July 30, 2019 | 3 minute read
This blog was guest authored by Richard G. Albery, John Crane Senior Staff Engineer, Gas Seals.
Rotating liquid pump shafts that are originally sealed with soft packings mostly use contacting face seals, also known as mechanical seals. Typical rotational speeds are roughly between 1,000 and 3,600 revolutions per minute (rpm). In turbomachinery—such as compressors and expanders—the rotational speeds are higher and mechanical seals would not immediately appear to be an option, due to greater rubbing speeds along with a lack of liquid cooling and lubrication.
The first, and still the bulk, of these dry gas seal applications are through-shaft or beam machines where the shaft, carrying multiple closed impellers, is supported at each end, also known as “between-bearings” compressors.
Shaft speeds range from near 20,000 rpm down to under 3,000 rpm for the biggest machines. The latter have shaft sizes up to 350 millimeters (mm), approximately 13 3/4 inches diameter.
Smaller, between-bearing machines run at higher speeds. The fastest machines are single-ended or overhung shaft types with open impellers in individual casings, typically referred to as integrally geared compressors (see Image 1). Such machines vary from single-stage to multiple-stage designs with several compressor casings driven by more than one shaft, usually off a central bull gear wheel. They are used in refineries and chemical plants for a range of processes from simple fuel gas requirements to complex multi-pressure applications. There is presently a growth area in the compression of carbon dioxide (CO2), which is a difficult-to-seal critical fluid.
For many reasons, as far as the shaft seal is concerned, the established between-bearing compressor applications may be seen as straightforward compared to those for overhung machines. The typical layout of a dry gas seal stage is as shown in Image 2.
Once the seal is pressurized, a thin fluid film of the sealed gas separates the stationary primary ring and rotating mating ring. The gap between the faces is typically 2 to 4 micrometer (µm) thick, but depends on the pressure, speed of the application and nature of the gas. This thin film must continue to separate the faces during significant axial movements of the shaft. Fortunately, the fluid dynamics involved are advantageous. As the gap tries to close, the fluid film becomes stiffer and increasingly resistant to the faces coming into contact.
There are two important implications associated with using a gas film seal in this way. First, there is some leakage to be considered, but this is small (measured in single or tens of standard-liters/minute) because of the small gap.
Second, because contamination is the most common cause of seal failure, the inboard area must be supplied with conditioned process gas, or an alternative, to ensure the seal is operating on clean dry gas. For the same reason, it is necessary to avoid contaminating the seal with liquid lubricant from the bearing adjacent to the seal. A separation seal is usually provided. These range from a simple labyrinth to a number of contacting or noncontacting devices, usually of a double gas-buffered design.
Dry gas seals are supplied in cartridge form, most commonly in a tandem arrangement with the inboard stage followed by an outboard stage. A double, back-to-back, inert gas buffered arrangement is sometimes used. It is possible to have a single-stage seal. This is most commonly the case in high-speed, overhung machines where space for the seals is limited.
With a single or unbuffered seal, careful thought must be given to the nature and destination of the leakage. A separation device may help.
Although there is a wide range of parameters, a typical between-the-bearings compressor may have a shaft size of 120 mm (4 3/4 inches) diameter and a rotational working speed of around 12,000 rpm. This gives a passing speed at the seal faces of about 100 meters per second (m/s) on a shaft supported at both ends and driven through a coupling with a degree of flexibility.
The rotor dynamics in turbomachinery are usually monitored by measuring the radial vibrations outboard of the shaft support bearings. Gas seals can withstand most normal levels of vibration in a turbo compressor, including brief transients into surge at levels acceptable to the compressor.
The noncontacting face seal is intrinsically tolerant of radial vibration, as the faces separated by a film of gas can easily slide eccentrically against each other. The seal does not provide much damping force to, and absorbs little energy from, such movements.
The stability of the seal under the axial component of any vibration is maintained by the stiffness of the fluid film and the responsiveness incorporated into the seal design. The shaft seal assembly must take up any relative axial movement between the rotor and the casing of the compressor.
Between-the-bearing machines usually have a thrust bearing at one end of the shaft. This means that, at the thrust-bearing end of the machine, the amount of sudden axial movement is limited in the seal (typically around 0.1 mm to 0.2 mm in operation).
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