U.S. patent application number 13/511470 was filed with the patent office on 2013-07-04 for low emission dry gas seal system for compressors.
The applicant listed for this patent is Stefano Meucci, Paolo Susini. Invention is credited to Stefano Meucci, Paolo Susini.
Application Number | 20130170961 13/511470 |
Document ID | / |
Family ID | 42316102 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130170961 |
Kind Code |
A1 |
Meucci; Stefano ; et
al. |
July 4, 2013 |
LOW EMISSION DRY GAS SEAL SYSTEM FOR COMPRESSORS
Abstract
Systems and methods according to these exemplary embodiments
provide sealing mechanisms 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 remain low enough that a
dedicated compressor for supplying the sealing gases is not needed.
Additionally, the risk of process gas being released into the
atmosphere in case of seal failure is limited.
Inventors: |
Meucci; Stefano; (Florence,
IT) ; Susini; Paolo; (Signa, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Meucci; Stefano
Susini; Paolo |
Florence
Signa |
|
IT
IT |
|
|
Family ID: |
42316102 |
Appl. No.: |
13/511470 |
Filed: |
November 15, 2010 |
PCT Filed: |
November 15, 2010 |
PCT NO: |
PCT/EP2010/067456 |
371 Date: |
September 18, 2012 |
Current U.S.
Class: |
415/170.1 ;
415/1 |
Current CPC
Class: |
F01D 25/22 20130101;
F16J 15/3404 20130101; F01D 25/16 20130101; F16J 15/406 20130101;
F04D 29/122 20130101; F16J 15/3484 20130101 |
Class at
Publication: |
415/170.1 ;
415/1 |
International
Class: |
F01D 25/16 20060101
F01D025/16 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 23, 2009 |
IT |
CO2009A000051 |
Claims
1. A turbo machine comprising: a rotor assembly including at least
one impeller; a bearing connected to, and for rotatably supporting,
the rotor assembly; a stator; and a sealing mechanism disposed
between the rotor assembly and the bearing, said sealing mechanism
including: a first dry gas seal, disposed proximate an inboard side
of said sealing mechanism, and having a primary seal gas supplied
thereto at a first pressure; a second dry gas seal, disposed
adjacent to said 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 said second dry gas seal and having a buffer
gas supplied thereto at a third pressure.
2. The turbo machine of claim 1, wherein said primary seal gas is a
process gas which is being pressurized by said turbo machine, said
primary buffer gas is fuel gas, and said buffer gas is
nitrogen.
3. The turbo machine of claim 1, further comprising: a first
venting mechanism which is configured to vent primary seal gas
which is recovered downstream of said first dry gas seal to a
recovery system within said turbo machine; a second venting
mechanism which is configured to vent primary buffer gas and buffer
gas which is recovered downstream of said second dry gas seal to a
flare associated with said turbo machine; and a third venting
mechanism which is configured to vent buffer gas which is recovered
downstream of said third dry gas seal into the atmosphere.
4. The turbo machine of claim 1, wherein a first pressure zone
associated with said process gas coming from said turbo machine has
a pressure P1, a second pressure zone disposed between said first
pressure zone and said first dry gas seal has a pressure P2, a
third pressure zone disposed within said first dry gas seal has a
pressure P3, a fourth pressure zone disposed between said third
pressure zone and said second dry gas seal has a pressure P4, a
fifth pressure zone disposed within said second dry gas seal has a
pressure P5, a sixth pressure zone disposed between said fifth
pressure zone and said third dry gas seal has a pressure P6, a
seventh pressure zone disposed within said third dry gas seal has a
pressure P7, and P1>P2>P4>P3>P6>P5>P7.
5. A method for sealing a turbo machine having a rotor assembly
including at least one impeller, a bearing connected to, and for
rotatably supporting, the rotor assembly, and a stator, the method
comprising: blocking a process gas, which is pressurized by said
turbo machine, from reaching said bearing by using a combination of
first, second and third dry gas seals in sequence; supplying said
first dry gas seal with a primary seal gas at a first pressure;
supplying said second dry gas seal, disposed adjacent to said first
dry gas seal, with a primary buffer gas at a second pressure; and
supplying said third dry gas seal, disposed adjacent to said second
dry gas seal, with a buffer gas at a third pressure.
6. The method of claim 5, wherein said primary seal gas is a
conditioned process gas, said primary buffer gas is fuel gas, and
said buffer gas is nitrogen.
7. A dry gas sealing control system comprising: a first sealing gas
input control mechanism which is configured to provide a first
sealing gas to a first dry gas seal at a first pressure; a second
sealing gas input control mechanism which is configured to provide
a second sealing gas to a second dry gas seal at a second pressure;
and a third sealing gas input control mechanism which is configured
to provide a third sealing gas to a third dry gas seal at a third
pressure, wherein said first, second and third sealing gases are
different from one another.
8. The dry gas sealing control system of claim 7, wherein said
first sealing gas is a process gas, said second sealing gas is fuel
gas, and said third sealing gas is nitrogen.
9. The dry gas sealing control system of claim 7, wherein each of
said first, second and third pressures is less than 51 Bar.
10. The dry gas sealing control system of claim 7, further
comprising at least one gas conditioning element associated with
said first sealing gas input control mechanism to perform at least
one of heating, cooling and filtering of said first sealing gas.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a national stage application under 35 U.S.C.
.sctn.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.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the present invention relate generally to
compressors and, more specifically, to the provision of dry gas
seal systems in compressors.
[0004] 2. Description of the Prior Art
[0005] 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".
[0006] 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.
[0007] 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.
[0008] 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
FIG. 1, a tandem-type dry gas seal system includes a first seal 2
and a second seal 4, both contained in a single package. During
normal operation of the compressor, the first seal 2 operates to
contain the total pressure of gas processed, while the second seal
4 acts as a back-up which is designed to operate only if the first
seal 2 fails or leaks excessively. Generally a conditioned gas flow
coming from compressor discharge is injected upstream of seal 2 to
isolate the dry gas seal from process gas. In the applications with
highly toxic process gases (e.g., gas having high contents of
H.sub.2S) and high sealing pressure, an external sealing gas source
having a low sulfur content, e.g., a so-called "sweet" gas is
usually provided to isolate the process gas from the surroundings.
Due to the high sealing pressure a dedicated reciprocating
compressor 6 that operates independently of the centrifugal
compressor is used to feed the sealing gas system. The second seal
4 in the tandem may receive a lower pressure (e.g., below 10 Bar)
of nitrogen as secondary sealing gas via a source 8 to ensure that
no toxic/flammable gas escapes to the surroundings.
[0009] 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.
[0010] 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.
BRIEF SUMMARY OF THE INVENTION
[0011] 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.
[0012] 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.
[0013] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings illustrate exemplary embodiments,
wherein:
[0015] FIG. 1 illustrates a tandem sealing mechanism;
[0016] FIG. 2 is a schematic view of a multistage-type centrifugal
compressor, provided with sealing mechanisms according to exemplary
embodiments;
[0017] FIG. 3 is a partial sectional view of an exemplary dry gas
seal used in sealing mechanisms according to exemplary
embodiments;
[0018] FIG. 4 is a sectional view of a sealing mechanism including
three dry gas seals according to an exemplary embodiment;
[0019] FIG. 5 illustrates a sealing mechanism including input and
output fluid controls according to exemplary embodiments; and
[0020] FIG. 6 is a flowchart illustrating a method for sealing a
compressor according to exemplary embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0021] 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.
[0022] To provide some context for the subsequent discussion
relating to sealing systems according to these exemplary
embodiments, FIG. 2 schematically illustrates a multistage,
centrifugal compressor 10 in which such sealing systems may be
employed. Therein, the compressor 10 includes a box or housing
(stator) 12 within which is mounted a rotating compressor shaft 14
that is provided with a plurality of centrifugal impellers 16. The
rotor assembly 18 includes the shaft 14 and impellers 16 and is
supported radially and axially through bearings 20 which are
disposed on either side of the rotor assembly 18.
[0023] 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 FIG. 2 is a balance drum 27 which
compensates for axial thrust generated by the impellers 16, the
balance drum's labyrinth seal 28 and a balance line 29 which
maintains the pressure on the outboard side of the balance drum 27
at the same level as the pressure at which the process gas enters
via duct 22.
[0024] 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 FIG. 3.
Therein, a dry gas seal 30 is seated on the compressor shaft 14 to
block the flow of the process gas along the gas path 32 from the
inboard side to the outboard side of the centrifugal compressor 10.
Each dry gas seal 30 includes a rotating seat 34 which rotates with
the compressor shaft and mates with a stationary ring 36. During
operation, grooves formed in at least one of the rotating seat 34
and the stationary ring 36 have a pressurized fluid pumped therein
which has the effect of generating a fluid dynamic force which
causes the stationary ring 36 to separate from the rotating seat
34. This creates a gap between the two rings, the combination of
which operates as a seal that generally prevents leakage of the
process gas, without friction between the rotating seat 34 and the
stationary ring 36. Examples of these types of dry gas seals can be
found in U.S. Pat. Nos. 5,492,341 and 5,529,315, the disclosures of
which are incorporated here by reference.
[0025] FIG. 4 illustrates a portion of a rotary machine, e.g., a
centrifugal compressor, having a triple dry gas seal system 26
according to exemplary embodiments. Therein, the triple dry gas
seal system 26 includes three individual dry gas seals 40, 42 and
44 arranged in series along the compressor shaft 14. In this
exemplary embodiment, a labyrinth seal 45 is disposed upstream of
the triple dry gas seal system 26 (on the inboard side proximate
the impellers) and a barrier seal 48 is disposed downstream of the
triple dry gas seal system 26 on (i.e., on the outboard side
proximate the bearings 20), although those skilled in the art will
appreciate that this particular configuration is purely
illustrative and that the labyrinth and/or barrier seals may be
omitted from other embodiments. Each of the three dry gas seals 40,
42 and 44 has respective inlets and outlets for their respective
sealing gases. More specifically, the first stage (primary) seal 40
has an inlet 46 and an outlet 48, the second stage (secondary) seal
42 has an inlet 50 and an outlet 52, and the third stage (tertiary)
seal 44 has an inlet 54 and an outlet 56. Similarly, each of the
three dry gas seals 40, 42 and 44 has rotating seats 58, 62, 66 and
stationary rings 60, 64 and 68, respectively, and each of the three
dry gas seals 40, 42 and 44 is designed to handle the maximum
sealing pressure associated with the process gas.
[0026] FIG. 5 depicts a triple dry gas sealing system 26 according
to exemplary embodiments from a fluid pressure point of view.
Therein, according to this exemplary embodiment, the primary seal
stage 40 is provided with conditioned (i.e., appropriately
filtered, heated and controlled) process gas as the sealing gas.
This sealing gas may, for example, be delivered at a pressure of
70-400 Bar, and provided to the first seal 40 at a pressure higher
than process gas of, for example, 300 Bar via a pressure control
valve (PCV) 70 and associated gas conditioning elements 72 (which
are not necessarily limited to the heater and filter indicated in
FIG. 5, e.g., cooling elements or other gas conditioning elements
could be provided as additional or alternative elements). The seal
gas is automatically controlled in flow or in differential pressure
by the PCV 70 in order to ensure the flow of conditioned gas in all
operating conditions (e.g., pressurization, start-up, normal
operation, shut down etc.).
[0027] 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
FIG. 4) via the PCV 74 in a manner which ensures a positive
separation between sour and sweet seal gas. Similarly, the tertiary
seal stage 44 may be provided with nitrogen as its buffer gas from
a source which delivers the gas at a pressure of 4-10 Bar, e.g.,
higher than the flare system pressure, and which can be
controllably provided to the third dry gas seal 44 at 4 Bar by PCV
78 and associated gas conditioning elements 80. Note, however, that
the provision of nitrogen to this third dry gas seal 44 is optional
and, therefore, gas pathway elements 79 to the third dry gas seal
44 may be omitted. Moreover, the buffer gas (nitrogen in this
example) may also be supplied to the barrier seal 81.
[0028] Those skilled in the art will appreciate that the specific
gas pressures described above and illustrated in FIG. 5 are purely
exemplary and that other pressures may be used. More generally, the
gas pressures through the sealing system 26 should generally be set
such that P1>P2>P4>P3>P6>P5>P7, referring to the
pressure zones illustrated in the figure, such that a stepwise
decrease in pressure is exhibited through the zones. Note, however,
that although these pressure values are exemplary, they are
sufficiently low that none of the sealing gas supply sources
require the provision of an auxiliary (e.g., reciprocating)
compressor to supply the sealing gas, e.g., as a non-limiting
example supply pressures ranging from 1-50 Bar or, stated somewhat
differently, lower than 51 Bar. This, in turn, renders exemplary
embodiments more cost efficient and requires less maintenance than
conventional compressor systems.
[0029] FIG. 5 also illustrates pressure controlled venting
mechanisms for each of the first two of the three sealing stages of
the sealing system 26. For example, the primary seal 40 includes a
venting mechanism 82 which returns process gas that escapes from
the primary seal 40 back to a recovery system. The venting
mechanism 82 includes, among other things, an optional PCV 84 set
to an appropriate pressure level given the sealing gas pressure, in
this example 10 Bar. The recovered seal gas venting mechanism 82 is
also equipped with flow and pressure monitoring instrumentation
which can monitor variations of flow and pressure (higher or lower)
along the return path, which parameters may be indicative of a
malfunction of the seals. These values are detected and can be used
to generate system alarm or shutdown signals. The recovered process
gas is then routed to a recovery system and injected into the
process gas loop.
[0030] 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.
[0031] 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 FIG. 6. Therein, at step 100,
a process gas, which is pressurized by the centrifugal compressor,
is blocked from reaching the bearing by using a combination of
first, second and third dry gas seals in sequence. This further
involves supplying the first dry gas seal with a primary seal gas
at a first pressure (step 102), supplying the second dry gas seal,
disposed adjacent to the first dry gas seal, with a primary buffer
gas at a second pressure (step 104), and supplying the third dry
gas seal, disposed adjacent to the second dry gas seal, with a
buffer gas at a third pressure.
[0032] 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.
[0033] 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.
* * * * *