2-3 Drinking water production. This section provides a functional description of how each ROWPU produces drinking
water from saltwater. While each unit normally operates independently, the two systems contain several crossover points
to provide backup during periodic maintenance procedures. These are described in Chapter 13.
2-3.1 An electrically powered seawater pump (Figure 2-2) draws seawater from a seachest mounted in the barge bottom
and through a duplex seawater strainer. The strainer (first of four strainer or filtering steps) removes large foreign
particles from the seawater. The seawater pump, in void 2 port, discharges the seawater through a seawater filter
(commercially referred to as a separator) which removes additional particles from the seawater. The seawater,
pressurized up to 150 psi, then flows to the pretreatment skid in the ROWPU space.
2-3.2 As seawater flows into the pretreatment skid (Figure 2-3), its quality and flow rate are shown on a flow rate indicator
on top of the pretreatment skid. In addition, the temperature of the incoming seawater is shown on a temperature gauge
mounted aft of the flow rate indicator. As seawater enters the pretreatment skid, coagulant (Hydrapol-50) is added by a
chemical metering pump. This chemical assists the media filters in removing fine particles and colloids (clouds of fine
particles suspended in water). Normal dosage is 4 parts per million (ppm). Dosage can be increased, however, if a
seawater sample contains above average impurities.
2-3.3 Seawater containing coagulant then flows through three-way valves to three media filters. In these filters, seawater
flows from the top downward through five different layers of filtering media where fine particles and colloids are removed
from the seawater so it is suitable for processing by the RO block assembly.
2-3.4 Seawater discharged from media filters is collected into a single stream where scale inhibitor (Hydrapol-1 00) is
added. A small chemical metering pump, similar to the one used to add Hydrapol-50, normally adds 4.0 ppm of inhibitor.
This inhibitor limits formation of scale on the RO pressure tube membranes.
2-3.5 Seawater, with scale inhibitor (Hydrapol-100), then flows through the pretreatment skid cartridge filter assembly. At
this stage the water is still being pumped through the system by the electrically powered seawater pumps in void 2
starboard. The cartridge filter assembly removes any foreign matter such as oil, soap, and detergents that would be
harmful to the RO tube membranes and the diesel-driven HP pump.
2-3.6 Seawater next flows to the diesel-driven HP water pump where water pressure is increased to the 835 psi
(maximum) required for reverse osmosis processing in the RO block.
2-3.7 The pressurized seawater enters the RO block inlet manifold which divides the flow among the 16 pressure tubes.
Each pressure tube separates seawater by reverse osmosis into a high purity product water stream and a brine
concentrate stream. Product water from each pressure tube flows to a common manifold. It then flows through a flow
meter (Figures 2-4 and 2-5), maximum output 108 gpm, and either into the drinking water storage tanks or overboard.
2-3.8 The chlorination unit in void 2 port produces chlorine and adds it to the drinking water just before it flows into the
storage tanks. One chlorination unit serves both ROWPU's.
2-3.9 Brine from each RO pressure tube also flows to a common manifold and then through a throttling valve and
pressure gauge. This routing allows pressure and flow rate adjustment. Brine is then discharged directly overboard
through the void 5 port shell.
2-3.10 An electrically powered discharge pump takes drinking water from storage tanks and pumps it through a flexible
discharge hose to a water distribution point ashore. Drinking water can also be discharged to other vessels through
piping on the port side.