The growth of marine micro-organisms in the ship’s sea water piping is prevented by the injection of chlorine at a controlled rate. The chlorine is injected into the sea suction chests and is circulated throughout the sea water system by the operating pumps.
Chlorine is manufactured in the Marine Growth Prevention System (MGPS) by the electrolysis of the salt (sodium chloride) in the sea water. A number of different chlorine compounds are formed during the electrolysis but they all have the effect of preventing the growth of marine micro-organisms. A supply of sea water to the MGPS unit is provided through a flow meter from the auxiliary cooling sea water pump discharge line.
Sea water is supplied to the reaction tank at a rate of 6.0-8.0m3/h. The electrolysis cell consists of two copper electrodes and two aluminum electrodes . An electric current is applied to the electrodes from the control unit.
The copper (MG or Marine Growth) electrodes are for the control of marine growth and the aluminum (TC or Trap Corrosion) electrodes reduce corrosion in the sea water pipes by the introduction of an alkaline coating. A further reaction takes place at the aluminum TC electrodes which results in the formation of aluminum hydroxide; this is a highly gelatinous substance to which minute impurities in the sea water adhere and carries them through the system for discharge overboard.
In service half of the electrodes act as anodes and the other half as cathodes, the polarity of the electrical supply dictating which electrodes act as the anode and which as the cathode; the polarity of the supplied current is reversed variable but are typically set every 2-6 hours in order to clean the hydroxide from the cathodes and to even the consumption of the electrodes.
The sea water flow rate to the reaction tank is monitored by a flow meter which is connected to the control unit; should the flow fail an alarm is activated and the current supplied to the electrodes is shut off. The sea water supply to the reaction tank must not exceed the maximum value and the flow meter alarm gives warning if the flow rate goes above the maximum allowed level.
During operation it is important to vent the top of the reaction tank regularly to prevent the build of air pockets inside. It is also a requirement to drain the tank once per month to remove sediment and the build up of any hydroxide compounds.
Care must be taken not to do any burning or welding to the outside of the reaction tank as it has an internal synthetic rubber lining that may be damaged.
From the MGPS reaction tank the treated sea water containing the chlorine is distributed to the two sea suction chests and hence to the ship’s sea water system. The rate of flow to each sea chest must be adjusted depending upon which sea suction chest is operational. If only one sea chest is open then that sea chest must receive 90% of the treated water flow from the reaction tank and the closed sea chest should receive 10% of the flow. If both sea chests are open they must both receive 50% of the sea water flow from the reaction tank.
Procedure for Operating the Marine Growth Protection System
a) Ensure that one of the sea water sea suction chests is open and that one of the auxiliary cooling sea water pumps is operating.
b) Open the reaction tank inlet valve after ensuring that the reaction tank drain valve is closed. Open the MGPS treated water supply valves to the sea chests . Set the flow rate control valves to the sea chests from the MGPS in order to give 90% water flow to the operating sea chest and 10% of the water flow to the closed sea chest.
c) Adjust the flow meter on the inlet side of the MGPS reaction tank to a value above the minimum for the unit (between 6.0-8.0m3/h).
d) Vent the reaction tank until all trapped air has been removed.
e) Ensure power is available to the MGPS control panel.
f) Switch on the mains power to the unit using the switch in the bottom left hand corner of the panel. A green neon light will illuminate. Check the supply voltage.
g) Set the anode currents to the levels specified in the operator’s manual by turning the control knobs. The high output setting must be made first and then the selector is turned to the low output for further setting. A read-out will be shown in the digital ammeter display.
h) Turn the mode selector switch to the AUTO position. It is possible to select MANUAL mode but the system will normally be operated in automatic mode.
i) At the main control panel set the polarity change timer; a setting of 3 hours should be initially used but this can be changed if any problems are detected with the system.
The MGPS is now fully operational. Water from the MGPS is pumped to the sea chests. There should be a flow to the sea chest which is not operating to prevent marine growth within that sea chest. The water flow to each sea chest may be adjusted by means of the flow valves .
The MGPS unit is designed for automatic control.
After a period of operation, about 6 months, a section of the sea water pipe work should be inspected to assess the performance of the marine growth prevention system. If there is any indication of marine growth infestation the anode current should be increased by 0.2A but if no infestation is detected the current may be reduced by the same amount. This routine should be repeated at intervals.
The initial current applied to the anodes should not be changed during the first year of operation of the ship. After that time the anode current can be reduced by 0.1 A if there is no indication of corrosion.