Note: Descriptions are shown in the official language in which they were submitted.
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P1107/Ke
Sulzer Management AG, CH-8401 Winterthur (Schweiz)
Pump for conveying a highly viscous fluid
The invention relates to a pump for conveying a highly viscous fluid in
accordance with the preamble of the independent claim.
Pumps for pumping highly viscous fluids are used in many different industries,
for example in the oil and gas processing industry for conveying hydrocarbon
fluids. Here, these pumps are used for different applications such as
extracting the crude oil from the oil field, transportation of the oil or
other
hydrocarbon fluids through pipelines or within refineries. But also in other
industries for example the food industry or the chemical industry there is
often
the need for conveying highly viscous fluids.
The viscosity of a fluid is a measure for the internal friction generated in a
flowing fluid and a characteristic property of the fluid. Within the framework
of
this application the term "viscosity" or "viscous" is used to designate the
kinematic viscosity of the fluid and the term "highly viscous fluid" shall be
understood such, that the fluid has a kinematic viscosity of at least 10-4
m2/s,
which is 100 centistokes (cSt).
For the pumping of highly viscous fluids it is known to utilize centrifugal
pumps. Pumping highly viscous fluids with centrifugal pumps requires
considerably more pump power than for example pumping water. The higher
the viscosity of the fluid becomes the more power the pump needs to deliver
the required pumping volume. Especially in the oil and gas industry the main
focus ¨ at least in the past ¨ has been on pumping volume, i.e. the flow
generated by the pump, and on the reliability of the pump rather than the
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efficiency of the pump. However, nowadays a more efficient use of the pump
is strived for. It is desirable to have the highest possible ratio of the
power,
especially the hydraulic power, delivered by the pump to the power needed for
driving the pump. This desire is mainly based upon an increased awareness
of environment protection and a responsible dealing with the available
resources as well as on the increasing costs of energy.
To improve the efficiency of a pump for pumping highly viscous fluids it is
known to use specific impeller designs, especially impellers with high head
coefficients. The head coefficient of the impeller can be increased for
example
by increasing the blade outlet angle or the number of blades or the impeller
outlet width. Despite of these measures there is still a need to even more
improve the efficiency of a pump for pumping highly viscous fluids.
Therefore, it is an object of the invention to propose a new pump for
conveying highly viscous fluids that has a better efficiency, i.e. an
increased
ratio of the power delivered by the pump when pumping the fluid to the power
that is supplied to the pump for driving the pump
The subject matter of the invention satisfying this object is characterized by
the features of the independent claim.
Thus, according to the invention a pump for conveying a highly viscous fluid
is
proposed, comprising a casing with at least a first inlet and an outlet for
the
fluid, an impeller for conveying the fluid from the inlet to the outlet,
wherein the
impeller is arranged on a rotatable shaft for rotation around an axial
direction,
and comprises a front shroud facing the first inlet of the pump, wherein the
casing is provided with a stationary impeller opening for receiving the front
shroud of the impeller and having a diameter, wherein the front shroud and
the stationary impeller opening form an gap having a length in the axial
direction, wherein the ratio of the length of the gap and the diameter of the
impeller opening is at most 0.092.
The invention is in particular based upon the finding that the pump efficiency
may be increased when pumping highly viscous fluids by designing the gap
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between the front shroud of the impeller and the stationary impeller opening
considerably shorter than it has been done in the prior art.
The gap which is sometimes also designated as the labyrinth is needed for
sealing the high pressure side of the impeller, more particular the side room,
against the inlet of the pump. The impeller is arranged in the stationary
impeller opening which is a part of the pump that is stationary with respect
to
the casing and adapted to receive the impeller. In the mounted state the
impeller is located in said impeller opening such that there is the gap or the
labyrinth between the outer circumferential surface of the impeller's front
shroud and the inner circumferential surface of the stationary impeller
opening. This gap has a length in the axial direction which provides a sealing
between the side room on the high pressure side of the impeller and the inlet
of the pump, which is the low pressure side of the pump.
During operation of the pump a back flow is generated flowing from the high
pressure side of the impeller, which is for a single stage pump the region
near
the outlet of the pump, through the side room, and through the gap between
the front shroud and the stationary impeller opening back to the low pressure
side of the impeller.
The gap or the labyrinth, respectively, is designed as a radial clearance seal
or labyrinth, i.e. it provides a clearance with respect to the radial
direction.
Therefore the main flow through the gap is in axial direction, i.e. parallel
to the
shaft. This has to be differentiated from an axial clearance seal or labyrinth
that extends perpendicularly or obliquely to the shaft, thus the main flow
through an axial clearance seal is in radial direction or oblique with respect
to
the radial direction. In an axial clearance seal the clearance in axial
direction
changes upon a relative movement of the stationary part and the rotating part
in axial direction, wherein in a radial clearance seal the clearance in radial
direction changes upon a relative movement of the stationary part and the
rotating part in radial direction.
An essential finding is that by the short axial length of the gap (i.e. the
labyrinth) proposed by the invention the power losses across the gap are
decreasing inter alia due to the reduced drag in the side room. On the other
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hand one may expect that the shortening of the gap would result in a reduced
sealing action thus increasing the back flow in the pump. However an
increase in the back flow rate reduces the pump efficiency and thus
contravenes an improved efficiency. Therefore the unexpected finding is that
by shortening the gap with respect to the axial direction the overall pump
efficiency increases despite the risk of an enhanced back flow rate.
According to the invention the length of the gap shall not exceed 0.092 times
the diameter of the impeller opening.
The optimal length of the gap depends on several factors for example the
viscosity of the fluid. Thus, depending on the specific application it may be
preferred that the ratio of the length of the gap and the diameter of the
impeller opening is at most 0.073 and preferably at most 0.055.
There are also applications for which it is advantageous when the ratio of the
length of the gap and the diameter of the impeller opening is at most 0.037
and preferably at most 0.019.
For practical reasons there is also a preferred lower limit for the length of
the
gap. According to the preferred design, the ratio of the length of the gap and
the diameter of the impeller opening is at least 0.0001.
In order to generate the desired sealing effect by the gap it is preferred to
have a radial clearance between the front shroud and the impeller opening
which is at most 0.0045 times the diameter of the impeller opening. The radial
clearance is the extension of the gap with respect to the radial direction,
i.e.
perpendicular to the axial direction, and may be considered as the width of
the
gap. This radial clearance is the minimum distance between the outer
circumferential surface of the impeller's front shroud and the inner
circumferential surface of the stationary impeller opening along the gap.
The two surfaces delimiting the gap may be designed as even surfaces.
According to another embodiment the gap comprises a plurality of lands
consecutively arranged with respect to the axial direction, wherein two
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adjacent lands are respectively separated by a groove. In such an
embodiment the two surfaces delimiting the gap are not even. The part of the
outer circumferential surface of the impeller's front delimiting the gap or
the
part of the inner circumferential surface of the stationary impeller opening
delimiting the gap may be provided with a plurality of lands and grooves there
between. In such an embodiment the length of the gap in axial direction is
defined as the sum of the lengths of all individual lands in the axiai
direction.
The grooves do not contribute to the overall length of the gap in axial
direction.
According to a preferred embodiment the stationary inlet opening comprises a
wear ring delimiting the gap with respect to the radial direction, the wear
ring
being arranged stationary with respect to the casing.
Supplementary or as an alternative measure it is also possible that the
impeller comprises a wear ring delimiting the gap with respect to the radial
direction, the wear ring being arranged stationary with respect to the
impeller.
The invention is especially suited for many types of centrifugal pumps. The
pump may be designed for example as a single suction pump or a double
suction pump, as a single stage pump or as a multistage pump. When the
pump is designed as a single suction pump it may have a rear shroud on the
impeller in addition to the front shroud. In such a design it is also possible
that
the rear shroud of the impeller forms a gap with a part being stationary with
respect to the casing. This gap at the rear shroud may be designed in an
analogously same manner as it is explained with respect to the gap at the
front shroud of the impeller.
According to a preferred embodiment the pump is designed as a double
suction pump, having a second inlet for the fluid being arranged oppositely to
the first inlet of the pump, wherein the impeller is designed as a double
suction impeller comprising vanes for conveying the fluid both from the first
inlet and from the second inlet to the outlet.
For such a design as a double suction pump it is preferred, that the impeller
comprises a second front shroud facing the second inlet of the pump, wherein
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the casing is provided with a second stationary impeller opening for receiving
the second front shroud of the impeller and having a diameter, wherein the
second front shroud and the second stationary impeller opening form a
second gap having a length in the axial direction, and wherein the ratio of
the
length of the second gap and the diameter of the second impeller opening is
at most 0.092.
Depending on the specific application it may be preferred that also the ratio
of
the length of the second gap and the diameter of the second impeller opening
is at most 0.073 and preferably at most 0.055.
There are also applications for which it is advantageous when the ratio of the
length of the second gap and the diameter of the second impeller opening is
at most 0.037 and preferably at most 0.019.
Also for the second gap it is advantageous, when there is a radial clearance
between the second front shroud and the second impeller opening which is at
most 0.0045 times the diameter of the second impeller opening.
It is an especially preferred measure when the gap and the second gap are
designed essentially in an identical manner.
According to an essential application the pump is designed for the use in the
oil and gas industry.
Further advantageous measures and embodiments of the invention will
become apparent from the dependent claims.
The invention will be explained in more detail hereinafter with reference to
the
drawings. There are shown in a schematic representation:
Fig. 1: a cross-sectional view of an embodiment of a pump according to
the invention,
Fig.2: an enlarged representation of detail I in Fig. 1,
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Fig. 3: a sketch of the front shroud and a wear ring as part of the
stationary impeller opening,
Fig. 4: as Fig. 3, but for a variant of the embodiment,
Fig. 5: a second variant for the design of the gap between the front
shroud
and the stationary impeller opening, and
Fig. 6: an illustration of a comparison of a pump according to the
invention
with prior art pumps.
Fig. 1 shows a cross-sectional view of an embodiment of a pump according to
the invention which is designated in its entity with reference numeral 1. Fig.
2
shows an enlarged representation of detail I in Fig. 1. The pump 1 is designed
for conveying a highly viscous fluid, whereas the term "highly viscous" has
the
meaning that the kinematic viscosity of the fluid is at least 10-4 m2/s, which
is
100 centistokes (cSt).
In this embodiment the pump 1 is designed as a double suction single stage
centrifugal pump. This design is one preferred embodiment which is in
practice useful for many applications. Of course, the invention in not
restricted
to this design. A pump according to the invention may also be designed as a
single suction centrifugal pump or as a multistage centrifugal pump or as any
other type of centrifugal pump. Based upon the description of the embodiment
shown in Fig. 1 and Fig. 2 it is no problem for the skilled person to build a
pump according to the invention, that is designed as another type of pump,
especially centrifugal pump, for example a single suction pump.
The double suction pump 1 comprises a casing 2 with a first inlet 3, a second
inlet 3' and an outlet 4 for the fluid to be pumped. The fluid may be for
example crude oil, oil or any other hydrocarbon fluid being highly viscous.
The
pump 1 has an impeller 5 with a plurality of vanes 51 for conveying the fluid
from the first inlet 3 and the second inlet 3' to the outlet 4. The impeller 5
is
arranged on a rotatable shaft 6 for rotation around an axial direction A. The
axial direction A is defined by the axis of the shaft 6 around which the
impeller
5 rotates during operation. The shaft 6 is rotated by a drive unit (not
shown).
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The direction perpendicular to the axial direction A is referred to as the
radial
direction.
The first inlet 3 and the second inlet 3' are arranged oppositely to each
other
with respect to the axial direction A. Thus, according to the representation
in
Fig. 1, the fluid is flowing both from the left side and from the right side
in axial
direction A to the impeller 5, whereas the fluid from the first inlet 3 is
flowing in
opposite direction to the impeller 5 as the fluid from the second inlet 3'.
The
impeller 5 conveys both the fluid coming from the first inlet 3 and the fluid
coming from the second inlet 3' into the radial direction to the outlet 4 of
the
pump.
The impeller 5 comprises a front shroud 7 covering the vanes 51 and facing
the first inlet 3 of the pump 1. Since in this embodiment the impeller 5 is
designed as a double suction impeller 5 it comprises a second front shroud 7'
facing the second inlet 3' and covering the vanes 51 on the side of the
impeller 5 which faces the second inlet 3'.
The casing 2 is provided with a stationary impeller opening 8 for receiving
the
front shroud 7 of the impeller 5. The stationary impeller opening 8 is
stationary
with respect to the casing 2 of the pump 1 and has a circular cross-section
with a diameter D, whereas the diameter D designates the smallest diameter
of that part of the stationary impeller opening 8 which receives the front
shroud 7.
In an analogous manner the casing 2 comprises a second stationary impeller
opening 8' for receiving the second front shroud 7' of the impeller 5.
In the mounted state the impeller 5 is arranged coaxially within the
stationary
impeller opening 8 such that the outer circumferential surface of the front
shroud 7 faces the inner circumferential surface of the stationary impeller
opening 8. Thus, the front shroud 7 and the stationary impeller opening 8 form
a gap 9 (see also Fig. 3) between the front shroud 7 and the stationary
impeller opening 8. The gap 9 is also called labyrinth. It has an essentially
annular shape and provides sealing action as will be explained hereinafter.
The gap 9 has a length L which is the extension of the gap 9 in the axial
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direction A. The gap 9 extends parallel to the shaft 6 or parallel to the
axial
direction A, respectively. Thus, the back flow is flowing through the gap 9
parallel to the shaft 6 and in the opposite direction as the fluid flowing
through
the respective inlet 3. Thus, viewed in the main flow direction of the fluid
entering through the respective inlet 3 the starting position of the gap 9,
i.e.
the opening through which the fluid enters the gap 9, is arranged behind the
ending position of the gap 9, i.e. the opening through which the fluid leaves
the gap 9.
In an analogous manner a second gap 9' is formed between the second front
shroud 7' and the second stationary impeller opening 8'. The second gap 9'
has a length L' in the axial direction A and the second stationary impeller
opening 8' has a diameter D'. The gap 9' extends parallel to the shaft 6 or
parallel to the axial direction A, respectively. Preferably, but not
necessarily,
the length L' equals the length L and the diameter D' equals the diameter D.
Since the design and the arrangement of the second gap 9' may be identical
as the gap 9 the following description will only refer to the gap 9. It shall
be
understood that this description applies in an analogously same manner also
for the second gap 9'.
The gap 9 or the labyrinth 9 seals a side room 10 located on the high
pressure side of the impeller 5 against the low pressure side of the impeller
5
which is located at the inlet 3. The side room 10 is located at the high
pressure side of the impeller 5 near the outlet 4 of the pump 1 and delimited
by the front shroud 7 of the impeller 5 as well as by the casing 2 of the pump
1. During operation of the pump 1 a back flow is generated from the region of
the outlet 4 through the side room 10. The back flow passes the gap or the
labyrinth 9 flowing essentially in the axial direction A, i.e. parallel to the
shaft 6
and reaches the low pressure side of the impeller 5 next to the first inlet 3.
It is
obvious that the back flow reduces the efficiency of the pump 1.
Thus, it is one of the functions of the gap 9 to provide some sealing action
to
limit the back flow. That is the reason why the gap 9 is also called
labyrinth.
It is the basic idea of the present invention to shorten the lengths L (see
Fig. 2
and Fig. 3) of the gap 9 in the axial direction A as compared to solutions
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known from the prior art. Although one could expect that a shortening of the
length L would result in an increased back flow which in turn reduces the
pump efficiency, it has been realized that by shortening the length L of the
gap 9 the overall efficiency of the pump 1 may be increased.
Referring to Fig. 2 and Fig. 3 the design of the gap 9 will now be explained
in
more detail. In the embodiment according to Fig. 1 the stationary inlet
opening
8 comprises a wear ring 11 delimiting the gap 9 with respect to the radial
direction. The wear ring 11 faces the outer circumferential surface of the
front
shroud 7 that is inserted in the stationary inlet opening 8. The wear ring 11
is
fixedly mounted to the casing 2, thus, the wear ring 11 is stationary with
respect to the casing 2.
Fig. 3 shows a sketch of the front shroud 7 and the wear ring 11 as part of
the
stationary impeller opening 8 to more clearly understand the dimensions of
the gap 9.
According to the invention the length L of the gap 9 is designed such that the
ratio of the length L and the diameter D of the impeller opening 8 is at most
0.092, i.e. L/D 5 0.092. As already said, the diameter D designates the
smallest diameter of the stationary impeller opening 8, i.e. the diameter at
that
location were the wear ring 11 comes closest to the outer circumferential
surface of the front shroud 7. The length L of the gap 9 is the extension in
axial direction A of that region where the stationary impeller opening 8 and
the
front shroud 7 come closest to each other.
In the arrangement shown in Fig. 3 the wear ring 11 is designed with a
protrusion 111 in radial direction. Accordingly the length L of the gap 9 is
equal to the extension of the protrusion 111 in the axial direction 9.
The second parameter defining the geometry of the gap 9 is the radial
clearance R between the front shroud 7 and the stationary impeller opening 8
or the wear ring 11, respectively, along the axial extension of the gap 9. The
radial clearance R designates the minimum radial clearance along the gap 9.
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In practice it has been proven as advantageous, when the radial clearance R
does not exceed 0.0045 times the diameter D of the stationary inlet opening
8, i.e. preferably the condition RID 0.0045 is fulfilled.
The optimal length L of the gap 9 depends on the respective application.
There are several factors influencing an appropriate choice of the length L of
the gap 9, for example the kinematic viscosity of the specific fluid to be
pumped, the pressure increase generated by the pump, the flow through the
pump or other operational parameters of the pump 1.
For a given set of operational parameters of the pump 1 the lengths L of the
gap 9 should preferably be reduced with increasing viscosity of the fluid to
be
pumped.
In practice and depending on the application it may be preferred that the
ratio
LID does not exceed 0.073 or more preferred does not exceed 0.055, or even
more preferred does not exceed 0.037 or specifically preferred does not
exceed 0.019.
According to the preferred embodiments of the pump 1 the minimum ratio LID
is 0.0001, i.e. the length L of the gap 9 is preferably at least 0.0001 times
the
diameter of the stationary impeller opening 8 or the wear ring 11,
respectively.
Fig. 4 shows in a similar representation as Fig. 3 a variant of the embodiment
of the pump 1. According to this variant the impeller 5 and more particular
the
front shroud 7 of the impeller 5 comprises a wear ring 11' delimiting the gap
9
with respect to the radial direction. The wear ring 11' is fixedly connected
to
the impeller 5 and rotating with the impeller 5. In this variant the
stationary
impeller opening 8 may comprise a wear ring 11, too, but may also be
designed without a wear ring.
Fig. 5 illustrates a second variant for the design of the gap 9 between the
front
shroud 7 and the stationary impeller opening 8. According to the second
variant the stationary impeller opening 8 or the wear ring 11, respectively,
or
as an alternative (not shown) the front shroud 7 is designed such that the gap
9 comprises a plurality of lands 12 consecutively arranged with respect to the
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axial direction A, wherein two adjacent lands 12 are respectively separated by
a groove 13. In such a design the total length L of the gap 9 is the sum of
the
individual lengths L1, L2, L3, L4, L5 of all lands 12 in the axial direction.
The
extension of the grooves does not contribute to the total lengths L of the gap
9, i.e. L=L1+L2+L3+L4+L5. It shall be understood that the number of lands
and grooves as well as their geometric design shown in Fig. 5 has only
exemplary character.
The pump 1 according to the invention has a better pump efficiency as
compared to pumps known from the state of the art. The pump efficiency
designates the ratio of the power delivered by the pump and the power input
for the pump, i.e. the power that is used to drive the pump. The power
delivered by the pump is usually the hydraulic power generated by the pump
1.
Fig. 6 illustrates a comparison of a pump according to the invention with
prior
art pumps. The graph shows the pump efficiency P as a function of the
viscosity V of the fluid conveyed by the pump. For the purpose of a better
understanding the graph is standardized such that the pump efficiency P of
the prior art pumps equals the horizontal viscosity axis V, i.e. the pump
efficiency P for the pump according to the prior art lies always on the V-axis
for each viscosity. Thus, the graph directly shows the increase of the pump
efficiency of the pump 1 according to the invention as compared to a prior art
pump. The pump efficiency of the pump according to the invention is
represented by the curve K. As can be clearly seen, as soon as the viscosity
of the fluid is greater than a specific value V1 the pump 1 according to the
invention has an increased pump efficiency compared to the prior art pump.
The efficiency gain is increasing with the viscosity of the fluid. The
specific
value V1 of the viscosity where the pump 1 according to the invention
becomes more efficient than the prior art pump is usually smaller than the
value of 10-4 m2/s. Thus, for a highly viscous fluid the pump 1 according to
the
invention has a higher pump efficiency than the prior art pump.
Although specific reference has been made for the purpose of explanation to
an embodiment, where the pump 1 is designed as a double suction single
stage centrifugal pump the invention is in no way restricted to such
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embodiments. The pump according to the invention may also be designed as
any other type of centrifugal pump, for example as a single suction pump or
as a multistage pump. In particular, the invention is applicable both to
centrifugal pumps with a closed impeller, i.e. an impeller having a front
shroud
and a rear shroud, and to centrifugal pumps with a semi-open impeller, i.e.
having a rear shroud but no front shroud. In such designs where the impeller
has a rear shroud or a rear shroud only, the design of the gap 9 according to
the invention may be used for the rear shroud in an analogously same
manner as herein described with reference to the front shroud.
