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CHEMICAL TANKER MANUAL

CH. 07 NITROGEN HANDLING / INERTING AND GAS FREEING

Rev. No: 1

Issue : 15-Apr-16

Page : 1 of 26

7.1. NITROGEN HANDLING / INERTING AND GAS FREEING

NITROGEN HANDLING AND MONITORING OF NITROGEN OPERATIONS

Nitrogen operations such as Purging, Padding and Gas Freeing are to be closely monitored by ship’s

officers to ensure that as a minimum the following are dealt with adequately:

1. Warning signs on tanks and at access points to the vessel.

2. Access Control to the vessel during Nitrogen operations.

3. Setting up of lines, valves and other equipment.

4. Venting and gas freeing of tanks containing Nitrogen.

5. Tank Pressures and oxygen content during operations and during the voyage.

7.1.1. Purpose

Cargo tanks on chemical tankers are generally purged or carried under inert conditions for reasons of

quality control and/or for safety reasons so as to:

a. Prevent fire / explosion hazard.

b. Prevent unwanted and hazardous reaction between cargo / air.

c. Prevent moisture pick-up by cargoes.

Additionally shippers may have their own special requirements to ensure cargo quality.

USAGE OF NITROGEN

The Inert gas requirement for the Oil and Chemical tankers has been amended .

Amendment apply to Oil and chemical tanker constructed on or after 01st January 2016.

The amendments do not have any retro-active effect for existing tankers constructed before

01st January 2016

Amendment has been included in SOLAS CH II,IBC Code,and the FSS code ch.15

General for all tankers

Deadweight limit for inert gas requirement has been lowered from 20,000tonnes deadweight to 8000

tonnes deadweight. Therefore, all oil and chemical tankers of 8000 dwt and above must be provided

with a fixed inert gas system

The oxygen limit for inert gas supplied to cargo tanks has been changed from 8% to 5%. This may

affect the way in which the oxygen detection alarm is set.

Uncontrolled when Printed



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Specific requirement applicable for chemical tankers

The exemption for existing chemical tankers having cargo tank volumes’ not exceeding 3000 m3 and

having tank cleaning machine throughput not exceeding 17.5 m3/h per nozzle and total throughput

not exceeding 110 m3/h per tank, does not apply to new chemical tankers constructed on or after 1st

January 2016.

For tankers that are arranged with an exhaust gas based inert gas system, inerting must be carried

out during loading, on voyage, during unloading, tank cleaning and for purging prior to gas freeing

with air.

For chemical tankers, it is however accepted that inert gas need only be applied before commencing

the unloading. It is understood that this exemption clause was accepted as a matter of convenience to

reduce cargo handling time (as pre-loading procedures for chemical tankers always require in-tank

cargo surveys).

A very important condition for the above alternative is that nitrogen is the only accepted inert gas

medium.

Although nitrogen does not need to be applied until before commencing the unloading, it is still

required to be applied during discharging, during tank cleaning and for purging prior to gas freeing

with air.

The application of inert gas before commencing the unloading can be performed through the normal

inert gas padding connections in way of P/V-valve risers.

The previous exemption for chemical tankers related to inert gas capacity still exists. When carrying

flammable chemicals it is therefore acceptable that the unloading rate is reduced to 80% of the inert

gas system capacity.

Note also that the operational requirements to gas freeing in the IBC code have been amended in line

with the requirements for oil tankers. In situations where chemical tankers are required to use inert

gas , the cargo tanks shall be purged with inert gas, using approved gas-freeing arrangements, until

the concentration of flammable vapors in the cargo tanks has been reduced to less than 2% by

volume. As the normal inert gas padding connections in way of P/V-valve risers are not suitable for

purging cargo tanks, the use of inert gas for purging before gas freeing with air will normally have to

be carried out via the cargo system.

For chemical tankers that are required to be inerted and that are carrying products containing an

oxygen-dependent inhibitor, the use of inert gas shall not take place before loading or during the

voyage. Instead it shall be applied before commencing the unloading procedure. The minimum level

of oxygen required in the vapour space of the tank for the inhibitor to be effective shall be specified in

the Certificate of Protection provided by the cargo manufacturer.




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This information should be taken into account in the operation of the inert gas system to ensure the

oxygen level does not fall below the level indicated on the certificate.

In lieu of inert gas, Nitrogen is used onboard chemical tankers to control the cargo tank environment

and in the following circumstances:

o Drying: In case cargo is sensitive to moisture.

o Padding: To prevent any oxidation or As per local / terminal / shipper requirement.

o Inerting: To prevent flammability hazard.

Nitrogen ‘Drying, Purging, Padding or Air-blow are high risk operations and if not properly

carried out may result in significant damage to Cargo tanks due to over pressurization

QUANTITY AND QUALITY OF NITROGEN REQUIRED

The voyage orders will indicate whether the cargo nominated requires environmental control or not. If

the cargo requires use of nitrogen, following considerations must be taken into account in order to

estimate the quantity and quality of Nitrogen the vessel needs to carry.

a) Length of the voyage.

b) Ambient temperature and weather expected during the voyage.

c) Volume of vapour space in each cargo tank.

d) Quality of Nitrogen required.

e) The number of tanks that require maintenance of nitrogen blanket.

f) The production / storage capacity of nitrogen generator (if installed).

In case vessel is required to arrive in inert condition, initial notification from the charterer will be sent

along with voyage orders.

If nitrogen bottles are carried there should be fixed piping to connect the bottles to the tank and the

piping must be maintained clean, dry and in good condition at all times.

Records must be maintained for the nitrogen operations.

It is prudent to cater for extra bottles for the voyage in case of loss of nitrogen from the tank

containment.

7.1.2. Sources of Nitrogen Supply

Following nitrogen supply sources exist:

Liquid Nitrogen storage from shore (normally 99.9 percent purity).

Nitrogen generator plant (normally 98-99 percent purity).




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Portable nitrogen cylinders (99.9% Purity or 99.999% ultra high purity).

QUALITY OF NITROGEN

A. Purity 99.9%

In most cases the nitrogen is used to provide a blanket over the cargo to eliminate direct

contact with air and moisture. This utilisation will require the tanks to be maintained within

about 5% oxygen with positive pressure. For such usage nitrogen let into the tank will be in

the range of 99.9% purity

N5030 - Purity - 99.9% Colour – Green

. Purity 99.999%

There are several cargoes, such as Hexene 1, which deteriorate drastically in quality upon contact

with air and require oxygen content to be as low as 50 ppm in the tank during carriage of the cargo.

This will require the use of high purity Nitrogen N 5050 with purity of 99.999% and require stricter

atmosphere control in the tank.

N5050 - Purity 99.999% - Colour Green / Orange

Tank pressure, Oxygen content and cargo temperature must be continuously monitored while the

tanks are under nitrogen padding.

Master should be alert in case such cargoes are offered in the voyage orders and also be aware of

the stipulations under Charter Party for carriage or right of refusal for carriage of such high quality

cargoes.




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However, if carried, due care MUST be taken that the correct grade and sufficient quantity of Nitrogen

is procured and stored on the vessel for the voyage.

The vapour tightness of the tank lids should also be checked and ensured prior carriage of such high

purity cargoes.

PROPERTIES OF NITROGEN

· It Is a relatively inert gas and slightly lighter than air with a vapor density of 14.4.

· It is colourless, odour-less and tasteless.

· Liquid nitrogen is also colourless and odour-less, and is similar in appearance to water.

· Water dissolves nitrogen in ratio of 1 : 22.

HAZARDS OF NITROGEN

Following are the major hazards associated with Nitrogen Handling.

BREATHING PURE NITROGEN

Breathing is stimulated and controlled by carbon dioxide (CO2) present in the lungs. As the CO2 level

increases, the brain sends a message to increase respiration. When the CO2 level drops, the rate of

respiration will also decrease in order to maintain the proper balance.

One deep breath of pure Nitrogen can be fatal. Pure Nitrogen will displace CO2 and O2 completely

and in the absence of CO2 signal to the brain, the stimulus to breath no longer exists. The person

immediately stops breathing.




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ASPHYXIATION

Nitrogen rich atmosphere creates Oxygen deficiency which can be fatal. Following table illustrates

symptoms and influence of Oxygen deficient atmosphere on Human body.

Oxygen

Concentration Symptoms / Influence on Human body

20.8 % Normal, No effects.

19.5 % Impaired coordination. Some unnoticeable adverse physiological effects.

16.0 % Increased pulse and breathing rate, impaired thinking and attention, reduced

coordination.

14.0 % Abnormal fatigue upon exertion, emotional upset, faulty coordination, poor

judgment, blue lips.

12.5 %

Very poor judgment and coordination, impaired respiration that may cause

permanent heart damage, nausea and vomiting, loss of consciousness, blue

lips.

8.0 % Inability to move, loss of consciousness, convulsions, death unless recovery

with treatment within 4 minutes.

6.0 % Coma within 40 seconds, convulsions, respiration stops, death.

0 – 6 % Fainting, almost immediate coma, convulsions, respiratory arrest, death, Brain

damage even if resuscitated.

OPENING A TANK UNDER NITROGEN VAPOUR

Opening a pressurised tank opening possesses great risk of crew injury, in addition to exposure to the

harmful tank atmosphere contents.

Care shall be exercised while opening tanks under nitrogen purged / padded tank openings or

entering spaces which may contain Nitrogen as detailed below:

1. Crew member should be aware of the risk of opening a tank under pressure. The tank lid may fly

open, There is possibility of flying debris, rust or pain scale jettisoned from the tank at great velocity

which can cause injury. Crew member should position himself in a safe position prior opening the lid.

Adjacent area must be cordoned off.

2. It is recommended that an area of 1 meter should be cordoned off around the tank which is being

nitrogen purged or ventilated after being inerted with Nitrogen. Although one meter is considered safe

distance for nitrogen gas to diffuse to safe limits in air however master must allocate a larger area to

be cordoned off in case the ship’s structure and construction prevents free flow of air. If required to




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operate on deck within the cordoned off area, It is only permitted for brief period. Two-man team will

be required. SCBA must be worn while operating in this area.

3. Check tank pressure and the tank opening shall be opened only after releasing tank pressure

through PV valve (Prohibited for toxic cargoes) or VRL as directed by the Responsible Officer.

4. The assigned crew member should stand on the windward side to minimize exposure to tank

atmosphere.

5. Whenever entering any locker on main deck adjacent to cargo tanks, or locker storing nitrogen

bottles. The spaces must be adequately ventilated prior entering.

6. Ship staff must equip themselves with operational personal Gas Monitors that detect the level of

Oxygen and sound out an alarm, when the level drops below the normal level of 20.8%.

Note: Cargo tank opening shall not be opened if the presence of toxic vapours within cargo

tank spaces cannot be completely ruled out.

TANK OVER PRESSURIZATION

A. Structural damages due to over-pressurization of tanks while using Nitrogen

B. Causes of tank over-pressurization

Over-pressurization can occur during inerting, loading, discharging, ballasting or internal transfer of

cargo or ballast. It can be caused by one of the following:

a. Overfilling of the tank with liquid.

b. Inadvertent closing or failure of the tank’s vapour or inert gas isolating valve to the vapour line

or inert gas line.

c. Failure or seizure of the venting valve.

d. Choked flame arresters / screens.

e. Loading the tank at a rate, this exceeds the maximum venting capacity.

f. Ice forming on vents during cold weather conditions.

g. Ice on the surface of the ballast.

h. Restriction in the vapour lines caused by wax, residues or scale.

i. Inert Gas supply at high flow rate / pressure from shore for atmosphere control of cargo tanks

compared to the venting arrangement.

The consequences may result in serious structural deformation of the tank structure and its peripheral

bulkheads or catastrophic failure; which could seriously affect safety of lives, the structural integrity of

the vessel and can lead to fire, explosion and pollution.




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C. Precautions To Prevent Over-Pressurization

Over-pressurization of tanks can be prevented by complying with the following precautions:

On ships without an inert gas system:

Establish a procedure to control the setting of the high velocity vent valves on the vent lines. Check lift

these valves on every occasion prior receiving N2 from shore to ensure free movement.

Record the status of all valves in the system and prevent them from being incorrectly or casually

operated.

Restrict the operation of the valves to authorized personnel only.

Tanks to be dried or inerted with Nitrogen have open vents with a greater flow rate capacity than the

inlet, such that the tank cannot be over- pressurized. This is achieved by opening the butterworth

hatches or tank dome of the tank, while the tank is being purged. If the terminal does not permit deck

level tank openings,alternate arrangement should be discussed and agreed with the terminal to

ensure that the tank are never over pressurized.

On vessels with inert gas systems:

Where isolating valves are fitted to the branch line to each tank, SOLAS requires these valves be

“provided with locking arrangements which shall be under the control of the responsible ship’s officer”.

This statement should be taken to mean that the valves must be locked open to prevent any change

in the valve setting being possible without application to the responsible officer to obtain the means of

releasing the locking system on the valve.

Prevent failure or seizure of isolating valves, pressure / vacuum valves or high velocity vents by

regular maintenance, pre-operational testing and operator awareness to detect failure during

operation.

To protect against over-pressurization through filling tanks too quickly, all ships should have

maximum filling rates for each individual tank available for reference onboard by ship’s personnel.

This information should be posted in the cargo control room.

Tank vents should be checked to ensure that they are clear when the operation commences, and

during freezing weather conditions they should be inspected at regular intervals to ensure they are ice

free throughout the operation.




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PRECAUTIONS FOR NITROGEN PURGING / PADDING OPERATION

Cargo tank purging / padding using shore Nitrogen may result in tank over pressurization. Shore

nitrogen flow-rate should be agreed prior commencing operation and continuous monitoring is to be

carried out through out Nitrogen purging / padding operation to avoid structural damages to the cargo

tank due to excessive pressure.

Appropriate pressure gauge (0-15 bar) should be fitted at the manifold to monitor the pressure.

The pressure gauge fitted at the tank gauge should have a calibration from minus 0.5 to plus 1.0 bar

Appropriate pressure gauge should be fitted at the manifold to monitor the pressure

Purging / padding procedures are discussed in details in below sections which are to be referred to

and complied with along with “Nitrogen Handling Checklist (OP-CHEM-14)”.

In addition the following is to be complied with:

1. Tanks to be purged, padded or which contain Nitrogen are to be tagged.

2. Placard is to be placed at Gangway that Nitrogen purging / padding is in progress.

3. Tanks which are to be purged / padded are to be cordoned off.

4. No tank entry is to be permitted anywhere on the Vessel while padding / purging operations

are in progress.

GUIDELINES ON NITROGEN PURGING

Nitrogen purging is carried out prior loading to bring the tank atmosphere to the desired level. This is

normally done by connecting up the loading arm / hose to the cargo manifold and passing Nitrogen

through the cargo lines into the empty cargo tanks.

Venting shall be allowed only through butter-worth pocket or purge pipe or though VRL to shore.

During venting it should be ensured that outlet diameter is greater than inlet diameter of the gas.

The flow rate of gas in m3/hr is a function of the pipe diameter and pressure in the pipe.

Hence for the same flow rate the pressure in a larger pipe will be much less as compared to that in a

smaller diameter pipe.

Below table shows the volume of Nitrogen that can be received in one minute through a known pipe

diameter at known pressure.




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Hose Dia /

Pressure 200 mm (8”) 150mm (6”) 100mm (4”) 50mm (2”)

25mm (1”)

5.2 bar

(75 psi) 1,771 (106,000) 914 (55,000) 343 (20,600) 67 (4,000) 12 (740)

3.4 bar

(50 psi) 1,286 (77,000) 662 (39,700) 243 (14,600) 48 (2,900) 9 (530)

2.1 bar

(30 psi) 886 (53,000) 457 (27,400) 171 (10,300) 33 (2,000) 6 (360)

0.7 bar

(10 psi) 471 (28,300) 214 (12,900) 80 (4,800) 16 (1,000) 3 (170)

Table 1 - Cubic metres of gas at various gauge pressures received in 1 minute (and 1 hour)

through hoses of various sizes.

Table 2 illustrates the time taken to receive gas into a tank at different pressures and hose sizes. The

example used assumes a cargo tank of 1,250 cubic meters requiring four atmosphere changes, i.e.

5,000 cubic meters of nitrogen, to flow through.

Hose Dia / Pressure 200mm(8”) 150mm(6”) 100mm(4”) 50mm(2”) 25mm(1”)

5.2 bar 3 min. 5½ min. 15 min. 1 ¼ hrs. 7 hrs.

3.4 bar 4 min. 7 ½ min. 21 min. 1 ¾ hrs. 10 ½ hrs

2.1 bar 5 ½ min. 11 min. 29 min. 2 ½ hrs

0.7 bar 11 min. 24 min 63 min. 5 ¼ hrs.

Table 2 - Time to receive 5,000 m3 of gas with various gauge pressures and hose sizes.

Refer to “Nitrogen Handling Checklist (OP-CHEM-14)” for relevant Nitrogen purging precautions.

GUIDELINES ON NITROGEN PADDING

Whenever a cargo is required to be carried under a pad of Nitrogen, and it is necessary to use

nitrogen supplied from shore, it is strongly preferred to purge the entire tank before loading.

After such purging of the tank, loading the cargo under closed conditions will automatically




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create the needed pad within the tank. Risk of over-pressurization can be substantially reduced by

avoiding padding with shore supplied nitrogen.

However, if the cargo is required to be carried under additional Nitrogen pad after loading or if

shippers insist on padding their cargo after loading, the required nitrogen pad will have to be supplied

by the vessel or shore using a low volume / low pressure source. As padding is very often done

against a small or limited ullage space (volume), pressure can therefore build up very quickly if

something fails.

To control the shore nitrogen flow rate, a hose having a maximum diameter of one inch (1”) or

Orifice of one inch (1”) is to be connected between the shore supplied nitrogen and the ships

manifold. This will ensure that the flow rate will be maintained at a minimum.

In case of bubbling through toxic cargoes using 1” hose / Oriface connected at the manifold, tank

openings other than vapour return line shall be kept closed.

NITROGEN MAINTENANCE DURING VOYAG

1. Tank pressure / O2 content should be checked everyday and recorded in the logbook, whenever

the weather permits.

2. Cargo tank padding instructions are be maintained in line with voyage instructions.

PURGING / PADDING FOR INHIBITED CARGOES

1. Polymerizing cargoes such as monomers, Acrylates and some cyanides which are transported

inhibited may also at times be subject to tank atmosphere control (reduced oxygen content).

2. It is the responsibility of the Master to check from the inhibitor certificate the level of oxygen

dependency required by the inhibitor to remain effective and ensure that the oxygen content in the

vapor space of these tanks does not fall below the level required in accordance with that

stated in the inhibitor certificate.

3. Inhibitors such as TBC added to Styrene Monomer preferably require vapour space oxygen

percentages in the range of 6 to 8% though oxygen levels as low as 2% may be acceptable for

carriage.

4. The Master must seek confirmation from Charterers on the level of atmosphere control required for

inhibited cargoes as soon as the voyage orders are received. The requirement for atmospheric control

of inhibited cargoes must be discussed with all personnel concerned during the pre-cargo conference.




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5. The level of oxygen dependency and final oxygen content upon completion of purging/padding is to

be indicated in

“Nitrogen Handling Checklist (OP-CHEM-14)”. In case the final oxygen content required on

completion of purging / padding is less than the level of oxygen required by the inhibitor to

remain effective, operations must not commence without express permission from the Office.

6. Under no circumstances should nitrogen be bubbled through inhibited cargoes.

LOSS OF NITROGEN

Loss of nitrogen can have serious consequences in case of certain cargoes, which can react

dangerously with air, e.g. Propylene Oxide, etc. Quality of certain cargoes can also be seriously

compromised in case of loss of nitrogen. Such cargoes must be very closely monitored during all

stages of the operation.

1. DURING PURGING OPERATION:

During purging operations if any stoppage in Nitrogen supply is experienced from the terminal end,

the respective tanks and all connected valves are to be closed immediately. Manifold valves are to be

closed and the terminal is to be informed of closure of the manifold valves.

2. DURING PADDING OPERATION:

In case Shore Nitrogen supply ceases whilst padding, all respective cargo tank valves including vent

or manifold valves to be closed. Any openings on the tank including purge pipe covers etc are to be

closed tight and the tank pressures monitored. Inform terminal regarding manifold valve status. For

tanks which have been loaded with a homogenous cargo, being padded using a single vapour return

line, the vapour return manifold valves must be closed. On resumption of shore Nitrogen supply, the

manifold, tank valves and openings are to be opened and terminal informed to start supply of Nitrogen

slowly and increase the flow rate gradually. The pressure readings of concerned tanks are to be

monitored carefully until completion of padding.

3. DURING THE VOYAGE:

Any loss of nitrogen must be immediately reported to the company as this has now become an

emergency situation. Depending on the cargo, a decision will be made whether it is necessary to

deviate the vessel to receive nitrogen or if it is safe to continue the voyage. In very severe conditions,

it may even become necessary to jettison the cargo. However this is a decision which will be taken by

the company in discussion with cargo owners, Ship owners, regulatory authorities and chemical

experts.




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The voyage orders will indicate the whether the cargo nominated for the vessel requires

environmental control or not. Nitrogen is used with the cargo for safety and quality reasons. If the

cargo requires use of nitrogen, following considerations must be taken into account in order to

estimate the quantity and quality of Nitrogen the vessel needs to carry. The vessel may or may not be

fitted with a Nitrogen generator.

1) Length of the voyage.

2) Ambient temperature and weather expected during the voyage.

3) Volume of vapour space in each cargo tank.

4) Quality of Nitrogen required.

If nitrogen bottles are carried there should be fixed piping to connect the bottles to the tank. The

piping must at all times be maintained clean, dry and in good condition. Records must be maintained

for the nitrogen operations. Tanks must be checked at regular intervals to ensure these are

maintained in conditions required by the shipper’s instructions. It is prudent to cater for extra bottles

for the voyage in case of loss of nitrogen from the tank containment.

Loss of nitrogen can have serious consequences in case of certain cargoes, which can react

dangerously with air, e.g. Propylene Oxide, etc. Quality of certain cargoes has to be seriously

compromised in case of loss of nitrogen. Such cargoes must be very closely monitored.

7.2. INERT GAS SYSTEM

7.2.1. General

The procedures laid down in this section are applicable for Chemical tankers > 20K carrying:

1. Annex I cargoes.

2. This section does not apply to chemical tankers which are exempted as per Section 7.2.2.

Hydrocarbon gas cannot be ignited in an atmosphere containing less than 11% oxygen by volume.

Cargo tanks or holds which are kept inerted to less than 8% oxygen thus have an adequate margin of

safety from risks of fire and explosion.

With an inert gas system the protection against a tank explosion is achieved by introducing inert gas

into the tank to keep the oxygen content low and reduce to safe proportions the hydrocarbon gas

concentration of the tank atmosphere




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7.2.2. Requirement of Inert Gas System

SOLAS requires tankers of 20,000 tonnes deadweight and upwards to be equipped with by a fixed

Inert Gas System in accordance with the requirements of the FSS code, except that, in lieu of the

above, the Administration, after having given consideration to the ship’s arrangement and equipment,

may accept other fixed installations if they afford protection equivalent to the above.

Chemical tankers are exempted from the above requirements, when carrying IBC cargoes and

meeting the following criteria:

1. Capacity of the cargo tank does not exceed 3000m3.

2. Individual nozzle capacity of tank washing machines is less than 17.5 m3/hr.

3. Total combined throughput of machines in the tank is less than 110 m3/hr.

7.2.3. Company Policy

a) The cargo spaces and slop tanks are to be kept fully inerted at all times except when these spaces

are required to be gas fee for entry or for dry-dock.

b) If for any reason the inert gas plant cannot be operated or the oxygen content of cargo tank

atmosphere cannot be reduced to below 8%, the Master will ensure that all cargo and tank cleaning

operations are immediately suspended.

c) Before any cargo tank is gas freed, it should be purged with inert gas until the hydrocarbon content

of the tank atmosphere is below the critical dilution line.

d) When a ship is in a gas-free condition, before arrival at loading port, tanks should be inerted prior

to loading.

e) In case of port authority requirement of more stringent requirement with regards to the maximum

acceptable oxygen level in inerted cargo tanks, such requirement should be strictly adhered to.

f) The fixed oxygen analyser and recorder shall be calibrated prior each usage of Inert Gas Plant.

g) The Chief Engineer is responsible for the operation and maintenance of the inert gas plant. Regular

inspection and testing of the components must be undertaken by the vessel’s senior officers. IGS

spare parts must be maintained at designed levels.




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7.2.4. Hazards of Inert Gas

1. TOXICITY OF FLUE GAS

Flue gas contains Sulphur dioxide, carbon monoxide etc which are toxic gases. Prior entering into

enclosed spaces which earlier contained inert gas, toxic gas checks should be carried out prior man

entry.

2. OXYGEN DEFICIENCY

Exposure to an atmosphere containing less than 5% oxygen results in immediate unconsciousness. If

resuscitation is delayed for more than about four minutes, irreversible brain damage will occur.

Further delay will cause death. Exposure to an atmosphere containing less than 21% oxygen is not

necessarily incapacitating, but can affect the sense of judgement and balance and could eventually

cause unconsciousness. In some circumstances it could also result in permanent brain damage. An

oxygen deficiency is thus much more serious than exposure to hydrocarbon vapours, and for this

reason a full scale reading on a portable oxygen analyzer (21%) should be obtained throughout the

tank before entry is allowed.

3. ELECTROSTATIC HAZARDS

Soot particles carried in flue gas can be electrostatically charged. The level of charge is usually small

but it increases significantly with water mists formed during tank washings. Possibility of electrostatic

ignition has to be considered only if the oxygen content of the tank atmosphere rises as a result of air

ingress.

4. TANK PRESSURE

1. When an inerted cargo tank is maintained at a positive pressure, such pressure must be

adequately reduced before any tank-lids, ullage plugs or tank washing openings are opened.

2. Severe damage can be caused by over or under pressure in tanks beyond their design limitations.

To avoid danger of over / under pressurizing tanks when loading or ballasting, it is essential the PV

valves and PV riser provided are operational. All flame screens must be clean and in good condition.

5. PYROPHORIC OXIDATION

Refer to ISGOTT 1.5.




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7.2.5. Inert Gas Precautions

1. The low oxygen content of inert gas causes rapid asphyxiation. At the time of ullaging care should

be taken to avoid standing in the path of venting gas.

2. To avoid possibility of flue gases escaping to areas adjacent to ventilation intakes, inert gas piping

must be inspected regularly to ensure that it is free of leaks. Suspected areas should be soap tested

while inert gas system is in operation.

3. Scrubber effluent water which tends to collect in the distribution pipes, particularly the deck main is

acidic and highly corrosive. Care should be taken to avoid skin contact with either effluent or

condensate water

7.2.6. Inert Gas Operations

BEFORE USE OF INERT GAS

The inert gas system must be available for use at all times. To ensure its availability it must be

operated, tested, and maintained at regular intervals and in accordance with the vessels Planned

Maintenance System. The stock of spare parts carried must be maintained at the appropriate level

and whenever critical parts are used they must be re-ordered immediately.

All inert gas safety systems must be checked before use to ensure that correct operation is possible

when required. These tests are described in the Inert Gas Operations and Equipment Manual and the

manufacturer’s instructions, and must also include the following:

· Oxygen monitoring equipment must be calibrated and alarm points checked, this includes both

portable and fixed equipment.

· Automatic shut-down systems and valves checked.

· Deck seal alarms tested.

· Non return valves should be checked.

· Hydrocarbon gas detectors should be checked and alarms tested.

An inert gas maintenance record is to be maintained during all above checks and testing.

7.2.7. Inerting Before Loading

Prior to arrival at the loading port all tanks are to be fully inerted and the oxygen content of the tanks

reduced to below 8%. Adequate time is to be allowed for this operation to ensure the tanks are

properly inerted.




Rev. No: 1

Issue : 15-Apr-16

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Using a portable oxygen analyzer, a check is to be made that the atmosphere in all tanks is below 8%

O2 just prior to arrival at the loading port.

When ballast is discharged from any tanks / holds prior to loading the Inert Gas Plant must be

operated to fully inert those spaces.

DURING LOADING

During the loading operation the Inert Gas Plant will be shut down and the main isolating valve

closed.

LOADED PASSAGE

It is important to maintain a slight positive pressure in the ullage spaces of cargo tanks / holds to

avoid the ingress of air through the P/V valve.

Safety checks during voyage:

Following safety checks should be carried out in on daily basis except when weather conditions

renders it too dangerous to go onto deck;

a. Oxygen content of cargo tanks.

b. The water supply and water level in the deck seal should be ascertained.

c. Check the water level in water loops installed in pipework for gas, water or pressure transducers,

to prevent the backflow of hydrocarbon gases into gas-safe spaces.

d. In cold weather, ensure that the arrangements to prevent the freezing of sealing water in the deck

seals, pressure / vacuum breakers etc are in order.

e. Positive pressure to be maintained on all tanks, pressure should never be allowed to fall below

100mm H2O when in an inert condition.

f. Tank low pressure alarms should be tested at an interval not exceeding one week

INERTING DURING DISCHARGE

The Inert Gas Plant should be started prior to the commencement of discharge and run continuously

until the completion. IG is not to be supplied to deck unless the Oxygen content is below 5%.

The main blower capacity is normally 25% in excess of the combined capacity of all cargo pumps and

a slight positive pressure should be maintained throughout. All cargo operations shall be

suspended if the oxygen content exceeds 8% in the cargo tanks. Operations should not be

resumed unless the quality of the gas is improved.




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Issue : 15-Apr-16

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It is recognized that starting and stopping cargo pumps and other major machinery can cause sudden

changes to the boiler load resulting in momentary changes in flue gas oxygen levels until the

combustion control stabilizes. When this occurs, cargo handling, crude oil washing, ballast handling or

tank washing need not be stopped, as long as the inert gas oxygen level does not exceed 5% and it

returns to below 5% within few minutes.

On completion of discharge all cargo and slop tanks should be in a fully inerted condition.

Ensure tank atmosphere is checked:

i. Prior arrival at the discharge port

ii. Periodically during discharge operations

INERT GAS DURING TANK CLEANING

The Inert Gas System is to be used throughout tank cleaning. Oxygen content of each cargo tank

must be checked prior commencing tank cleaning to ensure that oxygen level is below 8%. Ensure

tank atmosphere is checked:

i. Prior commencing tank cleaning.

ii. During tank cleaning.

iii. After completion of tank cleaning.

Tank cleaning should be suspended if any rise in oxygen content above 8% is noted until tank

atmosphere has been purged down to an acceptable level. Testing should be done from at least three

levels in each tank / hold and, if possible, from more than one sampling point.

The supply of inert gas must be maintained to all tanks / holds being cleaned or to be cleaned and

also to the slop tanks. A slight positive pressure should be maintained by keeping the Pressure /

Vacuum valve by-passes closed. The supply of inert gas to a tank/hold that has been cleaned may be

stopped only after the tank cleaning machines have been withdrawn, stripping or educator suctions

closed and the tank cleaning plates secured in position.

In the event of a failure of the Inert Gas Plant all tank cleaning operations must be suspended. On the

completion of repairs to the plant, any tanks the oxygen content of which has risen to 8% or above

must be re-inerted. During re-inerting no ullaging, dipping or sampling equipment or tank cleaning

machines may be inserted until it has been established that the tank is inert. This should be done by

monitoring the efflux gas from the tank/hold being inerted until the oxygen content is reduced to less

than 8% and always provided that it is known that the efflux gas is fully representative of the gas

within the tank. If this cannot be satisfactorily established and it is necessary to introduce an oxygen




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sampling line into the tank this should only be done after a delay of 30 minutes following the cessation

of inert gas injection.

The maintenance of positive inert gas pressure in tanks / holds during a suspension of washing

operations is of particular importance when hot water is being used. During an overnight suspension

the temperature of the tank/hold atmospheres and of the accum

ulated slops will usually drop and could result in air being drawn into tanks/holds.

INERTING DURING DE-BALLASTING OF CARGO TANKS

The procedures regarding the deployment of inert gas during de-ballasting or partial de-ballasting of

cargo tanks will be the same as for discharge of cargo.

PURGING

When it is necessary to gas free a tank or tanks after washing, the concentration of hydrocarbon

vapour must be reduced to less than 2% by volume, by purging the cargo tank with inert gas. This will

ensure that no portion of the tank atmosphere is brought within the flammable range during gas

freeing.

Purging cargo tanks with IG is also required to remove or reduce content the hydrocarbon and toxic

vapours. Purging also acts as a counter measure for hazard due to Pyrophoric oxidation. Vessels

must be clear regarding the most efficient and cost effective method of completely purging their

vessel.

BALLAST PASSAGE

During a ballast passage, cargo tanks other than those required to be gas free should remain in the

inert condition and under positive pressure to prevent ingress of air. Whenever pressure falls to the

low pressure alarm level, the inert gas plant should be restarted to restore the pressure.

INERTING DOUBLE HULL TANKS

As per SOLAS, Tankers required to be fitted with Inert Gas systems (i.e tankers of 20,000 tonnes

deadweight and above, and tankers using COW), shall have double hull spaces fitted with suitable




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Page : 20 of 26

connections for supply of inert gas. This could be by permanent connections to IGS, or by using

suitable portable connections like hoses or ducts.

Inerting of double hull tanks is required in the event of a leak of hydrocarbons into a double hull or

double bottom tank. Inerting shall be carried out in accordance with class approved ‘Loading

Operation Manual’.

1. Flexible hoses used for inerting double hull tanks should be clearly identified. Such hose

electrical continuity should be checked prior each usage.

2. Where available, PV valve should be placed on double hull tank to maintain inert atmosphere.

Any other openings to this tank shall be blanked.

3. In case of non-availability of PV valve, over pressurization of double hull tank shall be avoided

by connecting such tank to PV breaker using flexible hoses.

4. Upon completion of inerting operation, IG inlet hose should be left connected to IG line for

breathing through deck PV breaker.

5. Once inerted, the tank should be kept topped up as necessary to ensure that a positive

pressure is maintained and oxygen content does not exceed 8% by volume.

6. The exhaust vapour from the tank during inerting should be ventilated through an opening at

least 2 meters above the deck. Portable standpipes should be used where necessary.

IG BRANCH VALVE STATUS

IG Branch valves shall always be kept locked in ‘OPEN / CLOSED’ position as per specific

requirements and keys shall be under the control of Chief Officer. Operation of these valves should be

carried out only upon Chief Officer’s instructions. An updated status of IG branch valve status should

be maintained in CCR.

FAILURE OF THE INERT GAS SYSTEM

In the event of total failure of the inert gas system, following actions must be taken:

a. All cargo tank operations / tank cleaning operations must be stopped.

b. Shut deck isolating valve.

c. Open the vent between deck isolating valve and the gas pressure regulating valve.

d. Inform terminal / harbour authority of the failure of an inert gas system.

e. In case of carrying static accumulating cargoes in inert condition, minimum 30 minutes of

relaxation time should be allowed prior introducing any object into a tank for ullaging / sampling etc.




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Cargo operations should not resume until RA is done and express office permission is granted or inert

gas system is repaired.

If cargo or ballast discharging or tank washing is required on vessels carrying only products when IGS

is inoperative, conventional closed venting system procedures must be used which displaces cargo or

ballast being discharged by air drawn into tanks through masthead vent risers and deck vent piping.

The precautions contained in ISGOTT should be complied with.

7.3. GAS FREEING

After a tank has been washed there is likely to be enough hydrocarbon gas in it to create a flammable

atmosphere during a subsequent gas-freeing operation. Therefore, in order to avoid passing through

the flammable condition, the hydrocarbon gas should be reduced by further purging with inert gas

before commencing gas-freeing.

Purging should continue until the atmosphere is below the Critical Dilution Line. As a thumb rule, fresh

air should never be introduced in a tank until the hydrocarbon concentration in the tank is less than

2% by volume.

The following systems are used for gas freeing cargo tanks:

a. For Ships fitted with IGS: The IGS is run on Gas freeing / Fresh Air mode, where the IG blowers

take direct suction from the atmosphere and the air is led into the tanks through the IG line.

b. Fixed centrifugal motor driven gas freeing fans: Some ships are equipped with very high

capacity fans which are located in the pump room or other similar location from where fixed piping is

led to the cargo tanks and or manifold. Air from these fans can be either fed from the top of the tank

when dilution method is used or sent in from the bottom through the cargo pipelines when

displacement method is used.

c. Portable gas freeing fans: Portable fans are pneumatic or water driven and are capable of being

operated in supply or suction mode. Gas freeing is most effective when two fans are used in one tank

with one fan on supply mode and the other in exhaust mode using a suction duct at different height.

Many times, Gas freeing operation is concurrent to the purging operation, where Inert gas system is

still in use in the IG mode. To ensure effective gas freeing, the tank to be gas freed should be isolated

from the IGS by blanking of the IG inlet line of the tank. Also due consideration has to be made,




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regarding the wind direction, to ensure that the vented air from other inert tanks is not being drawn

into the tank being gas freed.

A tank is to be considered Gas Free, only after oxygen content of 20.8%, Hydrocarbon

concentration of 0% LEL and absence of Toxic gases have been established.

7.3.1. Gas-Freeing – Where Open Venting is not Permitted

Open venting is not permitted for Gas freeing of tanks containing flammable and or toxic

products.

Arrangements for gas-freeing cargo tanks used for cargoes for which open venting is not permitted,

shall be such as to minimize the hazard due to dispersal flammable or toxic vapours to the deck

level/surrounding atmosphere with the ship vicinity or to flammable or toxic vapours mixtures in the

cargo tanks.

Not more than four tanks should be gas freed simultaneously.

The gas-freeing operations shall be carried out such that the vapour is initially discharged:

1. Through the vent outlets provided onboard as follows:

a. At a height of not less than 6.0 meters above the weather deck or above the raised walk-way,

if fitted, within 4.0 meters of raised walkway

b. At a distance of at least 10.0 meters, measured horizontally from the nearest air intake or

opening to accommodation, service and machinery spaces and ignition sources.

OR

2. Through outlets at least 2.0 meters above the cargo tank deck level with a vertical exist velocity of

at least 30 m/s maintained during the gas freeing operations.

OR

3. Through outlets at least 2.0 meters above the cargo tank deck level with a vertical exit velocity of at

least 20 m/s which are protected by suitable devices to prevent the passage of flame.

The flammable vapour concentration at the outlets has been reduced to 30% of the lower flammable

limit and, in case of toxic product, the vapour concentration has been reduced below the TLV of the

cargo, gas freeing may thereafter be continued at cargo tank deck level.




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Issue : 15-Apr-16

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The number of cargo tanks that can be simultaneously gas freed while maintaining 20 m/s exit

velocity at vent pipe will depend upon capacity of gas freeing fan, vent diameter and pressure drop

against particular tank.

Following table provides guidance on maximum number of tanks that can be simultaneously gas freed

maintaining 20 m/s exit velocity.

Gas Freeing Fan Outlet

Capacity

Pipe Diameter - 4" Pipe Diameter - 6" Pipe Diameter - 8"

Number of tanks Number of tanks Number of tanks

120 m3/min 12 5 3

80 m3/min 8 3 2

60 m3/min 6 2 1

40 m3/min 4 1 1

Following table provides guidance on maximum number of tanks that can be simultaneously gas freed

maintaining 30 m/s exit velocity.

Gas Freeing Fan Outlet

Capacity

Pipe Diameter - 4" Pipe Diameter - 6" Pipe Diameter - 8"

Number of tanks Number of tanks Number of tanks

120 m3/min 8 3 2

80 m3/min 5 2 1

60 m3/min 4 1 1

40 m3/min 2 1 0

Care should be taken to investigate that the rated flow capacity of the portable or fixed fans are not

affected by the presence of obstruction in the pipelines. The number of vents obtained by this formula

should be rounded off to the lower value to allow for frictional losses in the pipeline.

7.3.2. Precautions During Gas Freeing

1. During the gas freeing operations other hazardous operations on the vessel which may cause

a dangerous situation should be suspended. The engine room and all personnel involved

should be informed.

2. Where cargo tanks are gas freed by means of permanently installed fans, air is introduced

into the cargo tank through the cargo lines. The entire line system should be thoroughly

drained before venting to avoid any obstruction of the airflow or tendency for water or cargo

residues to be blown into a cargo tank.




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3. Valves on the systems, other than those required for ventilation, should be closed and

secured. The fans should normally be blanked or disconnected from the cargo tank system

when not in use.

4. Fixed gas freeing equipment should not be used for gas freeing of a tank while

simultaneously being used to ventilate another tank in which washing is in progress,

regardless of the capacity of the equipment.

5. Portable fans should only be used if they are water driven, or hydraulically or pneumatically

driven. Their construction materials should be such that no hazard of incendiary sparking

arises if, for any reason, the impeller touches the inside of the casing. The manufacturer’s

recommendations for maintenance should be followed. Guards should be in place to prevent

accidental contact with fans blades.

6. Portable fans, where used, should be placed in such positions and the ventilations openings

so arranged that all parts of the tank being ventilated are effectively and equally gas freed.

Fans should be generally be as remote as possible from the ventilation outlets. They should

be so connected to the deck that an effective electrical bond exists between the fan and the

deck.

7. The wind direction may cause cargo vapours to pass near air intakes for accommodation

spaces or engine room ventilation, and necessitate additional precautions. Central air

conditioning or mechanical ventilation system intakes should be adjusted to prevent the entry

of gas, if possible by using recirculation of air within the spaces.

8. Air conditioning should be on recirculation mode. If at any time it is suspected that gas is

being drawn into the accommodation block, the central air conditioning and any mechanical

ventilating systems should be stopped and the intakes covered or closed.

9. If the tanks are connected by a common venting system, each tank should be isolated to

prevent the transfer of gas to or from other tanks.

7.3.3. Checks after Gas Freeing

1. When a tank appears to have been gas freed and all mechanical ventilation has been

stopped, a period of about ten minutes should elapse before taking final gas measurements.

This allows relatively stable conditions to develop within the tank space. Tests should then be

made at several levels and, where the tank is sub divided by a wash bulkhead, in each

compartment of the tank. In large compartments such tests should be made at widely




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separate positions. If satisfactory gas readings are not obtained, the tank should be checked

for cargo residues and then ventilation resumed.

2. On completion of all gas freeing and tank washing, the gas venting system should be carefully

checked, particularly attention being paid to the efficient working of the P/V valves and any

high velocity vent valves. If the valves or vent risers are fitted with devices designed to

prevent the passage of flame, these should also be checked, and cleaned if found necessary;

Gas vent risers and their drains should be checked to ensure that they are free of any

blockage.

3. On completion of gas freeing, attention should be given to all equipment that has been used,

and to enclosed or partially enclosed spaces that can retain or contain cargo residues or

vapours, so that no unsuspected dangerous pockets can remain. Places where such cargo

traces may exist include cargo lines, cargo valves, cargo pumps, stripping lines and valves,

venting lines and P/V valves, vapour return lines, ullaging or sounding arrangements, heating

coils, cargo handling equipment store rooms, protective clothing store rooms and cargo

sample store rooms. Additional considerations to take into account when the tank is inerted.

7.3.4. Prevention of Sweat in Cargo Tank

Air contains a certain amount of moisture (vapour of water) depending on its temperature and

surroundings. If the air contains the maximum amount of moisture allowed at the temperature, the

relative humidity is 100%. If the air contains no moisture, the relative humidity is 0%. Higher the

temperature, more the air can contain moisture, which means if air containing a certain amount of

moisture is cooled down, the relative humidity will increase. If air contacts a cold surface and gets

cooled down below the temperature of 100% of relative humidity (so called "Dew Point") the amount

of moisture contained in the air in excess of 100% relative humidity will condensed and become dew

(sweat).




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Issue : 15-Apr-16

Page : 26 of 26

Following table can be used as guidance for ventilation:

Air Temp.(°C) Dew

point

(°C)

Sea temp. or

Surrounding cargo

tank (°C)

Empty cargo

tank

(°C)

Outcome

Sweat Ventilation

Dry Wet

A B C D E

26 24 23 24 >24

No (C)

No necessary

26 25 25 24 26

Yes (C>D

and C)

/span>

Must not (A=E)

26 25 25 24 35

Yes (C>D,

and C)

/span>

Vent. Until to be

A=E

18 16 15 25 14 No (C)

/span> No necessary


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