15 August 2002
Wet pit pumps may be similar to main dewatering pumps of the axial
Dry pit pumps may be a mixed flow or centrifugal unit.
Axial discharge pumps are the least expensive in that no suction piping
and only a relatively small amount of discharge piping is required.
Head and Motor. The required pumping head of drainage pumps is
higher than that for the main dewatering pumps and is not so variable. Motors are
generally located on the dewatering pump motor drive floor.
Environmental Requirements. Local regulatory authorities may have
specific requirements for drainage pumping discharges. Refer to local regulations.
Sump Pumps. Sump pumps, vertical drive wet pit (submerged)
centrifugal type, should have a capacity of 283.88 L/min (75 gpm) each, and should be
installed in duplicate.
Pump Drives. Use vertical motors directly connected by line shafts to
pump impellers. Main dewatering pumps generally are of the dry pit type. Install the
motors and electrical switchgear on a floor only sufficiently below the top of the dock to
allow for equipment and operating headroom. Place drainage pump motors and the
drive mechanism for control of the main valves on the motor drive room floor. Drainage
pump motors may be at a lower elevation to shorten the shafts, as long as they are
above the possibility of being flooded due to a flooding pumpwell, or are submersible.
Speed. Speeds of dewatering pump motors are dictated by the specific
speed of the pump impellers; generally, low speed motors are required.
Driving Shaft. The entire weight of revolving parts of the vertical pumping
unit should be carried by a thrust bearing in the base of the motor at the top of the shaft.
Specific requirements include the following:
. Use self-aligning thrust bearings.
. Guide bearings must
be of sufficient strength and
adequate design to prevent vibration for lateral deflection of the vertical shaft. Use self-
aligning and adjustable; the bearing surfaces may be of bronze or babbitt.
Lubrication. Provide adequate lubrication. Generally, use water for this
purpose where the bearings are submerged in the pump discharge stream. Provide
clean, cold, fresh water of ample volume and pressure.
Elevation of Pumps. Determine pump elevation by the vertical position
of the dewatering pump impeller centerline. For vertical type centrifugal or mixed flow
dewatering pumps, refer to the centerline of the pump discharge. For vertical mixed