Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Chemical Pumps Metallic and Nonmetallic 29S Figure 15-10A. Casing volute with centerline discharge nozzle. Figure 15-1 OB. Casing volute with tangential discharge nozzle.
296 Centrifugal Pumps: Design and Application This makes it easier to manufacture and eliminates the problem of the angle machining. This usually results in the back wall of the impeller being cast on an angle. Studies have been made comparing angles of 0° to 8° in the forward position relative to the back position to determine if there was any difference in performance, efficiency, and NPSH. The investigation showed no difference in the various performance criteria. Volute The design of the volute for hydraulics depends upon the stiffness of the shaft system. Most of the systems can absorb the radial thrust developed by a single volute. When the bearing loading becomes excessive, a fall double volute can be employed. The radial thrust developed by the double volute is about 16% that of a single volute resulting in longer bearing life. A partial splitter, which is between a single and double volute, reduces the thrust to approximately 33% of the single volute and results in bearing life and deflection within the parameters of ANSI specifications. The length of the splitter is dependent on the specific speed. A partial splitter is easier to cast because its core support is not as long and is easier to remove after pouring. On specific speeds of 500 or less, circular volutes are sometimes employed. Typical volute designs and the method used to calculate radial load are described in Chapter 5. Gasketing The gasketing is usually a flat semi-confined design or an Oring design. Flat gaskets take more bolting because the bolt load must compress the gasket as well as resist the hydraulic force. With the O-ring only the hydraulic force acts at the centerline of the O-ring. Fiat Gasket The flat uncompressed gasket is l /32 inch to Vie inch in thickness and has 27 % compression due to the bolt force. For many years it was made from asbestos which was universal for most chemicals; however, with the changeover to nitrile synthetic material, two different types of base materials are required to handle the spectrum to which chemical pumps are applied. The new material has the same compression rate and hardness as the asbestos materials. O-Rings O-rings can be fitted radially, axially, or in a corner. Radial O-rings require more control of the machining of the concentricity of the casing
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296 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
This makes it easier to manufacture <strong>and</strong> eliminates the problem of the<br />
angle machining. This usually results in the back wall of the impeller being<br />
cast on an angle. Studies have been made comparing angles of 0° to<br />
8° in the forward position relative to the back position to determine if<br />
there was any difference in performance, efficiency, <strong>and</strong> NPSH. The investigation<br />
show<strong>ed</strong> no difference in the various performance criteria.<br />
Volute<br />
The design of the volute for hydraulics depends upon the stiffness of<br />
the shaft system. Most of the systems can absorb the radial thrust develop<strong>ed</strong><br />
by a single volute. When the bearing loading becomes excessive, a<br />
fall double volute can be employ<strong>ed</strong>. The radial thrust develop<strong>ed</strong> by the<br />
double volute is about 16% that of a single volute resulting in longer<br />
bearing life. A partial splitter, which is between a single <strong>and</strong> double volute,<br />
r<strong>ed</strong>uces the thrust to approximately 33% of the single volute <strong>and</strong><br />
results in bearing life <strong>and</strong> deflection within the parameters of ANSI specifications.<br />
The length of the splitter is dependent on the specific spe<strong>ed</strong>. A<br />
partial splitter is easier to cast because its core support is not as long <strong>and</strong><br />
is easier to remove after pouring. On specific spe<strong>ed</strong>s of 500 or less, circular<br />
volutes are sometimes employ<strong>ed</strong>. Typical volute designs <strong>and</strong> the<br />
method us<strong>ed</strong> to calculate radial load are describ<strong>ed</strong> in Chapter 5.<br />
Gasketing<br />
The gasketing is usually a flat semi-confin<strong>ed</strong> design or an Oring design.<br />
Flat gaskets take more bolting because the bolt load must compress<br />
the gasket as well as resist the hydraulic force. With the O-ring only the<br />
hydraulic force acts at the centerline of the O-ring.<br />
Fiat Gasket<br />
The flat uncompress<strong>ed</strong> gasket is l /32 inch to Vie inch in thickness <strong>and</strong><br />
has 27 % compression due to the bolt force. For many years it was made<br />
from asbestos which was universal for most chemicals; however, with<br />
the changeover to nitrile synthetic material, two different types of base<br />
materials are requir<strong>ed</strong> to h<strong>and</strong>le the spectrum to which chemical pumps<br />
are appli<strong>ed</strong>. The new material has the same compression rate <strong>and</strong> hardness<br />
as the asbestos materials.<br />
O-Rings<br />
O-rings can be fitt<strong>ed</strong> radially, axially, or in a corner. Radial O-rings<br />
require more control of the machining of the concentricity of the casing