HIGH EFFICIENCY DOUBLE SUCTION IMPELLER

Abstract

A new double suction impeller, in particular for centrifugal pumps and hydraulic power recovery turbines, wherein the flow-path arrangement has inter-blade channels intersecting each other at the impeller's outer diameter so that the equivalent number of blades is doubled with respect to a conventional configuration obtained by the coupling of two single suction impellers.

Description

TECHNICAL FIELD

The present disclosure relates to a high efficiency double suction impeller, e.g. one that may be used for centrifugal pumps.

BACKGROUND

Radial flow turbo machinery devices are particularly adapted to convert shaft power to kinetic energy (and vice versa) by accelerating (or decelerating) a fluid in a revolving device called impeller. When used as power-absorbing machines, impellers are commonly used to raise the pressure of a fluid or induce a fluid flow in a piping system.

The impeller is the device, within the turbo machinery, that, rotating, exchanges energy with the fluid. In its simplest implementation the impeller comprises a plurality of blades fitted onto a hub plate. The shape and the geometry of impeller blades can be of many different types depending on the use, the rating, the performance of the turbo machinery.

Having defined the specific speed, NS, of a pump as follows:

$\mathrm{NS}=\frac{{\mathrm{nQ}}^{1/2}}{1.1618H^{3/4}}$

where,
n=rotating speed in revolution per minute
Q=volumetric flow rate in [m³/h]
H=differential head [m]
for centrifugal pumps of capacity larger than 10 m̂3/h designed with low or medium specific speed values (e.g. NS<1600) and a double suction configuration, an impeller with a small number of blades is required in order to keep the head vs flow rate stable and continuously rising towards zero flow. This requirement is very important especially in case of more than one centrifugal pump employed in parallel, each working with a fraction of the available flow. Furthermore, centrifugal pumps of large capacity designed for low or medium specific speed values and for medium or high values of hydraulic head, require impellers having large diameters and narrow exit width. Double suction impellers are usually composed by two single suction impellers each elaborating half of the total flow and arranged in a back-to-back configuration.

In the state-of-the-art, centrifugal pumps having impellers provided with a center rib and staggered blades, the ratio between the impeller exit width b2 and the impeller diameter D2 can be well lower than 0.05. Impellers of this kind often show an unstable head vs flow rate characteristic curve. In addition to that, another drawback of this kind of impellers lies in the low blade exit angles (normally between 15° and 20°) and corresponding large wrap angles (normally between 120° and 270°) that are required to maintain acceptable slip factor values. As a result, the hydraulic efficiency of the state-of-the-art impeller of this kind is typically smaller than 95%.

Furthermore, the low blade load typical of this kind of impellers (normally corresponding to head coefficients “psi” lower than 1, psi being equal to:

$\mathrm{psi}=\frac{2\mathrm{gH}}{u_2^2}$

where
g=gravity acceleration in [m/ŝ2]
H=differential head in [m]
u2=peripheral speed of the impeller in [m/s]) increases the required diameter of the impeller, thus increasing the disk friction losses by 1%-2% when compared to impellers having head coefficients greater than 1.

The achievable head coefficient can be increased by employing conventional split blades impellers, but this choice does not solve the problem of the narrow b2/D2 and poor head curve stability. Moreover, the number of leading edges of conventional split blades impellers is doubled, causing additional hydraulic losses.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention therefore relate to a double suction impeller having the channels between the blades starting from both inlets and crossing the median axis of the impeller exit in such a way that, as a result, the equivalent blades number is doubled with respect to a conventional configuration obtained by the coupling of two single suction impellers.

In the new impeller reducing the slip factor by increasing the equivalent number of blades permits reducing the diameter of the impeller, thus reducing the size and therefore the manufacturing cost of the pump installing said impeller.

Compared to a conventional impeller provided with splitter blades, embodiments of the new impeller do not introduce any additional leading edge and corresponding losses.

For low specific speed pumps the new shape of the inter-blade channels of the impeller is such that the hydraulic diameter is increased and the length of each channel reduced, thus reducing the hydraulic losses with respect conventional impellers.

Furthermore, the reduction of the impeller diameter brings also along a significant reduction of the losses due to disk friction, thus increasing the overall pump efficiency.

The advantages and benefits associated to the new double suction impeller, with respect to an equivalent state-of-the-art impeller, increase as the operating speed of the new double suction impeller is decreased.

The main application for the new double suction impeller is within centrifugal pumps and hydraulic power recovery turbines especially, but not exclusively, intended for refinery, petrochemical and pipelines. However, other applications are possible and contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of embodiments of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a meridional section of a state of the art double suction impeller;

FIG. 2 illustrates a view of the inter-blade channel of the state of the art double suction impeller;

FIG. 3 illustrates a detail of the outer diameter arrangement of a state of the art double suction impeller;

FIG. 4 illustrates a detail of the section of an embodiment of a new double suction impeller;

FIG. 5 illustrates a view of the inter-blade channel of another embodiment of the new double suction impeller;

FIG. 6 illustrates a detail of the outer diameter arrangement of another embodiment of the new double suction impeller;

FIG. 7 illustrates a first front view of another embodiment of the new double suction impeller;

FIG. 8 illustrates a meridional cross section of an embodiment of the new double suction impeller; and

FIG. 9 illustrates a side view of an embodiment of the new double suction impeller.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the enclosed drawings and according to an exemplary embodiment, embodiments of the present invention relate to new a double suction impeller, in particular for centrifugal pumps, wherein the flow-path arrangement is characterized by inter-blade channels intersecting each other at the impeller outer diameter.

In one embodiment, the new double suction impeller comprises channels between the blades starting from both inlets and intersecting at the outer diameter of the impeller in such a way that the equivalent blade number is doubled with respect to a conventional configuration obtained by simply adjoining to a central rib two single suction impellers, as illustrated in enclosed FIGS. 1, 2 and 3.

In greater detail, and with reference to enclosed FIGS. 4, 5, 6, 7, 8 and 9, the new double suction impeller includes a shrouded impeller 10. The shrouded impeller 10 may further include a hub 11 associated with a tubular center bore 12. The tubular center bore 12 may be adapted to receive the impeller drive shaft which is drivingly connected thereto, generally by a key and a keyway.

The shrouded impeller 10 can be made either of one single piece—or assembly—or it can be made of a plurality of assemblies, e.g. comprising one left shroud, one right shroud and a central core.

In one embodiment, the new impeller is made of one single assembly, and the hub 11 further includes a plurality of blades 13 integrally attached to the hub 11 and to a pair of integral shrouds, a left side shroud 14 and a right side shroud 15. Each one of the integral shrouds 14, 15 is provided with a center aperture 16, 31 that constitutes the impeller eye. The impeller eye is adjacent to said tubular center bore 12 and comprises an aperture edge 17 with an aperture edge radius and an aperture rim 18 with an aperture rim radius.

The left side shroud 14 defines the left side aperture and the right side shroud 15 defines the right side aperture of the double suction impeller according to the present invention.

The outer edge of said left side shroud 14 and the outer edge of said right side shroud 15 define the impeller exit, said impeller exit having a width 19 and a median plane 20.

In greater detail, each pair of adjacent blades 13 of said plurality of blades 13 define a plurality of inter-blade channels, referred to in enclosed FIG. 5. Said inter-blade channels are adapted to connect a plurality of input apertures, located within the center apertures of both said left side shroud 14 and said right side shroud 15, to a plurality of output apertures located on said impeller exit.

With reference, in particular, to enclosed FIGS. 4 and 6, said inter-blade channels comprise left side inter-blade channels 21, having their respective input apertures 22 located within the center aperture of said left side shroud 14, and right side inter-blade channels 24, having their respective input apertures 25 located within the center aperture of said right side shroud 15.

Advantageously, said left side inter-blade channels 21 and said right side inter-blade channels 24 are such as intersecting the median plane 20 of said impeller exit in a way to dispose the output apertures 23 of said left side inter-blade channels 21 aligned and alternated with the output apertures 26 of said right side inter-blade channels 24, on said impeller exit.

In operation, when rotated, fluid will be drawn axially into the impeller as indicated by the arrows 27, 28, impelled by the plurality of blades 13 passing between the hub 11 and said left and right shrouds 14, 15 and finally expelled radially through said exit as indicated by the arrows 29. The impeller runs in the direction of arrow 30 in a suitable housing having axial inlets and a circumferential volute or diffuser outlet passage.

In the double suction impeller according to the present invention, the effect connected to the crossing by said inter-blade channels of the median axis of the impeller exit is such that the equivalent blades number is doubled with respect to a conventional configuration obtained by the coupling of two single suction impellers.

With reference to enclosed FIG. 2, the section 33 corresponds to the inlet of a channel of an impeller of the state of the art, and area 32 corresponds to the outlet of a channel of an impeller of the state of the art.

With reference to enclosed FIG. 5, the section 35 corresponds to the inlet of a channel of the new impeller, and area 34 corresponds to the outlet of a channel of the new impeller.

It is apparent that, when comparing inter-blade channels of a double suction impeller of the prior art with the inter-blade channels of the new double suction impeller, the outlet area 34 of the inter-blade channels of the new double suction impeller has a rectangular shape with an aspect ratio much closer to 1 with respect to that of a state of the art impeller.

Finally, benefits introduced by the new double suction impeller include, inter alia, reduction of the slip factor and reduction of hydraulic losses. These reductions may translate to an efficiency increase of about 3% to 4% and, ultimately, to a lower operating expenditure and lower capital expenditure.

The above description of exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. The above detailed description does not limit the scope of the claimed invention. Instead, the scope of the invention is defined by the appended claims.

Claims

1. A double suction impeller, comprising:

a hub associated to a tubular center bore, the hub comprising:

a plurality of blades integrally attached to the hub and to a left side shroud and to a right side shroud, the left side shroud and the right side shroud being provided with a center aperture adjacent to the tubular center bore, the outer edge of the left side shroud and the outer edge of the right side shroud defining the impeller exit, the impeller exit having a width and a median plane wherein each pair of adjacent blades of the plurality of blades define a plurality of inter-blade channels adapted to connect a plurality of input apertures, located within the center apertures of both the left side shroud and the right side shroud, to a plurality of output apertures located on the impeller exit, the inter-blade channels comprising left side inter-blade channels, having their respective input apertures located within the center aperture of the left side, and right side inter-blade channels, having their respective input apertures located within the center aperture of the right side shroud,

characterized in that the left side inter-blade channels and the right side inter-blade channels intersect the median plane of the impeller exit in a way to dispose the plurality of output apertures of the left side inter-blade channels to be aligned and alternated with the output apertures of the right side inter-blade channels, on the impeller exit.

2. The double suction impeller according to claim 1, characterized in that the double suction impeller is made of a single piece.

3. The double suction impeller according to one or more of claims from 2, characterized in that the center bore is adapted to receive the impeller drive shaft, the impeller drive shaft being drivingly connected to said center bore by a key and a keyway.

4. The double suction impeller according to claim 3, characterized in that the center aperture comprises an aperture edge and an aperture rim.

5. The double suction impeller according to claim 4, characterized in that it further comprises: a suitable housing having axial inlets and a circumferential volute or diffuser outlet passage.

6. A centrifugal pump comprising a double suction impeller according to claim 5.

7. A hydraulic power recovery turbine comprising a double suction impeller according to claim 5.