1. Field of the Invention
Exemplary embodiments relate generally to compressor seals and, more specifically, to the provision of abradable seals with axial offset for reducing leakage.
2. Description of the Prior Art
A compressor is a machine which increases the pressure of a compressible fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications and in a large number of industrial processes, including power generation, natural gas liquification and other processes. Among the various types of compressors used in such processes and process plants are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration, for example, by rotating a centrifugal impeller.
Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for gas to be compressed, an impeller which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy.
A multistage compressor 100 is illustrated in FIG. 1. Compressor 100 is included in casing 110 within which is mounted a shaft 120 and a plurality of impellers 130. The shaft 120 and impellers 130 are included in a rotor assembly that is supported through bearings 190 and 195.
The multistage compressor operates to take an input process gas from an inlet duct 160, to increase the process gas pressure through operation of the rotor assembly, and to subsequently expel the process gas through an outlet duct 170 at an output pressure which is higher than its input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof.
Seals or sealing systems 180, 185 and 188 are provided between impellers 130 and bearing 190 and 195 to prevent the process gas from flowing through to the bearings. Seals 188 are impeller eye seals.
Each of the impellers 130 increases the pressure of the process gas. Each of the impellers 130 may be considered to be one stage of the multistage compressor 100. Additional stages, therefore, result in an increase in the ratio of output pressure to input pressure.
The rotor assembly includes stationary portions known as stators and rotating portions known as rotors. The overall operating efficiency of a compressor is adversely affected by leakage flow of the working fluid or gas between the stator and rotor due to differential axial pressure of the rotor.
In centrifugal compressors, abradable seals can be used to reduce the leakage flow (of the working gas) to improve stage efficiency via clearance reduction without the risk of harming the rotor portion.
A sealing system using an abradable seal is illustrated in FIG. 2. Sealing system 200 includes a rotor 210 (i.e., a rotating portion) and a stator 220 (i.e., a stationary portion). Rotor 210 rotates around a longitudinal axis of the compressor. Rotor 210 includes a plurality of rotor teeth 215. Rotor teeth 215 can be radial. Stator 220 includes a cavity or housing 230 for housing a stator seal 223.
The stator seal includes an abradable portion or coating 225. Stator seal 223 can be an insert ring with an abradable coating on the inner circumference in some arrangements. In other arrangements, the (entire) insert ring can also be made of an abradable material.
The stator seal circumferentially encloses the rotor; the rotor can rotate within along the inner circumference of the stator seal. A stator seal is located on either side of each stage of a multistage compressor.
Rotation of the rotor 210 causes the rotor-mounted radial teeth 215 to create rub grooves 227 along the inner circumference of the stator-mounted abradable seal 223 and can cause debit (i.e., an increase in the leakage flow) to sealing performance.
In order to reduce leakage flow, a stator seal within housing 230 as illustrated in FIG. 2 can be offset radially from a top portion 235 of the housing 230 by a spring activated mechanism 240. The insert ring is typically partitioned into two or four equal sized sections. The spring activated mechanism permits the stator seal to radially move toward or away from the rotor 210. The two or four equal-sized section of the insert ring facilitate the radial movement.
Stator seal 223 with the spring activated mechanism may also be referred to as a compliant seal, a spring-backed seal or a spring-energized abradable seal. A compliant seal, when compared to a non-compliant seal (i.e., a seal not having a spring activated mechanism and hence no radial offset) provides a smaller radial clearance and a reduction in the leakage flow.
It would be desirable to design and provide an improved sealing mechanism for reducing the leakage flow debit further.