Page 147 - Introduction to Marine Engineering

Pumps and pumping systems 133

closed off. Where tanks are arranged for either oil or ballast a
change-over chest must be Fitted in the pipeline so that only the ballast
main or the oil transfer main is connected to the tank.

Domestic water systems

Domestic water systems usually comprise a fresh water system for
washing and drinking and a salt water system for sanitary purposes
(Figure 6.20). Both use a basically similar arrangement of an automatic
pump supplying the liquid to a tank which is pressurised by compressed
air. The compressed air provides the head or pressure to supply the
water when required. The pump is started automatically by a pressure
switch which operates as the water level falls to a predetermined level.
The fresh water system has, in addition, a calorifier or heater which is
heated, usually with steam.
Fresh water supplied for drinking and culinary purposes must meet
purity stndards specified by the Department of Transport. Water
produced from most evaporator/distillers will not meet these standards
and must be treated to ensure it is biologically pure and neutral or
slightly alkaline.
A treatment plant suitable for a general cargo ship is shown in Figure
6.21. The water is sterilised by an excess dose of chlorine provided as
hypochlorite tablets. It is then dechlorinated in a bed of activated
carbon to remove the excess chlorine. Any colour, taste and odour which
was present in the water will also be removed by the carbon. Excess
chlorine is originally applied to ensure that complete sterilisation
occurs,