U.S. patent application number 14/680212 was filed with the patent office on 2015-10-29 for impeller for a centrifugal pump, a centrifugal pump and a use thereof.
The applicant listed for this patent is c/o Sulzer Management AG. Invention is credited to Jussi AHLROTH, Matti KOIVIKKO, Kalle TIITINEN, Sami VIRTANEN.
Application Number | 20150308446 14/680212 |
Document ID | / |
Family ID | 50542871 |
Filed Date | 2015-10-29 |
United States Patent
Application |
20150308446 |
Kind Code |
A1 |
KOIVIKKO; Matti ; et
al. |
October 29, 2015 |
IMPELLER FOR A CENTRIFUGAL PUMP, A CENTRIFUGAL PUMP AND A USE
THEREOF
Abstract
An impeller for a centrifugal pump, the impeller includes a
front shroud, a rear shroud, and one or more working vanes
therebetween, the front shroud having a front face opposite to the
face having the working vanes, the rear shroud having a rear face
opposite to the face having the working vanes, the front shroud
having an outer circumference and a plurality of front pump-out
vanes attached to the front face of the front shroud, the rear
shroud having a plurality of rear pump-out vanes attached to the
rear face of the rear shroud, the front pump-out vanes being
dimensioned in accordance with an equation:
.SIGMA..sub.i=1.sup.z(l.sub.i)/D>8, where Z is the number of
front pump-out vanes, l is the vane length measured along the
leading surface of each front pump-out vane, D is the outer
diameter of the front shroud.
Inventors: |
KOIVIKKO; Matti; (Kotka,
FI) ; TIITINEN; Kalle; (Inkeroinen, FI) ;
VIRTANEN; Sami; (Kotka, FI) ; AHLROTH; Jussi;
(Halli, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
c/o Sulzer Management AG |
Winterthur |
|
CH |
|
|
Family ID: |
50542871 |
Appl. No.: |
14/680212 |
Filed: |
April 7, 2015 |
Current U.S.
Class: |
415/1 ;
415/208.3 |
Current CPC
Class: |
F04D 31/00 20130101;
F04D 29/2216 20130101; F04D 29/2266 20130101; F04D 7/04
20130101 |
International
Class: |
F04D 29/24 20060101
F04D029/24; F04D 29/44 20060101 F04D029/44; F04D 1/00 20060101
F04D001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2014 |
EP |
14165689.2 |
Claims
1. An impeller for a centrifugal pump, the impeller comprising: a
front shroud, a rear shroud, and one or more working vanes
therebetween, the front shroud having a front face opposite to the
face having the working vanes, the rear shroud having a rear face
opposite to the face having the working vanes, the front shroud
having an outer circumference and a plurality of front pump-out
vanes attached to the front face of the front shroud, the rear
shroud having a plurality of rear pump-out vanes attached to the
rear face of the rear shroud, the front pump-out vanes being
dimensioned in accordance with an equation:
.SIGMA..sub.i=1.sup.z(l.sub.i)/D>8, where Z is the number of
front pump-out vanes, l is the vane length measured along the
leading surface of each front pump-out vane, D is the outer
diameter of the front shroud.
2. The impeller as recited in claim 1, wherein each front pump-out
vane has a backward angle of inclination at the outer circumference
of the front shroud equalling to less than 25.degree..
3. The impeller as recited in claim 1, wherein the front pump-out
vanes have a height of less than 2% of the diameter of the front
shroud of the impeller.
4. The impeller as recited in accordance with claim 1, wherein the
front pump-out vanes have a height of 0.5-1.5% of the diameter of
the front shroud of the impeller.
5. The impeller as recited in accordance with claim 1, wherein the
front pump-out vanes are of equal length, the vane length being
l=0.9 . . . 1.1*D.
6. The impeller as recited in accordance with claim 1, the number
of front pump-out vanes is 10.
7. The impeller as recited in claim 2, wherein the backward angle
of inclination of each front pump-out vane at the outer
circumference of the front shroud is 22.degree..
8. The impeller as recited in accordance with claim 1, a
cylindrical extension of the front face of the front shroud of the
impeller, the cylindrical extension being configured to cooperate
with a wear ring arranged to a volute casing of a centrifugal
pump.
9. A centrifugal pump using the impeller of claim 1.
10. Use of the centrifugal pump of claim 9 for pumping liquids and
solids-containing liquids.
11. Use of the centrifugal pump of claim 9 for pumping fibrous
suspension of pulp and paper or board industry.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to EP Patent Application
14165689.2, filed Apr. 23, 2014, the contents of which is hereby
incorporated herein by reference.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to an impeller for a
centrifugal pump, a centrifugal pump and a use thereof. The present
invention relates especially to a novel closed impeller structure
for a centrifugal pump. The centrifugal pump utilizing the impeller
of the present invention is suitable for pumping both clean liquids
and solids-containing liquids like for instance fibrous suspensions
of pulp and paper or board industry.
[0004] 2. Background Information
[0005] Energy saving, in other words efficiency, is an important
factor in the development and design of all kinds of machines and
machine elements including centrifugal pumps and their impellers.
It has always been a known fact that the work the impeller of a
centrifugal pump subjects to the fluid it pumps is not totally
converted to kinetic and/or potential energy but a part of it is
wasted in phenomena taking place between the fluid and both the
rotary impeller and the static pump volute or volute casing. Such
phenomena include, among others, surface friction between the fluid
and the surfaces of both the impeller and the pump volute, and
various leakage flows between the impeller and the volute
casing.
[0006] The energy aspects of pumping have also been taken into
account by the European Union a few years ago when they established
a framework for the setting of ecodesign requirements for
energy-related products. In 2012 the European Commission has
introduced implementing measures for products used in electric
motor systems, such as water pumps. In accordance with the EU water
pumps forming parts of electric motor systems are essential in
various pumping processes, and there is a total cost-effective
potential for improving the energy efficiency of these pumping
systems by approximately 20% to 30%. Even though the main savings
can be achieved by motors, one of the factors contributing to such
improvements is the use of energy-efficient pumps. Consequently,
water pumps are a priority product for which ecodesign requirements
should be established.
[0007] Therefore the EU has set a goal to pump manufacturers to
manufacture pumps having a certain efficiency as a function of
specific speed of the pump. FIG. 1 illustrates schematically two
efficiency curves in relation to the specific speed, and FIG. 2 the
specific speed and its relation to basic pump construction. What
FIG. 2 in practice teaches is that the specific speed is the higher
the larger is the capacity of the pump. In other words, small sized
pumps have a low specific speed.
[0008] Specific speed (n.sub.s) means a dimensional value
characterizing the shape of the pump impeller by head (H), flow (Q)
and speed (n). Specific speed is calculated by using the following
equation:
n.sub.s=n*Q.sub.BEP.sup.1/2/H.sub.BEP.sup.3/4[min-1],
where [0009] head (H) means the increase in the hydraulic energy of
water in meters [m], produced by the pump at the specified point of
operation, [0010] rotational speed (n) means the number of
revolutions per minute [rpm] of the shaft, [0011] flow (Q) means
the volume flow rate [m.sup.3/s] of fluid through the pump, and
[0012] best efficiency point (REP) means the operating point of the
pump at which it is at the maximum hydraulic pump efficiency
measured with clean cold water.
[0013] There is one more variable that needs to be specified, i.e.
hydraulic pump efficiency or mere efficiency (.eta.), which is the
ratio between the mechanical power transferred to the liquid during
its passage through the pump and the mechanical input power
transmitted to the pump at its shaft.
[0014] Now coming back to FIG. 1 the solid curve A shows the
efficiency required by the EU, and the dashed curve B the
efficiency of a series of today's pumps having a semi-open
impeller. By a series of pumps is meant pumps having the same basic
construction but a differing capacity/flow designed to cover, more
or less, all the pumping needs (in view of flow) of the customers.
What is noteworthy is that for the most part of the operating range
(specific speed) of the pump series the semi-open impellers have an
efficiency well above that required by the EU. However, at the
lower end of the specific speed range the efficiency curve B drops
below the EU-curve A.
[0015] Thus it appears that in order to fulfill the requirements of
the EU, the efficiency of pumps having a low specific speed has to
be improved. Since it was already above explained that both the
surface friction and the leakage flows are clearly the causes of
the reduction of the pumping efficiency, they have to be considered
in more detail.
[0016] It has also been customary practice to use, for pumping pure
water, centrifugal pumps having closed impellers, shrouds with
smooth faces opposite to the working vanes and wear rings. However,
since the specific speed of a centrifugal pump correlates to
efficiency, it has been understood now when studying the pumps
having a low specific speed that they have low efficiency due to
two impeller-related factors having a relatively high impact to
efficiency. The first factor being high leakage flow, in relation
to the total flow, via the wear rings. And the second factor is the
energy wasted on the smooth faces of the shrouds in relation to the
total power used by the pump.
[0017] The leakage flows appear in the case of open impellers at
the opposite side edges of the impeller vanes, as there has to be a
certain running clearance between the side edges of the vanes and
the walls of the volute casing, whereby a part of the fluid to be
pumped is able to pass via such a clearance from a preceding vane
cavity to a succeeding vane cavity.
[0018] In the case of semi-open impellers the above mentioned
leakage flow appears only on one side of the impeller as at the
other side, usually the rear side of the impeller, the working
vanes are attached to a rear shroud, also called as a hub, of the
impeller. However, another type of leakage flow may be found in
semi-open impellers, as the pumped fluid has such a high pressure
at the radially outer edge of the rear shroud of the impeller that
it is capable of forcing the fluid round the impeller circumference
to the rear side of the impeller between the rear shroud and the
rear wall of the volute casing.
[0019] In the case of closed impellers, i.e. impellers having both
rear and front shrouds fastened to both the rear and front side
edges of the working vanes, the leakage flow round the side edges
of the working vanes is naturally prevented, but the leakage flows
round the radially outer edges or circumferences of the shrouds are
a fact.
[0020] The further consideration based, on the one hand to the
EU-requirements, and on the other hand, to the properties and
construction of pumps having a low specific speed has now taught
that the efficiency of a small-sized semi-open impeller is very
hard, if not impossible, to improve to such an extent that the
efficiency would be above the EU-curve A in FIG. 1. Therefore, the
consideration led to taking the closed impeller in use at the lower
end of the specific speed range.
[0021] The closing of the side edges of the working vanes in closed
impellers not only creates a leakage flow round the radially outer
circumferential edge/s of the shroud/s but also subjects the face/s
of the shroud/s opposite to the working vanes to the pressure of
the pumped fluid. The pressure distribution at the rear side of the
shroud is parabolic, i.e. at its highest at the outer circumference
of the impeller from where it reduces gradually when moving towards
the shaft of the impeller. The pressure results, both with
semi-open and closed impellers, in an axial thrust pushing the
impeller towards the pump inlet, as the full area of the rear
shroud is subjected to the fluid pressure. The axial thrust is
clearly greater in semi-open impellers than in closed impellers,
as, in semi-open impellers there is no front shroud to the front
side of which the pressure could act like in closed impellers. Yet,
in both impeller types the impeller needs to be balanced such that
the bearings of the shaft of the pump are not subjected to a too
high axial load. Also, without any measures the pressure affects
the shaft sealing, and has to be limited for preventing the sealing
from deteriorating. The axial force is balanced by arranging to the
rear face of the shroud pump-out vanes the purpose of which is to
increase the speed of the fluid entering the rear side of the
shroud such that its pressure is reduced. Thus, the rear pump-out
vanes act somewhat like the impeller working vanes. However,
because they are normally much smaller, the pressure they develop
cannot overcome that developed by the working vanes. Instead, the
back pump-out vanes simply act to break down that discharge
pressure to a value between suction pressure and discharge
pressure. Another measure to affect the pressure at the rear side
of the rear shroud is to provide the shroud close to the shaft with
holes extending through the shroud via which holes the pressure is
able to be balanced.
[0022] At the front side of the closed impeller the situation is
different. There is no need to fight the pressure, which is one of
the major tasks of the rear pump-out vanes, as there is no reason
to try to lower the pressure due to the fact that the area of the
front shroud face opposite to the working vanes is much smaller
than the area of the rear shroud face opposite the working vanes.
The front face of the shroud has to be provided with means to
minimize the leakage flow round the impeller circumference to the
front side of the front shroud. At its worst there is a significant
recirculating leakage flow from the pressure side of the impeller
back to the suction side of the impeller through the gap between
the front shroud of the impeller and the volute casing. Such a
leakage flow takes a substantial amount of energy used for pumping,
whereby the efficiency of the impeller is decreased remarkably.
There are two ways that the leakage flow may be controlled, i.e.
either by arranging a sealing, most often called as a wear ring,
between the impeller and the volute casing, or by arranging front
pump-out vanes on the front face of the front shroud, i.e. on the
face opposite to the working vanes.
[0023] Wear rings, which function basically as a slide ring
sealing, restrict efficiently the amount of discharge fluid that
tries to circulate back to the suction side of the impeller. Wear
rings provide an adequate solution for applications that handle
clear water or occasionally handle light solids. However, as the
wear ring has a certain operating clearance, the wear ring must be
replaced, when the clearance becomes excessive. The flow
restriction created by the tight clearance between the stationary
and rotating wear ring faces causes very high local velocities and
hence a high wear rate. If the fluid to be pumped contains abrasive
particles, wear rings, because they are subject to a very high flow
velocity, will have an unacceptably short life span, even when made
of hard materials or when their surfaces have been specifically
treated in view of wear. Thus the use of a wear ring is not
desirable when pumping liquids containing solids.
[0024] Pump-out vanes offer a better alternative for handling
abrasive solids. The use of such pump-out vanes is known from
slurry pumps like, for instance, those discussed in
US-A1-20090226317. Pump-out vanes control the leakage through a
pumping action creating a head to prevent or at least counter any
leakage or recirculation from an outer high pressure peripheral
outlet of the impeller radially inwardly in-between the impeller
and the volute casing. The pump-out vanes are typically almost
radial, or arranged at an angle of 10-30 degrees from the radial
direction.
[0025] The disadvantage of known pump-out vanes is that they
consume considerable amount of power while controlling leakage.
When new, a pump impeller equipped with pump-out vanes will likely
have a lower efficiency than its wear ring counterpart. However, it
will come close to maintaining its "as installed" efficiency
throughout its operational life. An impeller with wear rings loses
efficiency rapidly as the rings wear. It is not uncommon to have
several outages to replace wear rings over the life of a single
impeller when wear rings are used in an aggressive solids
application. Thereby, the use of pump-out vanes on the front face
of the front shroud has been accepted especially in connection with
pumps designed to pump slurries or other abrasive liquids in spite
of their power consumption, as the energy efficiency is not the
main issue in slurry pumps.
[0026] A further known disadvantage of closed impellers is that the
smooth front and back shrouds (not having pump-out vanes), rotating
in close proximity to the casing walls, generate disc friction that
lowers the efficiency of the pump relative to that found in open
impeller designs.
[0027] Yet another disadvantage is that the closed impeller is more
easily plugged. Large solids that might otherwise be broken up by
the grinding action generated by a rotating open impeller and the
stationary casing wall, can easily become lodged in the eye of a
closed impeller. This may create a mechanical or hydraulic
imbalance that has the potential to damage the pump, or at the
least causes a pre-mature outage to remove the blockage. In other
words, there are two separate methods of restricting internal
recirculation that can lower the efficiency of the pump and
generate a lot of unwanted heat to the fluid to be pumped.
SUMMARY
[0028] Thus, an object of the present invention is to find a way to
improve the construction of the centrifugal pumps at least at the
lower end of the specific speed range of a series of pumps such
that the efficiency for the entire range of pumps is above the EU
efficiency curve.
[0029] Another object of the present invention is to change the
construction of the impeller such that the efficiency of an
impeller may be raised.
[0030] Yet another object of the present invention is to design the
impeller such that its pump-out vanes both prevent the leakage flow
and function in an energy efficient manner, i.e. the pump-out vanes
are to be designed such that they prevent the leakage flow in an
optimal way in view of the total efficiency of the impeller.
[0031] A still further object of the present invention is to design
a novel impeller that is able to prevent the recirculating leakage
flow of liquids containing solids without the use of wear
ring/s.
[0032] At least one of the above objects of the present invention,
among others, are fulfilled by an impeller for a centrifugal pump,
the impeller having a front shroud, a rear shroud, and one or more
working vanes therebetween, the front shroud having a front (first)
face opposite to the (second) face having the working vanes, the
rear shroud having a rear (first) face opposite to the (second)
face having the working vanes, the front shroud having an outer
circumference and a plurality of front pump-out vanes attached to
the front face of the shroud, the rear shroud having a plurality of
rear pump-out vanes attached to the rear face of the shroud,
wherein the front pump-out vanes are dimensioned in accordance with
an equation:
.SIGMA..sub.i=1.sup.z(l.sub.i)/D>8, where [0033] Z is the number
of front pump-out vanes, [0034] l is the vane length measured along
the leading surface of each front pump-out vane, and [0035] D is
the outer diameter of the front shroud.
[0036] Other characterizing features of the impeller of the present
invention become evident in the accompanying dependent claims.
[0037] The centrifugal pump impeller of the present invention
brings about several advantages in comparison to prior art
centrifugal pumps. At least the following advantages may be found
[0038] to prevent the leakage typical for a closed impeller, [0039]
to make it possible to use a closed impeller or vane passages for
pumping suspensions having solids, and [0040] to reduce the power
needed to win the friction between the shroud and the volute
casing. This is performed by optimizing the liquid flow in the
volume between the shroud and the volute casing to have a
circumferential velocity component that results in minimum power
loss.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] The impeller of the present invention is described more in
detail below, with reference to the accompanying drawings, in
which
[0042] FIG. 1 illustrates schematically a comparison between the
efficiency curves based on EU-regulations and on a present series
of centrifugal pumps,
[0043] FIG. 2 explains schematically the correlation between the
impeller type and the specific speed,
[0044] FIG. 3 illustrates schematically a partial axial cross
sectional view of a prior art centrifugal pump,
[0045] FIG. 4 illustrates schematically a partial axial cross
sectional view of another prior art centrifugal pump,
[0046] FIG. 5 illustrates schematically the basic functional
differences between the pump-out vanes of the impeller of the
present invention by comparing such in the total head vs. flow rate
coordinates with both the working vanes and the pump-out vanes of
the front shroud of a prior art impeller,
[0047] FIG. 6 illustrates the impeller in accordance with a
preferred embodiment of the present invention, and
[0048] FIG. 7 illustrates schematically a comparison between the
efficiency curves based on EU-regulations and on centrifugal pumps
utilizing the impeller of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0049] FIG. 3 is a schematical cross sectional illustration of a
prior art centrifugal pump having a closed impeller. The pump of
FIG. 3 comprises a volute casing 2, a rear wall 4, a shaft 6 and an
impeller 8 attached to the end of the shaft 6. The volute casing 2
comprises an inlet or suction duct 10, and an outlet or discharge
duct 12. The rear wall 4, which is fastened to the volute casing 2
comprises some kind of sealing means or device 14 for axially
sealing the shaft 6. Here a stuffing box type sealing is shown. The
impeller 8 is, as mentioned already above, a closed one, which
means that the working vanes 16 of the impeller 8 are at their both
sides covered by a shroud, a rear shroud 18 and a front shroud 20.
To the sides of the shrouds 18, 20 opposite to the working vanes 16
so called pump-out vanes 22, 24, respectively, have been arranged.
The vanes 22, 24 are usually radial though also somewhat (of the
order of 10-30 degrees from radial direction) inclined pump-out
vanes have been used. The impeller may also be provided with a
series of balance holes (not shown) arranged to run through the
rear shroud 18 close to the shaft 6. The impeller 8 is arranged to
run in the volute casing 2 at a small clearance, i.e. such that the
clearance between the rear pump-out vanes 22 and the rear wall 4 is
as small as practically possible, i.e. of the order of 0.4-1.0 mm.
The front side of the impeller 8 is sealed by means of a so called
wear ring 26 in relation to the volute casing 2. Usually the wear
ring 26 is a cylindrical sleeve arranged at the end of the inlet
duct 12 facing the impeller 8. The impeller 8 is provided with a
cylindrical extension 28 fitting within the wear ring 26 with a
small clearance. The cylindrical extension 28 may also be provided
with a specifically treated surface or a specific ring facing the
wear ring 26 of the volute casing.
[0050] FIG. 4 is a schematical cross sectional illustration of a
prior art centrifugal pump having a closed impeller. The
centrifugal pump of FIG. 4 is identical to the pump of FIG. 3
except for the front end of the impeller. Now that the impeller of
FIG. 3 included the lengthy cylindrical extension 28 cooperating
with the wear ring arranged to the casing surface, the casing
surface of the pump of FIG. 4 is not provided with any wear ring,
but the shorter cylindrical extension of the impeller is arranged
at a distance 30 from the counter surface of the volute casing such
that liquid to be pumped may flow relatively freely to or from the
volume between the front shroud and the volute casing.
[0051] To be able to improve the efficiency of the impeller, or
that of the pump, the treatment of the leakage flow has to be
thought over once again. And, since a centrifugal pump cannot be
designed merely for pumping pure water, liquid or suspensions
containing more or less solids has to be taken into account, too.
Thus, the use of the wear ring remains a secondary means for
fighting the leakage flow, as the wear ring is susceptible to
considerable wear and difficult maintenance operation if the liquid
to be pumped contains solids. Therefor the main concern is the
design of pump-out vanes in a novel way. In other words, the aim of
the invention is to design pump-out vanes such that they prevent
the leakage flow in an optimal way in view of the total efficiency
of the impeller. Since the main task of the front pump-out vanes is
to prevent the leakage flow, it has to be accepted that they
consume power, but their power consumption has to be minimized. In
view of their efficiency, it is also important to adjust the
pressure difference of the pump-out vanes to be correct at the
optimal flow of the pump at or close to the best efficiency point
(BEP). The pressure difference is considered to be correct when it
produces the smallest total loss of the rotor.
[0052] In view of the above, the front pump-out vanes in the volume
between the front shroud and the volute casing are designed to
improve the efficiency by means of following three mechanisms:
1. The velocity field thereof is dimensioned such that the friction
subjected to the shroud surface is as low as possible, preferably
lower than when using a smooth-faced shroud. 2. The pressure the
pump-out vanes create is dimensioned such that the pump does not
leak at its BEP (best efficiency point) from its outer
circumference to the suction duct. 3. The hydraulic energy
transferred via the front volume is kept at such a low level that
only a minimal flow is allowed via the front volume. Thereby, even
if the efficiency of the front pump-out vanes themselves is weak,
its effect on the total efficiency of the impeller is negligible.
Thus, substantially all of the hydraulic energy is produced by the
working vanes operating in high-efficiency closed liquid
passages.
[0053] The above represents fresh thinking as this far the front
pump-out vanes have been understood and accepted as a necessity
that is allowed to decrease the impeller efficiency significantly.
Now the front pump-out vanes have been designed in view of minimal
friction loss between the shroud and the volute casing. After
extensive testing it has been learned that the friction losses are
at their minimum when the circumferential velocity component of the
liquid in the volume between the front shroud and the volute casing
is one half of that of the front shroud.
[0054] When the impeller is constructed in accordance with the
above guidelines, the impeller has a front and a rear shroud and
liquid passages formed between the shrouds and each successive
pairs of working vanes. Both the front and the rear shrouds are
provided with front and rear pump-out vanes, respectively. The
pump-out vanes create a field of pressure. When pumping liquid with
the pump a small or negligible flow compared to the flow via the
liquid passages is allowed to be guided to the effective area of
the front pump-out vanes. Thereby the losses based on the movement
of the impeller in relation to the volute casing are subjected to
the front pump-out vanes, which maintain potential energy, while a
major part or almost all of the energy of the pump is transferred
by the high-efficiency closed liquid passages between the
shrouds.
[0055] By connecting a wear ring arranged between the impeller and
the volute casing in series with the front pump-out vanes
maintaining the potential energy the energy transferred via the
front pump-out vanes may be minimized with all volume flows of the
pump.
[0056] However, the impeller should be designed to work without the
wear ring in case the liquid to be pumped contains solids.
[0057] Therefore the present invention introduces a manner by which
the total efficiency of the impeller may be raised in impellers
having a low specific speed.
[0058] In traditional pumps like the one cited earlier
(US-A1-20090226317) the purpose of the front pump-out vanes is to
create a mass flow between the front shroud and the volute casing.
However, the pumping of a mass flow takes place with a very low or
poor efficiency, as the front pump-out vanes form liquid passages
having a very low specific speed (narrow vanes in relation to their
length, see FIG. 2), which is not able to get even close to its
maximum efficiency. The reason for this is the energy spent by the
shroud in high friction in comparison to hydraulic energy recovered
from this kind of liquid passages. In pumps like the one cited
above the circumferential velocity component of the liquid in the
volume between the shroud and the volute casing is almost identical
to the velocity of the shroud, whereby the energy lost in friction
is nearly at its highest.
[0059] Based on the novel design of the pump-out vanes of the
impeller in accordance with the present invention the power needed
for running the front pump-out vanes is negligible compared to
traditional pump-out vanes. However, the pump-out vanes of the
present invention are still able to maintain rotation in the liquid
between the front shroud and the volute casing and prevent the
leakage flow with minimal power consumption.
[0060] The thinking behind the novel impeller design is that the
power consumption of the front pump-out vanes has to be kept low.
FIG. 5 is a schematic representation of the behavior of the front
pump-out vanes of the invention (curve C) compared to the working
vanes (curve D) and the pump-out vanes of conventional slurry pumps
(curve E) in total head vs. flow rate coordinates. FIG. 5
illustrates clearly that the pump-out vanes of the present
invention lose their ability to create head when the flow rate
increases.
[0061] The mass flow or flow rate is kept small so that the mixing
of liquids having different energies (meaning different speed and
different direction of speed) is minimized. Additionally, the aim
is that when the mass flows of the working vanes and the pump-out
vanes meet they would have as closely matching dynamic and static
energies as possible so that there is no need to convert static
energy to dynamic or vice versa in the energy interface area. If
there is a difference the equalizing of the energies means loss.
When it is a question of an impeller having no wear ring the
circumferential velocity component of the mass flow is kept in
about a half of that of the impeller, as has already been discussed
earlier in this specification. And when it is a question of an
impeller provided with a wear ring, the liquid has to be
accelerated to a circumferential velocity higher than a half of the
impeller circumferential velocity.
[0062] An exemplary impeller 40 of the present invention is shown
in FIG. 6. The impeller has a rear shroud 42, a front shroud 44 and
working vanes 46 therebetween. The rear shroud 42 has pump-out
vanes 48, and the front shroud 44 has pump-out vanes 50, too. The
front pump-out vanes 50 have a height h of at most 2%, preferably
between 0.5-1.5% of the diameter D of the front shroud of the
impeller. The front pump-out vanes may be of equal length, but they
may also be of variable length. An option is to have a certain
number of full-length vanes and an equal number of shorter vanes,
or shorter vanes twice the number of full-length vanes. The number
of front pump-out vanes 50 may be higher, the same or lower than
that of the working vanes 46. Here, in FIG. 6, the number of
pump-out vanes 50 is twice that of the working vanes 46. In
practice, the front pump-out vanes 50 of the present invention are
designed in accordance with the following guidelines:
[0063] The number of pump-out vanes 50 may be defined by using the
equation
.SIGMA..sub.i=1.sup.z(l.sub.i)/D>8,
[0064] where [0065] Z is the number of pump-out vanes 50, [0066] l
is the vane length measured along the leading surface of each front
pump-out vane 50, whereby the term .SIGMA..sub.i=1.sup.z(l.sub.i)
represents the sum of the lengths of the front pump-out vanes, and
[0067] D is the outer diameter of the front shroud 44.
Additionally, the angle of inclination of each pump-out vane 50 at
the outer circumference of the vanes .beta.<25 degrees, the
vanes being backwardly curved. Typically, the number of vanes Z=10,
the vane length l=+0.9 . . . 1.1*D (when the vanes are of equal
length), preferably l=D and .beta.=22.degree.. FIG. 6 also shows
the cylindrical extension 52 of the front face of the front shroud
44 of the impeller 40, the extension 52 being suitable for
cooperating, when in use, with a wear ring arranged to the volute
casing of a centrifugal pump.
[0068] When testing the front pump-out vanes 50 it has been learned
that such a vane may not extend radially outside the outer
circumference of the front shroud 44, as, if it does, the vanes 50
start acting like those of a side channel pump, which is known to
have a very low efficiency. However, in view of the working of the
present invention the front pump-out vanes 50 should, preferably
but not necessarily, irrespective of their length, extend radially
to the outer circumference of the front shroud 44, i.e. to the same
outer diameter as the working vanes.
[0069] FIG. 7 shows the efficiency curve F of the impellers in
accordance with the present invention. In other words, the
impellers of the pumps having a low specific speed in the series of
pumps have been manufactured in the manner described above, and the
result is that the entire series of pumps has an efficiency higher
than what the EU ecodesign requires.
[0070] As can be seen from the above description a novel
centrifugal pump impeller construction has been developed. While
the invention has been herein described by way of examples in
connection with what are at present considered to be the preferred
embodiments, it is to be understood that the invention is not
limited to the disclosed embodiments, but is intended to cover
various combinations and/or modifications of its features and other
applications within the scope of the invention as defined in the
appended claims.
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