Inert Gas System 4f164d

Inert Gas System/Crude Oil Washing COW/IGS

Introduction • In the late sixties, and until today, the shipping world has been shocked by several severe tanker explosions. In most cases, it is believed that a proper use of the Inert Gas in the tanks might have saved the ships and many lives.

Introduction • Consequently, national authorities and the classification societies established rules and specifications for the construction and use of inert gas system a first on optional bases, but now on compulsory for larger ship carrying combustible in bulk.

Introduction • Onboardship there will always known and unknown potential ignition sources, such as sparks, flames, hot sports, electrostatic charges, etc., powerful enough to the ignition of gases.

Introduction • The three main conditions for releasing fire or explosions are: • Presence of combustibles • Presence of oxygen • Presence of an ignition source.

Introduction • Of these three elements, the easiest to control no doubt is the atmosphere of the tanks by removing the oxygen and replacing it with inert gas.

Introduction • Even when the ship is fitted with inert gas facilities, there may be moments and operational modes when explosive mixtures exist for shorter and longer periods.

Introduction • For the safety of the ship and its crew it is obviously of the great importance that the inert gas plant itself, the maintenance and adjustment, as well as the control and understanding of the operation of both the ship and the inert gas are adequate anytime.

Introduction

• The explosion triangle is used as a symbol to warm of the simultaneous occurrence of:

FLAMMABLE GAS OXYGEN IGNITION SOURCES • Crude oil and sediments from crude oil gives off hydrocarbon gases. When crude oil or sediments are ignited and burn, hydrocarbon gases are releases from the surface and react with the gaseous oxygen in the air and heat is generated.

FLAMMABLE GAS OXYGEN IGNITION SOURCES • During a fire the strong generation of heat will tend to release increasing amounts of hydrocarbon gases from the surface, thus making fire extinguishing more difficult.

FLAMMABLE GAS OXYGEN IGNITION SOURCES • When hydrocarbon gases from crude oil or sediments in a cargo tank are mixed air, an explosion may occur if the mixture is ignited.

Explosive Limits: • Whether a mixture of hydrocarbon gas and air will be ignited depends on the ratio of the mixture.

Explosive Limits: • Flammable gas mixed with is Too rich • UEL Upper Explosion Limit • Flammable (Explosive) • LEL Lower Explosion Limit Too lean

Explosive Limits: • “Too rich” denotes a mixture where the concentration of gas is so high that it cannot be ignited. “Too lean” denoted a mixture where the concentration of gas is so light that it cannot be ignited.

Explosive Limits: • For hydrocarbon gases from crude oil and sediments, it is usually assumed that the upper explosion limit lies at about 10 percent by volume of hydrocarbon gas in the air and the lower at about 1 percent.

Difference between explosion and an oil fire is: • In explosion, the hydrocarbon gases and the oxygen in the air are already mixed before the ignition take place, so that the combustion takes place very rapidly. The temperature and pressure increase until the pressure is so great that the tank is burst open.

Difference between explosion and an oil fire is: • In an oil fire in the open air the hydrocarbon gases are given off from the surface in limited amount and the supply of oxygen is limited by the air circulation.

HYDROCARBON/OXYGEN MIXTURES

Difference between explosion and an oil fire is: • The ultimate in safety would be achieved using a pure Inert Gas, e.g. Nitrogen or Helium, indeed certain cargoes require inerting with Nitrogen to avoid contamination, but this imposes cost for specific installations and the need to find space on board on which to house them.

Difference between explosion and an oil fire is: • At this time the most economical approach is to use a mixture of gases with inert characteristics and an oxygen content below 8%. Most Inert Gas System are designed to achieve a normal oxygen content below 5% , and are monitored to indicate a high oxygen content of 8%.

Hydrocarbon gases • Crude oil has been from organic matter, that is from animal and plant residues and contains several thousand different chemical compounds, from the simplest compounds to some very complex ones. Most of this materials are composed of only the hydrogen (H) and carbon (C), and these are called by the common name hydrocarbons.

Hydrocarbon gases • A carbon atom has altogether four bonds which can link the atom to other atoms, while a hydrogen atom has only one bond which can link the atom to other atoms.

Hydrocarbon gases

Hydrocarbon gases Hydrocarbon gases: • METHANE • ETHANE • PROPANE • BUTANE

CH4 C2H6 C3H8 C4H10

Hydrocarbon gases • Hydrocarbons with up to four carbon atoms are gases at room temperature and atmospheric pressure. Hydrocarbons with from 5 to 16 atoms are liquids, and solid materials such as wax and asphalt contains more than 16 carbon atoms per molecules.

Table for Boiling Point for Hydrocarbons METHANE

CH4

-16°C

ETHANE

C2H6

-89

PROPANE

C3H8

-44

BUTANE

C4H10

-1

PENTANE

C5H12

+36

HEXANE

C6H14

+68

Hydrocarbon gases • When crude oil is forced or pumped out of the well, both hydrocarbon gases and solid materials are dissolved in it. When the pressure is reduced, gases will bubble out. To separate these gases the crude oil must through one or more process units (stabilizer)

Hydrocarbon gases

• This crude is called stabilized crude

Hydrocarbon gases • Even a stabilized crude will continuously gives off hydrocarbons from the surface to the atmosphere above. It includes gaseous hydrocarbons (methane, ethane, propane, butane and liquids, pentane, hexane, etc.).

Hydrocarbon gases • Inert means inactive, and the term Inert Gas is used for gases which do not react chemically. When the hydrocarbon gas burns in the air, it is the oxygen in the air which reacts while the nitrogen gas (inert gas) does not take part in the reaction.

Hydrocarbon gases • Examples of inert gas are Nitrogen (N2), Carbon Dioxide (CO2) or combustion gases.

Hydrocarbon gases Composition of Inert Gas

Good quality Inert Gas

N

80% by vol

Nitrogen

77% by volume

CO2

14 by vol

Carbon Dioxide

13% by volume

O2

2-5 by vol

Oxygen

5 % by volume

H2O

5 by vol

H2O

CO

0.01 by vol

Carbon Monoxide

.01% by volume

NO2

0.02 by vol

Nitrogen Oxide

.02% by volume

SO2

0.30 by vol

Sulfur Dioxide

.30%

Solid particles 300 mg/m3

5 – 10 mg/m3

Temp

27 – 30 deg. C

450 Deg C

Regulation that requires Inert Gas System For Tanker Ships

• The International Conference on Tanker and Pollution held in February 1978 ed resolution 5 recommending that the International Maritime Organization develop Guidelines

Regulation that requires Inert Gas System For Tanker Ships

• to supplement the requirements of amended Regulation 62 of Chapter 11-2 of the 1974 SOLAS convention by taking into the arduous operating conditions of inert gas systems and the need to maintain them to a satisfactory standard. In addition, Regulation 62.1 requires that an inert gas system shall be designed, contracted and tested to the satisfactions of the istration.

Regulation that requires Inert Gas System For Tanker Ships • These Guidelines have accordingly been developed to supplement and complement the Convention requirements for inert gas systems. They are offered to istration to assist them in determining appropriate design and constructional parameters and in formulating suitable operational procedures when inert gas systems are installed in ships flying the flag of the States.

Application • The status of these Guidelines is advisory. They are intended to cover the design and operation of: – inert gas systems that are required on new tankers by Regulation 60 of the Chapter II – 2 of the 1978 SOLAS Protocol and in accordance with Regulation 62.

Application – inert gas systems that are required on existing tankers by Regulation 60 of Chapter II-2 of the Protocol and in accordance with Regulation 62.20; – inert gas system which are fitted but not required to comply with the requirements of Regulation 60 Chapter II – 2 of the 1978 SOLAS Protocol.

Application • However, for existing inert gas systems the Guidelines are directed primarily of operational procedures and are not intended to be interpreted as requiring modifications to existing equipment other than those which are required on ships to which Regulation 62.20 applies.

Application • The content of these Guidelines is based on current general practice used in the design and operation of inert gas systems using flue gas from the uptake of the ship’s main or auxiliary boilers, and installed on crude oil tankers and combination carriers.

Application • The guidelines do not exclude other sources of inert gas, such as systems incorporating independent inert gas generators, other designs, materials or operational procedures. All such divergence should be carefully assessed to ensure that they achieve the objectives of these Guidelines.

Definitions of • Inert gas means a gas or a mixture of gases, such as flue gas, containing insufficient oxygen to the combustion of hydrocarbons. • Inert condition means a condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8% or less by volume by addition of inert gas.

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

Definitions of • Inert gas distribution system means all piping, valves and associated fittings to distribute inert gas from the inert gas plant to cargo tanks, to vent gases atmosphere and to protect tanks against excessive pressure or vacuum.

Definitions of • Inert gas system means an inert gas plant inert gas distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instruments and control devices.

Definitions of • Inerting means the introduction of inert gas into a tank with the object of attaining the inert condition. • Gas-freeing means the introduction of fresh air into a tank with the object of removing toxic, flammable and inert gases and increasing the oxygen content to 21% by volume.

General • Certification of inert gas system requirements under SOLAS 1974, as amended (MSC/Cir. 485)

I.G. Generator provided an independent of gas

INERT GAS GENERATOR SYSTEM

General • The Committee noted that the actual compliance dates in the 1978 SOLAS Protocol differ from the actual compliance’s date in SOLAS 74, as amended in 1981, however all of this compliance date have ed and an inert gas system would be required on tankers as per Regulation II – 2/60,

General • regardless of deadweight and date of construction using a crude washing system to clean cargo tanks and tankers of 20,000 tons DWT and upwards contracted for after 1 June 1979 or delivered after 1 June 1982 to have an inert gas system installed.

General • For tankers contracted for and delivered the above dates, the 1978 SOLAS Protocol requires an inert gas system on all such tankers of 20,000 tons DWT and upward except that:

General 1. A tanker of 20,000 tons DWT and above but less than 40,000 tons DWT carrying crude oil is not required to have an inert gas system: – if it is not fitted with tank washing machine having an individual throughput of greater than 60 cubic meters per hour; and

General - if the istration determines it is unreasonable and impractical due to the ship’s design characteristics, to install an inert gas system;

General A tanker of 20,000 tons DWT and above but less than 40,000 tons DWT carrying oil other than crude oil is not required to have inert gas system if it is not fitted with tank washing machine having an individual throughput of greater than 60 cubic meters per hour.

General • The inert gas system consist of three distinct parts: – the part that produces the inert gas (Boilers, Generators) – the part the cool and clean the gas (Scrubber) – the part that distribute the gas (Piping and valves)

General • The inert gas is cooled and clean in the scrubber where: – the undesirable by-products and water are remove it – its temperature is reduced to approximate 30 deg. C.

General • The inert gas must be kept under positive pressure to prevent the ingress of air. This means that it must be above atmospheric pressure. • The distribution system takes the inert gas to the cargo tanks.

General • A precaution must be taken to prevent the backflow of gas from the cargo tank to the generating plant because: – the gas may cause explosion in the generating plant.

General • The dangers of malfunction of the inert gas system are as follows: – a rise in the oxygen content of the gas – a drop in the supply pressure – insufficient cooling and cleaning of the scrubber – back flow of the hydrocarbon gas to the generating plant

Inert gas plant • The sources of inert gas in an oil tankers are: – The main and auxiliary boilers (Flue gas) – An independent inert gas generator – A gas turbine plant with after burner

Inert gas plant • The main purpose of the plant is to produce a good quality inert gas with a low oxygen content (5% by volume or less) – In the case of flue gas, the quality or inert gas depends on the boiler load. – In port, the boiler load depends mainly on the number of cargo pumps in service.

Inert gas plant – If the discharge rate gas to be reduced a way to increase the boiler load may have to be found. This may be achieved by circulating seawater through appropriate piping by means of the ballast pump. – The inert gas is let into the system by means of the inert gas uptake valve or valves.

Scrubber • The purpose of the scrubber is to cool the flue gas and remove most of the sulfur dioxide and particulate soot. All three actions are achieved by direct between the flue gas and large quantities of seawater.

Scrubber • The scrubber should be of a design related to the type of the tanker, cargoes and combustion control equipment of the inert gas supply source and be capable of dealing with the quantity of inert gas required by Regulation 62 at the designed differential of the system.

Scrubber • The internal part of the scrubber should be constructed in corrosion resistant materials in respect of the corrosive effect of the gas. Alternately, the internal parts may be line with rubber, glass fiber epoxy resin or other equivalent materials.

HOT GAS ENTRY • Hot gas is fed to the base of the Scrubber tower from the uptake and may immediately enter a wet seal which is used to protect against tank atmosphere entering the uptake and flue gas entering the tower when the plant is shut down.

HOT GAS ENTRY • In a dry bottom scrubber this protection may be achieved by a double flange air sealed uptake valve. In addition all hot gas lines may be fitted with blanking arrangements.

HOT GAS ENTRY • It is essential to drop the gas temperature very quickly on entry to the Scrubber to bring down the level at which H2SO4 is produce. In some designs, the water of the seal is used whilst in others a precooling spray system is fitted in the hot gas line. The bottom of the hot gas line may also be extended to form an elephants foot to increase gas water .

TYPICAL SCRUBBER INTERNAL LAYOUT

HOT GAS ENTRY • The design should be such that under normal condition of trim and list, the scrubber efficiency will not fall by more than 3% nor will the temperature rise at the gas outlet exceed the designed gas outlet temperature by more than 3°C.

HOT GAS ENTRY • The location of the scrubber above the load waterline should be such that the drainage of the effluent is not impaired when the ship is in the fully loaded condition.

Inert gas blowers • Blowers are use to deliver the scrubbed flue gas to the cargo tanks. Reg. 62.3.1 requires that least two blowers shall be provided which together shall be capable to delivering inert gas to the cargo tanks at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume.

Inert gas blowers • In practice, installations vary from those which have one large blower and one small blower, whose combined total capacity complies with Regulation 62, to those in which each blower can meet this requirement.

Inert gas blowers • The advantage claimed for the former, is that it is convenient to use small capacity blower when topping up the gas pressure in the cargo tank at sea; the advantage claimed for the latter is that if either blower is defective, one is capable of maintaining a positive gas pressure in the cargo tanks with-out extending the cargo discharge.

Inert gas blowers • The deck water seal and mechanical non return valve together from the means of automatically preventing the backflow of cargo gases from the cargo tanks to the machinery space or other safe area in which the inert gas plant is located.

Inert gas blowers • The blower must have the capability to be used for gas-freeing operation as per Pa. 3.12. 2.3.

Inert gas blowers • The blower casing should be constructed in corrosion resistant material or alternatively of mind steel but then its internal surfaces should be stove coated, or lined with rubber or glass fiber epoxy resin or other equivalent material to protect it from corrosive effect of the gas.

Inert gas blowers • Substantial drains, fitted with adequate water seals, should be provided in the casing to prevent damage by an accumulation of water. Sufficient openings in the casing should be provided to permit inspection.

Inert gas blowers • An audible and visual shall be provided to indicate: – low water pressure or low water flow rate to the flue gas scrubber – high water level in the flue gas scrubber – high gas temperature – failure of the inert gas blower – oxygen content in excess of 8 percent by volume

Inert gas blowers – failure of the power supply to the automatic control system for the gas regulating valve and to the indicating devices. – low water level in the water seal

Inert gas blowers – gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the pressure in the slop tanks in combination carriers can be monitored at all times; and – high gas pressure

Inert gas blowers • In the system with gas generations audible and visual alarms shall be provided in accordance with 19.1.1, 19.1.3, 19.1.5, to 19.1.9 and additional alarms to indicate:

Inert gas blowers – insufficient oil supply – failure of the power supply to the generator – failure of the power supply to the automatic control system for the generator.

Inert gas blowers • A means shall be provided for continuously indicating the temperature and pressure of the inert gas at the discharge side of the gas blowers, whenever the gas blowers are operating.

Inert gas blowers • Automatic shutdown of the inert gas blowers and gas regulating valve shall be arranged on predetermined limits being reach in respect to low water pressure or low flow rate in the scrubber, high water level in the scrubber and high gas temperature.

Inert gas blowers • The blower pressure/volume characteristics should be matched to the maximum systems requirements. The characteristics should be such that in the event of the discharge capacity of the ship expressed as in volume, a minimum pressure of 200 mm water gauge is maintained in any cargo tank after allowing for pressure loss due to:

Inert gas blowers • the scrubber tower and demister; • the piping conveying the hot gas to the scrubbing tower; • the distributing piping downstream of the scrubber; • the deck water seal; and • the length and diameter of the inert gas distribution;

Inert gas pressure regulation valve • Pressure control arrangement should fitted to fulfill two function: – To regulate the flow of inert gas to the inert gas deck main.

Inert gas pressure regulation valve – To prevent automatically any backflow of gas in the event of either failure of the inert gas blower, scrubber pump, etc. or when the inert gas plant is operating correctly but the deck water seal and mechanical non-return valve failed and the pressure of gas in the tank exceed the blower discharge pressure, during simultaneous stripping and ballasting operation.

Inert gas pressure regulation valve • The different ways or arrangement for controlling the inert pressure in the inert gas main: – throttling the regulating valve – Re-circulating the inert gas to the scrubber – leading inert gas to the atmosphere

Inert gas pressure regulation valve

• Systems with automatic pressure control and gas re-circulating line.

Inert gas pressure regulation valve • These installation permit control of inert gas pressure pump in the deck main without having to adjust the inert gas blower control speed.

Inert gas pressure regulation valve • Gas not required in the cargo tanks is recirculating valves are fitted in the both the main and re-circulating lines; one is controlled by a gas pressure transmitter and regulator, while the other may be controlled either in a similar manner or by a weight operated valve.

Inert gas pressure regulation valve • The pressure transmitter is cited downstream of the isolating valve; this enables a positive pressure to be maintained in the cargo tanks during discharge. However, it does not necessarily ensure that the scrubber is not overloaded during inerting and purging operations.

Non-return devices • The deck water seal and mechanical nonreturn valve together from the means of automatically preventing the backflow of cargo gas from the cargo tank to the machinery space or other space area in which the inert gas plant is located.

Non-return devices • Deck water seal is the principal barrier fitted 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 shutdown.

Non-return devices • Wet type seal – 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 line, but if the tank pressure exceeds the pressure in the inert gas inlet pipe, the water is pressed up into this inlet pipe and thus prevent backflow.

Non-return devices • Semi dry type – Instead of bubbling through the water trap the inert gas flow draws the sealing water into separate holding chamber by venturi action thus avoiding or at least reducing the amount of water droplets being carried over.

Non-return devices • Dry type – In this type the water is drained when the inert gas plant is in operation and filled with water when the inert gas plant is either shut down or tank pressure exceeds the inert gas blower discharge pressure.

Non-return devices • Dry type – Filling and drainage are performed by automatically operated valves controlled by the levels in the water seal and drop tanks and by the operating state of the blowers. The advantage of this type is that water carry is prevented.

Non-return devices • Dry type – The drawback could be the risk of failure of the automatically controlled valves which may render the water seal ineffective.

Inert gas distribution and venting • The inert gas distribution system consist of: – the inert gas main, which run from the deck isolating valve forward along the cargo tanks area. – branch lines, which run from the inert gas main to the individual tanks.

Inert gas distribution and venting • For gas-freeing and tank entry, some valve or blanking arrangement is always fitted to isolate individual cargo tanks from the inert gas main deck. • Inert gas piping may also serve as vent piping; and in this case, inert gas main ends in the vent riser.

Inert gas distribution and venting • The inert gas and venting system must allow for: – gas freeing – purging – inerting – breathing – cargo and ballast handling – tank entry

Inert gas distribution and venting • For these purposes, the following provision must be made: – blanks or valves to isolate tanks – vent stack or vent risers – p/v valves – liquid-filled p/v breakers

Inert gas distribution and venting • State a typical inert and vent piping arrangement illustrating the location of the above provision and describe the above operations.

Gas analyzing recording and indicating equipment • Arrangement for oxygen analyzer, recorder, and indicating equipment should be as follows:

Gas analyzing recording and indicating equipment • The sampling point for the oxygen analyzer and recorder unit should be located at a position in the pipework after the blowers and before the gas pressure regulating valve. At the chosen position turbulent flow conditions prevail at all outputs of the blowers. The sample point should be easily accessible and be provided with suitable air or steam cleaning connections.

Gas analyzing recording and indicating equipment • The sampling probe should incorporate a dust filter in accordance the instruments manufacturers advice. The probe and filter should be capable of being withdrawn and cleaned or renewed as necessary.

Gas analyzing recording and indicating equipment • The sensing pipe from the sampling probe to the oxygen analyzer should be so arranged that any condensation in the sensing pipeline does not prevent the gas sample reaching the oxygen analyzer. ts in the pipeline should be kept to a minimum to prevent the ingress of air.

Gas analyzing recording and indicating equipment • Any cooler required in the sensing pipe should be installed at the coldest point in the system; alternatively, in certain instances it may be prudent to heat the sensing pipes to prevent consideration.

Gas analyzing recording and indicating equipment • The position of the analyzer should be so chosen that it is protected from heat and adverse ambient conditions, but it should be placed as close as practicable to the sampling point to educe the time between the extraction of a sample and its analysis to a minimum.

Gas analyzing recording and indicating equipment • The recording unit and repeater indication required by Regulation 62.16 should not be located in positions subject to heat and undue vibration.

Gas analyzing recording and indicating equipment • The resistance of the connecting cables between the analyzer and the recorder should be in accordance with the instrument manufacturer’s instruction. • The oxygen analyzer should have an accuracy of +/ - 1% of the full scale deflection of the indicator.

Gas analyzing recording and indicating equipment • Dependent on the principle of measurement, fixed zero and / or span calibration arrangements should be provided in the vicinity of the oxygen analyzer fitted with suitable connection for portable analyzers.

Gas analyzing recording and indicating equipment • A sampling point should be provided between the automatic gas pressure valve and deck water seal for use with portable instruments.

Gas analyzing recording and indicating equipment • In accordance with Regulation 62.17 portable instruments shall be provided for measuring oxygen and flammable vapor concentration. With regard to the hydrocarbon vapor meter, it should be borne in mind that meters working the catalytic filament principle are unsuitable for measuring concentration in oxygen deficient atmospheres.

Gas analyzing recording and indicating equipment • All metal parts of portable instruments and sampling tubes requiring to be introduced into tanks should be securely earthed to the ship structure while the instrument and sampling tube are being used. These portable instruments should be of an intrinsically safe type.

Operations • Though flue gas system differ in detail certain basic principle remain the same.

Operations • These are: 1. Starting up the inert gas plant; 2. Shutting down the inert gas system; 3. Safety check when the inert gas plant is shutdown.

Operations • Start up procedure should be as follows: – Ensure boiler is producing flue gas with an oxygen content of 5% by volume or less (for existing ship 8% by volume or, whenever practicable, less). – Ensure that power is available for all control, alarm and automatic shutdown operations.

Operations – Ensure that quantity of water required by the scrubber and deck water seal is being maintained satisfactory by the pump selected for this duty. – Test operation of the alarm and shutdown features of the system department upon the throughout of the water in the scrubber and deck seal.

Operations – Check that the gas freeing fresh air inlet valves, where fitted, are shut and the blanks in position are secure. – Shut off the air to any air sealing arrangement for the flue gas isolating valve. – Open the flue gas isolating valve.

Operations – Open the selected blower suction valve. Ensure that the other blower suction and discharge valves are shut unless it is intended to use both blowers simultaneously. – Start the blower. – Test blower ‘failure” alarm. – Open the blower discharge valve.

Operations – Open the recirculating valve to enable plant to stabilize. – Open the flue gas regulating valve. – Check the oxygen content is 5% by volume or, whenever practicable, less then close the vent to atmosphere between the gas pressure regulating valve and the deck isolating valve. – The inert gas system is now ready to deliver gas to the cargo tank.

Shutdown procedure: – When all tank atmosphere have been checked for an oxygen level of not more than 8% and the required in-tank pressure has been obtained, shut the deck isolating/non-return valve. – Open vent to atmosphere between the gas pressure regulating valve and the deck isolating/non-return valve.

Operations – Shut the gas pressure regulating valve. – Shut down the inert gas blower. – Close the blower suction and discharge valve. Check that the drains are clear. Open the water washing system on the blower while it is still rotating with the power supply of he driving motor turned off, unless otherwise recommended by the manufacturer. Shut the water washing plant after a suitable period.

Operations – Close the flue gas isolating valve and open the air sealing system. – Keep the water supply on the scrubber tower in the accordance with the manufacturer’s recommendation. – Ensure that the water supply to the deck water seal is running satisfactorily, that an adequate water seal is retained and that the alarm arrangements for it are in order.

Safety checks when the inert gas plant is shut down: • The water supply and the water in the level in the deck seal should be ascertained at regular intervals, at least once per day depending on water conditions. • Check the water level loops installed in pipework for gas, water or pressure transducers, to prevent the back flow of hydrocarbon gas into gas-safe spaces.

Safety checks when the inert gas plant is shut down: • In cold weather, ensure that the arrangements to prevent the freezing of sealing water in the deck seals, pressure/vacuum breaker etc. are in order. • Before the pressure inerted cargo tanks drops to 100 mm they should be repressured with inert gas.

Cleaned and gas-freed tank should be inerted prior to loading, ensuring that: • purge pipes and vents are opened to the atmosphere. • those openings are closed when the oxygen content has fallen below 8% by volume.

Safety checks when the inert gas plant is shut down: • tanks are pressurized in excess of 100 mm water gauge and kept in common with the inert gas main. • re-inerting after the breakdown follows the same procedure.

Safety checks when the inert gas plant is shut down: “ that no sounding ullaging or sampling equipment should be lowered into tank during inerting.”

Before discharge of cargo tank ballast is undertaken, the following condition should be checked.

• All cargo tanks are connected up to the inert gas system and all isolating values in the deck pipework are locked open. • All other cargo tank openings are shut. • All valves isolating the mast risers from the inert gas system are shut.

Before discharge of cargo tank ballast is undertaken, the following condition should be checked.

• The requirements required by Regulation 62.13.4.1 are used to isolate the cargo main from the inert gas main. • The inert gas plant is producing gas of an acceptable quality. • The deck isolating valve is open.

Precaution to be taken when loading cargo • During the loaded age a positive pressure of inert gas of at least 100 mm water gauge should be maintained in the cargo tanks and topping up of the pressure may be necessary. When topping up the inert gas pressure in the cargo tanks, particular attention should be paid to obtaining an oxygen concentration of 5% or less in the inert gas supply before introducing the gas supply into the cargo tanks.

Precaution to be taken when loading cargo • On motor tankers, the boiler loading may have to be increased in order that the low oxygen concentration in the inert gas supply can be achieve. It may also be necessary to restrict the output of the inert gas blowers to prevent air being drawn the uptake during the topping up operation.

Ballasting of cargo tanks • The conditions for ballasting of cargo tanks are the same of those for loading in 5.3. When, however, simultaneous discharge and ballasting is adopted, then a close watch should be kept on the inert gas main pressure.

Ballast condition • During the ballast voyage, tanks other than those required to be gas free for necessary tank entry should be kept inerted with the cargo tank atmosphere at a positive pressure of not less than 100 mm water gauge having oxygen level not exceeding 8% by volume especially during tank cleaning.

Ballast condition • Before any inert gas is introduced into cargo tanks to maintain a positive pressure it should be established that the inert gas contains not more than 5% by volume of oxygen.

Tank Cleaning • Tank cleaning should be washed in inert condition and under positive pressure. The procedures adopted to tank cleaning with water should follow those for crude oil washing.

Tank Entry • The entry of personnel to the cargo tank should be carried out only under the close supervision of a responsible ships officer and in accordance with national rules and or with the normal industrial practice laid down in the ISGOTT.

Tank Entry • Regulation 55 (a) (i) of Part E, Chapter II2, 1974 SOLAS Convention as amended by the 1978 Protocol implies, inter alia, that Reg. 60 and 62 do not apply to tankers carrying petroleum products having a flashpoint exceeding 60ºC.

Tank Entry • In order words, products carriers may carry bitumen, lubricating oils, heavy fuel oils, high flashpoint jet fuels and some diesel fuels, gas oils and special boiling point liquids without inert gas system having to be fitted, or, if fitted, without tanks containing such cargoes having to kept in the inert condition.

Tank Entry • If cargoes with a flashpoint exceeding 60oC. Whether heated or otherwise, are carried at temperatures near or to or above their flashpoint a flammable atmosphere can occur. When cargoes with a flashpoint exceeding 60oC are carried at a temperature higher than 5oC below their flashpoint they should be carried in an inerted condition.

Product Contamination • Contamination of a product may effect its odor, acidity or flashpoint specifications, and may occur in several ways; those relevant to ships with an inert gas main interconnecting all cargo tanks are:

Product Contamination 1. Liquid contamination due to overfilling of a tank. 2. Vapor contamination through the inert gas main. This is, largely a problem of preventing vapor from low flashpoint cargoes, typically gasoline, containing the various high flashpoint cargoes listed in 6.1. 1, plus aviation gasoline and most hydrocarbon solvents. This problem can be overcome by:

Product Contamination – Removing vapors of low flashpoint cargoes prior to loading; and – Preventing ingress of vapors of low Flashpoint cargoes during loading and during the loaded voyage.

Combination Carriers • The basic principle of inerting are exactly the same on a combination carrier as on tanker. However, difference in design and operation of this vessels and relevant considerations must be made:

Combination Carriers • Slack Holds – It is particularly important for combination carriers to have their holds inerted because whenever a hold in a OBO carrier is partially filled with clean or oily ballast, water agitation of this ballast can occur at small angles of roll and this can result in the generation of static electricity.

Combination Carriers • Leakage – To ensure that leakage of tank gas, particularly through the hatch center-line ts, is eliminate or minimizes, it is essential that the hatch covers are inspected frequency to determine the state of the seals, their alignment, etc.

Combination Carriers • Ballast and void spaces – The cargo holds of combination carriers are adjacent to ballast and void spaces. Leakage may occur in pipelines or ducts in these spaces, or by fracture in the boundary plating; in this event there is a possibility that oil, inert gas and hydrocarbon gas may leak into the ballast and void spaces.

Combination Carriers • Inert gas distribution system – Due to the special construction of combination carriers, the vent line from the cargo hatchway coaming is situated very close to the level of the cargo surface. In many cases, the inert gas main line ing along the main deck may be below the boil level in the hold.

Combination Carriers • Inert gas distribution system – During rough weather oil or water may enter these lines and completely block the opening and thus prevent an adequate supply of inert gas during either tank cleaning or discharge. Vent lines should therefore have drains fitted at their lowest point and these should always be checked before any operation takes place within the cargo hold.

Combination Carriers • Applications when carrying oil – On combination carriers the inert gas system should be utilized in the manner described in Section 5 when the ship is engaged exclusively in the carriage of oil.

Meters, indicators and alarms • Meters and indicators in the inert gas system: – Means shall be provided for continuously indicating the temperature and pressure of the inert gas at the discharge side of the blowers, whenever the gas blowers are operating.

Meters, indicators and alarms – means for continuously indicating and recording inert gas pressure forward of non return devices. – means for continuously indicating and recording the oxygen content at the discharge side of the gas blowers.

Meters, indicators and alarms • The above devices shall be placed in the cargo control where provided. If no cargo control room is provided, they shall be placed in a position easily accessible to the officer in watch.

Meters, indicators and alarms • In addition, meters shall be fitted in the navigating bridge to indicate at all times the pressure in the slop tanks of combination carriers whenever those tanks are isolated for the inert gas supply main; and • In the machinery control room or in the machinery space to indicate oxygen content.

Emergency procedure • In the event of total failure of the inert gas system to deliver required quality and quantity of inert gas and maintain a positive pressure in the cargo tanks and slop tanks, action must be taken immediately to prevent any air being drawn into the tank.

Emergency procedure • All cargo tank operations should be stopped, the deck isolating valve should be closed and the vent between it and the gas pressure regulating valve should be opened and immediate action should be taken to repair the inert gas system.

Emergency procedure • In the case of tankers engaged in the carriage of crude oil it is essential that the cargo tanks be maintained in the inerted condition to avoid the hazard of pyrophoric iron sulfide ignition.

Emergency procedure • If it is assessed that the tanks cannot be maintained in an inerted condition before the inert gas system can be repaired, an external supply of inert gas should be connected to the system through the arrangements required by Regulation 62.11.5 as soon as Practicable, to avoid air being drawn into the cargo tanks.

Emergency procedure • In the case of product carriers, if it is considered to be totally impracticable to effect a repair to enable the inert gas system to deliver the required quality and quantity of gas and maintain a positive pressure in the cargo tanks.

Emergency procedure • Cargo discharge and deballasting may only be resumed provided that either an external supply of inert gas is connected to the system through the arrangement required by Regulation 62.11.5 or the following precautions are taken:

Emergency procedure 1. In the case of tankers built on or after 1 September 1984, the venting system is checked to ensure that approved devices to prevent the age of flame into cargo tanks are fitted and that these device are in a satisfactory condition.

Emergency procedure 2. In the case of tankers built before 1 September 1984 the flame screens are checked to ensure that they are in a satisfactory condition. 3. The valves on the vent must risers are opened.

Emergency procedure 4. No free fall of water or slop is permitted. 5. No dripping, ullaging, sampling or other equipment should be introduced into the tank. If it is necessary for such equipment to be introduced into the tank, this should be done only after at least 30 minutes have elapsed since the injection of inert gas ceased.

Maintenance and testing

List of inspection and checks to be made on:

Inert gas scrubber • Inspection may be made through the manholes. Checks could be made for corrosion attracts, fouling and damage to; – Scrubber shell and bottom – Cooling water pipes and spray nozzles (fouling); – Float switches and temperature sensor; – Other internals such as trays and demister filter

Inert gas scrubber • Checks should be made for damages to non-metallic part such as: 1. internal linings; 2. demisters; 3. packed beds.

Inert gas blowers • To a limited degree, interval visual inspection will reveal damages an early stage. Diagnostic monitoring system should be used as they great assist in maintaining the effectiveness of the equipment.

Inert gas blowers • Inspection of the inert gas blowers should include: – internal inspection of the blower casing for soot deposits or signs of corrosive attack; – examination of fix or portable washing system; – inspecting of the functioning of the fresh water flushing arrange where fitted.

Inert gas blowers – inspection of the drain lines from blower casing to ensure that they are clear and operative; and – observation of the blower under running conditions for signs of excessive vibrations, indicating too large an imbalance.

Deck Water Seal • Inspection of the deck water seal should include: – Opening for internal inspection to check for: – blockage of the Venturi lines in semidry type water seals – corrosion of inlet pipes and housing – corrosion of heating coils

Deck Water Seal – corrosion or sticking floats for water drain supply valves and level monitoring – Testing for Function: – Automatic filling and draining; check with local level gauge if possible – presence of water carry over during operation

Non-return valve • The non-return valve should be opened for inspection to check for and the valve seat. Functioning of the valve should be tested in operation.

Scrubber effluent line • The scrubber effluent line cannot normally be inspected internally except when the ship is in dry dock. The shipside stub piece and the overboard discharge valve should be inspected at each dry-docking period.

Types of Water Seals

•

DISPLACEMENT TYPE, WET SEAL WITH NON RETURN VALVE

Types of Water Seals

• DISPLACEMENT TYPE “SEMI-DRY” WITH NON RETURN VALVE

DRY TYPE WITH NON-RETURN VALVE

INERT GAS SYSTEM

INERT GAS SYSTEM 1. Boiler gas uptake or inert gas generator

10. Deck line isolating valve

2. Gas uptake valve

11. Tank isolating valve

3. Scrubber

12. Ventilation mast (riser)

4. Fan isolating valve suction side

13. Pressure/Vacuum breaker (common)

4a. Fresh air intake valve

14. P/V valves (individual)

5. Fans

15. P/V valve in ventilation line

6. Fan isolating valves pressure side

16. By- valve

7. Pressure control valve

17. Tank hatch

8. Deck water seal

18. Level indicator

9. Non-return valve

19. Purge pipe

INERT GAS SYSTEM • Including checking: 1. all alarm and safety function 2. the functioning of the flue gas isolating valves 3. operation of all remotely or automatic controlled calves 4. the functioning of the water seal and non-return valve

INERT GAS SYSTEM 5. the vibration levels of the inert gas blowers 6. for leakage 7. the interlocking of soot blowers 8. oxygen measuring equipment, both portable and fixed, for accuracy

THE INERT GAS SYSTEM Position No.

Component

Main Function

1

Gas uptake or generator

Stack gas supply

2

Gas uptake isolating valves

Isolating of inert gas plant from boilers generators

3

Inert gas scrubber

Cooling of gas and removal of SO2 and solid particles

3a

Seawater inlet

Cooling and washing inlet to scrubber

3b

Seawater outlet

Cooling and washing water outlet to scrubber

THE INERT GAS SYSTEM Position No.

Component

Main Function

4

Isolating valve suction side

Fan isolating valve suction side

5

Fan

Fan for transport of inert gas to tanks

6

Isolating valve

Fan isolating valve pressure side

7

Deck Water Seal

8

Deck Water Seal

Prevent backflow of hydrocarbon gases from tanks to engine room during shutdown

THE INERT GAS SYSTEM Position No.

Component

Main Function

9

Non-return valve

Prevent backflow of hydrocarbon gases or crude oil in the event of over filling

10

Tank isolating valve

For isolating of tanks from inert gas system

11

Tank isolating valve

For filling of cargo tanks

12

Mast Ventilation

For ventilation or relieving gas pressure from tanks

THE INERT GAS SYSTEM Position No.

Component

Main Function

13

Pressure/vacuum A common pressure vacuum valve in breaker addition to individual breaker p/v – valves on tanks. Prevention of damage in case of over pressure or under pressure in tanks.

14

Gas analyzer

Control of the inert gas with respect to high oxygen content.

Gas analyzer

Oxygen content meter with recorder

Pressure meter

Pressure meter with indicator and recorder in deck main line

14a 15

THE INERT GAS SYSTEM • In this paper the principles for the operation of inert gas plants for the different operational modes of oil tankers will be discussed.

THE INERT GAS SYSTEM • On board oil tankers required to have an inert gas system the cargo tanks should preferably at all times be inerted and have a tank atmosphere with an oxygen content not exceeding 8 percent by volume except when the tanks need to be gas free.

THE INERT GAS SYSTEM • This means that during normal operation of oil tankers the following operational modes frequently take place: – Inerting of empty tanks – Inerting during loading and simultaneous discharge of ballast – Inerting during loaded sea voyage

THE INERT GAS SYSTEM – Inerting during discharging and ballasting – Inerting during tank cleaning – Purging prior to gas freeing and use of the IGS during gas freeing

THE INERT GAS SYSTEM • On board vessels without purge pipes but with stand pipes (i.e. vent pipes from deck level and about 2,500 mm up vertically from deck level), these pipes are to be opened instead of the purge pipes mentioned in this paper.

THE INERT GAS SYSTEM • The vessel is not fitted with SBT (Segregated Ballast Tank) capacity according to MARPOL 73/79, and ballast water has to be carried in cargo oil tanks on ballast voyages.

Inerting of Tanks

Inerting of empty gas free tanks • Start up the inert gas plant according to instruction. Close all tank hatches and check proper function of O2 analyzer. Open lids or hatch covers on stand pipes or purge pipes on tanks to be inerted.

Inerting of empty gas free tanks

• Open fan discharge valve (6) and main isolating valve (10)

Inerting of empty gas free tanks • If the gas pressure control valve (7) is on “automatic,” these is now a risk of “overload” of the scrubber due to a limited pressure drop in pipes and tanks. The “overload” of the scrubber may result in “carry over” of water or high temperature trip of the plant.

Inerting of empty gas free tanks • Because of the above, it is therefore sometimes necessary to control the pressure manually to reduce the gas flow during the first part of the inerting, until a minimum gas back pressure is established in the tanks.

Inerting of empty gas free tanks • When the minimum gas pressure for stable operation is established, the inert gas plant can be operated automatically at full capacity.

Condition: Inerting of tank filled with air

Inerting of empty gas free tanks 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valve 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Inerting of empty gas free tanks • The oxygen-content in the tanks should be checked frequently, at least every half hour. The inerting should be continued until the gas leaving the tank has had a O2 content of less than 8 percent for a minimum of 30 minutes. This is because of the possibility of local air pockets in the tanks.

Inerting of empty gas free tanks • When all tanks are satisfactorily inerted the tanks should be put under a slight overpressure, normally 300-600 mm W.G., and the plant closed down according to instruction and the tank isolating valves closed.

Inerting during loading and simultaneous discharge of ballast water • The vessel is supposed to arrive at the loading port with all cargo tanks inerted. • If the ship is fitted with a central gas vent outlet, all tanks to be loaded are connected to the vent system.

Inerting during loading and simultaneous discharge of ballast water • In case only the local P/V valves are fitted, the valves are checked and adjusted for gas evacuation through the high speed valves. • It should be checked that all tank hatches are closed and possible float level indicators are operable.

Discharge of ballast • Discharge of ballast can be done either before or during the loading. During discharge of ballast, before loading is commended, the inerting procedure is the same as during discharge of cargo.

Discharge of ballast • At simultaneous loading of cargo and discharge of ballast the loading capacity. This means that the inert gas volume available in the ullage space above the oil level in the tanks being loaded is more than sufficient for the inert gas needed in the ballast tanks during discharge.

Discharge of ballast • By simply connecting the ballast tank to the cargo tanks with the inert gas lines on deck, inert gas will flow from the cargo tanks to the ballast tanks. • The inert gas system can be operated if required, but will normally not deliver any gas to the deck lines.

Discharge of ballast • If by chance the ballast discharge rate is higher than the loading rate, the inert gas system must be on operation and the deck pressure adjusted sufficiency high to give a positive outflow or inert gas through the ventilation mast (or the individual P/V – valves on the tanks).

Discharge of ballast • This is to avoid air being sucked into the tank system by a possible under pressure in the ballast tanks.

Condition: Simultaneous loading and deballasting

Simultaneous loading and deballasting 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Loading • During loading without deballasting it is normally not necessary to operate the inert gas system.

Loading or ballasting

Loading or ballasting 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Loading or ballasting • The ventilation mast – P/V valve should be in open position or by ed during this operation. This is to allow free flow of the inert gas above the oil level to the atmosphere during the loading.

Loading or ballasting • When the loading operation is finished, the byed valve must be closed and the P/V – valve adjusted to normal operating conditions. If the ship is fitted with individual P/V valves on the tanks, these should be adjusted to normal operating conditions accordingly.

Loading or ballasting • At this moment the “ullage” volume in the tanks is filled with inert gas mixed with hydrocarbon-gases at about atmosphere pressure. If this inert gas and HC-gas mixture has an oxygen content of 8 percent or below, this atmosphere is nonexplosive and safe.

Loading or ballasting • In order to start the loaded voyage with positive pressure in the tanks, the IGS has to be started and tanks purged to a pressure of 300 – 600 mm W.G.

Loaded sea voyage • During the loaded voyage the cargo tanks should as far as possible be kept inerted with a positive pressure. • This positive pressure may, however, be disturbed by several factors.

Loaded sea voyage • The most common are: – leakages in valves and hatch covers – change of pressure in the tanks due to temperature variations (i.e. day and night and sea/air temperature changes) – rolling and having in rough sea

Loaded sea voyage • The effect of the different conditions as mentioned above is partly a pressure drop in the tank (escape of inert gas) and partly the risk of ingress of air into the tanks (temperature aspiration and local underpressure in tanks due to rolling and heaving)

Loaded sea voyage • Consequently, the oxygen content and the tank pressure should be frequently checked during the sea voyage. The frequency should depend on weather and deck equipment conditions.

Loaded sea voyage • “Topping up” of the tank inert gas pressure may be done by starting up of the inert gas system, or by using a special “topping up” inert gas generator, if fitted, or by starting up the IGS. The volume needed for this topping-up operation is normally small in loaded condition.

“Topping –up” of tanks

“Topping –up” of tanks 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Discharging and ballasting

Discharging • When the vessel arrives at the discharge port, the inert gas system should be tested and ready for operation in due time for discharge. • Before cargo oil pumping is started the inert gas system should be in operation and connected to the deck line, with pressure control in the automatic position.

Discharging • Since the ullage volume is normally small in loaded condition, the desired overpressure is reached in a short time (minutes). • All deck openings and hatches should be closed, all P/V - valves in the operating position and by- valves closed.

Discharging • When the pumping (discharge) starts the pressure in the ullage volume will drop. Now the control valve (7) will start operating and open for inert gas to compensate the pressure drop and keep a constant preselected pressure in the tanks.

Discharging • After some time the required pressure is established. At this moment the volume delivered from the inert system is equal to the cargo pump delivery.

Discharging

Discharging • If for any reason access to the cargo tanks is necessary during discharge the following procedure should be followed: 1. Reduce the inert gas pressure by adjusting the pressure control valve set point.

Discharging 2. When the tanks inert gas pressure is reduce to near atmospheric pressure, a suitable ullage hatch has to be opened carefully. 3. When reading is finished, the hatch may be closed and pressure raised if desired.

Discharging • When discharge and stripping are finished, the tank should be put under the desired positive pressure.

Ballasting during discharge of cargo • Normally, time be saved by taking in ballast while discharging. This is normally done in the last period (during stripping) of the discharge.

Ballasting during discharge of cargo • This means that some tanks displace inert gas while some consume inert gas. During the stripping of the last tanks, the discharge rate is normally less than the ballast rate. In this case there is a surplus of inert gas, and the inert gas system may, if desired, be stopped.

Simultaneous discharge and ballasting

Simultaneous discharge and ballasting 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Ballasting during discharge of cargo • The procedure for these operations is the same as for discharging cargo. In case the ballasting rate exceeds the stripping rate, however, the by valve in the ventilation must be open (or in the case of individual P/V – valves, the valves must be opened).

Ballasting during discharge of cargo • When the vessel is ballasted the by- valve should be closed and the tank inert gas pressure raised to the desired pressure before the main isolating valve is closed and the inert gas pant stopped.

Ballast Voyage • After the vessel has left the discharge port, the inert gas system should be in operation for purging of all cargo tanks to reduce the HC-gas concentration.

Ballast Voyage • The operation should be controlled by checking the HC-gas concentration in the gas leaving the purge pipes, or with tests taken at different levels in the tanks.

Ballast Voyage • When the tanks have been purge and the HC-gas concentration checked to be below ab. 2.5 percent by volume, the tanks should be pressurized and the inert gas system closed down.

Tank Cleaning • The oxygen content in the tank atmosphere should always be checked before any tank cleaning is stated. No tank cleaning, either with the cargo oil - Crude Oil Washing (COW) – or with water, should be started unless the oxygen content is 8 percent by volume or less measured in the tanks.

Tank Cleaning • For tankers with Crude Oil washing Systems the following procedure is to be complied with.

Tank Cleaning • Before each tank is crude oil washed, the oxygen shall be determined at a point 1 meter from the deck and at the middle region of the ullage space and neither of these determinations shall exceed 8percent by volume.

Tank Cleaning • Where tanks have complete or partial wash bulkheads, the determination should be taken from similar levels in each section of the tank. The oxygen level of the inert gas being delivered during the washing process shall be continuously monitored. If during crude oil washing:

Tank Cleaning (i) the oxygen level of the inert gas being delivered exceed 8 percent by volume; or (ii) the pressure of the atmosphere at the tanks is no longer positive, then the washing must be stopped until satisfactory conditions are restored.

Tank Cleaning

Gas freeing • When access to the cargo tanks is necessary to inspection, repair a.s.o. the inert gas or inert gas/HC-gas mixture has to be replaced with fresh air. This replacement is called “gas freeing.”

Gas freeing • The gas freeing is normally carried out by one or more of the following three methods: – By portable tank ventilators – By permanently installed tank ventilators blowing air to tanks through the cargo oil piping system. – By using the inert gas system fans with suction from fresh air instead of the scrubber.

Gas freeing • Whenever a method is being used for gas freeing the following steps should be taken: 1. Hydrocarbon gas concentration to be measured in each tank to be gas freed. 2. If the HC-gas concentration is 2.5% by volume or less ventilation with fresh air may start immediately.

Gas freeing 3. If the HC-gas concentration is above 2.5% by volume the tanks should be purged with inert gas until HC-gas concentration is 2.5% by volume or less before ventilation with fresh air starts. 4. Ventilation should continue until HCgas concentration is 5% LEL or less and the oxygen level is 21% by volume before the tank is certified gas free.

Gas freeing • Gas freeing can take place either by a dilution method or by a displacement method. • Using the dilution or mixing method the fresh air is blown into the tank at deck level, forced down to the bottom at the tank and ventilated out at deck level

Gas freeing • By the displacement method the air is blown in at deck and is forced out of the tank near the bottom through the pipe.

Gas freeing • When a turbulent mixing flow is wanted using the dilution method, the opposite is wanted using the displacement method. Consequently the different methods require different air inlet nozzle arrangements where the fresh air enters the tank.

Gas freeing by the dilution method

• Fresh air is blown in by a fan on the tank deck

Gas freeing by the displacement method • Introduction of fresh air by moderate blowing from the tank top and discharge through purge pipe.

Gas freeing with portable tank ventilators • If the vessel is fitted with standpipes, the displacement method for gas freeing should be used.

Gas freeing with portable tank ventilators • This means that no particular requirement has to be put on the blowers except that they should give as little turbulence in the air inlet area as possible, for minimum mixing between the tank atmosphere and the fresh air.

Gas freeing with portable tank ventilators • If the mixing method is used, the inlet air should have an inlet speed sufficient to force the air jet down to the bottom of the tank (30-40 m/sec). If it is not possible the blower should be fitted with canvas hoses or similar to bring the fresh air outlet to the tank bottom.

Ventilation with permanently installed ventilators

Ventilation with permanently installed ventilators 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

Gas freeing with the inert gas fans • The modern IGS is generally fitted with a fresh air suction duct to the suction side of the fans. This can be connected to the fans instead of the normal suction line to the scrubber.

Gas freeing with the inert gas fans • When the fresh air ducts are connected to the fans, the inert gas system is started the normal way. Closed the openings on deck, but open the purge pipelines or adjust local P/V valves in open position. The by- valve in the ventilation mast should be closed.

Gas freeing with the inert gas fans

Gas freeing with the inert gas fans 1. Boiler gas uptake or inert gas generator 2. Gas uptake valve 3. Scrubber 4. Fan isolating valve suction side 4a. Fresh air intake valve 5. Fans 6. Fan isolating valves pressure side 7. Pressure control valve 8. Deck water seal

9. Non-return valve 10. Deck line isolating valve 11. Tank isolating valves 12. Ventilation mast (riser) 13. Pressure/Vacuum breaker (common) 14. P/V valves (individual) 15. P/V valve in ventilation line 16. By- valve 17. Tank hatch 18. Level indicator 19. Purge pipe

END INERT GAS