9 INERT GAS SYSTEM (IGS) (STCW Code, section A-V/1 paragraph 10) Learning objectives - which oil tanker must be provided with an inert gas system - the inert gas system - the inert gas plant - the scrubber - the inert gas blowers - the inert gas pressure regulating valve - the non-return devices - the inert gas distribution and venting - gas-analysing, recording and indicating equipment - operations - meters, indicators and alarms - emergency procedures - maintenance and testing 9.1 G ENERAL

Fire and explosion are among the greatest threats to the mariner; a tank ship carrying crude oil, refined

petroleum, or chemicals is an even greater threat. Fortunately, a properly designed, installed, operating, and

maintained inert gas system (IGS) will completely prevent fire and explosion in an intact ship tank.

Combustion is impossible without oxygen; if there is some way to keep the oxygen below about 8 percent, the

ship will be free of danger from explosions in intact tanks. Typically, this is done by adding to the tank

atmosphere a gas that has less oxygen (often 5 percent or less) than air, which has an oxygen concentration of

21 percent. Of course, when a tank is opened, as in a collision, oxygen can enter the tank regardless of the IGS.

In the 1920's and 1930's, one American petroleum company suffered several tank ship fires and decided to

inert the cargo tanks of its ships. Reportedly, its vessels have not had any intact cargo tank fires since that time.

In the mid1970' s, a series of tank ship accidents led to the International Conference on Tanker Safety and

Pollution Prevention (TSPP) of 1978 and the passage of the Port and Tanker Safety Act of 1978 (PTSA).

International rules are contained in Chapter II-2, Regulation 62 (Inert Gas Systems) of the International

Convention for the Safety of Life at Sea (SOLAS), 1974. The first set of amendments to SOLAS 74 were

adopted and came into force on 1 September 1984; under them, a ship must satisfy applicable requirements to

receive a SOLAS Safety Certificate. The second set of amendments to SOLAS 74 come into effect on 1 July

1986; they have only a minor effect on the IGS requirements.

Regulation 60 of SOLAS Chapter II-2 Cargo tank protection 1

For tankers of 20,000 tonnes deadweight and upwards the protection of the cargo tanks deck area and cargo tanks shall be

achieved by a fixed deck foam system and a fixed inert gas system in accordance with the requirements of regulations 61 and 62, except

that, in lieu of the above

installations, the Administration, after having given consideration to the ship's arrangement and equipment, may accept other

combinations of fixed installations if they afford protection equivalent to the above, in accordance with regulation I/5.

2 To be considered equivalent, the system proposed in lieu of the deck foam system shall: .1 be capable of extinguishing spill fires and also preclude ignition of spilled oil not yet ignited; and .2 be capable of combating fires in ruptured tanks. 3 To be considered equivalent, the system proposed in lieu of the fixed inert gas system shall:

.1 be capable of preventing dangerous accumulations of explosive mixtures in intact cargo tanks during normal service throughout the

ballast voyage and necessary in-tank operations; and

.2 be so designed as to minimize the risk of ignition from the generation of static electricity by the system itself. 4

Tankers of 20,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of

carrying crude oil shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than:

.1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and

.2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later except that

for tankers of less than 40,000 tonnes deadweight not fitted with tank washing machines having an individual throughput of greater than

60 m3/h the Administration may exempt such tankers from the requirements of this paragraph, if it would be unreasonable and

impracticable to apply these requirements, taking into account the ship's design characteristics.

5

Tankers of 40,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of

carrying oil other than crude oil and any such tanker of 20,000 tonnes deadweight and upwards engaged in the trade of carrying oil other

than crude oil fitted with tank washing machines having an individual throughput of greater than 60 m3/h shall be fitted with an inert gas

system, complying with the requirements of paragraph 1, not later than:

.1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and .2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later. 6

All tankers operating with a cargo tank cleaning procedure using crude oil washing shall be fitted with an inert gas system

complying with the requirements of regulation 62 and with fixed tank washing machines.

7 All tankers fitted with a fixed inert gas system shall be provided with a closed ullage system. 8 Tankers of less than 20,000 tonnes deadweight shall be provided with a deck foam system complying with the requirements of regulation 61.

9.2 T HE I NERT G AS S YSTEM

There are several ways to inert a tank. The simplest would be to add a pure non- flammable gas, such as

nitrogen or carbon dioxide, to the tank atmosphere. Unfortunately, these pure gases tend to be expensive, the

costs of their storage aboard ship tend to be high, and re-supply in some ports is difficult. Thus, most "inerted"

ships use the gases from a fuel burner, from the ship's propulsion equipment (flue gas), or from a unit

dedicated to producing inerting gas (an inert gas generator (IGG)). The following general description of an IGS

includes components that may vary, depending upon the manufacturer. Use of combustion gases as the inerting

medium proves

advantageous due to its availability as needed and much lower cost, if sufficient fuel and properly adjusted and

operated equipment is provided. Its disadvantage is that the raw combustion gases are impure and must be

treated before use in the cargo tanks. This is especially important for product carriers, in which cargo purity is

critical and some cargoes may react with impurities in the inert gases. Each IGS has several components

intended to remove these impurities. For example, sulphur in the fuel appears in the inert gas in the form of

sulphur oxides, sulphurous acid, and sulphuric acid. If not removed, they will attack the metal of the tank and

gradually destroy it. Passing the inert gas through a water-filled device called a scrubber removes most of these

acids.

installations, the Administration, after having given consideration to the ship's arrangement and equipment, may accept other

combinations of fixed installations if they afford protection equivalent to the above, in accordance with regulation I/5.

2 To be considered equivalent, the system proposed in lieu of the deck foam system shall: .1 be capable of extinguishing spill fires and also preclude ignition of spilled oil not yet ignited; and .2 be capable of combating fires in ruptured tanks. 3 To be considered equivalent, the system proposed in lieu of the fixed inert gas system shall:

.1 be capable of preventing dangerous accumulations of explosive mixtures in intact cargo tanks during normal service throughout the

ballast voyage and necessary in-tank operations; and

.2 be so designed as to minimize the risk of ignition from the generation of static electricity by the system itself. 4

Tankers of 20,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of

carrying crude oil shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than:

.1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and

.2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later except that

for tankers of less than 40,000 tonnes deadweight not fitted with tank washing machines having an individual throughput of greater than

60 m3/h the Administration may exempt such tankers from the requirements of this paragraph, if it would be unreasonable and

impracticable to apply these requirements, taking into account the ship's design characteristics.

5

Tankers of 40,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of

carrying oil other than crude oil and any such tanker of 20,000 tonnes deadweight and upwards engaged in the trade of carrying oil other

than crude oil fitted with tank washing machines having an individual throughput of greater than 60 m3/h shall be fitted with an inert gas

system, complying with the requirements of paragraph 1, not later than:

.1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and .2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later. 6

All tankers operating with a cargo tank cleaning procedure using crude oil washing shall be fitted with an inert gas system

complying with the requirements of regulation 62 and with fixed tank washing machines.

7 All tankers fitted with a fixed inert gas system shall be provided with a closed ullage system. 8 Tankers of less than 20,000 tonnes deadweight shall be provided with a deck foam system complying with the requirements of regulation 61.

9.2 T HE I NERT G AS S YSTEM

There are several ways to inert a tank. The simplest would be to add a pure non- flammable gas, such as

nitrogen or carbon dioxide, to the tank atmosphere. Unfortunately, these pure gases tend to be expensive, the

costs of their storage aboard ship tend to be high, and re-supply in some ports is difficult. Thus, most "inerted"

ships use the gases from a fuel burner, from the ship's propulsion equipment (flue gas), or from a unit

dedicated to producing inerting gas (an inert gas generator (IGG)). The following general description of an IGS

includes components that may vary, depending upon the manufacturer. Use of combustion gases as the inerting

medium proves

advantageous due to its availability as needed and much lower cost, if sufficient fuel and properly adjusted and

operated equipment is provided. Its disadvantage is that the raw combustion gases are impure and must be

treated before use in the cargo tanks. This is especially important for product carriers, in which cargo purity is

critical and some cargoes may react with impurities in the inert gases. Each IGS has several components

intended to remove these impurities. For example, sulphur in the fuel appears in the inert gas in the form of

sulphur oxides, sulphurous acid, and sulphuric acid. If not removed, they will attack the metal of the tank and

gradually destroy it. Passing the inert gas through a water-filled device called a scrubber removes most of these

acids.

9.2.1 Sources Possible sources of inert gas on tankers and combination carriers are:

Uptake gas from the ship’s main or auxiliary boilers.

An independent inert gas generator.

A gas turbine fitted with an afterburner. 9.2.2 Quality

A final oxygen level of 8% or less will be more easily achieved if the oxygen content of the inert gas in the

inert gas main is considerably less than 8%. Ideally the inert gas should not contain oxygen but this is not

possible in practice.

When using flue gas from a main or auxiliary boiler, an oxygen level of less than 5% can generally be

obtained, depending on the quality of combustion control and the load on the boiler.

When an independent inert gas generator or a gas turbine plant with afterburner is fitted, the oxygen content

can be automatically controlled within finer limits, usually within the range 1.5% to 2.5% by volume and not

normally exceeding 5%.

Whatever the source, the gas must be cooled and scrubbed with water to remove soot and sulphur acids before being supplied to the cargo tanks.

The International Convention for the Safety of Life at Sea (SOLAS 1974), as amended, requires that inert gas

systems be capable of delivering inert gas with an oxygen content in the inert gas main of not more than 5% by

volume at any required rate of flow; and of maintaining a positive pressure in the cargo tanks at all times with

an atmosphere having an oxygen content of not more than 8% by volume except when it is necessary for the

tank to be gas free. Existing systems are only required to be capable of

producing inert gas with an oxygen content not normally exceeding 5% by volume, and of maintaining the

tank inerted at all times except when it is necessary for the tank to be gas free.

9.3 I NERT G AS P LANT

The inert gas system referred to in SOLAS regulation 60 shall be designed, constructed and tested to the

satisfaction of the Administration. It shall be so designed and operated as to render and maintain the

atmosphere of the cargo tanks non-flammable at all times, except when such tanks are required to be gas-free.

In the event that the inert gas system is unable to meet the operational requirement set out above and it has

been assessed that it is impracticable to effect a repair, then cargo discharge, deballasting and necessary tank

cleaning shall only be resumed when the “emergency conditions” laid down in the Guidelines on Inert Gas

Systems are complied with.

The system shall be capable of:

Inerting empty cargo tanks by reducing the oxygen content of the atmosphere in each tank to a level at which

combustion cannot be supported;

Throughout this regulation the term cargo tank includes also slop tanks.

Maintaining the atmosphere in any part of any cargo tank with an oxygen content not exceeding 8% by volume

and at a positive pressure at all times in port and at sea except when it is necessary for such a tank to be gas-

free;

Eliminating the need for air to enter a tank during normal operations except when it is necessary for such a tank to be gas free;

Purging empty cargo tanks of a hydrocarbon gas, so that subsequent gas- freeing operations will at no time

create a flammable atmosphere within the tank.

The system shall be capable of 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.

The system shall be capable of delivering inert gas with an oxygen content of not more than 5% by volume in

the inert gas supply main to the cargo tanks at any required rate of flow.

The inert gas supply may be treated flue gas from main or auxiliary boilers. The Administration may accept

systems using flue gases from one or more separate gas generators or other sources or any combination

thereof, provided that an equivalent standard of safety is achieved. Such systems should, as far as practicable,

comply with the requirements of this regulation. Systems using stored carbon dioxide shall not be permitted

unless the Administration is satisfied that the risk of ignition from generation of static electricity by the system

itself is minimized.

9.4 S CRUBBER

A flue gas scrubber shall be fitted which will effectively cool the volume of gas and remove solids and sulphur

combustion products. The cooling water arrangements shall be such that an adequate supply of water will

always be available without interfering with any essential services on the ship. Provision shall also be made for

an alternative supply of cooling water.

Filters or equivalent devices shall be fitted to minimize the amount of water carried over to the inert gas blowers. The scrubber shall be located aft of all cargo tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces of category A. 9.4.1 Boiler Uptake

These valves are located near the main boiler uptake to isolate the IGS scrubber from the boiler uptake.

Alternatively, if a dedicated IGG is used, this valve will be located near the IGG; it is closed when the IGS

system is not operating. Associated with each boiler uptake valve is a steam soot blowing system. A spectacle

blank is also fitted between the boiler uptake valve and the IGS scrubber to ensure complete isolation of the

IGS plant and cargo. This is very important to ensure that inert gas is not introduced into the system during

maintenance.

9.4.2 IGS Scrubber

The raw inert gases are hot and contain soot, sulphur oxides, sulphurous acid, and sulphuric acid; all of these

can be harmful to the cargo and the cargo tanks. The scrubber cools the gases and removes the contaminants

by bubbling the gases through large quantities of seawater (there must be two sources of water for the

scrubber). The gases are then sprayed with additional quantities of water, or rise through a packed bed of

ceramic forms, plastic shapes, or metal trays through which seawater falls, increasing the efficiency of the

water in cleaning the inert gas. The warm, acidic, dirty seawater is then piped overboard. If the scrubber is

provided with a heater to prevent the water from freezing, an automatic control system is installed to prevent

its overheating. The scrubber also acts as an automatic safety device by preventing a backflow of gas to the

boiler uptake or the IGG.

9.4.3 Demister Units

The gas from the scrubber has significant amounts of moisture, both from the burning process and from

bubbling through the seawater in the scrubber. The demister is located close to the scrubber to remove

entrained water from the IGS gas stream. If this water is not removed, it increases the corrosion rate in the

system's piping, valves, and cargo tanks. Water may also contaminate the cargo. The demister may consist of

"pads" or "mattresses" of woven polypropylene or fibreglass, or centrifuge separation (cyclone dryers). There

are many designs, which vary considerably.

Figure 9.4.4

9.5 I NERT G AS B LOWERS

At least two blowers shall be fitted which together shall be capable of delivering to the cargo tanks at least the

volume of gas required by paragraph 3. In the system with a gas generator the Administration may permit only

one blower if that system is capable of delivering the total volume of gas required by paragraph 3 to the

protected cargo tanks, provided that sufficient spares for the blower and its prime mover are carried on board

to enable any failure of the blower and its prime mover to be rectified by the ship's crew.

Two fuel oil pumps shall be fitted to the inert gas generator. The Administration may permit only one fuel oil

pump on condition that sufficient spares for the fuel oil pump and its prime mover are carried on board to

enable any failure of the fuel oil pump and its prime mover to be rectified by the ship's crew.

The inert gas system shall be so designed that the maximum pressure, which it can exert on any cargo tank,

will not exceed the test pressure of any cargo tank. Suitable shutoff arrangements shall be provided on the

suction and discharge connections of each blower. Arrangements shall be provided to enable the functioning of

the inert gas plant to be stabilized before commencing cargo discharge. If the blowers are to be used for gas

freeing, their air inlets shall be provided with blanking arrangements. The blowers shall be located aft of all

cargo tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces of category A.

9.5.1 Fan Units

Two or more independent blowers are located near the demister to draw the inert gas through the scrubber and

the demister and deliver it to the IGS distribution system at the required pressure. Since the greatest need for

inert gas is during offloading, the blower capacity is set at 125 percent of the maximum rated capacity of the

cargo pumps. This provides a margin of safety to ensure that no air enters the cargo tanks. This capacity may

be provided by two blowers of equal size, or by one large and one small blower. Separate inlet and discharge

valves are fitted to each blower unit. On most plants, the valves are hand operated, but on some the discharge

valves are combined with the main and auxiliary pressure regulating valves.

Figure 9.2

(3) The dry seal operates normally dry and is filled with water when the IG plant is shut down or when tank

pressure exceeds the IG pressure. This system requires more operating parts, reacts slower and is not deemed

as reliable as the wet type. Dry seals are not approved for use on U.S. vessels. See Figure 9.3 for a more

complete description.

In the dry type seal, the water is drained from the seal when the IG plant is in operation (gas flowing to the

tanks), and filled with water when the IG plant is either shut down, or the tank pressure exceeds the IG blower

discharge pressure. Filling and drainage are performed by automatically operated valves controlled by the

levels in the water seal and the drop tank, and by the operating state of the blowers. Vessels must be equipped

with seals that are completely passive in operation, so that failure of sensors, control systems, or moving parts

cannot cause failure to establish a seal. Active seals, such as the dry seal shown above, are not acceptable.

Figure 9.3

9.8 I NERT G AS DISTRIBUTION AND VENTING 9.8.1 General

The inert gas main may be divided into two or more branches forward of the non-return devices. The inert gas

supply main shall be fitted with branch piping leading to each cargo tank. Branch piping for inert gas shall be

fitted with either stop valves or equivalent means of control for isolating each tank. Where stop valves are

fitted, they shall be provided with locking arrangements, which shall be under the control of a responsible

ship's officer. The control system operated shall provide positive indication of the operational status of such

valves.

9.8.2 Gas Distribution

When the inert gas is clean, cool, and water-free, it is pressurized and sent to the various cargo tanks. The

distribution system contains backflow prevention devices, control valves, alarms, and automatic shutdowns.

These alert personnel to a malfunction and, when necessary, shut down the system before a dangerous

condition is developed in the tanks. Such conditions involve the oxygen concentration, the temperature and

pressure of the inert gas, the water flow to and water level within the scrubber, and the backflow prevention

devices (water seals). Because IG systems are complex and subject to malfunction, it is important for ships'

crews to inspect and maintain them in good working order.

9.8.3 Combination carriers

In combination carriers, the arrangement to isolate the slop tanks containing oil or oil residues from other tanks

shall consist of blank flanges which will remain in position at all times when cargoes other than oil are being

carried except as provided for in the relevant section of the Guidelines on Inert Gas Systems.

9.8.4 Overpressure or vacuum

Means shall be provided to protect cargo tanks against the effect of overpressure or vacuum caused by thermal

variations when the cargo tanks are isolated from the inert gas mains. Piping systems shall be so designed as to

prevent the accumulation of cargo or water in the pipelines under all normal conditions.

Suitable arrangements shall be provided to enable the inert gas main to be connected to an external supply of inert gas. 9.8.5 Venting, inerting, purging

The arrangements for the venting of all vapours displaced from the cargo tanks during loading and ballasting

shall consist of either one or more mast risers, or a number of high velocity vents. The inert gas supply main

may be used for such venting.

The arrangements for inerting, purging or gas-freeing of empty tanks shall be to the satisfaction of the

Administration and shall be such that the accumulation of hydrocarbon vapours in pockets formed by the

internal structural members in a tank is minimized and that:

On individual cargo tanks the gas outlet pipe, if fitted, shall be positioned as far as practicable from the inert gas/air inlet. The inlet of such outlet pipes

may be located either at deck level or at not more than 1 m above the bottom of the tank;

The cross-sectional area of such gas outlet pipe shall be such that an exit velocity of at least 20 m/s can be

maintained when tanks are being simultaneously supplied with inert gas. Their outlets shall extend not less

than 2 m above deck level;

Each gas outlet shall be fitted with suitable blanking arrangements;

If a connection is fitted between the inert gas supply mains and the cargo piping system, arrangements shall be

made to ensure an effective isolation having regard to the large pressure difference, which may exist between

the systems. This shall consist of two shutoff valves with an arrangement to vent the space between the valves

in a safe manner or an arrangement consisting of a spool-piece with associated blanks.

The valve separating the inert gas supplies main from the cargo main and which is on the cargo main side shall be a non-return valve with a positive means of closure. One or more pressure/vacuum-breaking devices shall be provided to prevent the cargo tanks from being subject to:

A positive pressure in excess of the test pressure of the cargo tank if the cargo were to be loaded at the

maximum rated capacity and all other outlets are left shut; and

A negative pressure in excess of 700 mm water gauge if cargo were to be discharged at the maximum rated

capacity of the cargo pumps and the inert gas blowers were to fail.

Such devices shall be installed on the inert gas main unless they are installed in the venting system or on individual cargo tanks.

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.

This subsystem consists of a single inert gas main running the entire length of the cargo deck, starting at the

deck isolating valve aft and ending at the vent valve forward. One or more pressure/vacuum devices are fitted

to the inert gas main to prevent the cargo tanks from being over or under-pressurized. The inert gas main

contains a means for receiving an outside source of inert gas when the IGS is not functioning. From the inert

gas main, individual branch lines run to the cargo tanks. Stop valves or equivalent closures are fitted at each

branch line, so that each cargo tank can be isolated from the inert gas system. If there is a connection between

the inert gas main and the cargo piping system, there must be valves or similar closures to isolate the systems

from one another.

Each tank vessel has a venting system that is capable of venting vapours displaced from the cargo tanks during

loading and ballasting. There are also pressure/vacuum (P/V) valves to protect the cargo tanks from

overpressure or vacuum resulting from thermal variation when the cargo tanks are isolated from the inert gas

mains.

Some of the possible arrangements are:

(1) A single common venting system, using the IGS deck main and branch lines from each tank and venting to

the atmosphere through one or more mast risers or high velocity vents. Precautions must be taken to prevent an

arrangement by which tank vents can be blocked off;

(2) A common venting system, using a separate vent main and vent lines from each tank, with the system

venting to the atmosphere through one or more mast risers or high velocity vents; or

(3) Individual vents on each tank, using either standpipes (vent stacks) or high velocity vents. 9.9 G AS-A NALYSING, RECORDING AND INDICATING EQUIPMENT Instrumentation shall be fitted for continuously indicating and permanently recording, when the inert gas is being supplied:

The pressure of the inert gas supply mains forward of the non-return devices; and

The oxygen content of the inert gas in the inert gas supply mains on the discharge side of the gas blowers.

The devices shall be placed in the cargo control room where provided. But where no cargo control room is

provided, they shall be placed in a position easily accessible to the officer in charge of cargo operations.

In addition, meters shall be fitted:

In the navigation bridge to indicate at all times the inert gas pressure forward of the non-return devices and the

pressure in the slop tanks of combination carriers, whenever those tanks are isolated from the inert gas supply

main; and

In the machinery control room or in the machinery space to indicate the oxygen content of the inert gas in the

inert gas supply mains on the discharge side of the gas blowers

Portable instruments for measuring oxygen and flammable vapour concentration shall be provided. In addition,

suitable arrangement shall be made on each cargo tank such that the condition of the tank atmosphere can be

determined using these portable instruments.

Suitable means shall be provided for the zero and span calibration of both fixed and portable gas concentration measurement instruments. For inert gas systems of both the flue, gas type and the inert gas generator type, audible and visual alarms 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 blowers; Oxygen analyser

Oxygen content in excess of 5% by volume;

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;

Gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the

pressure in slop tanks in combination carriers can be monitored at all times; and

High gas pressure forward of the non-return devices. For inert gas systems of the inert gas generator type, additional audible and visual alarms shall be provided to indicate:

Insufficient fuel oil supply;

Failure of the power supply to the generator;

Failure of the power supply to the automatic control system for the generator. Automatic shutdown of the inert gas blowers and gas-regulating valve shall be arranged on predetermined limits being reached. Automatic shutdown of the gas-regulating valve shall be arranged in respect of failure of the inert gas blowers.

In respect of the oxygen content of the inert gas exceeds 5% by volume, immediate action shall be taken to

improve the gas quality. Unless the quality of the gas improves, all cargo tank operations shall be suspended so

as to avoid air being drawn into the tanks and the isolation valve shall be closed.

Alarms shall be fitted in the machinery space and cargo control room, where provided, but in each case in such

a position that they are immediately received by responsible members of the crew.

In respect of low water level in the water seal the Administration shall be satisfied as to the maintenance of an

adequate reserve of water at all times and the integrity of the arrangements to permit the automatic formation

of the water seal when the gas flow ceases. The audible and visual alarm on the low level of water in the water

seal shall operate when the inert gas is not being supplied.

An audible alarm system or automatic shutdown of cargo pumps shall be provided to operate on predetermined limits of low pressure in the inert gas main being reached.

Tankers constructed before 1 September 1984, which are required to have an inert gas system, shall at least

comply with the requirements of regulation 62 of chapter II-2 of the International Convention for the Safety of

Life at Sea, 1974.

Detailed instruction manuals shall be provided on board, covering the operations, safety and maintenance

requirements and occupational health hazards relevant to the inert gas system and its application to the cargo

tank system. The manuals shall include guidance on procedures to be followed in the event of a fault or failure

of the inert gas system.

9.10 OPERATION 9.10.1 Introduction The purpose of an IG system is to keep the oxygen content of the vapour space below the level needed for combustion. For crude carriers, the oxygen content of the inert gas delivered to the cargo tanks should be no more than 5 percent, to ensure that there is not enough oxygen in the tank to support combustion.

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% by volume except when the

tanks need to be gas-free.

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

Inerting during discharging and ballasting

Inerting during tank cleaning

Purging prior to gas freeing and use of the IGS during gas freeing. The vessel used as an example in this paper is fitted with so-called “purge pipes” for ventilation and purging. (see Fig. 1 on page 19 of this chapter)

On board vessels without purge pipes but with stand-pipes (i.e. vent pipes from deck level and about 2500 mm

up vertically from deck level), these pipes are to be opened instead of the purge pipes mentioned in this paper.

The vessel is not fitted with SBT (Segregated Ballast Tank) capacity according to MARPOL 73/78, and ballast

water has to be carried in cargo oil tanks on ballast voyages.

IG operation panel 9.10.2 Inerting of empty gas-free tanks

Start up the inert gas plant according to instructions. Close all tank hatches and check proper function of O2

analyser. Open the lids or hatch covers on standpipes or purge pipes on tanks to be inerted.

Open fan discharge valve (6) and main isolating valve (10) (see Fig. 1 on page 19 of this chapter)

If the gas pressure control valve (7) is on “automatic”, there 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.

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 backpressure is established in the tanks. When

this minimum gas pressure for stable operation is established, the inert gas plant can be operated automatically

at full capacity.

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 an O2 content of less than 8% for a minimum of 30 minutes.

This is because of the possibility of local air pockets in the 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 instructions and the tank isolating valves closed.

Fig. 1 indicates the plant in operation Figure 1 Condition: Inerting of tanks filled with air 9.10.3

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

9.10.4 Discharge of ballast

Discharge of ballast can be done either before or during the loading. During discharge of ballast, before

loading is commenced, the inerting procedure is the same as during discharge of cargo. See below.

At simultaneous loading of cargo and discharge of ballast, the loading capacity is normally higher than the

ballast water discharge capacity. This means that the inert gas volume available in the usage 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. 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. See Fig. 2.

The inert gas system can be operated if required, but will normally not be required to

deliver any gas to the deck lines.

If by chance the ballast discharge rate is higher than the loading rate, the inert gas system must be in operation

and the deck pressure adjusted sufficiently high to give a positive outflow of inert gas through the ventilation

mast (or the individual P/V valves on the tanks). This is to avoid air being sucked into the tank system by a

possible under pressure in the ballast tanks.

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;

Gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the

pressure in slop tanks in combination carriers can be monitored at all times; and

High gas pressure forward of the non-return devices. For inert gas systems of the inert gas generator type, additional audible and visual alarms shall be provided to indicate:

Insufficient fuel oil supply;

Failure of the power supply to the generator;

Failure of the power supply to the automatic control system for the generator. Automatic shutdown of the inert gas blowers and gas-regulating valve shall be arranged on predetermined limits being reached. Automatic shutdown of the gas-regulating valve shall be arranged in respect of failure of the inert gas blowers.

In respect of the oxygen content of the inert gas exceeds 5% by volume, immediate action shall be taken to

improve the gas quality. Unless the quality of the gas improves, all cargo tank operations shall be suspended so

as to avoid air being drawn into the tanks and the isolation valve shall be closed.

Alarms shall be fitted in the machinery space and cargo control room, where provided, but in each case in such

a position that they are immediately received by responsible members of the crew.

In respect of low water level in the water seal the Administration shall be satisfied as to the maintenance of an

adequate reserve of water at all times and the integrity of the arrangements to permit the automatic formation

of the water seal when the gas flow ceases. The audible and visual alarm on the low level of water in the water

seal shall operate when the inert gas is not being supplied.

An audible alarm system or automatic shutdown of cargo pumps shall be provided to operate on predetermined limits of low pressure in the inert gas main being reached.

Tankers constructed before 1 September 1984, which are required to have an inert gas system, shall at least

comply with the requirements of regulation 62 of chapter II-2 of the International Convention for the Safety of

Life at Sea, 1974.

Detailed instruction manuals shall be provided on board, covering the operations, safety and maintenance

requirements and occupational health hazards relevant to the inert gas system and its application to the cargo

tank system. The manuals shall include guidance on procedures to be followed in the event of a fault or failure

of the inert gas system.

9.10 OPERATION 9.10.1 Introduction The purpose of an IG system is to keep the oxygen content of the vapour space below the level needed for combustion. For crude carriers, the oxygen content of the inert gas delivered to the cargo tanks should be no more than 5 percent, to ensure that there is not enough oxygen in the tank to support combustion.

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% by volume except when the

tanks need to be gas-free.

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

Inerting during discharging and ballasting

Inerting during tank cleaning

Purging prior to gas freeing and use of the IGS during gas freeing. The vessel used as an example in this paper is fitted with so-called “purge pipes” for ventilation and purging. (see Fig. 1 on page 19 of this chapter)

On board vessels without purge pipes but with stand-pipes (i.e. vent pipes from deck level and about 2500 mm

up vertically from deck level), these pipes are to be opened instead of the purge pipes mentioned in this paper.

The vessel is not fitted with SBT (Segregated Ballast Tank) capacity according to MARPOL 73/78, and ballast

water has to be carried in cargo oil tanks on ballast voyages.

IG operation panel 9.10.2 Inerting of empty gas-free tanks

Start up the inert gas plant according to instructions. Close all tank hatches and check proper function of O2

analyser. Open the lids or hatch covers on standpipes or purge pipes on tanks to be inerted.

Open fan discharge valve (6) and main isolating valve (10) (see Fig. 1 on page 19 of this chapter)

If the gas pressure control valve (7) is on “automatic”, there 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.

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 backpressure is established in the tanks. When

this minimum gas pressure for stable operation is established, the inert gas plant can be operated automatically

at full capacity.

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 an O2 content of less than 8% for a minimum of 30 minutes.

This is because of the possibility of local air pockets in the 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 instructions and the tank isolating valves closed.

Fig. 1 indicates the plant in operation Figure 1 Condition: Inerting of tanks filled with air 9.10.3 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. In case only local P/V valves are fitted, the valves are checked and adjusted for evacuation of gas through the high-speed valves. It should be checked that all tank hatches are closed and possible float level indicators are operable.

9.10.4 Discharge of ballast

Discharge of ballast can be done either before or during the loading. During discharge of ballast, before

loading is commenced, the inerting procedure is the same as during discharge of cargo. See below.

At simultaneous loading of cargo and discharge of ballast, the loading capacity is normally higher than the

ballast water discharge capacity. This means that the inert gas volume available in the usage 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. 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. See Fig. 2.

The inert gas system can be operated if required, but will normally not be required to deliver any gas to the deck lines.

If by chance the ballast discharge rate is higher than the loading rate, the inert gas system must be in operation

and the deck pressure adjusted sufficiently high to give a positive outflow of inert gas through the ventilation

mast (or the individual P/V valves on the tanks). This is to avoid air being sucked into the tank system by a

possible under pressure in the ballast tanks.