Gas Seals for Improving Reliability & Eliminating Emissions from High-Speed Centrifugal Compressors
Dynamic seals for high-speed centrifugal compressors have been evolving/migrating from oil-based seals to aerodynamic gas bearing type seals over the last 50 years. Drew Devitt of New Way Air Bearings has demonstrated aerostatic bearing technology that improves compressor reliability and avoids having to vent or flare fugitive emissions.
Dynamic seals for high-speed centrifugal compressors have been evolving/migrating from oil-based seals to aerodynamic gas bearing type seals over the last 50 years. These seals are known as Dry Gas Seals for a reason, because if any liquid gets drawn across the seal face it boils or burns from the shear heat likely causing seal failure. Drew Devitt, Founder and CTO of New Way Air Bearings has demonstrated aerostatic bearing technology that improves compressor reliability and avoids having to vent or flare fugitive emissions.
Externally Pressurized Porous (EPP) Gas Bearings
By leveraging the pressures that can be generated in the gaps of Exter-nally Pressurized Porous (EPP) gas bearings, a new class of seal offers capablities not preivously available. EPP gas bearings have been used in other industries for decades as bearings but are just now being ap-plied in seal applications for rotating equipment. This effort should: (1) increase the pressures that can be sealed against, (2) reduce the flow of buffer, barrier and seal gases, (3) eliminate friction, heat and ware in seal applications and (4) eliminate fugitive emissions from seals.
First, a quick review of enabling gas bearing technology.
There are two type of gas bearings:
Aerodynamic and Aerostatic. Most large pump and turbine rotors are supported on hydrodynamic oil bearings. Dry Gas Seals (DGS) in compressors are examples of aero-dynmaic gas bearings and many mi-cro turbines are supported on foil type aerodynamic bearings. In all these cases it is the relative motion between the surfaces that draws the fluid or gas into the bearing gaps, so motion is required to generate separation force. This means there will be contact wear at start up, shut down and in slow-speed conditions.
In aerostatic type bearings, motion is not required to generate separation force between the faces, instead a source of externally pressurized flu-id or gas is injected directly between the bearing or seal faces. This pressurized flow is introduced though the faces via precision orifices, grooves, steps or porous compensation techniques, see Figure 1. It is this idea of compensation that is key but not yet well appreciated in the rotating equipment industries.
Compensation is the first restriction, holding back source pressure in re-serve. This enables the faces to run very close together without touching: the closer they get together the higher the gas pressure between them gets. This is because the gap is the second restriction and as it is reduced (increasing the gap restriction) the compensation restriction is relatively less. This allows the pressure gap to increase, creating a separating force. The gas pressure in the gap can be almost as high as the source pressure because the small gap is so restrictive. With flow being a cubed function of the gap, gas consumption is dramatically reduced by the small gaps that become possible.
Diffusion With Porous Media
An elegant method for providing compensation is to diffuse the seal gas though a porous material. This is because the ideal seal design would supply pressure equally across the whole face of the seal and automatically restrict and dampen the flow of air to the face at the same time. The stability of porous media compensation is due to the damping effect from the torturous passageways the gas must flow through to reach the face. This damping effect makes it difficult for the volume of air in the gap to change quickly, resulting in a naturally stable gas film that can-not be plugged by particulates. Even if the supply tubes and/or ports be-come completely full of particulates (sand, dust, teflon tape, etc.), it still does not create as much restric-tion as the porous media itself and none of these contaminants can get though the porous media and into the seal gap. These characteristics make Externally Pressurized Porous (EPP) compensation a good choice for industrial seal applications.
The porous media used is made from familiar materials, such as graphite, carbon or silicon carbide that have proven their plain bearing tribology as contact seal faces, bearings and brushes over decades of use in rotat-ing equipment. So even if the source pressure is lost there is little to no ware and small gaps are maintained.
These materials also have very high heat capability, having been sintered during manufacturing; they resist oxidation to at least 400°C without melting like Babbitt, or burning like polymers. When made from Ceramic Matrix Composites (CMCs) they may operate over 800°C.
EPP gas bearing technology may be applied in many seal applications. Examples of conventional seals that could be replaced with EPP gas seals include packing seals, labyrinth seals, steam seals, bearing isolators, injection-type seals, mechanical seals and DGS.
Figure 1: Externally Pressurized Porous (EPP) seal face under water.
Comparing Dry Gas Seals (DGS)
Likely the most advanced seal is the Dry Gas Seal (DGS). These seals are very expensive and require ex-tensive gas supply services. They are typically used in high speed, multi-megawatt centrifugal compressors that are often compress-ing noxious or flammable gases that need to be contained. Although they are preferred to the oil-based seals they are replacing, they still are one of the main reliability issues for these compressors and their design requires that some of the gas flow across the seal face and so this flow must be captured or sent to a flare.
Let us use the aerodynamic bearing DGS example to show the difference in the gas bearing technologies and the new functionalities available from EPP or aerostatic based seal technology.
Conventional Dry Gas Seals (DGS), see Figure 1, have a flow across their face, from the high pressure to the low-pressure side. Moisture or oils from the process are naturally carried into the seal gap by the flow from this pressure differential, where they carbonize or boil from the shear and cause reliability issues. Trying to stop this with buffer gas is like trying to stop water from flowing downhill. Although this leakage is small, it is coming under closer scrutiny from the EPA and other regulatory bodies.
Externally Pressurized Porous (EPP) Gas Sealing Technology diffuses seal gas through a porous seal face to create a pressure in the seal gap that is higher than the process pres-sure. The flow to vent is the same, but about the same amount that is vented flows back into the process. The advantage is that moisture will not enter the gap because the gap is at a higher pressure. It would be like water running up hill; it is just not natural for a lower pressure to flow into a higher pressure. So, there is no flow across the seal face from the process, increasing reliability. The process gas required for the externally pressurized bearing is 1/100 of the buffer gas used with Convention-al DGS, and the higher-pressure differential makes it easier to condition the seal gas reliably.
Figure 2: Conventional Dry Gas Seal. Figure 3: Externally Pressurized Porous Seal. Figure 4: Zero Emissions Seal.
Zero Emissions Seal (ZES)
A ‘Vent-less Seal’ may be arranged that has all the advantages of the EPP Seal, but additionally can segregate gases in a single seal face. This enables a Zero Emissions Seal (ZES). This is where all the process gas stays in the compressor and all the barrier gas exits the compressor. By using two externally pressurized bearing gases, (process gas is used on the process side of the seal face and a barrier or inert gas on the vent side) their relative pressure may be adjusted to steer the highest pressure point in the gap to be between the gases. At this point, all process gas flows back to process and all the barrier gas exits the compressor, eliminating fugitive emissions and flaring, see Figure 4. The balance point is determined by a gas detector in the vent that looks for any process gas molecules (say 10 to 100 parts per million as a threshold) with a control to slightly increase the barrier gas pressure. This maintains the balance at the separation between the gases and also pro-vides a log proving there have been no fugitive emissions.
About the Author
Drew Devitt is the Founder and CTO of New Way Air Bearings. He can be reached at (610) 364-3460; firstname.lastname@example.org.