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Despite the hermetically sealed nature of the typical turboexpander, seal gas systems are still normally required. For example, it is undesirable for the gas that is passing through the expander wheel to enter into the bearing housing.
This article contains excerpts from the paper, "Tutorial on cryogenic turboexpanders" by Jigger Jumonville presented at the 2010 Turbomachinery Symposium.
The gas in the expander is typically very cold and has not been filtered for small particles. If this gas were directly admitted into the bearing housing, the oil would become dirty, very cold and possibly freeze solid. Likewise, it is also undesirable for the oil from the bearing housing to enter the expander housing, since it would tend to freeze inside the machine and in downstream heat exchangers causing problems.
It should be noted that it is very difficult if not impossible to get all of the oil out of a typical cryogenic cold box (heat exchanger) once it has gotten in. With magnetic bearings, there are many parts that should be protected from the cold gas of the expander, so seals are also used in these applications. For both oil and magnetic bearing systems, the most common type of seal used to separate the process from the bearing housing is the buffered labyrinth seal. Clean and filtered buffer gas is injected near the middle of the seal, and is forced to flow toward both the bearing housing and the expander housing (for the expander end seal). This prevents fluids in one housing from entering the other housing.
The use of a stepped tapered labyrinth further enhances the ability of the seal to reject oil from the bearing housing, as well as lowers the buffer gas flow required by the seal. By using a stepped tapered laby instead of simply a tapered laby, the seal clearance can be maintained regardless of the seal’s axial location. The flow through a normal tapered laby seal will be dependent on the axial location of the rotating and stationary parts, since the radial clearance changes with axial motion of the parts.
In order to maintain an effective barrier even if the seal wears, the buffer gas (or “seal gas”) should be differential pressure controlled, not flow controlled. If the buffer gas is flow controlled and the seal wears, then the ability of the seal to provide an effective barrier is compromised. With a differential pressure controller, the worn seal can still be an effective barrier, since additional flow will be provided when needed to maintain the differential pressure. The source of the seal gas (buffer gas) can also be important. This source needs to be sufficient pressure to provide adequate buffering of the seals under all expected operating conditions. It should be warm, clean, and dry by the time it enters the bearing housing.
The hermetic nature of the lube oil system, like the refrigerator in your home, means that the oil viscosity will be changed due to the presence of the process gas. Also like your refrigerator at home, it means the actual oil viscosity inside the system will be lower than the viscosity of pure oil at the same pressure and temperature conditions. This is referred to as “dilution,” since the seal gas is “dissolved” in the lube oil. To deal with dilution, it is often necessary to increase the oil viscosity so that the resulting diluted mixture is sufficiently viscous to provide the necessary stiffness and damping that the bearings need to control the shaft motion.
Obviously, with magnetic bearings, there are no concerns about oil dilution. However, in some applications, additional cooling gas must be put into the magnetic bearing housing to remove heat due to windage, resistance, and eddy current losses in the magnetic bearings. When needed, this cooling gas is normally supplied from the same source as the seal gas supply. The last thing to mention here is that seal gas should be turned on before the lube oil is turned on when starting a machine with oil bearings, to ensure that oil is not accidentally admitted into the expander and compressor housings. Likewise, when shutting down the machine, the lube oil should be turned off prior to shutting down the seal gas system.