Inert Gas System

INERT GAS SYSTEM

Components of plant

Boiler uptake valve- Provides a take off point for the flue gas, A cleaning arrangement is fitted to prevent soot build up.

Scrubber- Flue gas passes to the scrubber via the uptake valve, here it enters at the bottom via a waterseal and passes up through a series of sea water sprays and baffle plates being cooled and cleaned before exiting via a demister

The water is supplied via the scrubber pump, the sprays reduce the temperature to within 2^oC 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 following alarms are fitted;
• High water level alarm and trip
• Low water level alarm
• Low water level alarm

Fans-Two types of fans are fitted, a steam turbine driven one of sufficient capacity to supply I.G requirements during cargo ops, and an electrically driven unit with sufficient capacity for topping up only

Shut down of the fans occurs due to;
• High gas temperature after the blowers
• Low scrubber water pressure, high scrubber water lvl.
• Blower failure.

A blank on a stub pipe may be removed to allow the fans to blow fresh air up the I.G. main when gas freeing

Recirculating and regulating valves- The pressure within the tanks is controlled by automatic or manual operation of the regulating v/v, if the demand is low and the regulating v/v nearly closed then the recirculating v/v opens thereby reducing the possibility of the fan overheating by passing the gas back to the scrubber tower.

Oxygen monitor- Fitted just upstream of the regulating v/v and initiates an alarm if oxygen content above 8%.

Deck water seal- The inert gas leaving the engine room to deck passes through the deck water seal whose purpose is to prevent gases from passing back to the engine room from the cargo tanks. A demister is fitted on the outlet side.

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.

Non-return v/v- Purpose similar to the deck seal

Deck isolating valve-Allows the isolation of the deck system from the engine room system

Pressure/vacuum breaker ( PV)- This is the safety valve for the system and prevents both under and over pressurisation of the tanks. Flame screens are fitted to the PV breaker vent. The PV breaker may be of the mechanical or liquid type. The liquid type is filled with a liquid of the correct specific gravity, such as monoethylene Glycol and water.

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

- The tanks are always kept at a positive pressure

- The volume of gas acts as a buffer for variations in loading/discharging rates

A disadvantage is that HC's can reenter a purged tank

Scrubber

	Gas at inlet	Gas at outlet
Temperature	170oC	5oC above sea water
O2	3-4%	less than 5%
SO2 & SO3	0.2 - 0.3%	less than 0.02%
CO2	12 - 14%	15-16%
N2	77%	17%
Solids	150 mg/m3	8 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;

Wet

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 

Semi-Dry

Dry

GAS VENTING SYSTEM

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