Turbo Tips: Challenges of Dry Gas Seals in Modern Large-Size Low-Temperature Turbocompressors

Turbocompressors have often been used in low-temperature services for many different applications, including LNG. They have been unique turbomachines; however, little has been published about these machines and their unique operational characteristics. Modern turbocompressors for low-temperature gas services present some unique challenges and difficulties. Dry gas seals for these turbocompressors require a sophisticated configuration, material selection, and technology.

The challenge of low-temperature applications is intensified by large diameters in some of these seals. It is not possible to simply upscale a smaller dry gas seal for larger applications. The engineering and operation of these seals should consider the challenges and difficulties in such harsh low-temperature conditions to ensure a reliable sealing function, even under punishing operation conditions such as transient cases.

Challenges & Core Issues

The dry gas seal should always have the ability to maintain a gap of only a few micrometers between the rotating seal face and the stationary seal face. This is a key requirement of dry gas seals for minimizing the gas leakage. Often, large seals with a nominal diameter of 300mm or 400mm (or even more), are required for modern turbocompressors in low-temperature services. This can be for operation at low temperatures—sometime as low as -170°C.

Dry gas seals play a significant role in low-temperature turbocompressors. Some of the well-known seal models for these applications were originally developed for high-pressure applications; however, further investigations showed that such seals offer other advantages such as a reliable operation at very low temperatures. To assure a near-zero leakage, a tandem seal configuration is usually selected which often includes an intermediate labyrinth. The opposing seal faces should always form a parallel gap, ensure optimal stiffness of the gas film, and minimize the risk of contact while controlling leakage.

Material Selection

The proper selection of seal materials for sliding faces is essential to reliability and proper operation. Under extreme low-temperature conditions, standard sliding materials such as carbon tend to be affected by an extreme wear rate and lowers the reliability of seals. Modern sliding faces usually employ silicon carbide with a diamond-like carbon coating. Sintered silicon carbide has a high capacity to battle with extreme temperature differences. This material could be one of the best options (between all material candidates for low-temperature applications) to keep the sealing gap in low-temperature services.

Challenge of Tight Tolerances

In extreme-temperature applications, it is usually difficult to keep proper tolerances between various components to achieve an optimal seal gap and seal behavior. Large-size low-temperature dry gas seals have been challenging to manufacture because the geometrical tolerances are the same as those prescribed for smaller seals. Geometrical tolerances (such as planarity tolerances imposed to seal faces) should be kept ensuring the stability of the gas film. These tolerances have little to do with the size of the seal, as the planarity tolerance on a seal ring could be less than 0.5 μm. The thermal shrinkage at large sizes and significant temperature differences could be critical issues.

Key Concerns & Considerations

Key concerns and considerations for a dry gas seal in large-size, low-temperature turbocompressor applications include the seal lift-off speed (which usually translates to the minimum speed of operation); the seal operation and depressurization during a coast-down (a turbocompressor trip); the shaft axial shuttling and the ability of the seal to absorb and tolerate axial vibration or movements; and special situations or transient operating cases such as particle ingestion or reverse rotation.

All these key issues should be addressed in the engineering, reliability analysis, and testing campaign of dry gas seals for such applications. Attention should be paid to understand how seal conditions change during operational upsets or turbocompressor malfunctions.

Challenging Transient Cases

Many modern turbocompressors for low-temperature services have been massive turbomachines. During a trip (shutdown), because of high rotating masses of the large compressor, it takes a long time to come to rest and each dry gas seal should be robust enough to operate at a low speed for a significant period of time when the seal face elements begin touching (below the lift-off speed). The seal should accommodate all issues relating to extremely low operating temperatures and effects from relatively high linear speed at seals.

Other Considerations for Dry Gas Seals

In the case of changing temperatures, particularly during transient conditions, the shaft sleeve material could expand or contract more than the seal components. As an indication, the thermal contraction coefficient of the material used in the dry gas seal is usually about a third of that of the stainless steel (such as the shaft sleeve). The resulting relative movement can cause the ring torsion and could lead to a gap distortion.

As a result, the seal gas leakage could increase immediately. Because of these transient effects, a proper tapering of the seal face is recommended and used. This tapering is a critical part of the configuration of a dry gas seal for extreme temperature applications. The predicting of the seal-face deformation is a fundamental requirement for achieving a seal-face with a proper taper. The approach is to size the seal-face taper for an optimal geometry under normal operating conditions, and to get the best possible result for potential transient conditions.

ABOUT THE AUTHOR
Amin Almasi is a chartered professional engineer in Australia and the United Kingdom. He is a senior consultant specializing in rotating equipment, condition monitoring, and reliability.