seal faces. There are considerable differences between the three most common types of lubrication for contacting seals: full liquid film, which uses a lubricating oil and requires light-duty conditions (e.g., 20 psig at room temperature) for optimum performance; partial liquid/gas film, which requires at least a 50-percent balance of liquid at the seal face to preserve seal life (and which therefore can only be used under moderated pressures, temperatures and speeds); and dry gas film, which often presents a problem for contacting seals because the absence of liquid creates hot localized areas that expand and crack, causing major damage to the seal face material.

When non-contacting spiral groove gas sealing technology was first introduced, it was aimed primarily at correcting the problems inherent in a dry gas film environment by eliminating friction. However, it soon became obvious that the technology was also an ideal replacement other types of lubricated seals, thereby eliminating the need for costly oil support systems. Today, non-contacting dry gas seals are standard on compressors used for a variety of applications, including on pipelines, off-shore applications, refineries, and petrochemical and gas processing plants.

Two: Common problems associated with high-pressure operations have been eliminated.

Higher operating pressures can pose plenty of problems for sealing technology-increases in pressure and temperature can lead to secondary seal friction, deflection of the seal faces, explosive decompression and, ultimately, an increased chance of leakage. Over the years, the technology of the non-contacting spiral groove gas seal has evolved to provide a solution to these complex problems.

Explosive decompression occurs when the pressure on the secondary seal, typically an elastomeric o-ring, is suddenly decreased. Any gas absorbed by the material will try to escape, creating a blister in the material and causing the o-ring to lose its sealing capability. To solve this problem, John Crane Inc. created a variation on the design of the non-contacting spiral groove gas seal (to be used with pressures up to 450 bar g or 65,000 psig) that replaces the o-rings with spring-loaded polymeric seals. The design of the retainer (which provides the anti-rotation feature for the primary ring) and disc also are altered to allow the primary ring to be placed in compression by the process gas to be sealed. This design allows for better control of the deflection of the seal faces, thereby avoiding the problem of explosive decompression. As a result, compressor manufacturers have been able to increase the efficiency of their equipment by as much as 37 percent.

Three: Mean Time Between Failure (MTBF) has been improved.

With early seal designs, operating for an extended period of time just wasn't possible. High-pressure seal oil systems required considerable maintenance, which generally meant a good deal of downtime for owners-particularly since replenishment of oil beyond the normal consumption rate was a manual operation.

The development of non-contacting dry gas seals brought a breakthrough to the improvement of Mean Time Between Failure (MTBF). Because the technology eliminates the problem of wear due to friction, the seals can provide reliable system performance without any system upsets. In some refineries, they have run successfully for more than 10 years-a duration of time practically unheard of in the days before this technology was put into place.

A case study of a chemical plant that incorporated double gas seals with a nitrogen barrier fluid found that the seals, which had been in operation for six and a half years, demonstrated no change in performance than when they were brand new. Currently, the plant has scheduled maintenance for the seals every five years, but the goal is to extend the maintenance schedule to 10 years. The plant has seven other compressors in operation, some of which are approaching nine years in service, suggesting that this goal is well within reach.

In an environment where cost containment and improved equipment reliability are essential to doing business, non-contacting gas-lubricated seals have become the preferred solution for extending MTBF.

Four: The elimination of oil lubrication systems has resulted in significant savings.

The reduction in maintenance-related downtime as a result of non-contacting spiral groove gas sealing technology has also proved to have another significant benefit: cost savings. Contacting seals requiring oil lubrication systems are not only time-consuming for owners to maintain, but also quite expensive. In some cases, oil consumption, normally three 55-gallon barrels per day, could increase to 10 barrels a day before a unit was shut down for seal replacement. If a pipeline was used, the more oil consumed would reduce the efficiency of the pipeline. In the case of a refinery using a hydrogen recycle compressor, the oil would contaminate the catalyst, resulting in a major impact on operating costs.

One case study revealed that a high-pressure hydrogen recycle compressor fitted with dry gas seals saved the user $1.7 million in just one year, not including the replacement of the oil-contaminated catalyst. With regular maintenance scheduled every five years, the life of these seals has been as long as 10 years. Given the cost savings they represent, dry gas seals such as these have become the standard for high pressure hydrogen recycle compressors used in refinery services.

Five: Low-temperature liquids can be pumped easily.

Certain liquids, most commonly cryogenic fluids and liquefied hydrocarbon gases, must be pumped at or near their boiling point. This situation has long presented a challenge to seal engineers, as contacting seals would require the heat developed to be removed to preserve the liquid at the seal face. However, this approach is dependent on the amount of cooling and the efficiency of the seal design to dissipate the heat generated at the seal faces. If the liquid is allowed to turn to gas in a controlled manner, on the other hand, non-contacting gas-lubricated seals provide a perfect solution to this dilemma.

When cryogenic fluids (liquid oxygen, nitrogen, argon, hydrogen, helium, methane and ethane at temperatures below -100° F/-73° C) are pumped using contacting seals, the heat generated during the process often reduces the life of the seals to a matter of weeks, primarily because these fluids can turn from a solid to a gas in a small temperature range. When using a contacting seal, the temperature increase caused by friction at the seal faces is often sufficient to start the boiling process.

Eliminating the friction by using a non-contacting gas-lubricated seal ensures that the temperature rise at the seal face will only be a few degrees, eliminating the violent flashing of cryogenic liquid that causes wear on traditional contacting seals. In addition, the design of the seal is free of any distortion and is capable of handling all vibrations and motion from the tanker in which the liquids are stored.

One operator with a fleet of 25 such tanker trucks often experienced exorbitant costs due to repairs and downtime using contacting seals. The leaks that resulted from sealing problems cost up to $1,500 per repair, with approximately 100 failures per year. It was not uncommon for the owner to have some pumps run for only four to six weeks before maintenance was required.

After replacing the original pump seals with non-contacting dry gas seals, the owner saw a dramatic increase in equipment reliability. Seals examined after 6,000 hours of service showed remarkably little wear. The fleet of 25 tanks now has an average seal life of more than six years, and the owner's average savings in that same time period total $900,000.

Conclusion
The past 30 years have seen thousands of non-contacting dry gas seals installed on centrifugal compressors, pumps and similar rotating equipment. Throughout the world, they have become the industry standard for sealing rotating shafts, eliminating many of the problems caused by traditional contacting seal designs and saving owners millions of dollars in maintenance and downtime costs in the process. A firm understanding of the tribological problems of the technology has resulted in exceptional solutions to difficult sealing applications. As the industry moves forward, new and more complex sealing problems will undoubtedly arise. If the past 30 years have been any indication, the sealing technology pioneered by John Crane Inc. will continue to evolve to meet the challenges of the future.