INERT GAS SYSTEM

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Inert gas systems are used to inert empty cargo tanks (including slop tanks) by reducing the oxygen content to a level at which combustion cannot be supported.

The atmosphere in any cargo tank must not exceed 8% oxygen by volume and be at a positive pressure at all times.

The system should be able to purge tanks of hydrocarbon gas prior to gas freeing.

The inert gas system must be capable of supplying gas at a concentration of not more than 5% oxygen by volume at 125% of the maximum cargo discharge rate.

Many tankers have inert gas systems fitted for ballast tanks. This prevents the possibility of a build up of explosive gasses in the event of the ballast tank becoming connected (via crack) to the cargo tank atmosphere. A reduction in corrosion due to reduced levels of oxygen is also claimed.

The composition of the inert gas should be:

                                                O₂                                -           4 – 5 %

                                                CO₂                             -           14 – 15 %

                                                NO_x and trace gases    -           1 %

                                                N₂                                -           Remainder.

Flue Gas Isolating Valve.

            Valve has cleaning for seats – either steam or air – to ensure good sealing properties. The valve must have an indicator to show its position. Interlocks should be fitted to prevent soot blowing operations when open.

Scrubber.

            Main purpose is to cool the flue gas. Sulphur products of combustion and soot particles are also removed. There must be a water seal between the scrubber and the flue gas isolating valve. There must be two means of supplying cooling water. The scrubber usually has a corrosion resistant lining of neoprene rubber. Alarms are fitted for low water pressure, high water level and high gas outlet temperature – all of which will automatically shut down the plant. On leaving the scrubber the gas passes through a demister to prevent carry over of water.

Fans or Blowers.

            At least two blowers are required, together capable of supplying 125% of the maximum cargo discharge capacity. The maximum pressure in the system should not exceed the maximum test pressure of the cargo tanks. It is normal for each blower to be capable of the required delivery rate, often a third, smaller, blower is fitted for topping up purposes. The blowers have a fresh water wash to remove corrosive products and cleaning. Blower failure causes automatic closing of gas regulating valve.

Gas Regulating valve.

            Must be automatically operated and capable of controlling the flow of inert gas to the cargo tanks. This valve must be fitted in the machinery space, before the deck seal.

Oxygen Analyser.

            The oxygen content of the gas is continually analysed and recorded. If the O₂ content should rise to 8% then an alarm sounds and the isolating valve to the deck main closes to prevent supply of gas to the cargo tanks. Cargo operations should stop. The analyser should be regularly calibrated.

Deck Seal.

            At least two non-return devices must be fitted to prevent hydrocarbon vapours reaching the machinery space, one of which must be a water seal. The water seal must have two separate water supply pumps, be protected against freezing and siphoning. Low water level in the seal will cause alarms to sound.

Pressure ~ Vacuum Breaker.

            Protects cargo tanks against over / under pressure caused by thermal variations when the isolation valves are shut.

Audible & visual alarms for: ‑

(i)                 Low water level in scrubber

(ii)               High water level in scrubber

(iii)             High gas temperature

(iv)             Fan failure

(v)               Power failure to control systems for instruments & regulating valve

(vi)             Low level in deck water seal

(vii)           Oxygen content (at fan discharge) in excess of  8%

(viii)         Gas pressure lower than 100 mm water gauge

(ix)             High gas pressure