Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPELLER WHEEL
TECHNICAL FIELD
The invention concerns a pump impeller and more precisely a pump impeller
for centrifugal-or half axial pumps for pumping of fluids containing
pollutants.
BACKGROUND OF THE INVENTION
In literature there are lot of types of pumps and pump impellers for this
purpose described, all however having certain disadvantages. Above all, this
concerns problems with wear, clogging and low efficiency.
Sewage water contains a lot of different types of pollutants, the amount and
struct.ure of which depend on the season and type of area from which the
water emanates. In cities plastic material, hygiene articles, textile etc are
common, while industrial areas may produce wearing particles. Experience
shows that the worst problems are rags and the like which stick to the
leading edges of the vanes and become wound around the impeller hub. Such
incidents cause frequent service intervals and a reduced efficiency.
In agriculture and pulp industry different kinds of special pumps are used,
which should manage straw, grass, leaves and other types of organic
material. For this purpose the leading edges of the vanes are swept
backwards in order to cause the pollutants to be fed outwards to the
periphery instead of getting stuck to the edges.
Differ-ent types of disintegration means are often used for cutting the
material
and making the flow more easily. Examples are shown in SE-435 952, SE-
375 831 and LJS-A 4 347 035.
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In literature the designs of the pump impellers are described very generally,
especially as regards the sweep of the leading edges of open impeller wheels.
Attempts to define the sweep of the leading edges for an open impeller wheel
are described in e.g. EP 0916851 and EP 0916852.
Di.iring pumping where high lifting heights are required, for example in
mines, closed pump impellers are often used, i.e. such with two cover discs,
an upper and a lower as well as inteimediate vanes, because these types of
iinpellers reduce the axial load on the pump in comparison to the open type
impellers. On the other hand, closed impellers have a lesser lead-through,
which means higher risk for clogging.
Regarding closed impeller wheels they are generally designed with leading
edges placed in the central inlet such that they are rather straight as seen
in
the flow direction. It has been found that during use, and in particular when
the liquid contains rather large quantities of abrasive pollution, there is
considerable wear on the leading edges of the vanes. There is further a risk
that pollutants such as rags, plastic bags and other elongated objects get
stuck on the leading edges, thereby requiring cleaning of the pump.
BRIEF DESCRIF'TION OF THE INVENTION
The aim of the present invention is to remedy the above mentioned problems
and in particular in connection with closed impeller wheels.
This aim is solved according to a main aspect of the invention by what, is
characterized in claim 1. Preferable embodiments of the invention form the
dependent claims.
According to a main aspect of the invention, it is characterized by an
impeller
wheel of a centrifugal or half axial type to be used in a pump for pumping
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liquids, wherein the impeller is arranged with one or several vanes, that the
vanes are attached to cover discs on each side, forming a so called closed
impeller wheel, characterized in that the leading edges of the vanes are swept
backwards at the inlet opening in one of the cover discs of the impeller.
According to a fizrther aspect of the invention, it is characterized in that
the
leading edges co-operate with cutting means arranged in a central inlet
opening of the pump.
Preferably the leading edges protrude somewhat out of the inlet opening, and
that the cutting means constitute at least one narrowing section of the gap
between the central inlet opening of the pump and the leading ledges of the
impeller, whereby, when the leading edges sweep past these sections during
operation, a cutting action is obtained there between.
By designing the leading edges of the vanes strongly backswept it has been
found that the wear on these edges are reduced drastically. The design also
reduces the risk of material getting stuck on the edges. Rather such material
are fed towards the periphery.
The material that however tends to get stuck on the edges at the inlet
opening of the impeller will be cut by the cutting means. In a preferred
embodiment the leading edge of the vanes co-operate with two narrowing
parts of the inlet opening of the pump housing.
For a good performance regarding the pump function and transportation of
material towards the periphery, the sweep angle a should be in the region 40
- 60 and the sweeping of the leading edges may generally follow the
curvature of a logarithmic spiral.
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In all an impeller wheel with improved performance and wear resistance is
obtained, leading to long operation cycles between maintenance and service.
These and other aspects of and advantages with the present invention will
become apparent from the following detailed description and from the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following detailed description of the invention, reference will be made
to the drawings, of which
Fig. 1 is an axial cross-section through a pump according to the
invention,
Fig. 2a is a perspective view from above of an impeller wheel comprising
the present invention,
Fig. 2b is a view corresponding to Fig. 2a from below,
Fig. 3 is a view from below of the impeller wheel of Fig. 2, and
Fig. 4 is a bottom plate to be mounted in a pump and co-operating with
the impeller wheel according to Fig. 1.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 shows an impeller wheel of the closed type mounted in a pump
comprising a drive shaft 10 connected to an electrical motor (not shown) for
driving the pump. To the lower end of the shaft a pump impeller 12 is
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mounted, comprising upper 14 and lower 16 cover discs, vanes 18 and back
vanes 19. The above mentioned components are mounted in a pump housing
20, having a bottom wall 22, an inlet 24 and an outlet 26. The pump impeller
12 is mounted such in the pump housing that there is a gap 28 between the
5 peripheral surface of the lower cover disc 16 and an inner side wall of the
ptimp housing 20, a space 29 between the lower disc and the bottom wall as
well as a gap 30 between a lower surface of the lower cover disc 16 and an
upper surface of the bottom wall 22.
Aceording to the principles for a centrifugal pump, the liquid is sucked in
axially through the inlet 24 and leaves the pump through the outlet 26
according to flow arrows A, B and C.
According to the present invention the vanes 18 are designed such that the
leading edges 40 are swept strongly backwards at the inlet opening 42 of the
impeller wheel, in relation to the direction of rotation R. The sweep angle
coltld be increased from the hub and outwards. The sweep of the leading
edges could be defined by a logarithmic spiral where the sweep angle a is rc/2
-(3, where (3 is the angle of the logarithmic spiral. Preferably the sweep
angle
should be in the region 40 - 60 .
At the inlet opening of the impeller the leading edges are placed generally in
a
plane perpendicular to the axis of rotation 10 of the impeller. The advantage
with this design is to make sure that pollutants slide towards the periphery
instead of getting stuck on the edges. It has further been shown that the
wear on the leading edges are reduced when they have this configuration
instead of the conventional design.
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Further, the leading edges protrude somewhat out of the central opening of
the ilnpeller, Fig. 1. The edge parts 44 of the vanes protruding above the
bottom surface of the lower- cover- disc 16 can be arranged to co-operate with
the edge part 46 of the stationary inlet 24 of the housing arranged in a
bottom plate 22. The circular inlet is thus arranged with two oppositely
ar-ranged straight parts 50 whereby the gap between the vanes and the edge
part of the inlet becomes very small. Elongated pollution such as rags,
plastic
bags and the like that have been stuck on the leading edges of the vanes will
be cut here.
In total the design of the leading edges being swept and protruding
somewhat, and co-operating with the opening of the bottom plate, greatly
1-educes the r-isk of pollutants getting stuck and reduces the wear on the
leading edges.
It is to be understood that the embodiment described above and shown in the
drawings is to be regar-ded as a non-limiting example of the invention and
that it may be modified within the scope of the patent claims.
