U.S. patent application number 11/673148 was filed with the patent office on 2008-08-14 for screw pump rotor and method of reducing slip flow.
Invention is credited to David Deloyd Anderson, Vasanth Srinivasa Kothnur.
Application Number | 20080193309 11/673148 |
Document ID | / |
Family ID | 39356693 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080193309 |
Kind Code |
A1 |
Kothnur; Vasanth Srinivasa ;
et al. |
August 14, 2008 |
SCREW PUMP ROTOR AND METHOD OF REDUCING SLIP FLOW
Abstract
A pump rotor for a screw pump includes a shaft, a first set of
threads disposed on a portion of an outer surface of the shaft, at
least one thread of the first set of threads including a groove
disposed on an end portion thereof, and a ring seal disposed on the
groove such that the ring seal is configured to protrude outwardly
from the groove and to rest against an inner surface of a liner of
the screw pump, and the groove is sized so as to allow the ring
seal to move radially with respect to the plurality of threads as
the rotor is deflected.
Inventors: |
Kothnur; Vasanth Srinivasa;
(Clifton Park, NY) ; Anderson; David Deloyd;
(Glenville, NY) |
Correspondence
Address: |
GENERAL ELECTRIC CO.;GLOBAL PATENT OPERATION
187 Danbury Road, Suite 204
Wilton
CT
06897-4122
US
|
Family ID: |
39356693 |
Appl. No.: |
11/673148 |
Filed: |
February 9, 2007 |
Current U.S.
Class: |
418/1 ; 416/182;
418/143 |
Current CPC
Class: |
F04C 2210/24 20130101;
F04C 18/16 20130101; F04C 27/001 20130101 |
Class at
Publication: |
418/1 ; 416/182;
418/143 |
International
Class: |
F04D 3/02 20060101
F04D003/02; F04D 29/18 20060101 F04D029/18 |
Claims
1. A pump rotor for a screw pump, the pump rotor comprising: a
shaft; a first set of threads disposed on a portion of an outer
surface of the shaft, at least one thread of the first set of
threads comprising a groove disposed on an end portion thereof; and
a seal disposed on the groove.
2. The pump rotor according to claim 1, wherein the seal is a brush
seal.
3. The pump rotor according to claim 1, wherein the seal is a ring
seal.
4. The pump rotor according to claim 3, wherein the ring seal is
configured to protrude outwardly from the groove and to rest
against an inner surface of a liner of the screw pump, and the
groove is sized so as to allow the ring seal to move radially with
respect to the at least one thread as the rotor is deflected.
5. The pump rotor according to claim 3, wherein the ring seal is
configured to rotate with the shaft.
6. The pump rotor according to claim 5, further comprising: first
and second pins disposed in the groove, a first end of the ring
seal being disposed against the first pin and a second end of the
ring seal being disposed against the second pin.
7. The pump rotor according to claim 6, wherein the ring seal and
the first set of threads are helical.
8. The pump rotor according to claim 1, further comprising: a
second set of threads disposed on another portion of the outer
surface of the shaft, at least one thread of the second set of
threads comprising a groove disposed on an end portion thereof, the
first set being separate from the second set, and a second ring
seal being disposed on the groove of the at least one thread of the
second set; first and second pins disposed in the groove of the at
least one thread of the first set, a first end of the ring seal
being disposed against the first pin and a second end of the ring
seal being disposed against the second pin; and third and fourth
pins disposed in the groove of the at least one thread of the
second set, a first end of the second ring seal being disposed
against the third pin and a second end of the second ring seal
being disposed against the fourth pin.
9. The pump rotor according to claim 8, wherein circumferential
positions of the first and second pins and circumferential
positions of the third and fourth pins are different from each
other.
10. The pump rotor according to claim 3, wherein the ring seal is a
sacrificial wear component of the pump rotor.
11. A twin-screw pump, comprising: a casing having an inlet and an
outlet; a liner disposed inside of the casing; and two rotors
disposed inside of the liner, each rotor comprising, a shaft; a set
of threads disposed on a portion of an outer surface of the shaft,
at least one thread of the first set of threads comprising a groove
disposed on an end portion thereof; and a ring seal disposed on the
groove.
12. The pump according to claim 11, wherein each ring seal is
configured to protrude outwardly from the groove and to rest
against an inner surface of the liner, and the groove is sized so
as to allow each ring seal to move radially with respect to the at
least one thread of the first set of threads as the rotor is
deflected.
13. The pump according to claim 12, wherein the ring seal is
configured to rotate with the respective shaft.
14. The pump according to claim 13, wherein each rotor further
comprises first and second pins disposed in the groove, a first end
of the ring seal being disposed against the first pin and a second
end of the ring seal being disposed against the second pin.
15. The pump according to claim 14, wherein the ring seal and the
set of threads are helical.
16. The pump according to claim 11, wherein for each rotor the set
of threads is a first set of threads, each rotor further
comprising, a second set of threads disposed on another portion of
the outer surface of the shaft, at least one thread of the second
set of threads comprising a groove disposed on an end portion
thereof, the first set being separate from the second set, and a
second ring seal being disposed on the groove of the at least one
thread of the second set; first and second pins disposed in the
groove of the at least one thread of the first set, a first end of
the ring seal being disposed against the first pin and a second end
of the ring seal being disposed against the second pin; and third
and fourth pins disposed in the groove of the at least one thread
of the second set, a first end of the second ring seal being
disposed against the third pin and a second end of the second ring
seal being disposed against the fourth pin.
17. The pump according to claim 11, further comprising: variable
speed drives; and a clearance control logic configured to control
the speed of the drives.
18. A method of reducing slip flow in a screw pump having a casing
having a low-pressure inlet and a high-pressure outlet, a liner
disposed inside of the casing, and a rotor disposed inside of the
liner having a shaft and a first set of threads disposed on a
portion of an outer surface of the shaft, the method comprising:
forming a groove on end portions of each thread Of the first set of
threads; disposing a ring seal on the groove, the ring seal being
configured to protrude outwardly from the groove and to rest
against an inner surface of the liner of the screw pump, the groove
being sized so as to allow the ring seal to move radially with
respect to the first set of threads as the rotor is deflected, and
the ring seal being configured to reduce the slip flow from the
high-pressure outlet to the low-pressure inlet.
19. The method according to claim 18, further comprising rotating
the ring seal when the shaft is rotated.
20. The method according to claim 19, further comprising preventing
the ring seal to rotate with respect to the shaft by placing first
and second pins in the groove, a first end of the ring seal being
disposed against the first pin and a second end of the ring seal
being disposed against the second pin.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates in general to screw pumps,
and, more particularly, to improved screw pump rotors and methods
of reducing slip flow in screw pumps.
[0003] 2. Description of the Related Art
[0004] In the exploration for oil and gas the need to transport
fluids (oil, water, gas, and foreign solids) from a wellhead to
distant processing and/or storage facilities (instead of building
new processing facilities near the wellheads) is well understood.
Twin-screw pumps are increasingly being used to aid in the
production of these wellhead fluids, resulting in increased
production by lowering the pressure at the exit of the wellhead as
well as a greater total recovery from the reservoir by allowing
lower final reservoir pressures before abandoning production.
[0005] FIG. 1 illustrates a conventional twin-screw pump 10. This
figure is presented simply to illustrate the main components of a
twin-screw pump and should not be considered as limiting the
invention disclosed herein in any way. As illustrated, the
twin-screw pump 10 has two rotors 12 and 14 that are disposed
within a close-fit casing or pump housing 16. Each rotor has a
shaft 18A and 18B with one or more outwardly extending sets of
screw threads 20 for at least a portion of the length of the shaft.
The shafts 18A and 18B run axially within two overlapping
cylindrical enclosures, collectively, a rotor enclosure, or liner,
19. The two rotors 12, 14 do not touch each other, but they have
threads of opposed screws that are intertwined. Pump 10 will often
be driven by a motor (not shown), which rotates rotors 12 and 14. A
drive gear 22 on one of the shafts engages a second gear on the
other shaft, such that, when the pump motor turns rotor 12, rotor
14 is turned at the same rate, but in an opposite direction. In
operation, wellhead fluids, including particulate materials, are
drawn into pump 10 at inlet 24. As the rotors 12 and 14 are turned,
the threads 20, or more properly, rotor chambers 26 formed between
adjacent threads 20 displace the wellhead fluids along the rotor
shafts 1SA and 18B towards an outlet chamber 28, which is the point
of greatest pressure at the center of the rotors, from where the
wellhead fluids are finally discharged from an outlet 30 of the
pump 10. The rotor chambers 26 are not completely sealed, but under
normal operating conditions the normal clearance spaces that exist
between the rotors 12,14 and between each rotor and the rotor
enclosure 19 are filled with transport fluid. The liquid portion of
the transport fluid in these clearance spaces serves to limit the
leakage of the pumped fluids between adjacent chambers. The
quantity of fluid that does escape from the outlet side of the
rotor back toward the inlet represents the pump slip flow, which is
known to decrease the pump volumetric efficiency. As illustrated in
FIG. 2 and just explained, pump slip flow (illustrated by the
arrows in FIG. 2) can occur between each rotor and the rotor
enclosure 19. As understood by those of ordinary skill in the art,
other slip paths include slip between screw tip and adjacent rotor
and between faces.
[0006] As understood by those of ordinary skill in the applicable
arts, conventional twin-screw multiphase pumps face significant
challenges. Consider for example, the following exemplary problems.
First, assuming a fixed pressure rise per stage, as the total
pressure rise requirement increases, the rotor length has to
increase, resulting in an increased rotor deflection under the
imposed pressure loading thereby creating a more eccentric
alignment of the screws within the liner resulting in excessive
slip between the screw rotor and the pump liner, if not contact and
rubbing. Secondly, as the pump slip flow increases, sand
particulates trapped in the slip flow leads to increased
erosion/abrasion within the pump, particularly at the rotor tips by
a phenomenon referred to as jetting. Such erosion/abrasion further
leads to deterioration of the clearance profile and an increase in
the pump slip flow. Finally, during periods of operation in which
the transported fluids have a high gas-volume fraction, the
temperature of the flow exiting the pump rises due to the heat
generated during compression, leading to reduced clearances in the
last pump stages due to variations in thermal expansion of the
various pump parts, thereby possibly resulting in catastrophic
seizure.
[0007] It would therefore be desirable to develop a pump rotor that
will minimize or eliminate pump slip flow, resulting in a high
differential pressure boost multiphase pump with a compact rotor
length. In addition, better sealing between the edges of the rotor
and the pump casing will also insure a reduction in solid
particulate erosion/abrasion within clearances. Finally, having the
ability to accommodate differences in thermal expansion as may
occur when boosting high gas-volume fraction fluids may also reduce
the likelihood of catastrophic seizures.
BRIEF SUMMARY OF THE INVENTION
[0008] One or more of the above-summarized needs and others known
in the art are addressed by pump rotors for screw pumps, the rotors
including a shaft, a first set of threads disposed on a portion of
an outer surface of the shaft, at least one thread of the first set
of threads comprising a groove disposed on an end portion thereof,
and a ring seal disposed on the groove.
[0009] In another aspect of the disclosed inventions, twin-screw
pumps are disclosed that include a casing having an inlet and an
outlet, a liner disposed inside of the casing, and two rotors
disposed inside of the liner, each rotor having a shaft, a set of
threads disposed on a portion of an outer surface of the shaft, at
least one thread of the first set of threads comprising a groove
disposed on an end portion thereof, and a ring seal disposed on the
groove.
[0010] Methods of reducing slip flow in a screw pump are also
within the scope of the embodiments of the invention disclosed, the
screw pump having a casing having a low-pressure inlet and a
high-pressure outlet, a liner disposed inside of the casing, and a
rotor disposed inside of the liner having a shaft and a first set
of threads disposed on a portion of an outer surface of the shaft,
such methods including the steps of forming a groove on end
portions of at least one thread of the first set of threads, and
disposing a ring seal on the groove, the ring seal being configured
to protrude outwardly from the groove and to rest against an inner
surface of the liner of the screw pump, the groove being sized so
as to allow the ring seal to move radially with respect to the at
least one thread as the rotor is deflected, and the ring seal being
configured to reduce the slip flow from the high-pressure outlet to
the low-pressure inlet.
[0011] The above brief description sets forth rather features of
the present invention in order that the detailed description that
follows may be better understood, and in order that the present
contributions to the art may be better appreciated. There are, of
course, other features of the invention that will be described
hereinafter and which will be for the subject matter of the
appended claims.
[0012] In this respect, before explaining several preferred
embodiments of the invention in detail, it is understood that the
invention is not limited in its application to the details of the
construction and to the arrangements of the components set forth in
the following description or illustrated in the drawings. The
invention is capable of other embodiments and of being practiced
and carried out in various ways. Also, it is to be understood that
the phraseology and terminology employed herein are for the purpose
of description and should not be regarded as limiting.
[0013] As such, those skilled in the art will appreciate that the
conception, upon which disclosure is based, may readily be utilized
as a basis for designing other structures, methods, and systems for
carrying out the several purposes of the present invention. It is
important, therefore, that the claims be regarded as including such
equivalent constructions insofar as they do not depart from the
spirit and scope of the present invention.
[0014] Further, the purpose of the foregoing Abstract is to enable
the U.S. Patent and Trademark Office and the public generally, and
especially the scientists, engineers and practitioners in the art
who are not familiar with patent or legal terms or phraseology, to
determine quickly from a cursory inspection the nature and essence
of the technical disclosure of the application. Accordingly, the
Abstract is neither intended to define the invention or the
application, which only is measured by the claims, nor is it
intended to be limiting as to the scope of the invention in any
way.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0016] FIG. 1 illustrates a conventional twin-screw pump;
[0017] FIG. 2 illustrates the pump slip flow path between rotor
tips and the liner.
[0018] FIG. 3 illustrates a cross section view of a rotor in
accordance with an embodiment of the invention;
[0019] FIG. 4 illustrates a close up view of a rotor tip of the
rotor of FIG. 3;
[0020] FIG. 5 illustrates a ring seal disposed on the rotor of
FIGS. 3 and 4;
[0021] FIG. 6 illustrates a screw tip envelope of a rotor in
accordance with the invention with respect to a piston-ring seal
mounted on the rotor with the rotor aligned with the liner (FIG.
6A) and with the rotor deflected with respect to the liner (FIG.
6B);
[0022] FIG. 7 illustrates a perspective view of a rotor in
accordance with another embodiment of the disclosed invention;
and
[0023] FIG. 8 illustrates a cross sectional view of another rotor
seal in accordance with another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring now to the drawings, wherein like reference
numerals designate identical or corresponding parts throughout the
several views, several embodiments of the pump rotor according to
the disclosed invention will be described. One of the advantageous
aspects of the disclosed invention is the use of a rotating
inter-stage ring or brush seal to minimize and/or eliminate pump
slip flow, thus providing for higher pressure rise per stage while
being compliant to accommodate rotor deflections.
[0025] FIGS. 3-5 illustrate, respectively, a cross section view of
a rotor 40, a cross sectional view of one tip of the screw threads
of FIG. 3, and a ring seal 60 in accordance with an embodiment of
the disclosed invention. Throughout this disclosure, the terms
"ring seal," "piston-ring seal," "brush seal," "inter-stage seal,"
"split-ring seal," or "seal" will be used interchangeably. As shown
in FIG. 3, the rotor 40 includes a shaft 42, on the periphery of
which a plurality of screw threads 44 is disposed. At the tip 46 of
the screw threads 44 a groove 48 is provided, inside of which the
ring seal 60 is disposed. In one embodiment the ring seal 60, when
installed and under normal operation conditions, is designed so as
to spring outward, resting against an inside surface 49 of the pump
liner 51. In operation, as the rotor turns and a profile of
increasing pressure develops across the pump, the elimination
and/or minimization of pump slip flow is accomplished by an outer
surface 50 of the ring seal 60 being pushed against the inside
surface 49 of the pump liner 51 by the springing action of the
resilient ring seal 60 as well as a centrifugal load on the ring
seal 60 caused by the rotation of the rotor 40 while a side surface
52 of the ring seal 60 is pushed against an inner surface 54 of the
groove 48 by the pressure difference from one side of the ring seal
60 to the other. As shown, the seal is installed on the rotor
(unlike conventional applications elsewhere in gas turbine/steam
turbines where seals are disposed on the stator), generating a
rotating seal between the successive pressure rise stages of a
twin-screw pump.
[0026] The ring seal 60 is helical in structure and may have a
length to cover any specific amount of circumferential displacement
of the helical threads 44 of the rotor 40. FIG. 5 illustrates a
ring seal 60 covering a complete revolution of the threads 44.
[0027] In addition, as illustrated in FIGS. 6A and 6B, the
dimensions of the groove 48 and ring seal 60 are selected such that
the contact of the outer surface 50 of the ring seal 60 with the
inside surface 49 of the pump liner 51 is accomplished when the
rotor is aligned with the pump liner (as illustrated in FIG. 6A by
the outer edge of the ring seal 60) and with the rotor deflected
with respect to the liner (FIG. 6B). In FIGS. 6A and 6B, a screw
tip envelope 62 is illustrated with the fully deflected ring seal
60 disposed in the groove 48 at the tip of the screw threads
44.
[0028] As understood by those of ordinary skill in the applicable
arts, in pumps with a twin-screw architecture, rotor deflection
varies as the third power of the rotor length. As such, the
pressure rise per stage has been limited by the need to provide
sufficient clearance between the rotor and the surrounding liner
because as the pressure rise increases the rotor deflects
proportionately and this correspondingly requires a larger
circumferential clearance to prevent any catastrophic rubs. Current
technology limits the pressure rise to approximately 6-8 bars per
stage and achieving a higher pressure boost requires longer rotors
with significantly higher deflections. With the reduction and/or
elimination of slip pump flow provided by the ring seal of the
instant invention, the pressure rise per stage is increased,
allowing a more compact rotor to be designed for a desired overall
pressure rise.
[0029] As such, the pump rotor 40 according to the disclosed
invention will minimize and/or eliminate pump slip flow between the
rotor and the casing, resulting in a high-pressure differential
boost multiphase pump with a compact rotor length. In addition,
better sealing between the edges of the rotor and the pump casing
will also insure a reduction in solid particulate erosion/abrasion
of rotor tips as well as providing allowance for thermal expansion
mismatch when pumping transport fluids with a high gas-volume
fraction, thus also reducing the likelihood of catastrophic
seizures. In addition, the ride-through operation of the twin screw
pump when slugs of high gas volume fraction are present in the
well-head flows may be enhanced by using variable speed drives and
clearance control logic.
[0030] Another embodiment of a rotor 70 of the instant invention is
illustrated in FIG. 7. As shown, pins 72 are used to hold the ring
seal 60 in place inside and with respect to the grooves 48 when the
rotor 70 is rotated, such pins 72 acting as anti-rotation
constraints. As shown, the ring seals 60 are held in place by pins
72 disposed once per revolution (or any multiple or fraction there
of, depending on the circumferential length of the seals). In
addition, in embodiments having more than on set of threads 44, the
pins 72 are disposed in the first set of threads 44 at a
circumferential location opposite to the circumferential location
in which the pins 72 are disposed of the second set of threads 44
or otherwise optimal to insure proper balance when the rotor 70
rotates. In embodiments having multiple rings in each set of
threads, a first pin is disposed on a first end of the ring seal
and a second one at the second end thereof. The second ring is then
disposed against the second pin holding the second end of the first
ring and so on. As explained, during operation of the pump, the
shaft deflects and rubs against the side surfaces of the piston
ring, or ring seal 60. The outside diameter of the piston ring is
in constant contact with the liner bore, thus maintaining the seal.
Contact with the liner bore (in spite of seal wear) is maintained
by virtue of the ring seal's out-springing effect and/or
centrifugal loads on the ring as the rotor rotates.
[0031] As shown in FIG. 8, another version of the inter-stage seal
in accordance with the disclosed invention is a brush seal 80
installed on the screw rotor OD. As illustrated, the inter-stage
brush seal 80 includes front and back plates 82, 84 holding a
bristle pack 86 therebetween, the bristle pack 86 being held
against the casing 88 to minimizing the passage of flow from one
side of the brush to the other.
[0032] The thermal design of the rotor/liner interface which
enables operation of the twin screw pump under wet gas compression
conditions by using rotor materials which have low thermal
coefficient of expansion compared to the liner bore is also within
the scope of the disclosed invention. For example, the use of a
specific rotor material, such as invar, which has a low thermal
coefficient of expansion, enables the pump to ride through a gas
slug within a minimum amount of deflection due the thermal heating.
In another embodiment of the invention, a longer mean time between
failure, or MTBF, is achieved by selecting the material of the ring
seal 60 so as to allow the ring seal to be a sacrificial wear
component, while simultaneously guaranteeing the rated design
pressure/flow rate conditions.
[0033] Methods of reducing slip flow in a screw pump are also
within the scope of the embodiments of the invention disclosed, the
screw pump having a casing having a low-pressure inlet and a
high-pressure outlet, a liner disposed inside of the casing, and a
rotor disposed inside of the liner having a shaft and a first set
of threads disposed on a portion of an outer surface of the shaft.
Such methods include the steps of forming a groove on an end
portion of at least one thread of the first set of threads, and
disposing a ring seal on the groove, the ring seal being configured
to protrude outwardly from the groove and to rest against an inner
surface of the liner of the screw pump, the groove being sized so
as to allow the ring seal to move radially with respect to the at
least one thread of the first set of threads as the rotor is
deflected, and the ring seal being configured to reduce the slip
flow from the high-pressure outlet to the low-pressure inlet.
[0034] With respect to the above description, it should be realized
that the optimum dimensional relationships for the parts of the
invention, to include variations in size, form function and manner
of operation, assembly and use, are deemed readily apparent and
obvious to those skilled in the art, and therefore, all
relationships equivalent to those illustrated in the drawings and
described in the specification are intended to be encompassed only
by the scope of appended claims.
[0035] In addition, while the present invention has been shown in
the drawings and fully described above with particularity and
detail in connection with what is presently deemed to be practical
and several of the preferred embodiments of the invention, it will
be apparent to those of ordinary skill in the art who review this
disclosure that many modifications are possible (as for example,
but not as a limitation, variations in sizes, dimensions,
structures, shapes and proportions of the various elements, values
of parameters, mounting arrangements, use of materials, and
orientations, to name a few) without materially departing from the
novel teachings, the principles and concepts set forth herein, and
advantages of the subject matter recited in the appended claims.
Accordingly, all such modifications are intended to be included
within the scope of the present invention as defined in the
appended claims. The order or sequence of any process or method
steps may be varied or re-sequenced according to alternative
embodiments. In the claims, any means-plus-function clause is
intended to cover the structures described herein as performing the
recited function and not only structural equivalents but also
equivalent structures. Other substitutions, modifications, changes
and omissions may be made in the design, operating conditions and
arrangement of the preferred and other exemplary embodiments
without departing from the spirit of the embodiments of the
invention as expressed in the appended claims. Hence, the proper
scope of the present invention should be determined only by the
broadest interpretation of the appended claims so as to encompass
all such modifications and equivalents.
* * * * *