FIELD OF THE INVENTION:
The present invention generally relates to a vertical axis, single stage, mixed flow
type, centrifugal pump for pumping large volume of water in Lift Irrigation
Schemes. More particularly, the invention relates to an improved volute casing for
high capacity pumps to mitigate secondary flows and losses caused by twin
vortices and eliminate flow reversal at the exit of the volute casing .
BACKGROUND OF THE INVENTION
Thus, a volute Casing is an important hydraulic component of a centrifugal
pump (Fig 1). The purpose of Volute casing is to collect the liquid coming out of
the pump impeller and lead it away. Another important function of the Volute
casing is to convert into pressure, the high velocity kinetic energy of the flow
discharging from the impeller. In case of the volute casing, the liquid flows in a
direction adverse to pressure gradient i.e. from low pressure side to high
pressure side. It is known that once the liquid is discharged from the impeller, it
can no longer be influenced by the forced vortex. Hence, there is every tendency
for flow reversal specially in the exit portion of the volute casing which has larger
dimensions. Further, in a volute, the maximum velocity occurs at the periphery of
the impeller or just at the entrance to the volute. The liquid coming out of the
impeller has substantial velocities with both tangential and radial components.
Though the velocities are equal along the periphery of the impeller, they are not
equal along the width of the impeller.
Thus, a typical velocity pattern in any radial section of the volute comprises high
velocity core and decrease of velocity towards volute walls. This complex flow
pattern causes a spiral motion leading to creation of twin vortices.
Volute casing pumps are very common. Their characteristic feature is the volute-
shaped pump casing which, as a rule, makes this pump type recognizable from
the outside. Volute type casings can be built as part of a single stage or
multistage pump arrangement. In some multistage pumps a volute shaped casing
is provided only for the last stage. Single suction and double suction, double
volute casing pumps are both used frequently.
A volute shaped casing generally includes a chamber designed to house at least
one impeller being usually of the radial or mixed flow type and mounted on a
shaft for rotation when driven by a motor. The casing further includes a volute
shaped chamber to collect pumped medium and a channel and discharge section
to guide the medium out. The discharge can be arranged tangentially to the
volute casing, or arranged radially by providing a swan neck. A suction channel
section is favorably arranged axially in case of bearings arranged only at one side
of the impeller, and radially or tangentially in case of bearings at either side of
the impeller. In its simplest embodiment of a single volute, the casing can be
broadly sub-divided into two main sections consisting of a downstream chamber
section including a volute shaped chamber and the upstream channel and dis-
charge section. The plane or section at which the
volute and channel meet is generally defined as the throat. The leading edge of
the throat which separates or guides the flow from the chamber into the channel
is designated cutwater lip or cut water and for any given length the top and
bottom surface extending beyond the lip is termed the tongue. In the case of a
casing with a plurality of volutes and flow channels disposed around an impeller
the number of lips will usually be equal to the number of volutes or flow
channels. I.e., in case of a double volute there will be two lips.
The conventional pump casing with double volute and double discharge is
arranged in such a way that an outer or long channel connected to a first volute
is wrapped around a second or inner volute and over a short channel connected to
the second volute so that the discharge of the pumped medium is made through a
common discharge nozzle which can e.g. be provided in a flange. This provides
effective pumping means but imposes a large area envelope in radial direction
and, thus, an increased height which is disadvantageous for pump arrangements
requiring a more compact solution.
A volute shaped pump casing includes a chamber for housing at least one impeller
mounted on a shaft for rotation around an axis of rotation when for example driven by a
motor. The casing further includes a volute shaped chamber which forms a flow channel
divided into a first volute and a second volute typically extending each over about half or
less of the circle, and a first channel and a second channel connected to the first and the
second volute respectively to guide the pumped medium out.
The casing further includes at least one wall separating the first channel from the second
volute and/or from the second channel . A leading edge part of the casing which
separates or guides the flow from a volute into a channel is designated cutwater lip or
cut water and for any given length the top and bottom surface extending beyond the lip is
termed the tongue. In the case of a casing with double volutes and double flow-channels
there are two cutwater lips.
Hydraulic loss within the volute casing constitute a large part of the total
hydraulic loss within a pump, and hence, optimum design of pump casing
has a very significant influence to centrifugal pump performance. The
design of volute casing gains even more importance in case of Lift
Irrigation schemes since the ratings of the pumps involved are very large
and hence, even a small reduction in hydraulic losses means a large saving
of energy and money.
A significant amount of hydraulic loss occurring inside a pump volute casing can
be contributed to the twin vortices created inside the flow domain when viewed
along any radial plane. Hence, in order to improve the hydraulic performance of
the volute casing and the pump, the intensity of said twin vortices needs to be
completely suppressed or mitigated .
Also, since there is decelerating flow in a volute casing, a flow reversal occurs
towards the exit region specially for high capacity pumps used for Lift irrigation
schemes (specific speed Nq > 50 SI units) at off-design conditions or even at
design point.
Linearly diffusing, circular Volute casing (Fig 2(1), 3)
The most commonly used Volute casing in pumps comprises Uniformly diffusing
circular
volute casing.
This volute casing however suffers from the following drawbacks:
a) Twin vortices created at any radial section leading to loss of efficiency
(Fig 6).
b) Flow reversal occurring towards the exit of volute casing also leading
to a loss in efficiency (Fig 8).
Non-linearly diffusing, circular Volute casing (Fig 2(H)):
These prior art volute casings are also used in pumps but they also suffer from
the above two drawbacks of Linearly diffusing circular Volute casing.
Trapezoidal cross-section Volute casing Fig (2(IID):
Volute casings with cross-sections other than circular (like trapezoidal, rectangular)
have been used in prior art. However, their efficiency is comparative poor as
compared to the circular cross-sectioned casing.
OBJECTS OF THE INVENTION:
It is therefore an object of this invention to propose an improved volute casing for
high capacity pumps to mitigate secondary flows and losses caused by twin
vortices and eliminate flow reversal at the exit of the volute casing, which can be
adapted for lift irrigation schemes to reduce hydraulic losses.
Another object of the invention is to propose an improved volute casing for high
capacity pumps to mitigate secondary flows and losses caused by twin vortices
and eliminate flow reversal at the exit of the volute casing, which is configured with
derived geometry such that the intensity of these twin vortices is suppressed,
thus reducing the hydraulic losses..
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved volute casing for high capacity pumps
to mitigate secondary flows and losses caused by twin vortices and eliminate flow
reversal at the exit of the volute casing, the improvement is characterized in that
the circular section of the volute casing is reconfigured to have partially at least a
straight portion, and in that the ratio of critical velocities (V1,V2) is restricted
within 1.06, wherein the length of
said straight portion of the circular section of the volute is defined by L= v2R(R-
h), in which 'h' representing height of all the sections of the volute, 'R' is the
radius of the said circular section, and V is the length of the straight portion
connecting the annular walls and the circular section, and wherein V1 is the
velocity of the fluid at the volute section commencing flow-acceleration, and V2 is
the flow-velocity at the last section of the volute.
DETAIL DESCRIPTION OF THE INVENTION
Out of the total hydraulic losses occurring inside a centrifugal pump, more than
30% of the losses occur inside the volute casing for high capacity pumps used
for Lift Irrigation schemes. These losses basically comprise two constituents, for
example, the frictional losses occurring due to fluid-fluid friction and fluid-
structure friction, and losses due to secondary flows which disturb the primary
flow. Not much can be done to curb the frictional losses because it depends upon
the surface area in contact and the volume of flow, both of which are derived
from the desired application of the pump. However, the secondary flows can be
mitigated by optimizing the geometry of the volute casing. One pattern of the
secondary flow forms a pair of twin vortices when viewed along any radial
plane.
According to the invention, the area of flow is reduced significantly as compared
to a true volute casing, especially towards the zone where the volute casing
sections intersect with the diffuser annular walls, thus suppressing development
of the twin vortices and increasing the free-stream flow area.
According to this aspect of the present invention, the circular section of the
volute casing is re-configured such that a part of it is formed using a straight
line, the length of which is defined as

h, being known and constant for all the defining sections and
R, being the radius of the particular section in consideration and
L can be defined as the length of the straight line connecting the annular walls of
the diffuser to the circular segment of the section.
Therefore, a change in the flow pattern has been achieved by changing the
shape of the sections from a complete circular to a shape as shown in Figure 4,
wherein the shape of the section comprises , a circular section and two straight
lines connecting the end points of the circular section to the annular walls of the
diffuser. Thus, the core high velocity although remains unchanged, the velocity
gradient towards wall is reduced resulting in significant suppression of the twin
vortices created inside the volute casing (Fig.-7). It has been validated using
CFD Analysis that by suppressing the twin vortices, the strength of the secondary
flows decreases significantly, thus reducing the hydraulic losses inside the volute
casing.
Added advantage of suppressing of twin vortices is smooth flow in the volute
leading to reduction in pressure pulsations.
Conventionally, volute casings are designed for decreasing velocities from
the first section to the last section, i.e a decelerating nature of flow for
converting the residual kinetic energy to pressure energy. The chances of
flow separation or complete reversal increase significantly whenever the flow
is decelerating in nature. For high capacity pumps used in Lift Irrigation
schemes there is a tendency for reversal of flow towards the exit of volute
leading to losses (Fig 8).
The present invention follows an approach in which the velocity decreases
for a major portion of the casing, but changes its trend in the last few
sections and marginally accelerates (Fig 5) such that the chances of flow
reversal are eliminated. At the same time, this acceleration of flow should be
such that it more than compensates any loss of efficiency.
The acceleration given to the flow is selected such that the ratio of the
critical velocities vl and v2 does not exceed 1.06.

where, vl is the velocity at the section from which the flow begins to
accelerate and v2 is the velocity at the last section .
It is observed that in high capacity pumps being used for Lift Irrigation scheme,
there is a tendency of flow reversal towards the exit side thereby causing losses.
This tendency
increases with increase in specific speed of the pump (Fig 5). Accordingly
another aspect of the present invention is to eliminate this possibility of flow
reversal. According to the invention, the volute sections has been changed such
that the velocity gradually falls up to a value (9) depending on the specific
speed and thereafter it is allowed to increase leading to a small acceleration of
the flow towards the exit of the casing. This eliminates chances of flow reversal
towards the exit of casing for most flow conditions and more than compensates
any loss of efficiency (Fig 9).
High capacity lift irrigation pumps typically use disc type Valves at the exit. This
is to reduce the starting power/torque of pump by shutting off the flow and also
to regulate flow specially during load throw-offs. Thus, an added advantage of
this invention is that the size of the Valve is reduced leading to significant
reduction in cost
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1. shows a plan view of a typical volute casing defined by radial circular
sections showing the definition of '?' and '?end'
Figure 2. (I) shows a volute casing of the prior art having circular cross-section
and linear variation of velocity with ?
Figure 2. (II) shows a volute casing of the prior art having circular cross-section
and non-linear variation of velocity with ?
Figure 2. (Ill) shows a volute casing of the prior art having trapezoidal cross-
section and linear or non-linear variation of velocity with ?
Figure 3. shows the geometry of a typical radial circular section of a volute
casing as per the prior art showing the Diffuser leading and trailing edges and
also depicting the definition of 'R' and 'h'
Figure 4 shows a typical radial section as per the present invention, with
changed section shape, showing the dimension 'L'
Figure 5 (I), shows the trend of velocity variation inside a volute casing of
Nq~50 as per the present invention
Figure 5 (II). shows the trend of velocity variation inside a volute casing of
Nq~80 as per the present invention
Figure 6. Streamlines along a typical radial section (Prior Art) showing the high
intensity of the twin vortices
Figure 7. Streamlines along a typical section (Present Invention) showing the
mitigated twin vortices
Figure 8. Streamlines along a planar section (Prior Art) showing the tendency of
flow to reverse at the outlet of the volute casing resulting in eddies
Figure 9. Streamlines along a planar section (Present Invention) showing smooth
flow exiting from the volte casing
We Claim:
1. An. improved volute casing for high capacity pumps to mitigate secondary
flows and losses caused by twin vortices and eliminate flow reversal at the
exit of the volute casing, the improvement is characterized in that the
circular section of the volute casing is reconfigured to have partially at
least a straight portion, and in that the ratio of critical velocities (V1,V2) is
restricted within 1.06, wherein the length of said straight portion of the
circular section of the volute is defined by L= v2R(R-h), , in which 'h'
representing height of all the sections of the volute, 'R' is the radius of the
said circular section, and 'L' is the length of the straight portion
connecting the annular walls and the circular section, and wherein V1 is
the velocity of the fluid at the volute section commencing flow-
acceleration, and V2 is the flow-velocity at the last section of the volute.
2. An improved volute casing for high capacity pumps to mitigate secondary
flows and losses caused by twin vortices and eliminate flow reversal at the
exit of the volute casing, as substantially described and illustrated herein
with reference to the accompanying drawings.

ABSTRACT

The invention relates to an improved volute casing for high capacity pumps to
mitigate secondary flows and losses caused by twin vortices and eliminate flow
reversal at the exit of the volute casing, the improvement is the circular section of
the volute casing is reconfigured to have partially at least a straight portion, and
in that the ratio of critical velocities (V1,V2) is restricted within 1.06, wherein the
length of said straight portion of the circular section of the volute is defined by
L= √2R(R-h), , in which 'h' representing height of all the sections of the volute,
'R' is the radius of the said circular section, and 'L' is the length of the straight
portion connecting the annular walls and the circular section, and wherein V1 is
the velocity of the fluid at the volute section commencing flow-acceleration, and
V2 is the flow-velocity at the last section of the volute.