Page 331 - Centrifugal Pumps 2E

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 be-
ing 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 in-
vestigation 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 devel-
oped 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 vo-
lute, reduces the thrust to approximately 33% of the single volute and
results in bearing life and deflection within the parameters of ANSI spec-
ifications. 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, cir-
cular 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 de-
sign. 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
l
The flat uncompressed gasket is /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 hard-
ness 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