Page 153 - Centrifugal Pumps Design and Application

Vertical Pumps 131

mally insulate with jackets or in a "cold box." A thermal barrier, also known
as a "warming box," with a throttle bushing and a double mechanical seal or
a gas shaft seal, is located in the discharge head, just above the discharge
nozzle. Here the cryogenic liquid is flashed and bled back to the suction
source, while an inert gas blanket under the shaft seal prevents leakage to
atmosphere. Depending on the liquid pumped, pump materials with ade-
quate impact strength are bronzes, aluminum, and austenitic stainless steels.
Materials with similar thermal coefficient of expansion must be used where
tolerances are critical.

Loading Pumps

Normally installed immediately adjacent to large storage tanks used for
loading product into pipelines or transport vessels, these pumps are
mounted so that the storage tank can be emptied, even when the available
NPSH becomes zero at the bottom of the tank. Adequate provisions must
be made for venting the suction barrel and providing a minimum flow
bypass when applicable.

Pipeline Booster Pumps

These pumps typically operate unattended and must be designed for relia-
bility. The discharge and suction nozzle should both be located in the dis-
charge head for simplicity in piping and valve placement. When handling
liquids where leakage to the atmosphere is hazardous, a tandem or double
mechanical shaft seal with a buffer fluid should be provided for the stuffing
box, Pumps can be arranged to operate singly, in parallel, or in series.

Design Features

For comparison and evaluation of design features, the three basic as-
semblies of vertical pumps should be addressed, namely the bowl assem-
bly, the column assembly, and the head assembly. (See Figure 9-14).

The Bowl Assembly

The simplest bowl assembly configuration consists of a straight shaft
with taper collet mounted impellers, bowls that are joined together with
straight threads and furnished with a shrink fitted bearing (Figure 9-2).
The impellers can be either of the enclosed design, with both a front and
a back shroud, or the semi-open design without a front shroud (see Fig-
ure 9-15). The bottom case bearing is normally permanently grease lu-