Sketch and Explanation on Inert Gas of an Oil Tanker:
Inert gas system is the most important integrated system for oil tankers for safe operation of the ship.
Inert gas is the gas which contains insufficient oxygen (normally less then 8 %) to suppress combustion of flammable hydrocarbon gases.
Inert gas system spreads the inert gas over the oil cargo hydrocarbon mixture which increases the lower explosion limit LEL (lower concentration at which the vapors can be ignited), simultaneously decreasing the Higher explosion limit HEL (Higher concentration at which vapor explodes). When the concentration reaches around 10 %, an atmosphere is created inside tank in which hydrocarbon vapors cannot burn. The concentration of inert gas is kept around 5% as a safety limit.
Components and description of IG system:
The following components are used in a typical inert gas system in oil tankers:
- Exhaust gases source: inert gas source is taken from exhaust uptakes of boiler or main engine as contains flue gases in it.
- Inert gas isolating valve: It serve as the supply valve from uptake to the rest of the system isolating both the systems when not in use.
- Scrubbing tower: Flue gas enters the scrub tower from bottom and passes through a series of water spray and baffle plates to cool, clean and moist the gases. The SO2 level decreases up to 90% and gas becomes clear of soot.
- Demister: Normally made of polypropylene, it is used to absorb moisture and water from the treated flue gas.
- Gas Blower: Normally two types of fan blowers are used, a steam driven turbine blower for I.G operation and an electrically driven blower for topping up purpose.
- I.G pressure regulating valve: The pressure within the tanks varies with the property of oil and atmospheric condition. To control this variation and to avoid overheating of blower fan, a pressure regulator valve is attached after blower discharge which re-circulates the excess gas back to scrubbing tower.
- Deck seal: Purpose of the deck seal is to stop the gases to return back which are coming from the blower to cargo tanks. Normally wet type deck seals are used. A demister is fitted to absorb the moisture carried away by the gases.
- Mechanical non return valve: It is an additional non return mechanical device inline with deck seal.
- Deck isolating valve: The engine room system can be isolated fully with the deck system with the help of this valve.
- Pressure Vacuum (PV) breaker: The PV breaker helps in controlling the over or under pressurization of cargo tanks. The PV breaker vent is fitted with flame trap to avoid fire to ignite when loading or discharging operation is going on when in port.
- Cargo tank isolating valves: A vessel has numbers of cargo holds and each hold is provided with an isolating valve. The valve controls the flow of inert gas to hold and is operated only by a responsible officer in the vessel.
- Mast riser: Mast riser is used to maintain a positive pressure of inert gas at the time of loading of cargo and during the loading time it is kept open to avoid pressurization of cargo tank.
Safety and alarm system: The Inert gas plant is provided with various safety features to safeguard the tank and its own machinery.
Following are various alarms (with Shutdown) incorporated in the Inert Gas plant on board ship:
- High Level in scrubber leads to alarm and shutdown of blower and scrubber tower.
- Low pressure sea water supply (approx. 0.7 bar) to scrubber tower leads to alarm and shutdown of blower.
- Low pressure sea water supply (approx. 1.5 bar) to deck seal leads to alarm and shutdown of blower.
- High inert gas temperature (approx. 70 deg C) leads to alarm and shutdown of blower.
- Low pressure in line after blower (approx. 250mm wg) leads to alarm and shutdown of blower.
- Oxygen content high (8%) leads to alarm and shutdown of gas delivery to deck.
- Low level in deck seal leads to alarm and shutdown of gas delivery to deck.
- Power failure leads to alarm and shutdown of blower and scrubber tower.
- Emergency stop leads to alarm and shutdown of blower and scrubber tower.
Following are various alarms incorporated in the Inert Gas plant:
- Scrubber low level
- Deck seal High level
- Low O2 Content (1%)
- High O2 Content (5%)
- Low lube oil pressure alarm
Working of Inert Gas Plant:
The basis of inert gas production in the IG plant is the flue gas generated from the ship’s boiler. The high temperature gas mixture from the boiler uptake is treated in an inert gas plant which cleans, cools and supplies the inert gas to the individual tanks via PV valves and breakers to ensure safety of tank structure and atmosphere.
The system can be divided in to two basic groups:
a) A production plant to produce inert gas and deliver it under pressure, by means of blower(s), to the cargo tanks.
b) A distribution system to control the passage of inert gas into the appropriate cargo tanks at the required time.
Brief working procedure:
- Boiler uptake gases are drawn to the scrubber unit via flue gas isolating valve(s) to the scrubber unit.
- In the scrubber unit the gas is cooled, cleaned and dried before being supplied in to the tanks.
- Motor driven inert gas blowers supplies the treated gas from scrubber tower to the tanks through. They are mounted on rubber vibration absorbers and isolated from the piping by rubber expansion bellows.
- Regulation of gas quantity delivered to deck is taken care of by the gas control valves and the deck pressure is managed by pressure controller. If the deck pressure is lower than the set point the output signal will be raised to open the valve more, and vice versa if the deck pressure is lower than the set-point. These valves will then work in cooperation to keep both the deck pressure / blower pressure at their respective set point without starving or overfeeding the circuit.
- Before entering the deck line, the gas passes through the deck water seal which also acts as non-return valve automatically preventing the back-flow of explosive gases from the cargo tanks.
- After the deck seal the inert gas relief is mounted to balance built-up deck water seal pressure when the system is shut down. In case of a failure of both the deck seal and the non-return valve, the relief valve will vent the gases flowing from the cargo tank into the atmosphere.
- The oxygen analyser which is fitted after the blower separates the “production” and “distribution” components of the plant and analyzes the oxygen content of the gas and if it is more than 8%, it alarms and shut downs the plant.
Features of flammability diagram with respect of following: Purging:
- When it is required to gas free a tank after washing, it should first be purged with inert gas to reduce the hydrocarbon content to 2% or less by volume so that during the subsequent gas freeing no portion of the tank atmosphere is brought within the flammable range.
- The tank may then be gas freed.
- The hydrocarbon content must be measured with an appropriate meter designed to measure the percentage of hydrocarbon gas in an oxygen deficient atmosphere.
- The usual flammable gas indicator is not suitable for this purpose.
- If the dilution method of purging is used, it should be carried out with the inert gas system set for maximum capacity to give maximum turbulence within the tank.
- If the displacement method is used, the gas inlet velocity should be lower to prevent undue turbulence.
Features of flammability diagram with respect of following: Inerting
- Before the inert gas system is put into service the tests required by the operations manual or manufacturer’s instructions should be carried out. The fixed oxygen analyser and recorder should be tested and proved in good order. Portable oxygen and hydrocarbon meters should also be prepared and tested.
- When inerting empty tanks which are gas free, for example following a dry-docking or tank entry, inert gas should be introduced through the distribution system while venting the air in the tank to the atmosphere.
- This operation should continue until the oxygen content throughout the tank is not more than 8% by volume.
- The oxygen level will not thereafter increase if a positive pressure is maintained by using the inert gas system to introduce additional inert gas when necessary.
- If the tank is not gas free, the precautions against static electricity given in Section 10.6.7 of ISGOTT should be taken.
- When all tanks have been inerted they should be kept common with the inert gas main and the system pressurised with a minimum positive pressure of at least 100mm water gauge.
Features of flammability diagram with respect of following: Gas Freeing:
- In a gas freeing operation air is delivered into the tank, where it mixes with the existing tank atmosphere and also tends to mix together any layers that may be present.
- The resultant mixture is expelled to the outside atmosphere. Because the process is one of continuous dilution with the air, the highest hydrocarbon concentration is vented at the beginning of gas freeing and decreases thereafter.
- For example, on a non-inerted ship, gas freeing of a motor gasoline tank that has been battened down can give initial concentrations as high as 40% by volume, but in most circumstances the concentration in the vented gas is much lower, even at the start of the operations.
- On inerted ships, where purging to remove hydrocarbon vapour before gas freeing is a requirement, even the initial concentration will be low, 2% by volume or less.
Functions and maintenances of cargo related equipments on oil tankers using sketches and diagrams: Pressure Vacuum Valve (P/V Valve):
- The main components of this system are fixed washing machines which are fed from a 200 mm main line on upper deck.
- The washing fluid is supplied by any of the cargo oil pumps via a riser line from the pump discharge cross-over line.
- In the pump room, there is a steam heated washing water heater which is connected to the riser line in a by-pass arrangement with the water side isolated by means of stop valves and spectacle flanges.
- These flanges may be in open position only when required for hot water washing. The rated heating capacity is 280 m3h of sea water from 20OC to 80OC.
- During all washing, the pressure in the main line should be maintained at minimum 8.5 bar at the aft end in order to ensure satisfactory operating conditions also for the forward-most washing machines.
- A pressure less than 7.5 would considerably reduce the effectiveness of the washing operations. For pressure monitoring, there is fixed pressure gauge at the aft end of the main line as well as a boss for a portable pressure gauge at the forward end of the main line.
In order to ensure the correct functioning of PV valves the following should always be complied with:
- PV valves should be serviced and calibrated according to classification society requirements;
- Prior to loading and discharging, PV valves should be checked to ensure they function as designed;
- During cargo operations the correct functioning of PV valves should be monitored; and
- Pressure sensors fitted as the secondary system as a back up to the primary vent system should be checked to ensure that they function as designed and, where provided, that the alarms are correctly set.
Setting PV alarms:- High pressure alarms and low pressure alarms must be set to:
- Activate additional safety or other alarm systems;
- Support maintenance of correct positive inert gas pressure in tanks;
- Prevent air intake to tanks; and
- Comply with regulations.
Working of a Pressure Vacuum Breaker (PV Breaker):
Pressure Vacuum Breaker or usually known as PV Breaker is a safety measure used in the IG line on deck.
The major functions of a PV Breaker are:-
- Abnormal rise of Pressure in Cargo tanks when loaded specified rate of gas outlets.
- Abnormal rise of Pressure in Cargo tanks when cargo is unloaded beyond specified rate of the inert gas blower.
- Abnormal rise or drop of pressure in cargo tanks when the breather valve does not operate properly for the fluctuation of the pressure in cargo tanks due to variation in atmospheric and sea water temperatures.
- When Pressure Rises: – When the pressure in the cargo oil tanks rise, the seal liquid rises in the inner pipe. At this time , if the pressure beyond the specific capacity of the breaker, the seal liquid will push out of the pipe to let the pressure inside the be out.
- When Pressure drops: – When the pressure in the cargo oil tanks fall, the seal liquid rises in the outer pipe. If the pressure beyond the specific capacity of the breaker, the seal liquid is drown into the cargo oil tanks, and atmospheric air will be inhaled in the tank.
How to ensure that P/V Breaker is protecting the cargo tanks effectively:
- Every inert gas system is required to be fitted with one or more pressure/vacuum breakers or other approved devices. These are designed to protect the cargo tanks against excessive pressure or vacuum and must therefore be kept in perfect working order by regular maintenance in accordance with the manufacturer’s instructions.
- When these are liquid filled it is important to ensure that the correct fluid is used and the correct level maintained for the density of the liquid used. The level can normally only be checked when there is no pressure in the inert gas deck main. Evaporation, condensation and possible ingress or sea water must be taken into consideration when checking the liquid condition, density and level.
- In heavy weather, the pressure surge caused by the motion of the liquid in the cargo tanks may cause the liquid in the pressure/vacuum breaker to be blown out. When cold weather conditions are expected, liquid filled breakers must be checked to ensure that the liquid is suitable for low temperature use, and if necessary anti-freeze is to be added.
- The P/V breaker(s) are to be clearly marked with their high pressure and vacuum opening pressures and also with the type and volumetric concentration of antifreeze (if water filled type), and minimum operating temperature.
Pressure Vacuum Valve or PV Valve:
- Moderate pressures of 0.24 bar acting on large surfaces in liquid cargo tanks are sufficient to cause damage and rupture.
- The pressure on each unit of area multiplied by the total area gives a large loading on the underside of the top of a tank or other surface, which may then buckle or the metal plate may be torn.
- Similarly, pressure drop within a tank can cause damage due to greater atmospheric pressure on the outside.
- Pressure vacuum valve or pv valve in the ventilation system will prevent either over or under pressure. They are set usually so that tank pressure of about 0.14 bar will lift the main valve (The smaller valve will lift along with it) and release excess pressure. The vapour passes to atmosphere through a gauze flame trap. Drop in tank pressure compared with that of the outside atmosphere will make the small valve open downwards to equalize internal pressure with that outside.
- Pressure vacuum valve or pv valve can relieve moderate changes in tank pressure due to variations in temperature and vapour quantity. A drop towards vacuum conditions as the result of the condensation of steam will also be handled by the valve. Rapid pressure rise due to an explosion would not be relieved.
- The fast rate at which a tank is filled while loading produces a very rapid expulsion of the previous contents (vapour and inert gas). The pressure vacuum valve is not designed as a filling vent and neither should the tank hatch be left open. The latter method of venting can cause an accumulation of flammable vapours at deck level. Tanks should be vented while filling, through mast head vents or through special high velocity vents.