Salt water system
Function
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Hot Domestic service
Cold Domestic service
Fire main
Sprinkler system
Deck Wash
Ballast system
Engine and Equipment cooling
Special services (for example Watermaking etc)
“The word
manifold may
come from the
Old English
word
manigfeald
(from the
Anglo-Saxon
manig [many]
and feald [fold])
and refers to
the folding
together of
multiple
inputs and
outputs.”
Rule 1
In each section the water is drawn from a
skin fitting placed on the hull
• as low as possible
• in a position which will not affect or be
effected by any other intake or outlet
Rule 2
• skin fitting with a built in “Screw Lift Valve”
to simplify installation
• If it is not possible to fit a combined Sea
cock and skin fitting then a screw lift valve
should be placed in the suction line as
near to the skin fitting as possible
Often a manifold system is adopted
• Individual line valves are fitted for system
isolation ideally as close as possible to the
Suction Manifolds
• sometimes additional local valves may be fitted
adjacent to a particular piece of equipment for
ease of operation
• The supply valves, if possible, should be quick
action types (1/4 turn full on to fully off) although
due to the pressure involved with larger systems
they may well have to be conventional “screw
down” types.
Sea Water Intakes
• With sea water systems the main problem
is usually air entrainment which creates a
“false cavitation” in the pump with its
consequential corrosion problems
• This problem is created by the
design/placement of the intake
• The problem is also increased with a
vessel’s speed and also on vessels which
do a large amount of astern running
All intake boxes should be
designed to assist the de—aeration
of the water and remove
turbulence.
• Upward buoyancy of small air bubbles is
not great and, therefore, in most intake
boxes, the bubbles have only sufficient
time to travel a few inches in a vertical
direction.
• With fast liquid velocities the bubbles may
even be carried downwards
• The intake grid should be of longitudinal
slots and not holes or vertical slots which
are sometimes installed in error
• Intake grid of holes - much of the area is lost, due to
turbulence. If horizontal slots had been used only a small area
loss would have occurred, due to the turbulence at each end
of the slots only;
• The flow is upwards to
the suction pipe and all
the air travels into the
system.
• The intake grid is in the middle of the intake box, and so is the
suction pipe. Due to the forward motion, astern conditions are
sacrificial, the general liquid direction is aft/inboard, and it
therefore has to swirl forward to enter the suction pipe
incurring losses.
• Relative position of grill
suction pipe;
• Longitudinal baffle and
submerged suction pipe
to create rise and fall in
flow to assist de—
aeration;
• Box top—angled past
longitudinal baffle to
avoid flow picking up any
separate air;
• De—aeration pipe is in a
non—flow area;
• Steady baffle—difficult to
place correctly but
worthwhile and should
face general flow
direction.
Scoop Intakes
• There is usually less choice in where a scoop
intake can be positioned compared with a
normal intake but, again, the position should be
such that the entry of aeration from beneath the
vessel must be minimized
• Since the effectiveness of the scoop increases
rapidly with the speed of the vessel (scoop head
varies as the square of the vessel’s speed), the
use of a main circulating pump is still necessary
for low speed manoeuvring.
Salt Water Flow Rates
The choice of water speed in the system will
depend upon
• the main circulating pump characteristics
• the materials to be chosen for the
pipework
Salt Water Flow Rates
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Owing to the severe effect of
corrosion/erosion which sea water has on
materials the following maximum design
water speeds are given for guidance:
Galvanized steel
3m/sec
Copper
1m/sec
Aluminium brass
3m/sec
90/10 Copper—nickel 3.5m/sec
70/30 Copper—nickel 4m/sec
Q
V=----S
Strainers
• The simplest strainer consists of a box
with a removable lid in which a flat
perforated,
• Perforation sizes vary according to duty
and manufacture but are invariably in the
range 3—12 mm.
• Such strainers will be found close to sea
water suction valves and immediately
before bilge valves (strum boxes)
basket strainers
• These consist of a cylindrical container in which a
perforated metal or wire basket is suspended
Single strainer for high pressure water
Or oil service
Single strainer for low pressure water
or oil service