INERT GAS SYSTEM – IGS

System Arrangement

The arrangement and functions of the main components of the Inert Gas System are shown schematically. The system shown uses flue gas from the main – or auxiliary boilers as a source of inert gas. There are also systems using an inert gas generator i.e. an init producing gas by combustion of oil similar to the combustion chamber in the boiler. However, the functional descriptions and arrangements shown still apply in principle except that the scrubber and inert gas generator normally are built as an integral unit.


Main components and their functions in an inert gas system.

In the following sections a brief description of the main components is given. It must however, be observed that all details related to specific types and makes of inert gas system are not dealt with.

Inert Gas Scrubber

The following processes take place in the scrubber

Ø Cooling of the flue gas
Ø Soot extraction
Ø Removal of sulphur dioxide (SO2)


Cooling of flue gas

Cooling of flue gas can be achieved either by sea water being sprayed into the gas flow, or by the gas flow bubbling through a sea water bath.

Both methods are convenient for cooling, but there are definite design requirements to be satisfied for them to be effective. Spray cooling or a combination of bubble and spray cooling is most common method.

Extraction of soot

The following 4 different methods for soot extraction are considered here:

Spray scrubber tower
Packed bed scrubber tower
Plate stage scrubber tower
Venturi scrubber

The working principle for all there are based on the collision effect between soot particles and water drops.

Removal of Sulphur Dioxide (SO2)

The removal of sulphur is obtained by absorption in the cooling water. This effect takes advantage of the solubility of the sulphur dioxide in sea-water. The extraction of SO2 occurs by the contaminated gas being brought into contact with the sea-water. It is important to have a large contact surface and this achieved by leading the gas through a packed scrubber tower or a plate stage scrubber tower. The seawater runs through the packing in a center-flow in relation to the gas (or from plate to plate stage tower).

Example of scrubber designs

The figures 40 and 41 show examples of current designs of inert gas scrubbers. The first example shows a scrubber with a spray cooling a venturi for soot extraction and a packed bed tower for removal of SO2. The second example is a scrubber with spray nozzles in venture and bubble bath for cooling, venture for soot removal and plate tower for SO2 removal.

Closing Arrangement at Flue Gas Inlets

The flue gas valves operate in every sever conditions: high temperature (up to 450ºC). high soot and SO2 contents in the flue gas. This results in corrosion and clogging problems.

Valves intended for complete shut-off have a tendency to jam and deposits of soot make full closing difficult. Some valve types are therefore designed with a certain leakage (under-cut). To prevent flue gas leakage to the scrubber when inert gas system is shut down, the scrubber and flue gas line after the flue gas valve is pressurized with air from the boiler fans so that air is leaking past the valve into the flue gas uptake. A drawback with this arrangement is that maintenance work on the scrubber is difficult with the boilers in operation. The overpressure in the scrubber cannot be maintained in this situation and flue gas may flow into the scrubber. Moreover the high temperatures in the flue gas intake, when the boilers are in operation (turbine-vessels), makes inserting of blind flanges difficult. An alternative closing arrangement is to fit double shut-off valves with supply of pressurized air between the valves. Butterfly valves seem most convenient as flue gas valves. The valves must be equipped with an arrangement for purging with air or steam for removal of soot deposits.

Effluent Discharge from Inert Gas Scrubbers

The outlet water from scrubbers has a pH value between 2 & 4 depending on quantity of sea-water supplied, sulphur content in the fuel oil and the efficiency of the scrubber. Experience has shown that the scrubber effluent is far more aggressive than expected with mentioned acidity and content of sulphuric acid. The reason is probably to be found on the intermittent in sea-water and oxides in the flue gas. As the pH valus seems to have less influence on corrosion than presumed. The attacks will probably in the main take place during stand still periods. When the concentration of acid will increase owing to evaporation. If so a thorough flushing of the effluent line after the system has been stopped will have a reducing effect on the corrosion.

Inert Gas Blowers

Inert gas blowers may be either steam driven or electrically driven. These units have been a significant source for problems in some installation. These problems very often stem from deposits built up on the blower disc causing unbalance and eventually damage. Regular water washing has shown to improve the conditions and therefore water spray nozzles are usually fitted in the blower casing for this purpose.

Deck Water Seal

It is required that a water seal and a non-return valve are to be fitted in the main inert gas line after the blowers. The non-return valve is to be located on the (tankdeck). Normally the water seal is also located on the tank deck.

The task of the water seal and the non-return valve is to prevent return of gases from gas dangerous areas (tank area) to gas safe spaces, e.g. engine room.

Upon stop of inert gas plant the water seal should automatically be filled with sufficient quantity of water to be able to establish a water column sufficiently high to withstand the highest possible tank pressure higher than the opening pressure of the pressure/vacuum valves and liquid filled pressure / vacuum breaker. In principle, the filling should occur immediately when the gas flow will have a certain inertia at stop of the plant especially because of the dynamic energy of the blower impellers. Therefore a certain time for filling of the water seal may be accepted. However, there are classification societies that require a maximum filling time of 3 seconds.

Non-return Valve in Deck Line

The non-return valve in the deck line is an additional safeguard against back-flow of gas. A common design has been a valve with a loose disc whose weight provide the closing force or a hinged flap with weight for closing.

The main problem connection with this component has been of functional nature. During the design process of the non-return valve. Two conflicting aims appear. The first is to achieve as small pressure drop as possible over the non-return valve. This means a minimum of lifting power which in turn means the lowest possible weight of the valve disc or valve flap. On the other hand, the largest possible closing force is required to deposits or corrosion. This means the heaviest possible weight of the valve disc or flap. The design of the non-return valve is therefore a compromise between these two considerations.
A possible solution is to apply an external closing device to the valve upon stop of the inert gas system. This will assist in solving the problem when the plant is out of operation. During operation. However, the problem is still remaining.

Inert condition
A condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8 percent or less by volume by addition of inert gas.

Inert gas
A gas or a mixture of gases, such as flue gas, containing insufficient oxygen to support the combustion of hydrocarbons.

Inert gas distribution system
All piping, valves, and associated fittings to distribute gas from inert gas plant to cargo tanks, to vent gases to atmosphere and to protect tanks against excessive pressure or vacuum.

Inert gas plant
All equipment specially fitted to supply, cool, clean, pressurize, monitor and control delivery of inert gas to cargo tank systems.

Inert gas system (IGS)
An inert gas plant and inert gas distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instrument and control devices.

Inerting
The introduction of inert gas into a tank with the object of attaining the inert condition.