Page 325 - Centrifugal Pumps 2E

290 Centrifugal Pumps: Design and Application

The impellers employed by most manufacturers are semi-open even
though the specifications allow for both semi-open and closed impellers.
The specifications call for the wetted-end materials to be manufactured
from alloy steels, carbon steel, ductile iron, or cast iron. However, most
manufacturers stock ASTM A744 (similar to Type 316) or ASME A395
(cast ductile iron).
ANSI B.73.1M is for horizontal cradle pumps. Most pump manufac-
turers divide the 19 pump sizes into three groups. Many of the parts are
interchangeable.
ANSI B.73.2M covers the same hydraulic range up to the 6 in. suc-
tion x 4 in. discharge nozzle size and consists of a total of 15 pump sizes.
The vertical in-line pumps are designed so a user can obtain access to the
impeller and stuffing box area without disassembly of the pump from the
line nor disassembly of the motor. This is accomplished by use of a rigid
coupling and/or a separate bearing housing that fits between the casing
and the motor,
These pumps have the same deflection and total indicator runouts as
the horizontal pumps. When using a rigid coupling, these pumps employ
a P-face motor. This motor is called an in-line motor (NEMA MGI-
18.620). It was developed in conjunction with NEMA and employs dou-
ble-row or back-to-back deep groove thrust bearings to absorb the radial
and axial thrust developed by the pump. The construction of this motor is
such that for a radial load of 25 Ibs. at the end of the motor shaft the
radial deflection shall be no more than .001 in. with an axial load of 50
lbs,» the shaft movement is limited to .0015 in.
When a separate bearing housing design is employed for the vertical
pumps, a C-face motor is used (Figure 15-8). Because the hydraulic
thrust is absorbed by the bearing housing's bearings, the standard C-face
motors do not require special thrust bearings. A flexible coupling is used
between the bearing housing and the motor. The advantage of the bearing
housing design on the larger size impellers (over 10 in.) and the larger
size motors (used on 3 in. discharge nozzle and above) is that the pump
shaft system is more rigid and deflection is less because of the smaller
overhang. A disadvantage of the design is the problem of removing the
extra weight of the assembly from the casing and out of the support head
area without causing damage to the parts being removed.

General Construction
Impeller
Semi-open impellers develop higher axial thrust loads than do closed
impellers. However, with chemical pumps, the semi-open type impellers
are normally employed to facilitate cleaning of fibers or particles often
contained in the process liquid.