With the inert gas system, the protection against tank explosion is achieved by introducing inert gas into the tank to keep the oxygen content low and reduce the hydrocarbon gas concentration of the tank to a safe proportion.
A mixture of hydrocarbon gas and air cannot ignite unless its composition lies within the range of gas in air concentration known as the “flammable range”. The lower limit of this range known as the “lower flammable limit” is any hydrocarbon concentration below which there is insufficient hydrocarbon gas to support combustion. The upper limit of the range, known as the ‘upper flammable limit” is any hydrocarbon concentration above which there is insufficient air to support combustion. The flammable limit varies somewhat for different pure hydrocarbon gases and the gas mixture derived from different petroleum liquids. In practice however, the lower and upper flammable limit of oil cargoes carried in tankers can be taken for general purpose, to be 1% and 10% by volume respectively.
Effect of Inert Gas on flammability
When we dilute a Hydrocarbon gas and air mixture with air (having concentrations as shown at point F), air will replace the mixture till it becomes 100% air. FA is the path that will be followed which passes through the flammable mixture range. If any spark is applied during this passage, the mixture will explode. To avoid this, the mixture is diluted with inert gas along the line FH till point H is reached. At point H, if oxygen is let in freely, the dilution will take place along line HA, without passing through the flammable range causing no danger of fire or explosion.
Source of Inert Gas
Uptake from the ship’s boilers
Independent IG generators
A gas turbine plant equipped with afterburners
Quality of Inert gas
Good combustion control in the ship’s boilers is necessary to achieve an oxygen content of 5% by volume. To obtain this quality, it may be necessary to use automatic combustion control.
Inert Gas distribution system
Flue Gas Uptake Valve
This valve isolates the flue gas uptake from the boiler to the scrubber. The valve must be opened before the system is started. The gas flows through a black flow prevention system. A push-button switch on the I.G. panel in control room operates the valve pneumatically when the control mode switch is on I.G mode. The flue gas uptake valve can also be open manually using a detachable manual lever in case of pneumatic failure but remember to stop air supply pressure to the valve before opening it manually. Compressed air at 5.0 kg/cm2 keeps the valve gas tight when not in use. Cleaning steam at 4.0 kg/cm2 is used for cleaning the uptake valve.
In the Scrubber, the gas is cooled by bubbling it through seawater. It is then thoroughly washed under a water spray by taking through venturi slots, impingement plates and baffle trays which contain silica, ceramic chips, stones, etc. The gas almost becomes free of soluble gases like SO2, SO3, NO and NO2 as well as insoluble particles like soot, dust. Moisture is trapped by the demister pads and gas flows to the blowers. The scrubber is internally coated with a good polyurethane or epoxy coating to resist corrosion.
Adequate opening and sight glasses should be provided in the shell for inspection, cleaning and observation purposes. The sight glass should be reinforced to withstand impact and be of heat resistant type. This may be achieved by the use of double glazing.
The design should be such that in normal condition of trim and list, the scrubber efficiency will not fall by more than 3 % nor the temperature will rise at the gas outlet exceed the designed gas outlet temperature by more than 3%
Inert Gas Blowers
Blowers are used in delivering the scrubbed gas to the cargo tanks. The regulation requires that at least 2 blowers shall be provided w/c together shall be capable of delivering inert gas to the cargo tank at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume.
I.G pressure regulating valve
The pressure within the tanks varies with the property of the oil and atmospheric condition. To control this variation and to avoid overheating of the blower fan, a pressure regulator valve is attached after blower discharge which circulates the excess gas back to the scrubbing tower.
The oxygen concentration in the inert gas is detected from the fan (blower) outlet. When an Oxygen content of more than 8% is detected after the blower discharge valve, an alarm will be activated on the IG panel. At the same time, the interlock will open the exhaust valve to vent the inert gas to the atmosphere while the main IG valve will shut so as not to allow the delivery of more than 8% of IG into the tank.
When the O2 % is brought back below 8%, the exhaust valve will shut and the main IG valve will open allowing delivery of O2 of <8% into the cargo tanks.
Non Return Devices
The deck water seal and the mechanical non-return valve together form the means of automatically preventing the backflow of cargo gasses from cargo tanks to the machinery spaces or other safe areas where the inert gas plant is located.
Deck water seal
This is the principle barrier. A water seal is fitted which permits inert gas to be delivered to the deck main but prevents any backflow of cargo gases even when the inert gas plant shuts down. It is vital that supply water is maintained to the seal at all times, particularly when the inert gas plant is shut down. Also, drains should be led directly to overboard and should not pass through the machinery spaces. There are different designs but any of the 3 principal types may be adopted:
- Wet Type
- Semi Dry Type
- Dry Type
This is the simplest type of water seal. When the inert gas plant is operating, the gas bubbles through the water from the submerged inert gas inlet pipe and passes into the tank, but when the tank pressure exceeds the pressure in the inert gas line, the water is pressed up into this inlet pipe and thus prevents the backflow. The drawback of this water seal is that water droplets may be carried over with inert gas which, although does not impair the quality of inert gas but could increase corrosion. A demister should, therefore, be fitted in the gas outlet for the water seal to reduce any carryover.
Instead of bubbling through the water trap, inert gas flow draws the sealing water into the separate holding chamber by venture action thus avoiding or at least reducing the amount of water droplets being carried over. Otherwise, it is functionally the same as the wet type.
In the “dry” type, the deck seal unit consists of two chambers fitted with automatic flow control valves. In this system when the system is started and the flow of gas is delivered to the deck main system, the automatic drain valve of the main chamber is opened and the water seal drained out leaving a dry passage for the inert gas to be delivered to the deck main system. At the same time the filling valve for the upper chamber is opened and the upper chamber filled to its required level, before the filling valve is again closed.
When the inert gas system is stopped, the main chamber drain valve is closed and the drain or dump valve of the upper chamber (or drop tank) is opened releasing the water into the main chamber to create a water seal and prevent the back flow of gases from the cargo spaces. With this system the risk of water carry over is eliminated, but in the event of failure of the automatic valves could render the protection device ineffective.
Deck mechanical non-return and deck isolating valve
As a further precaution to avoid any backflow of gas from the cargo tanks, and to prevent any backflow of liquid which may enter the inert gas mainline if the cargo tanks are overfilled, non-return valves should be fitted forward of the deck seal and should operate automatically at all times.
This valve shall be provided with a positive means of closure, or alternatively, a separate deck isolating valve fitted forward of the nonreturn valve so that the inert gas deck main may be isolated from the non-return devices. The separate isolating valve has the advantage of enabling maintenance work to be carried out of the non-return valve.
Atmosphere vent valves
This valve should be opened when the inert gas plant is shut down to prevent leakage past the non-return devices from building up any pressure in the inert gas line between the gas pressure regulating valve and these non-return devices.
Design considerations of the Non-return devices
The material used in the construction of the non-return devices should be resistant to fire, and to the corrosive attacks from acids formed by the gas. Alternatively, low carbon steel protected by rubber lining or coated by fiberglass epoxy resin or equivalent material may be used. Particular attention should be paid to the gas inlet pipe to the water seal.
Tank Venting System
The P/V valve is the primary mechanism for the protection from the cargo tank overpressure.The valve is fitted to a vertical pipeline connected directly to the vapour space of a cargo tank above. The typical pressure setting for a P/V valve is traditionally measured in millimeters of water gauge and would be in the range from 1,400 to 1,800 mmWG.
These valves are supported on a connecting pipeline to the tank’s atmosphere by a 100 to 150 mm diameter pipeline and located at least 2 meters above the deck. Due to the requirements to prevent mechanical damage to these valves the closing pressure is controlled by a damping mechanism (to prevent hammering of the valve). As a result of the damping mechanism, the closing pressure of the valve will vary but will be in the range of 400-800 mmWG.
The conditions such as loading, unloading and thermal variation brings about over or under pressure of the cargo tank. In the above condition, this valve plays an important role in relieving excessive pressure in tank. For excessive pressures, the valve functions as follows;
By loading into the tank, the pressure inside the tank increases. When the tank pressure reaches the set pressure of the valve, pressure disc lifts automatically. During the lifting of pressure disc, excessive pressure is relieved to the atmosphere. When the tank pressure reaches below the setting pressure of the valve, pressure disc will close.
By discharging from the tank, the pressure inside the tank decreases below atmospheric pressure. When the inside pressure decreases till the vaccum setting of the valve, vaccum disc operates automatically. During lifting of the vacuum disc, inside of the tank gets charged by ingress of air. When the pressure inside equals the pressure outside, vacuum disc gets closed.
During the voyage of the ship, the pressure inside the cargo tank may vary by atmospheric temperature or by rolling and pitching of the ship due to bad weather conditions. The valve will automatically relieve the pressure or vacuum due to increase or decrease in the temperature.
Liquid filled vacuum breakers
Supporting the overpressure safety system of the P/V valve is the secondary safety mechanism of P/V breaker. When there is a rapid pressure fluctuation within the common vapour system the P/V breaker relieves such an overpressure.
The pressure setting in the P/V breaker is achieved by way of the internal water column.
The typical settings of the P/V breakers are +1,800 mmWG and -500 mmWG. The water column also isolates the vapour phase from external air ingress into the system.
In the event of an excessive pressure surge within the tank system, the water column would either be displaced out of the breaker onto the deck, in the event of excessive pressure, or drawn into the cargo tanks in the event of an under pressure. This will, therefore, open the total vapour system to the external environment and atmospheric pressure and, due to the equipment’s dimensions, will relieve the pressure in the system very quickly. Thus, this will only operate if the vessel tank’s P/V valves fail to operate or are not of sufficient capacity to relieve the pressure surge adequately