Axial diffusor for a centrifugal pump

Abstract: The invention provides a centrifugal pump having an axial diffusor (12) with plural blades (16) having such a specific incident angle (β G ) as to prevent appearance of revolving and stall of a treating liquid. The incident angle (β G ) of the blades (16) of the axial diffusor (12) is so determined that a value provided by a subtraction from the blade incident angle (β G ) by a treating liquid incident angle (α G ) is equal to or below a specific angle defined by a kind of treating liquids.

Description: The invention relates to a centrifugal pump with an axial diffusor for a treating liquid, and more particularly to an improvement in an axial diffusor with a plurality or blades having such an incident angle as to prevent revolving and stall to appear at an inlet portion of the diffusor. The structure of a conventional centrifugal pump with the axial diffusor will be described with reference to FIG. 1. In general, the centrifugal pump is provided with an impeller 10, an axial diffusor 12 with a plurality of blades 16. The impeller 10 is engaged with a rotation shaft 22 through a key 11 so as to make a rotation around a rotation axis together with the rotation shaft 22 where a treating liquid 14 exhibits rise of its flow rate. In contrast, the axial diffusor is so secured through a slide bearing 19 to the rotation shaft 22 that the axial diffusor 12 with the plural blades 16 does not rotate around the rotation axis. The plural blade 16 is placed at its one side on a side portion of the axial diffusor. The plural blade 16 is placed at is opposite side on an inner wall of a casing 20. The treating liquid 14 is subjected to a flow straightening by the blade 16 of the axial diffusor 12 where the treating liquid exhibits a reduction of the flow rate and a rise of its pressure. Such axial diffusor pump is necessarily engaged with the appearance of the following undesirable phenomenon in the flow of the treating liquid. When the treating liquid flows through the blade 16 of the axial diffusor 12 and thus is subjected to the flow rate reduction and the pressure rising, the following undesirable phenomenon occurs. When the discharge flow rate is in a high flow rate range, the effects of the flow rate reduction and the pressure rising are normally exhibited by the blades 16 of the axial diffusor 12. However, when the discharge flow rate is in a low flow rate range, the above normal effects of the flow rate reduction and the pressure rising are not exhibited. In replacement of the normal phenomenon, the treating liquid exhibits a revolving and a stall at the inlet portion of the axial diffusor 12. Such revolving and stall phenomenon cases a violent axial vibration which makes a life-time of the slide bearing 19 shortened. FIG. 2 illustrates a head-discharge curve. Further, this makes the flow rate and head property inferior, and thereby the pump efficiency is lowered. The revolting and the stall at the inlet portion of the axial diffusor 12 also cause hunting which has a tendency of increase thereof. Actually, an available flow rate range where the pump is operational receives a restriction by generation of the revolving and the stall of the treating liquid. From GB-A-604 121 it is known to provide a centrifugal pump and an axial diffusor with a certain angle. According to this document, the treating liquid shall enter the diffusor through a square area and shall be guided along the blades to allow straightening of the flow. According to this solution, the passage between the diffusor blades is made so that width and depths increase at a 7° taper angle. However, this centrifugal pump includes the generation of recirculation and stall of the treated liquid. Accordingly, it is an object of the present invention to provide a centrifugal pump with an improved axial diffusor with blades having such an incident angle as to prevent revolving and stall to appear at an inlet portion of the axial diffusor. The above and other objects, features and advantages of the present invention will be apparent from the following description. The present invention provides an axial diffusor with blades which has such an incident angle as to prevent the revolving and stall to appear at the inlet portion of the axial diffusor in which a subtractive value by an incident angle of the treating liquid from the blade incident angle is in the range form -1° to 8° and the blade incident angle is in the range from 3° to 10°. In the present invention, the centrifugal pump with the axial diffusor with the improved blades having the incident angle of 3° to 10° is so operated that the subtractive value by the incident angle of the treating liquid from the blade incident angle is in the range form -3° to 8°. In the axial diffusor centrifugal pump, the incident angle of the blade of the axial diffusor is so variably determined as to prevent the revolving and stall to appear at the inlet portion of the axial diffusor. Preferred embodiments of the present invention will hereinafter fully be described in detail with reference to the accompanying drawings. FIG. 1 is a fragmentary cross sectional elevation view illustrative of the axial diffusor with blades and the impeller in the centrifugal pump in the prior art. FIG. 2 is a diagram illustrative of the head-discharge curve of the head coefficient versus the flow rate coefficient. FIG. 3 is a fragmentary perspective view illustrative of an improved axial diffusor with blades and an impeller in a novel centrifugal pump of a first embodiment according to the present invention. FIG. 4 is a fragmentary cross sectional elevation view illustrative of an inlet portion of blades of an improved axial diffusor in a novel centrifugal pump of a first embodiment according to the present invention. FIG. 5 is a diagram illustrative of a blade property of pressure variation versus a stagger angle. FIG. 6 is a diagram illustrative of a property of a blade incident angle versus an incident angle of the treating liquid. A preferred embodiment according to the present invention will be described. The present invention provides a novel centrifugal pump with an improved axial diffusor with blades, which has the same structure as that of the prior art illustrated in FIG. 1, except for a specific angle of the blade of the axial diffusor. The structure of the centrifugal pump with the axial diffusor will thus be described with reference to FIG. 3. The centrifugal pump has an impeller 10 and an axial diffusor 12. The axial diffusor 12 comprises a body 18, a plurality of blades 16 placed on a side portion of the diffusor body 18 and a casing 20 accommodating both the impeller 10 and the axial diffusor 12. The impeller 10 is engaged with a rotation shaft 22 through a key so as to exhibit a rotation together with the rotation shaft 22. The impeller 10 has an opening serving as an inlet port for the treating liquid at its top portion. The treating liquid is introduced through the opening of the impeller 10 into the impeller 10 for receipt of a flow rate rising. Subsequently, the treating liquid is introduced into the axial diffusor 12 so as to flow through the blades 16 arranged on the side portion of the axial diffusor 12 where the treating liquid receives the flow straightening from the blades 16 thereby the pressure rising and the flow rate reduction of the treating liquid appear. The blades 16 of the axial diffusor 12 has a specific incident angle β G which will be described in detail with reference to FIG. 4 illustrative of first and second blades 16 and 16' adjacent to each other. The incident angle β G of the blades 16 and 16' is defined by an included angle formed by a line L a and a line L b where the line L a includes both a center point P of an inscribed circle O between the first and second blades 16 and 16' and an intersection point X of tangential lines L 1 and L 2 of the first and second blades 16 and 16'. An incident angle α G of the treating liquid is defined by both a peripheral velocity V uG of the inlet portion 16a of the blade 16 and a meridian velocity V mG of the treating liquid at the inlet portion 16a of the first blade 16 where the meridian velocity V mG has been subjected to a compensation concerned with a thickness of the blade 16 or 16'. The treating liquid incident angle α G is given by the following equation. α G = tan -1 (V mG /V uG ) A stagger angle  defined by the difference (β G - α G ) between the blade incident angle β G and the treating liquid incident angle α G is so determined as to be equal to or below a specific angle which is approximately 7° to 8° thereby the revolving of the treating liquid is prevented. Preferably, the specific angle is approximately 7° when the treating liquid is water. When the treating liquid is a liquefied gas, the specific angle is suitably approximately 8°. Results of examination and measurement concerned with the flow of the treating liquid at the inlet portion 16a of the blades 16 of the axial diffusor 12 will be described with reference to FIG. 5 and TABLE 1. The examination has been conducted under the following conditions which are represented in TABLE 1. DIFFUSOR d G (mm) b G (mm) Z G β G (°) D-1 15.4 17.1 4 4.0 D-3 16.1 17.1 4 6.5 D-5 21.0 17.1 4 8.2 D-6 24.5 17.1 4 8.9 D-7 27.9 17.1 4 9.5 A-1 17.7 17.1 8 14.0 O-1 15.7 17.1 8 9.5 E-1 13.8 14.0 8 7.7 E-2 14.3 14.0 8 9.3 d G : a diameter of the inscribed circle b G : a height of the blades Z G : the number of blades β G : the incident angle of blades The examination in case of each of the above nine types of the axial diffusors was conducted in a water testing machine. FIG. 5 illustrates a pressure variation curve 24 of pressure variation ΔP versus the stagger angle  as the results of the examination. The pressure variation curve 24 teaches that the pressure variation ΔP is rapidly increased at a stagger angle  of approximately 7°. This identifies the revolving and stall of the treating liquid appear at the stagger angle  of approximately 7°. On the other hand, in case of the liquefied gas the pressure variation ΔP is rapidly increased at a stagger angle  of approximately 8°, although the illustration of the pressure variation curve in this case is omitted. This identifies that the revolving and stall of the treating liquid appear at the stagger angle  of approximately 8°. From the above results of the examinations, it is cleared that the appearance of the revolving and stall of the treating liquid is in association with the stagger angle . The stagger angle depends upon a kind of the treating liquids. Thus, the stagger angle is so determined as to prevent the appearance of the rapid increase of the pressure variation thereby it is possible to suppress the appearance of the revolving and stall of the treating liquid. This makes the available flow rate range wide. In addition, since the incident angle α G of the treating liquid is a function of the discharge flow rate of the treating liquid of the pump, the measurement of the incident angle property of the blade 16 in the normal axial diffusor permits an expectation of a critical flow rate at which the revolving and stall of the treating liquid commences appearance. FIG. 5 expresses a relation between the blade incident angle β G and the treating liquid incident angle α G . When values of both the treating liquid incident angle α G and the blade incident angle β G take a ponit below lines labeled by β G = α G + 8° , and β G = α G + 7° , neither the revolving nor stall of the treating liquid appears. In contrast, when the values of both the treating liquid incident angle α G and the blade incident angle β G take a ponit above the above lines, the revolving and stall of the treating liquid appear. When the blade incident angle β G and the treating liquid incident angle α G are respectively 15° and 10° as illustrated by a ponit P, the revolving and the stall do not appear so that the axial diffusor is able to exhibit the normal performance. When the treating liquid incident angle α G is lowered by closing a discharge valve and then approaches an angle range below 8°, the tendency of the revolving and stall of the treating liquid commences appears. It is preferable that the centrifugal pump with the axial diffusor 12 having the blade incident angle β G of 3° to 8° is driven at such a treating liquid incident velocity that the stagger angle  takes -3° to 8°. Further, it is suitable that when the treating liquid incident angle α G is in the range from 0.1° to 2°, the centrifugal pump is so driven that the stagger angle  takes -1° to 8°. The blade incident angle β G and the thickness, height and the number of blades are variably so determined as to match various conditions.

Claims: Axial diffusor (12) for a centrifugal pump comprising a body (18) having a side portion and a plurality of blades (16), characterized in that a blade incident angle (β G ) is in the range from 3° to 10° so that, in use, a value defined by subtracting a treating liquid incident angle (α G ) from said blade incident angle (β G ) is in range from -1° to 8°.

Inventors: OGAWA MOTOYASU, OGAWA, MOTOYASU, OGAWA, MOTOYASU C/O NIKKISO CO., LTD.
Applicants: NIKKISO CO LTD, NIKKISO CO., LTD.
Application year: 1993
Main classification: F04D29
IPC codes: F04D29, F04D1