PREVIOUS | TABLE OF CONTENTS | NEXT

PART IV
OPERATION OF INERT GAS PLANT

29. (1) Flue gas systems may differ in detail, but certain basic principles remain the same; these are:

1. starting up the inert gas plant;
2. shutting down the inert gas plant;
3. safety checks when the inert gas plant is shut down;

in all cases the manufacturer’s detailed instructions should be followed.

Start-up procedures

(2) Ensure boiler is producing flue gas with an oxygen content of 5 per cent by volume or less (for existing ships 8 per cent by volume or, wherever practicable, less).

(3) Ensure that power is available for all control, alarm and automatic shutdown operations.

(4) Ensure that the quantity of water required by the scrubber and deck water seal is being maintained satisfactorily by the pump selected for this duty.

(5) Test operation of the alarm and shutdown features of the system dependent upon the throughput of water in the scrubber and deck seal.

(6) Check that the gas-freeing fresh air inlet valves, where fitted, are shut and the blanks in position are secure.

(7) Shut off the air to any air-sealing arrangements for the flue gas isolating valve.

(8) Open the flue gas isolating valve.

(9) Open the selected blower suction valve; ensure that the other blower suction and discharge valves are shut unless it is intended to use both blowers simultaneously.

(10) Start the blower.

(11) Test blower "failure" alarm.

(12) Open the blower discharge valve.

(13) Open the recirculating valve to enable plant to stabilize.

(14) Open the flue gas regulating valve.

(15) Check that oxygen content is 5 per cent by volume or less (for existing ships 8 per cent by volume or, wherever practicable, less); then close the vent to atmosphere between the gas pressure regulating valve and the deck isolating valve.

Note: Some oxygen analysers require as much as two hours to stabilize before accurate readings can be obtained.

The inert gas system is now ready to deliver gas to the cargo tanks.

Shutdown procedures

(16) When all tank atmospheres have been checked for an oxygen level of not more than 8 per cent and the required in-tank pressure has been obtained, shut the deck isolating/non-return valve.

(17) Open vent to atmosphere between the gas pressure regulating valve and the deck isolating/non-return valve.

(18) Shut the gas pressure regulating valve.

(19) Shut down the inert gas blower.

(20) Close the blower suction and discharge valve; check that the drains are clear; open the water washing system on the blower while it is still rotating, with the power supply of the driving motor turned off, unless otherwise recommended by the manufacturer; shut down the water-washing plant after a suitable period.

(21) Close the flue gas isolating valve and open the air sealing system.

(22) Keep the water supply on the scrubber tower full, in accordance with the manufacturer’s recommendation.

(23) Ensure that the water supply to the deck water seal is running satisfactorily, that an adequate water seal is retained and that the alarm arrangements for it are in order.

Safety checks when inert gas plant is shut down.

(24) Ascertain the water supply and water level in the deck seal at regular intervals, at least once per day depending on weather conditions.

(25) Check the water level in water loops installed in pipework for gas, water or pressure transducers, to prevent the backflow of hydrocarbon gases into gas safe spaces.

(26) In cold weather, ensure that the arrangements to prevent the freezing of sealing water in the deck seals, pressure vacuum breakers, etc. are in order.

(27) Re-pressurize the inerted cargo tanks with inert gas before their pressure drops to 100 mm.

Possible failures of inert gas system, and actions to be taken

(28) High oxygen content may be caused or indicated by the following conditions:

1. poor combustion control at the boiler, especially under low load conditions;
   
2. air drawn down the uptake when boiler gas output is less than the inert gas blower demand, especially under low load conditions;
   
3. air leaks between the inert gas blower and the boiler uptake;
   
4. faulty operation or calibration of the oxygen analyser;
   
5. inert gas plant operating in the recirculation mode; or
   
6. entry of air into the inert gas main through the pressure vacuum valves, mast risers etc. due to maloperation.

(29) If the inert gas plant is delivering inert gas with an oxygen content of more than 5 per cent, the fault should be traced and repaired; however, that all cargo tank operations shall be suspended if the oxygen content exceeds 8 per cent, unless the quality of the gas improves.

(30) Inability to maintain positive pressure during cargo discharge or deballasting operations may be caused by:

1. inadvertent closure of the inert gas valves;
   
2. faulty operation of the automatic pressure control system;
   
3. inadequate blower pressure; or
   
4. a cargo discharge rate in excess of the blower output.

(31) The cargo discharge or deballasting should be stopped or reduced, depending on whether or not the positive pressure in the tanks can be maintained while the fault is rectified.

PREVIOUS | TABLE OF CONTENTS | NEXT