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  • When petroleum is ignited, it is the Gas that burns as a visible flame.

  • The quantity of Gas given off by petroleum liquid depends on its volatility.

  • Petroleum gases will burn only when mixed with air in certain proportions. If there is too little or too much petroleum gas, the mixture cannot burn.

  • The limiting proportions are known as the Lower Flammable Limit and Upper Flammable Limit respectively. These limits vary according to the different components of petroleum gases.

  • For gas mixtures from petroleum liquids that are likely to be encountered in normal tanker trades, the overall flammable range is from a minimum Lower Flammable Limit of about 1% gas by volume in the air to a maximum Upper Flammable Limit of about 10% gas by volume in air.

  • As petroleum liquid is heated, the concentration of gas in the air above it increases. The temperature of the liquid at which this concentration reaches the Lower Flammable Limit is known as the Flashpoint.




  • The gases from most petroleum liquids are heavier than Air and Inert Gas.

  • These density differences diminish as the gases are diluted with air.  Flammable mixtures usually contain at least 90% by volume of air and consequently have densities almost indistinguishable from that of air.




  • Toxicity is the degree to which a substance or mixture of substances can harm humans.

  • Toxic substances can harm humans in three main ways:

  • by being swallowed (ingestion);

  • through skin contact; and

  • through the lungs (inhalation).

  • Toxic substances can have local effects such as skin or eye irritation, but can also affect more distant parts of the body.


  • Continued uncertainties in the availability of existing crude supplies have placed additional significance on the production and transportation of the difficult-to-handle waxy, high pour point crudes.

  • The low sulfur content of many waxy types of crude makes them even more desirable from an environmental standpoint.

  • However, numerous flow problems are related to the normal handling of waxy crudes:

(1) Temperature must be maintained substantially above the pour point to permit crude handling,

 (2) Transportation costs tend to be much higher because of special pumping and heating requirements, and

 (3) Waxy components are deposited in pipelines and storage tanks, which then must be pigged or scraped.




Static accumulator oil

  • An oil with an electrical conductivity of less than 50 picoSiemens/metre (pS/m), so that it is capable of retaining a significant electrostatic charge.


Static electricity

  • The electricity produced by movement between dissimilar materials through physical contact and separation.

  • Static electricity presents fire and explosion hazards during the handling of petroleum and during other tanker operations such as tank cleaning, dipping, ullaging and sampling.

  • Precautions as per ISGOTT must be taken.



  • IMO Resolution MSC.150 (77) adopted in June 2003 urges governments to ensure the supply and carriage of Material Safety Data Sheets for the cargoes.

  • This MSDS should be based on the format as suggested by the Resolution.

  • It is the responsibility of the Supplier to provide a Tanker that is to load a cargo or bunker fuel with a Material Data Safety Sheet (MSDS) before loading commences.

  • The MSDS should indicate the type and probable concentrations of hazardous or toxic components particularly H2S and Benzene.

  • Provision of an MSDS does not guarantee that all of the hazardous or toxic components of the particular cargo or bunkers being loaded have been identified or documented.

  • Absence of an MSDS should not be taken to indicate the absence of hazardous or toxic components.

  • Operators should have procedures in place to determine if any toxic components are present in cargoes that they anticipate may contain them.

  • It is the ship’s responsibility to provide the receiver with an MSDS for the cargo to be discharged.

The ship must also advise the terminal and any tank inspectors or surveyors if the previous cargo contained any toxic substances.

Gas Density
Waxy Content of Few Grades
Static Accumulation
Static Electricity


ADB - Hydrocarbon gas/air mixture (No Inert Gas Content)

CDE – Flammable Envelope

C – Lower Flammable Limit (LFL)  (1% by volume)

D – Upper Flammable Limit (UFL)  (10% by volume)

E – Minimum O2 required for combustion (~11% by volume)

flamibility diagram.jpg
  • A hydrocarbon gas/air mixture would only be flammable when the respective percentages of hydrocarbon gas and oxygen lie within the FLAMMABLE ENVELOPE.

  • On left side of line AB, as the inert gas content increases, the oxygen & hydrocarbon content reduces. Accordingly, the flammable range decreases in proportion with it. The lower flammable range progresses along the line CE while the upper flammable limit decreases rapidly along the line DE. Finally both these lines merge at point E.

  • Thus, when there is about 11.5% oxygen by volume, the flammable range ceases to exist due to insufficient O2 to support combustion.


  • LFL & UFL of oil cargoes carried in tankers can, for general purposes, be taken as 1% and 10% by volume respectively



  • The areas outside “flammable envelope” are safe with respect to flammability. They are marked as  “inert”, “too lean” or “too rich”






  • It is the process of introduction of Inert gas into a tank with purpose of reducing its Oxygen content well below 8%

  • Inerting will keep vapour/air mixture outside the flammable range.




  • It is evident from Flammability diagram that as inert gas is added to hydrocarbon gas/air mixtures, the flammable range progressively decreases until the oxygen content reaches a level of about 11% by volume, when no mixture can burn.


  • The figure of 8% by volume of oxygen allows a safety margin as specified in ISGOTT.


  • As the inert gas content increases, the flammable limit mixture changes as indicated by the lines CE and DE which finally converge at point E.


  • In already Inerted tank, gas/air mixture will follow typical path of  “F” to “H” while remaining out of explosive envelope.



  1. When inerting empty tanks that are gas free, inert gas is introduced through the distribution system while venting the air inside the tank to the atmosphere.


  1. This operation is continued until the oxygen content throughout the tank is not more than 8% by volume.


  1. Air/IG mixture will follow x-axis(Oxygen Axis)  from 21% value to another value under 8%.






The introduction of inert gas into a tank that is already in the inert condition with the object of raising the tank pressure to prevent any ingress of air is called Topping up.



Thus location of gas/air mixture does not change on the flammability diagram during Topping up.







  • It is the process of introduction of fresh air into the tank in order to bring up Oxygen level to 21% and at the same time reducing content of other toxic gases to acceptable minimum level.


  • Thus, Gas freeing is the process of creating normal atmospheric conditions inside the tank wherein oxygen level is 21%.


  • Purging is done prior gas freeing to keep out of Flammable Envelope.




  • When an inert mixture, such as that represented by point F, is diluted by air, its composition moves along line FA and enters the shaded area of flammable mixtures; this means that all inert mixtures in the region above line GA (critical dilution line) pass through a flammable condition.


  • However, since purging is done prior gas freeing, the mixture composition moves from “F” to “H” first. Thereafter, when fresh air is introduced, dilution line will remain below critical dilution line GA and will not enter Flammable Envelope.


  • Purging is introduction of inert gas into a tank already in the inert condition.


  • The object is to further reduce content of oxygen and/or hydrocarbon volume.


  • As per ISGOTT, tank is said to be purged when the HC content falls to 2% HC or less.


  • The Purging can also be called as dilution with additional inert gas.





  • Purging enables to move a mixture, such as that represented by “F” to another safe state like “H” which will remain well clear of the “Flammable Envelope” even if fresh air is subsequently introduced inside the tank for gas freeing.


  • Purging is also carried out after discharge operation to avoid vapour contamination with next grade of cargo to be loaded.




  1. This system is usually found on large crude carriers, where the cargo piping is not used for the discharge of cargo.

  2. Instead, gate valves are provided on the bulkheads of the tanks which when opened; allow the oil to flow freely in the aft most tank and into the COP.

  3. The advantages of this system are primarily the cost factor, it allows for fast drainage and efficient means of pumping the cargo tanks. Disadvantages are of single crude being shipped.

Free Flow.jpg




  1. This is the simplest type of pipeline system which uses fewer valves than the others.

  2. Direct line system. Used mainly on crude and black oil tankers where separation of oil grades is not so important.

  3. It takes oil directly from the tank to the pump and so reduces friction. This has an affect of increasing the rate of discharge, at the same time improving the tank suction.

  4. It is cheaper to install and maintain than the ring main system because there is less pipeline length and with fewer valves less likelihood of malfunction.

The disadvantages over the ring main system :

  1. Line washing is more difficult. Since there is no circular system and the washings must be flushed into the tanks

  2. The system has fewer valves which make pipeline leaks difficult to control

  3. As the system lacks versatility, there is problem with line and valve segregation

  4. This system provides the vessel to carry as many grades as there are tanks. The disadvantage is the cost factor having a multitude of pumps on board.   

Direct Line.jpg

    Ring Main System

• It is generally of a square or circular layout.

• It is used mostly on product tankers, as segregation of cargo is required.

• The system is expensive because more piping and extra number of valves are used.

• However if the vessel is carrying many grades of cargo, the advantages compensate for the extra cost of the original outlay.

Ring main.jpg


Inert Gas System

        As a matter of policy it is required that tanker ships that have inert gas systems should utilize to keep the tanks under inert conditions all the time which in turn means that whether the tanks are carrying cargo, ballast or whether it is about to arrive in port for loading, the tanks should have inert condition which means that oxygen content throughout the tanks is less than eight percent by volume.


  • It is a gas which contains insufficient Oxygen to support combustion.



  • Properly functioning Inert Gas system is required to maintain cargo tanks in a non-flammable condition.

  • Inert Gas System must deliver Inert Gas with Oxygen content below 5% by volume.

  • Oxygen content of the tanks must not exceed 8% by volume.

IG System.jpg

Components and description of IG system:

The following components are used in a typical inert gas system in oil tankers:

1)      Exhaust gases source: inert gas source is taken from exhaust uptakes of boiler or main engine as contains flue gases in it.


2)      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.

3)      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.

  •   Seawater flows downwards in the scrubber while the flue gases rise upwards and during this process of cross-flow the sea water dissolves the soot and sulfur dioxide as well as cools the gas.

  • There is an extensive arrangement of spray nozzles, perforated plates and venture nozzles inside the tower to maximize the contact between the water and gases. Therefore it acts like a heat exchanger and the exact design of the scrubber tower could vary depending on the make of the machine.

  •  The working conditions require that the scrubber is made out of highly corrosion-resistant material which can withstand the hot corrosive gases that pass through it and there should be ample provision for viewing and cleaning the inside portions of the chamber during maintenance routine.

  •  After the scrubber, the gas passes through a demister which removes excess of water from the flue gases which is done using materials such as polypropylene or cyclone dryers. This gas is now ready to be sent to inert gas blowers which pump this gas further to the required regions.

  •  It should be noted that there is an IG pressure regulating valve which returns any excess inert gas to the scrubber and then the gas passes to the deck water seal going through the vent valve. The vent valve should be opened when the main IG plant is shut down in order to prevent any backflow or pressure build up in the pipelines.

4)     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.

5)      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 the blower fan, a pressure regulator valve is attached after blower discharge which re-circulates the excess gas back to scrubbing tower

6)     The Deck Water Seal

  • A proper functioning deck water seal is the automatic device which permits inert gas to be delivered to the deck main but prevents any backflow of cargo gas, even when the inert gas plant is shut down.

  • Backflow of gas could result in Hydrocarbon gases from the cargo tanks reaching the boiler, leading to an explosion.

  • It is vital that a supply of water is maintained to the seal at all times, particularly when the inert gas plant is shut down.

  • In addition, drains should lead directly overboard and not pass through the machinery spaces.

  • Heating coils system is required to prevent water from freezing when ship operates in sub-freezing conditions.


The deck water seal is the main safeguard against the reverse flow of gases from the distribution system to the IG plant. Deck water seals come in three variants known as

       - Wet type seal,

       - Semi-wet seal and

       - Dry type seal.



  1. It consists of a chamber semi-filled with water

  2. Two pipes for inlet and outlet of flue gases

  3. Two small pipes denote inlet and outlet for sealing water.

  4. There is a demister pad to remove water droplets from gas.


  1. This is the simplest type of water seal

  2. When the inert gas plant is operating & inert gas is being delivered to cargo tanks, the gas bubbles through the water from the submerged inert gas inlet pipe

  3. But if the tank line water is pressed up into this inlet pipe, It will prevent backflow


  1. Water droplets may be carried over with the inert gas which could increase corrosion. A demister should, therefore, be fitted in the gas outlet from the water seal to reduce any carry-over.

Wet DWS.jpg


  1. It consists of a separate holding chamber.

  2. Two pipes for inlet and outlet of flue gases

  3. Two small pipes denote inlet and outlet for sealing water.



  1. When the inert gas plant is operating & inert gas is being delivered to cargo tanks, Inert gas flow draws the sealing water into a separate holding chamber by venturi action, thus avoiding or at least reducing the amount of water being carried over.

  2. But if the tank line water is pressed up into this inlet pipe, It will prevent backflow



  1. Water droplets may be carried over with the inert gas which could increase corrosion.

Semi DWS.jpg


  1. It consists of a chamber and a DROP TANK.

  2. Two pipes for inlet and outlet of flue gases

  3. Automatic valve control system                                                                               

  4. Note : This seal totally eliminates any water carry over and uses automated valve control to deliver water to the seal in case there is any back flow.


  1. The water is drained from the chamber when the Inert gas plant is operating & inert gas is being delivered to cargo tanks.

  2. The Chamber is filled with water when the inert gas plant is shut down.

  3.  Filling and drainage are performed by automatically operated valves which are controlled by the levels of the water seal and drop tank and by the operation of the blowers.


  1. The risk of failure of the automatically controlled valves that may render the water seal ineffective.


7)    Mechanical non return valve: It is an additional non return mechanical device inline with deck seal.


8)      Deck isolating valve: The engine room system can be isolated fully with the deck system with the help of this valve.

9)      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.

 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

1) Abnormal rise of Pressure in Cargo tanks when loaded specified rate of gas outlets
2) Abnormal fall of Pressure in Cargo tanks when cargo is unloaded beyond specified rate of the inert gas blower

3) 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


PV Breaker  Operation

1) 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.

2) 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


PV Breacker 2.jpg

10)     Cargo tank isolating valves: A vessel has numbers of cargo tanks  and each tank  is provided with an isolating valve. The valve controls the flow of inert gas to tank and is operated only by a responsible officer in the vessel.

11)     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.

Mast Riser.jpg

PV Valve is also called Breather Valve is a protection device mounted on a nozzle opening on the top of a fixed roof atmospheric storage tank.

   Its primary purpose is to protect the tank against rupturing or imploding.

  • Without an opening or a controlled opening, a fixed roof atmospheric tank would rupture under increasing pressure caused by pumping liquid into the tank or as a result of vapor pressure changes caused by severe thermal changes. Imploding, or the collapsing of a tank, occurs during the pumping out procedure or thermal changes.

  •  Imploding, or the collapsing of a tank, occurs during the pumping out procedure or thermal changes. As the liquid level lowers, the vapor space pressure is reduced to below atmospheric pressure.

PV Valve 2.jpg
PV Valve.jpg
PV Valve 3.jpg

High Velocity Vent

  • This vacuum condition must be alleviated through a controlled opening on the tank. In short, the tank needs to breathe in order to eliminate the possibility of rupturing or imploding. Because of its primary function, this Valve is called Breather Valve

  • Tank vapours can be released and sent clear of the decks during loading through large, high velocity vents. The type shown below has a moving orifice, held down by a counter weight to seal around the bottom of a fixed cone.

  • Pressure build up in the tank, as filling proceeds, causes the moving orifice to lift. The small gap between orifice lip and fixed cone gives high velocity to the emitted vapour.

  •  It is directed upwards with an estimated velocity of 30 meters per second. Air drawn in by the ejector effect dilutes the plume.

High Velocity PV Breaker.jpg
High Velocity PV Breaker 3.jpg
  • The conical flame screen fixed to the moving orifice to give protection against flame travel will, like the moving parts, require periodic cleaning to remove gummy deposit. The cover is closed (as shown) when the vessel is on passage. A simpler design of vent, having two weighted flaps which are pushed open by the pressure build up to achieve a similar nozzle effect.

Crude Oil Washing


  • COW stands for “CRUDE OIL WASHING”.

  • During the process of COW, cargo tanks are cleaned by means of high-pressure crude oil.

  • COW reduces the quantity of oil remaining on board after discharge.



  • Reduced sludge accumulation

  • Increased cargo out turn

  • Reduced manual cleaning

  • Reduced gas freeing time




  • Increased stay in the discharge port

  • Specialized man power required

  • Increased work load

  • Does not eliminate water washing

  • Need for additional equipments

  • Increases corrosion rate inside the tank



  • Every crude oil tanker above 20,000 DWT shall be fitted with cow system.

  • All ships fitted with COW must have on board a “Crude Oil Washing Manual” approved by the Administration.

  • All ships fitted with COW must be provided with I.G system.

  • Only those cargo tanks that have undergone COW, can be ballasted.

  • Aprox. 25 per cent of the cargo tanks need to be crude oil washed every voyage for sludge control purposes. However, no tank need be crude oil washed for sludge control purposes more than once in every four months



  • Characteristics of the crude oil

    • Wax content,

    • Specific gravity,

  • Temperature of oil

  • System pressure

  • Location & number of washing machines

  • Shadow sectors in the tank

  • Stripping effectiveness

  • Nozzle rotation pitch



  • Fixed washing machines

  • Pumps

  • Stripping system

  • Piping


  • 85% of the VERTICAL surface area inside a tank must be covered by the direct impingement of the jet

90% of the HORIZONTAL surface area inside a tank must be covered by the direct impingement of the jet






  • Notify Terminal about your intension regarding COW.

  • Terminal pre-arrival radio check list to comply with.

  • Communication system on board must be tried.



  • Fixed and portable O2 analyzing equipment tested & working.

  • COW system isolated from the heater.

  • Machine drive units checked.

  • Valves to fixed machines shut.

  • Hydrant valves blanked.

  • Pressure gauges checked.

  • Lines and pumps pressure tested.

  • Stripping system checked



  • COW Plan must be prepared, discussed in pre-arrival meeting and posted in prominent location(s).

  • It must include responsibilities & job description.




  • Pre-arrival checks confirmed to be in order.

  • COW plan discussed during cargo meeting between ship & shore staff.

  • Communication link within vessel & shore to be established.

  • Abort conditions & Procedures to be identified.

  • Fixed analyzer must be calibrated prior start of Inert Gas.

  • IG system & O2 content being delivered to be in good order.

  • O2 content of the tanks confirmed to be below 8%.

  • Positive pressure to be ensured inside the tanks.

  • Responsible person assigned to check leaks.

  • Drive units of machines must be readied.

  • Double check on valves & lines.

  • Ullage floats, if fitted, must be housed.


  • Responsible person stationed on deck

  • Frequent check on quality of IG & record maintained.

  • Prescribed wash pressure maintained.

  • Frequent checks for leaks.

  • Frequent checks to confirm working of machines.

  • Minimum recommended trim.

  • Frequent checks & monitoring of other tanks.

  • Frequent monitoring of tank draining.

  • Continuous monitoring of Slop tank(s).


  • Shut all valves

  • Drain COW line

  • Drain all pumps, tanks & associated pipelines.



  • Properly functioning Inert Gas system is required to maintain cargo tanks in a non-flammable condition.


  • Inert Gas System must be working properly during COW and the content of inert gas being delivered should be below 5% by volume.


  • Oxygen content of the tanks to be crude oil washed below 8% by volume has to be confirmed before washing operations commences.


  • All the cleaning tanks must have positive pressure? (preferably more than 200mmAq)


  • I.G. system, cargo tanks and openings must be tightly closed for avoiding gas leakage to the atmosphere

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