Centrifugal pump for very thick and/or viscous materials and products

A centrifugal pump used for conveying very thick and/or viscous materials and products which comprises a screw impeller of at least one start, of which the component blade, which is in the form of a helix (8) having a pitch which increases in the material feed direction, has a slightly conical outer profile, is wound on a highly conical core (5) smoothly joined to a base disc (6), and at its downstream terminal portion is twisted towards the core (5) to assume the configuration of a delivery vane (9) forming an acute angle with the disc (6) and disposed along a chord thereof, and supporting an overlying circumferential ring (10) which defines a closed delivery duct for the material.

In many industrial sectors, thick and viscous materials which are difficult 
to pump have to be circulated within respective plants. For example, in 
modern plants for the continuous concentration of liquid foodstuffs and in 
chemical process plants very high concentrations and/or thickness levels 
are attained, in particular in the last stages of the process, so as to 
make normal circulating pumps ineffective to the point of causing their 
total blockage. This is due to the fact that an increase in product 
concentration leads to a considerable reduction in its liquidity, and thus 
to a considerable increase in the friction which it encounters as it 
passes through the circulating pumps and conveying pipes. 
Furthermore, in such plants the product is very often concentrated in 
environments under vacuum. Consequently, circulating pumps for handling 
very thick and/or viscous products and materials must have high suction 
capacity and good hydraulic efficiency, and must have very small 
friction-generating surfaces and large product passages. 
For handling thick or viscous products, pumps are known comprising an 
impeller substantially in the form of a screw wound at a constant pitch on 
a conical core and having a conical outer profile, so that the head 
derives mainly from the reduction in the cross-section of the fluid 
passage duct between downstream and upsteam. 
In these known pumps, the helical screw blade reduces in height, by virtue 
of the different degree of taper between its outer edge and the core, 
until it disappears at the flat base of the latter, into which it smoothly 
joins. In this manner, a toroidal channel disposed at the end of the screw 
collects the liquid in order to enable it to be discharged tangentially. 
The toroidal channel, which can be compared to the header of a centrifugal 
pump, has however a mere collecting function, as the vanes which help to 
deliver the liquid at a sufficient velocity are lacking. 
The use of these known pumps has shown that they have good throughput and 
hydraulic efficiency characteristics, but they are subject to a 
considerable deficiency in hydraulic equilibrium, and thus to excessive 
mechanical unbalance and consequently high stresses. Furthermore, these 
pumps are difficult to adapt to the requirements of different plants 
because reducing the impeller diameter results in an unacceptable 
reduction in the main pump characteristics such as throughput, head, 
suction capacity and efficiency. 
Circulating pumps are also known comprising an impeller with three or more 
blades, i.e., comprising three or more equidistant helices which are wound 
at a more or less constant pitch on the same conical core. 
These known pumps have the drawback of passages which reduce in size as the 
number of helices increases, and thus have very large friction-generating 
surfaces which oppose the circulation of thick and viscous products. 
Vortex pumps, i.e., with their impeller set back, are also known and have 
proved particularly suitable for handling products of high viscosity but 
have a poor suction capacity, very low efficiency and excessively flat 
characteristic curves. 
Thus generally, the known types of circulating pumps have acceptable 
operation for materials of relatively low thickness or viscosity, whereas 
the use of the same pumps for very thick and viscous products which are 
obtained from modern multi-stage concentration plants results in a drastic 
reduction in their throughput, head and efficiency characteristics and, as 
stated, the pump can become blocked in certain cases thus leading to 
stoppage of the plant and its obvious consequences. 
Moreover, the best and most efficient known pumps are limited in their 
application to the extent that they can handle thick materials or 
suspensions containing a maximum of 30% of solid residue, and viscous 
materials or solutions having a maximum viscosity of 40.degree. Engler. 
The present application provides and protects a pump of special design, 
which is able to handle thick products containing up to 40-45% of solid 
residue, and viscous products having a viscosity of up to 60.degree.-65' 
Engler, while offering throughput, efficiency and head characteristics 
which are very close to pumps which convey to normal liquid products. 
The pump according to the present invention is of the centrifugal type, 
provided with an impeller comprising one or two blades, preferably two 
blades, which extend upstream in the form of two helical blades wound on a 
conical core at a pitch which decreases towards the vertex of the core. In 
other words, starting from the material inlet, the first portion of each 
blade is in the form of a helix having a pitch which increases from 
upstream to downstream and wound about a highly conical core, the helix 
having a slightly conical outer profile. In proximity to the base of the 
core where the latter smoothly joins into a plate orthogonal to the 
impeller axis, the helix is twisted so that its last portion is in the 
form of a delivery vane which forms an acute angle with the orthogonal 
plate, and is disposed along a chord of this plate which is very close to 
its circumferential edge. 
The vane lies between the plate and an upper ring, which define the 
delivery section for the pumped product. 
Because of the preferable use of two helices and respective vanes, the 
impeller is perfectly balanced, and its inlet part, in which the helices 
have their portions of smaller pitch, provides high suction capacity which 
enables it to operate correctly even when its suction side is connected to 
the environment under vacuum. 
In addition, the intermediate greater-pitch portions of the two helices 
form a member for axially thrusting the material, which is thus fed in 
compact form to the inclined chordal vanes which together with the upper 
ring constitute an extremely effective centrifugal delivery member. The 
fact that the outer profiles of the helices are slightly conical whereas 
the connecting core is highly conical means that two particularly large 
passages of decreasing cross-section are provided which, as stated, enable 
the pump to handle very thick and very viscous products without any 
significant alteration in its throughput, head and efficiency 
characteristics.

From the figures, and in particular FIG. 1, it can be seen that the 
invention comprises a stator element or casing 1 in which there is 
provided a frusto-conical chamber 2 comprising a suction port 3. At the 
opposite end to the suction port there is a normal volute 4 for collecting 
and evacuating the material or product. The two fixed bodies which form 
the operating chamber 2 and collection volute 4 are joined together by 
respective flanges, between which suitable sealing gaskets are interposed. 
In the frusto-conical chamber 2 at the end comprising the volute 4, there 
is coaxially disposed a conical core 5 which is smoothly joined at its 
base to a transverse disc 6, the disc being driven by a drive shaft 7 
which is idly mounted through the casing 1. 
The core 5 extends longitudinal substantially through 2/3 of the chamber 2, 
and the cone angle at its vertex is of the order of 30.degree.-40.degree., 
and preferably 36.degree.. 
The core 5 constitutes the shank from which two equal helically extending 
blades 8 branch, their winding pitch increasing in the direction from the 
suction mouth 3 to the delivery volute 4. Moreover, as can be better seen 
in FIG. 2, the front ends of said two helices 8 project beyond the vertex 
or point of the core 5, to terminate immediately to the side of the 
suction port 3, whereas their rear terminal portions gradually twist as 
they approach the disc 6 by being bent towards the core 5, in order to 
form two diametrically opposing vanes 9 which terminate in two chamfered 
portions 99 disposed in line with the circumferential edge of the disc 6. 
Said bending is clearly visible in FIG. 4. From FIGS. 3 and 4 it can be 
also seen that the two vanes 9 form an acute angle with the base disc 6 
and are disposed along two chords which are very close to the 
circumferential edge of said disc. 
In addition, said vanes 9 are disposed in front of the mouth of the volute 
4 and extend axially through a distance practically equal to the width of 
said mouth (FIG. 1). 
Immediately upstream of this latter, ie at the terminal downstream part of 
the operating chamber 2, there is provided a circumferential recess 11 
which forms a seat for receiving a ring 10, the inner surface of which 
forms a direct continuation of the operating chamber 2 and acts as the 
element by which the chamber is smoothly joined to the volute 4. Said ring 
10 is disposed overlying the two diametrically opposing chordal vanes 9, 
and has an inner transverse curvature which exactly follows the 
corresponding outer profile of the twisted portions of the helices 8, with 
which it is rigid. 
Again, as can be clearly seen in FIG. 1, the ring 10 and the terminal part 
of the vanes 9 are external to the conical surface defined by the 
operating chamber 2, to thus form a delivery member which is of large 
diameter and thus very efficient with regard to head. 
Finally, the outer edges of profiles of the two helices 8 exactly fit 
inside the chamber 2, of which the cone angle at the vertex is between 
13.degree. and 19.degree., and preferably 16.degree.. 
The direction of rotation of the impeller is shown by the arrow R in FIGS. 
2 and 4. 
It is apparent at this point that the double blading 8 means that the 
described impeller is perfectly balanced, and the small-pitch front 
portions of the two helices 8 provide high suction capacity which enables 
the pump to operate correctly even when its suction side is connected to 
environments under high vacuum. 
In addition, the intermediate large-pitch portions of increasing height of 
the two helices provide a structure for axially thrusting the material, 
which is fed continuously and/or in compacted form to the delivery member 
constituted by the two vanes 9 and ring 10, so as to enable the invention 
to handle very thick and/or very viscous products. 
This is also due to the fact that combining the small taper of the outer 
profile of the helices 8 with the accentuated taper of the core 5 provides 
a pair of large passages of decreasing cross-section (FIG. 1), which 
ensures the compacting of the material being pumped. 
Essentially, by virtue of the aforesaid characteristics, the pumped fluid 
is compelled to follow a path which is initially practically axial, and 
becomes increasingly more radial as it approaches the delivery zone 
defined by the ring 10. 
From tests carried out it has been found that the pump under examination is 
able to handle very thick and/or very viscous products while maintaining 
its main efficiency, throughput and its head characteristics are 
practically equal to those relative to materials which are much less thick 
and much less viscous, i.e., substantially liquid. 
The same tests have shown that the invention is able to handle thick 
products or suspensions containing up to 40-45% of dry residue and viscous 
products or solutions of viscosity up to 60.degree.-65.degree. Engler 
without problems. 
The same operating characteristics are obtained when the impeller according 
to the present invention comprises a single helix, obviously of the 
aforesaid type and combined with a ring 10. 
The invention is not limited only to the embodiments heretofore described, 
and modifications and improvements can be made thereto without departing 
from the scope of the present invention, the main characteristics of which 
are summarized in the following claims.