Page 156 - Centrifugal Pumps Design and Application

134 Centrifugal Pumps: Design and Application

The Column Assembly

The column assembly consists of three primary components:

• The outer column, which serves as the conduit and pressure boundary
for the flow from the bowl assembly.
• The column shaft, or line shaft, which transmits torque from the driver
to the impellers on the pump shaft and carries the hydraulic thrust from
the bowl assembly to the thrust bearing in the head/driver assembly.
• The shaft enclosing tube, or inner column, which houses the column
bearings, serves as a conduit for bearing lubrication, and protects the
shafting. The liquid pumped determines whether or not a shaft enclos-
ing tube is required.

Outer Column

The simplest outer column construction consists of pipe sections, nor-
mally of 10-feet length, with straight thread on both ends, and joined
with pipe couplings. This design is commonly used for 12-inch column
diameters or less. For handling relatively clean liquids, bearings of a rub-
ber compound are located in housings with a three-legged or four-legged
spider and a mounting ring, which is centered within the column coupling
and clamped between the column pipe ends. Metal to metal contact pro-
vides an adequate liquid seal. This configuration is often referred to as
open tineshaft construction,
For larger column sizes, or where corrosive or other properties of the
pumped liquid make threaded joints undesirable, flanged column joints
are used. Registered fits are used to provide alignment, with "O"-ring
gaskets for sealing because they provide metal-to-metal flange face con-
tact for alignment. Bearings housed in spiders can be clamped between
column faces; however, superior alignment is provided with a design in-
corporating spiders welded into the outer column, with the flange regis-
ter and spider bore machined in the same operation. When a shaft enclos-
ing tube is required, the larger column sizes require a metal stabilizing
spider clamped or welded at the column joint with a snug, machined fit
around the enclosing tube. Again a tensioning device is required at the
top end of the threaded enclosing tube,

Column Shaft
Shaft sections with three-inch shaft diameters or less are commonly
joined by threaded couplings, which transmit both torque and axial
thrust. For pumps with this construction, it is imperative that drivers be