15 August 2002
Periphery of Pit Opening. In dewatering a drydock, one or more of the
main pumps may be shut down as the water level approaches the dock floor, in order to
prevent loss of pump suction. As the water level continues to recede it generally
becomes necessary to throttle the discharge of the last operating pump. This condition
occurs when the pump capacity exceeds the quantity of water flow reaching the pump
suction from the dock chamber through the pump suction pit and conduits. The
elevation of the pumps with reference to the dock floor does not contribute to this
condition. To delay the time of shutdown and throttling, design the dock floor suction
inlet to have as large a perimeter as practicable.
Area of Pit Opening. The suction pit opening (free area) should be of
sufficient size to result in a flow velocity in the range of 1.07 to 1.37 m/s (3-1/2 to 4-1/2
ft/s). Base the flow on the pumping rate when the water level is 0.6 m (2 ft) above the
dock floor and discharging against mean high tide. The suction condition may be
greatly improved by providing openings on the opposite side of the dock chamber
connected by conduits under the dock floor.
Use of Sidewall Culverts
a sidewall culvert drainage system
used to facilitate removal of low-level water from the dock floor, it should drain to and
terminate in the main dewatering pump suction chamber.
Shape of Pump Suction Works. Design the configuration of a pump
suction pit and the conduit leading to the pump suction chamber, or suction bell, so that
the flow will have a constant or uniformly accelerated velocity. The surface of the
passages should be smooth and of such shape as not to produce eddies. There should
be no sharp turns or abrupt changes in a conduit section. It may be necessary to install
stream guide vanes in the suction chamber to effect good distribution and flow to the
pump suction bells.
Arrangement of Suction Bells
Design for Velocity. Where several pump suction bells draw from a
common chamber, the flow in the region of the bellmouth should be free from high
velocities and changes in direction that tend to cause vortices.
The velocity of approach should be in the range of 0.61 to 0.91 m/s (2 to 3
The designed bellmouth velocity should be in the range of 1.22 to 1.52
m/s (4 to 5 ft/s).
The water depth below the bellmouth should be approximately one-half
the bellmouth diameter.
Laterally there should be no obstruction to flow within one diameter of the
centerline of the bell.