Sunday, January 26, 2014

Inert Gas System


Components of plant

The water is supplied via the scrubber pump, the sprays reduce the temperature to within 2oC of the sea water temperature, the sulphur dioxide content is reduced 90%, and the gas is clear of soot.
The tower is rubber lined and other parts are made of inconel or glassfibre to protect against the SO2 .The water seal at the bottom is provided by the weir arrangement fitted to the drain system.
The seal is internally rubber lined and a heating coil fitted to allow use in low temperatures. The weir controlled water level has a low level alarm fitted.
As the seal is a primary safety feature it is supplied from the general salt water system when the scrubber pump is not in use. Checks are made on the pipe from the seal to the regulating v/v to ensure no HC prescience which would indicate corrosion damage.
Inert gas system

Deck distribution system

After the PV breaker the inert gas is led down a main and valveless distributors to all the tanks, excepting the slops tanks whose lines contain isolating valves. On the main there are three PV hi velocity safety vents, the slops tanks are fitted with individual safety vents of same design. On the for'd end of the main is the for'd pressure release which allows regulation of the inert gas pressure during loading
During loading the ballast is discharged at the same time as the cargo is loaded thereby limiting vapour release to deck, the I.G. plant is kept running in a state of readiness so in the event of sudden cargo stoppage pressure is not lost via the for'd pressure release; this may have to be throttled in to prevent overloading the fans.

Closed system-

This describes the common I.G. main connected to all the tanks, this has the following advantages A disadvantage is that HC's can reenter a purged tank
deck ig system


Gas at inletGas at outlet
Temperature170oC5oC above sea water
O23-4%less than 5%
SO2 & SO30.2 - 0.3%less than 0.02%
CO212 - 14%15-16%
Solids150 mg/m38 mg/m 3
Due to the corrosive nature of the atmosphere in the scrubber and the inside of the scrubber is specially considered and is normlly coated with a glass fibre product or similar The scrubber is supplied by two sea water pumps. A small capacity seal water pump which is run on a continuous bases and ensures that the bottom of the scrubber is continuouly full to the weir level thereby ensuring that there is a gas seal. A larger capacity spray water pump which is run when the system is in use supplying inert gas. The drains from the scrubber is very acidic and special cosideration is given to the pipe run which may be internally coated 

Deck Seals

There are thre types of deck seals in common use;
The seal is kep full using a continuously runing seal water pump which may be backed up with a crossover from a secondary system as required. Should the pressure on the downstream side exced the upstream side the water is pushed up the inlet pipe. The height of this pipe ensures that the head pressure generated is greater than either the pressure release valve or anywater seals 


Hi-jet gas vent
When loading cargo tanks at high rates, steps must be taken to avoid pressure build up which can cause structual damage. This requires the release of the vapours from the tank. This could be by pipes led up the mast with a release point via a flame trap. An alternative as shown provides a high velocity release which carries the gas well clear of the ship. Depending on the pressure, the area between the fixed cone and the orifice plate will change allowing the velocity to be maintained. At high pressures the orifice is forced upwards against the bellows force, thus increasing the flow area. At low pressure the counterweight pulls the orifce downwards thereby reducing the flow area and maintaining the gas velocity