1. Field of the Invention
Embodiments of the present invention relate generally to compressors and, more specifically, to the provision of dry gas seal systems in compressors.
2. Description of the Prior Art
A compressor is a machine which accelerates the particles of a compressible fluid, e.g., a gas, through the use of mechanical energy to, ultimately, increase the pressure of that compressible fluid. Compressors are used in a number of different applications, including operating as an initial stage of a gas turbine engine. Among the various types of compressors are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration which accelerates the gas particles, e.g., by rotating a centrifugal impeller through which the gas is passing. More generally, centrifugal compressors can be said to be part of a class of machinery known as “turbo machines” or “turbo rotating machines”.
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 accelerated, 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. Various types of gases are used in centrifugal compressors, some of which are toxic or dangerous to the environment and/or to workers in the plants. Accordingly, centrifugal compressors employ sealing systems, usually placed on the ends of the shaft that supports the impeller(s), to prevent the gas from escaping from the compressor and contaminating the surrounding environment. Single rotor centrifugal compressors are usually provided with two separate seals as part of this sealing system, i.e., one for each end of the shaft, while in a overhung centrifugal compressor it is usually sufficient to seal the shaft end, located immediately downstream of the impeller.
Recently there has been an increase in the use of so-called “dry” gas seals in sealing systems for centrifugal compressors. Dry gas seals can be described as non-contacting, dry-running mechanical face seals which include a mating or rotating ring and a primary or stationary ring. In operation, grooves in the rotating ring generate a fluid-dynamic force causing the stationary ring to separate and create a gap between the two rings. These seals are referred to as “dry” since they do not require lubricating oil which, among other things, greatly reduces their maintenance requirements.
For centrifugal compressors, such dry gas seals are available in different configurations, e.g., so-called tandem configurations which are primarily used in compressors that employ toxic or flammable gases as the input or process gas. As shown in
Centrifugal compressors equipped with these types of dry gas sealing systems thus also require additional compressors whose function is solely to provide the sealing gas, thus making the overall system more complex. In addition to simply adding complexity, reciprocating compressors 6 may have greater maintenance requirements than even the centrifugal compressors which they are intended to serve. Moreover, although the second seal 4 in the tandem configuration does provide a back-up capability, current dry gas seal systems are still not fault free, in which case they may undesirably release a certain amount of sealing gas into the atmosphere.
Accordingly, it would be desirable to design and provide a low emission, dry gas seal for compressors which overcomes the aforementioned drawbacks of existing sealing systems.
Exemplary embodiments provide sealing mechanisms usable, e.g., for centrifugal compressors. A sealing mechanism includes first, second and third dry gas seals arranged in series. Each seal receives its own sealing gas and has its own venting mechanism. Sealing gas pressures which separate the process gas from the venting system remain low enough that a dedicated compressor for supplying the sealing gases is not needed. Advantages according to exemplary embodiments described herein include, for example, better control over potentially hazardous process gas and lower complexity and maintenance requirements associated with sealing mechanisms for centrifugal compressors. However, it will be appreciated by those skilled in the art that such advantages are not to be construed as limitations of the present invention except to the extent that they are explicitly recited in one or more of the appended claims.
According to an exemplary embodiment, a centrifugal compressor includes a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, a stator, a sealing mechanism disposed between the rotor assembly and the bearing, the sealing mechanism including a first dry gas seal, disposed proximate an inboard side of the sealing mechanism, and having a primary seal gas supplied thereto at a first pressure, a second dry gas seal, disposed adjacent to the first dry gas seal and having a primary buffer gas supplied thereto at a second pressure, and a third dry gas seal, disposed adjacent to the second dry gas seal and having a buffer gas supplied thereto at a third pressure.
According to another exemplary embodiment, a method for sealing a centrifugal compressor having a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, and a stator includes the steps of blocking a process gas, which is pressurized by the centrifugal compressor, from reaching the bearing by using a combination of first, second and third dry gas seals in sequence, supplying the first dry gas seal with a primary seal gas at a first pressure, supplying the second dry gas seal, disposed adjacent to the first dry gas seal, with a primary buffer gas at a second pressure, and supplying the third dry gas seal, disposed adjacent to the second dry gas seal, with a buffer gas at a third pressure.
The accompanying drawings illustrate exemplary embodiments, wherein:
The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
To provide some context for the subsequent discussion relating to sealing systems according to these exemplary embodiments,
The multistage centrifugal compressor operates to take an input process gas from duct inlet 22, to accelerate the particles of the process gas through operation of the rotor assembly 18, and to subsequently deliver the process gas through outlet duct 24 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. Between the impellers 16 and the bearings 20, sealing systems 26 are provided to prevent the process gas from flowing to the bearings 20. The housing 12 is configured so as to cover both the bearings 20 and the sealing systems 26 to prevent the escape of gas from the centrifugal compressor 10. Also seen in
According to exemplary embodiments, each of the sealing systems 26 includes three dry gas seals which together cooperate to seal the process gas from escaping toward the bearings 20. Generally speaking, each of the three dry gas seals in the sealing system 26 can be implemented as, for example, shown in
According to this exemplary embodiment, the secondary seal stage 42 is provided with fuel gas or other suitable sweet gas source as a primary buffer gas, which is provided to the dry gas seal 42 at, for example, 20 Bar via pressure control valve 74 and associated gas conditioning elements 76. The primary buffer gas (normally sweet fuel gas or other suitable gas that is available in the plant) is injected into the compressor 10 (e.g., via port 50 in
Those skilled in the art will appreciate that the specific gas pressures described above and illustrated in
Similarly, the secondary seal 42 is equipped with a venting mechanism 86. The primary vent is equipped according to this exemplary embodiment, and like the recovered gas vent, with flow and pressure monitoring instrumentation, as well as with a PCV 88 to keep the pressure within a defined range. This pressure can be set to be higher than a pressure used in the plant flare system, to which venting mechanism 86 vents. The variation of flow and pressure (higher or lower) can also be used to detect and generate alarm or shutdown signals in the secondary seal venting system 86. The tertiary seal 44 also has a venting mechanism 90 which is sized to avoid high back pressure in case of a failure of the sealing mechanism 26, and which vents the nitrogen (or primary buffer gas) to the atmosphere.
Thus, according to one exemplary embodiment, a method for sealing a centrifugal compressor having a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, and a stator, includes the method steps illustrated in the flowchart of
Thus, based on the foregoing, it will be seen that exemplary embodiments provide for a sealing mechanism for a centrifugal compressor which is capable of preventing, or at least make it unlikely, that the potentially hazardous process gases will be released into the atmosphere. This is particularly useful, for example, in the presence of process gases such as hydrogen sulfide (H2S). In addition, these exemplary embodiments create seal mechanisms which are substantially impervious to dry gas for a centrifugal compressor that does not require the presence of another compressor which is dedicated to the generation of a highly pressurized sealing gas. Moreover, although sealing mechanisms as shown and described in the above exemplary embodiments have three dry gas seals, it will be appreciated that four or more dry gas seals provided in sequence could also be used according to other exemplary embodiments.
The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.
Number | Date | Country | Kind |
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CO2009A000051 | Nov 2009 | IT | national |
This is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending PCT patent application serial number PCT/EP2010/067456, filed on Nov. 15, 2010, which claims priority to Italian Patent Application Serial No. CO2009A000051, filed on Nov. 23, 2009, the entire contents of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2010/067456 | 11/15/2010 | WO | 00 | 9/18/2012 |