Page 147 - Centrifugal Pumps Design and Application

Vertical Pumps 125

Wet Pit Pumps

The most common applications for wet pit pumps are:

• Water supply pumps for municipalities and industry. The pumps are
mounted in intake structures on lakes or rivers.
» Condenser cooling water pumps for central power plants. The pumps
take suction from a natural body of either fresh or salt water.
• Cooling tower pumps. Take suction from a cooling tower basin and cir-
culate water through a closed system.
» Flood control pumps mounted at dams and in collection basins, often as
part of large flood control systems.
« Transfer pumps for central irrigation districts and water treatment facilities,

Water Supply Pumps

This pump type is normally installed as multiple, parallel operating
units in a simple intake structure or as a stand-alone pumping plant lo-
cated on a reservoir, lake, or river and discharging into a pipeline or an
open canal. Depending on the system requirements, multi-stage pumps
or single-stage pumps of the desired specific speed are used. A combina-
tion of fixed and variable speed drivers may be desirable to obtain opti-
mum system efficiency. While structural integrity and cost are critical
items in design of intake structures, hydraulic considerations and protec-
tion of the pumping equipment are equally important. The structure
should be physically located so that a minimum of debris and silt will be
diverted toward it. Trash racks and rotating screens, which can routinely
be cleaned, must be provided to keep foreign objects from entering the
pumps. The intake structure must be designed for low approach velocity
and with dividing walls forming individual bays as required [5]. The
pumps should be located within the structure in such a fashion that uni-
form velocity distribution is provided at each pump suction bell. Ob-
structions, changes in flow direction, or velocity changes that may cause
formation of vortices and air entrainment must be avoided. Flow patterns
within the structure, when one or more pumps are idle, must also be con-
sidered. Pump settings, the distance from the mounting floor to the suc-
tion intake, typically vary from 15 feet to 80 feet. The discharge may be
located above or below the mounting floor, depending on the system re-
quirement. In either case, the discharge pipe should be anchored down-
stream from the pump discharge flange to prevent pump misalignment
from pipe reaction forces. When a flexible discharge piping connector is
used, tie bars must be provided across the connector to restrain the hy-
draulic separating forces and prevent pump misalignment. Figure 9-11