ABRADABLE SEAL WITH AXIAL OFFSET
TECHNICAL FIELD
[0001] Exemplary embodiments relate generally to compressor seals
and, more specifically, to the provision of abradable seals with axial offset for
reducing leakage.
BACKGROUND
[0002] 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.
[0003] 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.
[0004] A multistage compressor 00 is illustrated in Figure 1.
Compressor 100 is included in casing 10 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.
[0005] The multistage compressor operates to take an input process
gas from an inlet duct 160, to inaease 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.
[0006] 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.
[0007] Each of the impellers 30 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.
[0008] 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.
[0009] 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.
[0010] 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 2 1 . Rotor teeth
2 can be radial. Stator 220 includes a cavity or housing 230 for housing a
stator seal 223.
[0011] 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.
[001 2] 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.
[0013] Rotation of the rotor 210 causes the rotor-mounted radial teeth
2 15 to create rub grooves 227 along the inner circumference of the statormounted
abradable seal 223 and can cause debit (i.e., an increase in the
leakage flow) to sealing performance.
[0014] 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.
[001 5 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.
[0016] It would be desirable to design and provide an improved sealing
mechanism for reducing the leakage flow debit further.
SUMMARY
[0017] Systems and methods according to these exemplary
embodiments provide improved sealing characteristics to reduce leakage flow
between a stator seal and a rotor of a rotor assembly by introducing a spring
mechanism for facilitating axial positioning control over the seal.
[0018] According to an exemplary embodiment, a sealing system for a
centrifugal compressor is disclosed. The sealing system includes a stator
having a seal housing, a seal disposed in the seal housing and having an
abradable portion along an inner circumference, a rotor having a plurality of
rotor teeth configured to rotate within the inner circumference of the seal and
configured to create rub grooves within the abradable portion, and a first
spring disposed between the stator and the seal and configured to facilitate
axial movement of the seal relative to the seal housing.
[001 ] According to another embodiment, a method for reducing
leakage flow between a seal housed within a stator seal housing and a rotor
of a centrifugal compressor wherein the rotor rotates within an inner
circumference of the seal is disclosed. The method includes biasing the seal
toward a high pressure side of the seal housing by a spring load to create an
axial gap between the seal and the seal housing, starting the compressor,
cutting radial rub grooves in an abradable portion of the seal, increasing a
speed of the compressor and moving the seal toward a low pressure side of
the seal housing.
[0020] According to a further embodiment, a centrifugal compressor
includes a stator having a seal housing, a seal disposed in the seal housing
and having an abradable portion along an inner circumference, a rotor having
a plurality of rotor teeth configured to rotate within the inner circumference of
the seal and configured to create rub grooves within the abradable portion, a
first spring disposed between the stator and the seal and configured to
facilitate axial movement of the seal, and a secondary seal located between a
lower pressure side of the housing and the seal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings illustrate exemplary embodiments,
wherein:
[0022] FIG. 1 illustrates a multistage compressor;
[0023] FIG. 2 illustrates a side view of a rotor/stator sealing assembly
utilizing an abradable seal;
[0024] FIG. 3 illustrates a side view of a rotor/stator sealing assembly
utilizing an abradable seal according to exemplary embodiments in an initial
state;
[002S] FIG. 4 illustrates a side view of a rotor assembly utilizing an
abradable seal according to exemplary embodiments in an operating state;
and
[0026] FIG. 5 illustrates a method in accordance with exemplary
embodiments.
DETAILED DESCRIPTION
[0027] 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.
[0028] In exemplary embodiments, leakage flow between rotor teeth
and stator rub grooves in a rotor assembly may be reduced by employing a
pressure activated mechanism to achieve active axial positioning control of
the rotor teeth relative to the rub grooves.
[0029] A computational fluid dynamics (CFD) analysis of rub groove
impact indicates that teeth/groove axial positioning plays an important role in
leakage performance. An axial offsetting of rotor teeth after initial formation of
the grooves may reduce the performance debit.
[0030] Exemplary embodiments may utilize a pressure-activating
mechanism to achieve active axial positioning control of the rotor teeth
relative to the rub groove as illustrated in sealing system 300 FIG. 3 . Sealing
system 300 may include rotor 310 and stator 320. Rotor 310 may include a
plurality of rotor teeth 3 5 which may be radial. Stator 320 may include a
stator seal 323 located within a seal housing 330.
[0031 ] As described above, the stator seal may include an abradable
portion or coating 325. Stator seal 323 may include an insert ring (two or
more portions) with an abradable coating on the inner circumference in some
arrangements. The insert ring may be made of steel. The abradable coating
on the inner circumference may be a porous metallic material such as porous
Aluminum, Cobalt or Nickel-based thermally sprayed coatings; it may
alternatively be plastic-based (e.g., Teflon, Polyester).
[0032] In other arrangements, the (entire) insert ring may also be made
of an abradable material. The abradable material may be Fluorosint-500, a
mica-reinforced Teflon for example.
[0033] Stator seal 323 may be placed within housing 330 and offset
radially from a top portion 335 of the housing by a spring activated
mechanism 340. Rotor teeth 315 may create rub grooves 327 in the
abradable stator portion 325. In exemplary embodiments, an axial gap 360
may be created or designed between stator seal 320 and seal housing 330 by
the use of an axially oriented spring 350.
[0034] While being assembled, stator seal 323 may be pushed (or set)
to the high(er) pressure side (P+) of housing 330 by the axially oriented spring
350. Spring 350 may be placed between seal 323 and the low(er) pressure
side (P-) of housing 330. Spring 350 may be in its (original) or slightly
compressed state at this time. The axially oriented spring 350 may bias the
stator seal 323 toward the higher pressure side.
[0035] During the usage, at start-up (i.e., when the pressure ratio is
low), rotor teeth 3 5 may cut into abradable portion 325 during critical
vibration and create rub grooves 327. At start up speeds, the difference in
pressure (between P+ and P-) is relatively low making it possible to maintain
the position of the seal at the higher pressure side. Rotor teeth 315 may ride
in the middle of rub grooves 327 at these speeds and stator seal position.
[0036] At design speed, the pressure differential (between P+ and P-)
increases. As illustrated in FIG. 4, the increased pressure ratio across seal
323 from the high pressure (or P+) side may force seal 323 to move axially
towards the low pressure (or P-) side. Spring 350 may start to contract (i.e.,
not in its original state) as the pressure increases.
[0037] This axial movement (from the higher pressure side P+ to the
lower pressure side P-) forms a smaller or tighter gap between rotor teeth 3 15
and rub grooves 327. Rotor teeth 315 are not riding in the middle of the rub
grooves 327 as illustrated.
[0038] The spacing between the (tips of the) rotor teeth and the part of
the abradable portion facing the rotor teeth of FIG. 4 is smaller in relation to
the spacing between the rotor teeth and the middle of rub grooves of FIG. 3 .
The amount of leakage may be decreased as a result. The leakage can be
controlled by spring stiffness and operating pressure ratio.
[0039] A secondary seal may also be included between seal 323 and
seal housing 330 in some embodiments to provide a further reduction in
leakage as illustrated in FIGs. 3 and 4 . Referring to FIG. 4 , secondary seal
370 may block leakage between seal 323 and seal housing 330 at design
speed. Secondary seal 370, therefore, further reduces the leakage.
[0040] While the axial offset as described in exemplary embodiments
may be additional to the radial offset, the axial offset may be implemented by
itself (i.e., without the radial offset) in some embodiments. The axial offset
reduces the radial gap and reduces leakage debit in either (i.e., with or
without radial offset) arrangements.
[0041] A method 500 in accordance with exemplary embodiments may
be described with reference to FIG. 5. During assembly, an axial gap may be
created between a seal and seal housing of a centrifugal compressor at 51
by moving the seal toward a high pressure side of the housing utilizing an
axially oriented spring (in its original or slightly compressed state). The
compressor may be started at 520. Rotor teeth may cut rub grooves in the
stator seal abradable portion at 530.
[0042] The compressor speed (and hence, pressure) may be increased
at 540. As the compressor operates toward the design speed, the seal spring
may become compressed with increased pressure and the seal moves toward
the low pressure side at 550. The gap between rotor teeth and the abradable
portion may decrease and secondary seal may provide additional leakage
protection as described above.
[0043] CFD analysis demonstrates that axial rub-groove positioning of
the abradable seal, achieved by pressure-activating mechanism in exemplary
embodiments, increases sealing performance.
[0044] Exemplary embodiments are directed to axial positioning control
of the abradable seal design. While previous efforts have focused on
minimizing radial clearance between rotor teeth and abradable seal,
embodiments as described herein introduce a pressure-activated axial
positioning mechanism, which leads to minimizing the sealing gap
independent of compressor critical vibration or thermal transients.
[0045] Exemplary embodiments as described herein provide multiple
advantages, A sealing system according to exemplary embodiments reduces
leakage caused by rub grooves to a non-compliant abradable seal. The
system also optimizes performance of the compliant (spring-energized)
abradable seal for an impeller eye of a centrifugal compressor.
[0046] Centrifugal compressor stage efficiency can be improved by
providing an active axial-positioning mechanism as described in exemplary
embodiments to achieve a minimum sealing gap.
[0047] In some embodiments, a plurality of axial positioning
mechanisms may be provided in the stator seal. The plurality of axial
positioning mechanisms may be distributed (or spaced apart) equally along
the seal for example.
[0048] The axial-positioning mechanism according to exemplary
embodiments is not limited to implementation in new compressors. Existing
compressors with seal rings can be retrofitted with the axial-positioning
mechanism, Furthermore, while the active-positioning mechanism of
exemplary embodiments has been described with respect to centrifugal
compressors, it may be equally applicable to turbo machines in general.
[0049] 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 an 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.

CLAIMS:
1. A sealing system (200) for a centrifugal compressor comprising:
a stator (220) having a seal housing (230);
a seal (223) disposed in the seal housing and having an abradable
portion (225) along an inner circumference;
a rotor (21 0) having a plurality of rotor teeth (21 5) configured to rotate
within the inner circumference of the seal and configured to create rub
grooves (227) within the abradable portion (225); and
a first spring (240) disposed between the stator (220) and the seal
(223) and configured to facilitate axial movement of the seal (223).
2 . The sealing system of claim 1, wherein the rotor teeth are oriented in a
radial direction.
3 . The sealing system of claim 1, wherein the first spring biases the seal
axially toward a high pressure side of the seal housing at startup of the
compressor.
4 . The sealing system of claim 3, wherein the first spring is in an
expanded state at the startup of the compressor.
5 . The sealing system of claim 3, wherein the seal is configured to move
toward a low pressure side of the seal housing at a design speed of the
compressor.
6. The sealing system of claim 5, wherein the first sp ng is in a
compressed state at design speeds of the compressor.
7. The sealing system of claim 1, further comprising:
a secondary seal located between a lower pressure side of the seal
housing and the seal.
8 . The sealing system of claim 1, further comprising:
a second spring between the seal and the seal housing and configured
to provide radial movement of the seal.
9 . The sealing system of claim 8, wherein the second spring is in an
expanded position during operation of the compressor.
10. The sealing system of claim 9, wherein the second spring is configured
to reduce a radial distance between the seal and the rotor.
11. The sealing system of claim 1, wherein the seal is an insert ring.
12. The sealing system of claim 1, wherein the insert ring is made of steel
and the abradable portion is made of porous metallic material.
13. The sealing system of claim 1, wherein the seal is an insert ring made
of abradable material.
14. A method for reducing leakage flow between a seal (223) housed
within a stator seal housing (230) and a rotor (210) of a centrifugal
compressor wherein the rotor (210) rotates within an inner circumference of
the seal (223), the method comprising:
biasing the seal (223) toward a high pressure side of the seal housing
(230) by a spring load (240) to create an axial gap between the seal (223) and
the seal housing (230);
starting the compressor;
cutting radial rub grooves (227) in an abradable portion (225) of the
seal (223);
increasing a speed of the compressor; and
moving the seal (223) toward a low pressure side of the seal housing
(230).
15. A centrifugal compressor comprising:
a stator (220) having a seal housing (230);
a seal (223) disposed in the seal housing (230) and having an
abradable portion (223) along an inner circumference;
a rotor (210) having a plurality of rotor teeth (21 5) configured to rotate
within the inner circumference of the seal (223) and configured to create rub
grooves (227) within the abradable portion (225);
a first spring (240) disposed between the stator (220) and the seal
(223) and configured to facilitate axial movement of the seal (223); and
a secondary seal (370) located between a lower pressure side of the
housing (230) and the seal (233).