Page 310 - Ship Construction.DJ Eyres 6Ed

Ch26-H8070.fm Page 299 Wednesday, October 18, 2006 7:01 AM
Pumping and Piping Arrangements 299
is effectively reduced. The shaft tunnel of the ship is drained by means of a
well located at the after end, and the bilge suction is taken from the main
bilge line (see Figure 26.1).
At the open ends of bilge suctions in holds and other compartments,
outside the machinery space and shaft tunnel, a strum box is provided. The
strum box is a perforated plate box welded to the mouth of the bilge line
(Figure 26.1) which prevents debris being taken up by the bilge pump
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suction. Perforations in the strum box do not exceed 10 mm in diameter,
and their total cross-sectional area is at least twice that required for the
bore of the bilge pipe. Strums are arranged at a reasonable height above
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the bottom of the bilge or drain well to allow a clear flow of water and to
permit easy cleaning. In the machinery space and shaft tunnel the pipe from
the bilges is led to the mud box which is accessible for regular cleaning. Each
mud box contains a mesh to collect sludge and foreign objects entering the
end of the pipe.

BILGE PUMPS AND PIPE SYSTEMS Cargo ships have at least two
power driven bilge pumping units in the machinery space connected to the
main bilge line, and passenger ships have at least three.
In passenger ships the power driven bilge pumps are where practicable
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placed in separate watertight compartments, so that all three are not
easily flooded by the same damage. Where the passenger ship has a length
in excess of 91.5 m it is a requirement that at least one of these pumps will
always be serviceable in reasonable damage situations. A submersible pump
may be supplied with its source of power above the bulkhead deck. Altern-
atively the pumps are so distributed throughout the length of the ship that it
is inconceivable that one might not be able to work in the event of reason-
able damage.
Suction connections are led to each hold or compartment from the main
bilge line. Valves are introduced to prevent one watertight compartment
from being placed in direct communication with another, and to prevent
dry cargo spaces and machinery spaces being placed in direct communica-
tion with tanks or pumps having sea inlets. These screw-down non-return
valves are often provided in a bilge valve distribution chest, or may be fitted
directly in the connections to the bilge main. The bilge pipes which are used
to drain cargo and machinery spaces are kept separate from the sea inlet
pipes and ballast pipes which are used for filling or emptying tanks where
the water and oil are carried. Often a separate ‘dirty ballast’ system is arranged
to overcome this problem.
If possible the bilge pipes are kept out of the double bottom tanks, and in
way of a deep tank are led through a pipe tunnel. If the peaks are used as
tanks then a power pump suction is led to each peak. Only two pipes are
permitted to pass through the collision bulkhead below the bulkhead deck
and a screw-down valve operated from above the bulkhead deck is provided