143.IG System

REGULATION

Tankers of 20,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of carrying crude oil shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than:
.1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and

.2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later except that for tankers of less than 40,000 tonnes deadweight not fitted with tank washing machines having an individual throughput of greater than 60 m3/h the Administration may exempt such tankers from the requirements of this paragraph, if it would be unreasonable and impracticable to apply these requirements, taking into account the ship's design characteristics.

Boiler Uptake

These valves are located near the main boiler uptake to isolate the IGS scrubber from the boiler uptake. Alternatively, if a dedicated IGG is used, this valve will be located near the IGG; it is closed when the IGS system is not operating. 

IGS Scrubber

The raw inert gases are hot and contain soot, sulphur oxides, sulphurous acid, and sulphuric acid; all of these can be harmful to the cargo and the cargo tanks. The scrubber cools the gases and removes the contaminants by bubbling the gases through large quantities of seawater (there must be two sources of water for the scrubber). The gases are then sprayed with additional quantities of water, or rise through a packed bed of ceramic forms, plastic shapes, or metal trays through which seawater falls, increasing the efficiency of the water in cleaning the inert gas. The warm, acidic, dirty seawater is then piped overboard.

Demister Units

The gas from the scrubber has significant amounts of moisture, both from the burning process and from bubbling through the seawater in the scrubber. The demister is located close to the scrubber to remove entrained water from the IGS gas stream.

Fan Units

Two or more independent blowers are located near the demister to draw the inert gas through the scrubber and the demister and deliver it to the IGS distribution system at the required pressure. Since the greatest need for inert gas is during offloading, the blower capacity is set at 125 percent of the maximum rated capacity of the cargo pumps. This provides a margin of safety to ensure that no air enters the cargo tanks. This capacity may be provided by two blowers of equal size, or by one large and one small blower. Separate inlet and discharge valves are fitted to each blower unit.

Gas-regulating valve

A gas-regulating valve shall be fitted in the inert gas supply main. This valve shall be automatically controlled to close as required. (SOLAS Chapter II-2 paragraphs 19.3 and 19.4.) It shall also be capable of automatically regulating the flow of inert gas to the cargo tanks unless means are provided to automatically control the speed of the inert gas blowers required. (SOLAS Chapter II-2 paragraph 7.9.2). The valve referred to shall be located at the forward bulkhead of the forward most gas-safe space through which the inert gas supply main passes.

The pressure-regulating valve is used to adjust the pressure between the IGS and the cargo tanks. An over-pressurized cargo tank can rupture if the pressure in the IGS value is too high. It is also important to prevent back flow from the cargo tanks, which could lead to inert or cargo gases entering the machinery spaces

ABOUT DECK SEAL, PV VALVE AND PV BREAKER REFER MY EARLIER NOTES