Offshore Safety and Survival Centre
Inert Gas
STUDENT HANDBOOK
Be ready for the challenge.
Inert Gas Systems
Table of Contents Outline 1.0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Regulations and Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Introduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Canada Shipping Act and Regulations . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 IMO Conventions and Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.0
Applied Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1 Physical and Chemical Properties of Crude Oil . . . . . . . . . . . . . . . . . . . . 10 2.2 Flammability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3 Sources of Ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Static Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Flammability and Explosion Hazards . . . . . . . . . . . . . . . . . . . . . . . . . 21 Tank Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4 Inert Gas Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.5 Inert Gas Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.0
Safety and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.1 Safety and Health Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Health Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Toxicity
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Cargo Hazard Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Enclosed Space Entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2 Gas Monitoring Equipment (Fitted and Portable) . . . . . . . . . . . . . . . . . . . 37 3.3 Personal Safety Equipment and Practices . . . . . . . . . . . . . . . . . . . . . . . 40 3.4 First Aid with Resuscitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.0
Design Parameters and Construction . . . . . . . . . . . . . . . . . . . . . . 45 4.1 General Components of an Inert Gas System . . . . . . . . . . . . . . . . . . . . . 46 4.2 Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.3 Isolating Valves, Water Seals, and other Non-Return Devices . . . . . . . . . . . . 53 4.4 Inert Gas Blowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.5 Inert Gas Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
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5.0
Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.1 Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.2 Audible and Visual Alarms and Interlocks . . . . . . . . . . . . . . . . . . . . . . 60 5.3 Testing
6.0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.1 Operational Cycling of Inert Gas Plants. . . . . . . . . . . . . . . . . . . . . . . . 66 Petroleum Carriers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Chemical and Product Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Gas Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Special Inerting Requirements - Combination Carriers . . . . . . . . . . . . . . . . 72 6.2 IG Plant Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.0
Vapour Emission Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 7.1 System Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 7.2 Controlling Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
8.0
Materials, Maintenance, Inspection and Testing . . . . . . . . . . . . . . . . 79 8.1 Maintenance Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 8.2 Inspecting and Testing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 81 8.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Appendix A - References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Appendix B - Course Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
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INERT GAS SYSTEMS TYPE AND PURPOSE:
The course will provide those responsible for cargo operations on vessels carrying highly volatile liquid cargoes with a thorough understanding of the principles behind the use of inert gas, its generation, quality control, and distribution and the safety aspects of its use.
CALENDAR ENTRY:
Regulations and Guidelines; Applied Science; Safety and Health; Design Parameters and Construction; Instrumentation; Operations; Vapour Emission Control; and Materials, Maintenance, Inspection, and Testing.
CLASS SIZE:
Maximum: 8
CERTIFICATE AWARDED:
Certificate of Achievement
PREREQUISITES:
Certificate of Competency as Watchkeeping Mate or 4th Class Engineer with Level 1 Endorsement or Watchkeeping rating with minimum of three months sea service on tankers First aid certificate.
SCHEDULE:
2.5 days (17 hours)
COURSE AIMS:
1)
To enhance students’ familiarity with the design, operation, and maintenance of inert gas systems for carriers of different types of product.
2)
To enhance students’ understanding of the relationship between the processes in the systems and the correct operation of associated equipment.
3)
To help students recognize the hazards associated with the use of inert gases.
4)
To enable students to comprehend the safeguards provided by inert gases in the handling of volatile liquids.
1.0
Regulations and Guidelines
2.0
Applied Science
3.0
Safety and Health
4.0
Design Parameters and Construction
5.0
Instrumentation
6.0
Operations
7.0
Vapour Emission Control
8.0
Materials, Maintenance, Inspection, and Testing
MAJOR TOPICS:
EVALUATION:
25 multiple-choice question examination 60% rating required to pass 100% attendance required to pass
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~NOTES~
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1.0 Regulations and Guidelines
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INERT GAS SYSTEMS Introduction In 1969 a series of explosions on the VLCC’s Mactra, Marpessa and Kong Haakon VII caused concern about safety. The common factor in these explosions was that they occurred when tank cleaning operations were being carried out. The investigation into the explosion on the Mactra recommended that VLCC’s, including ships with cargo compartments exceeding 10,000 cubic metres in capacity, should be fitted with an Inert Gas System (see Department of Trade and Industry, Press Notice, 29 January 1973). IMCO recommendation MSC/Circular 136 of 13 November 1972 proposed a revision to the Fire Safety Requirements for Construction and Equipment of New Tankers and Combination Carriers including the provision of an on-board Inert Gas System (see Annex X to MSC XXVI/19). Germanisher Lloyd, Lloyds, ABS, and DNV have subsequently laid down standards for the installation of Inert Gas Systems on tankers which must be adhered to before a vessel can be classified as INERT in their Registers of Ships. Great emphasis is now being given to providing a method for on-board inerting. It is difficult to understand why this should only now be the case since explosive cargoes have been carried on vessels for many years. There are probably two basic reasons for this. First is the impact on the shipping world of these three explosions on new ships in a very short space of time. Second is the growing world awareness of the effects of pollution. The explosions caused doubts to be raised about the degree of safety achieved by traditional methods of operation while the second brought pressure to bear on the shipping industry to tighten up on its pollution standards. With the advent of crude oil washing the need for an on-board Inert Gas System has been made mandatory and Regulations 58, 60, 61 and 62 of the International Conference of Safety of Life at Sea (SOLAS)1974 state the requirements for the design and operation of that system.
1.1
Canada Shipping Act and Regulations
Inert Gas Applications to Tankers (a)
Oil Pollution Prevention Act and Regulations
(See Figure 1 page 4). The Oil Pollution Prevention Regulations governs the prevention of the pollution of water by oil discharged from ships and from loading and unloading facilities for ships. These regulations include standards for construction, equipment, operations and inspections. They
• apply to any Canadian ship operating outside of waters under Canadian jurisdiction; • apply to any ship operating in waters under Canadian jurisdiction; and • do not apply to warships or ships used in non-commercial government services.
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Figure 1 - Regulations and Guidelines
Construction Plans and specifications for the following must be submitted to the Steamship Board for approval:
• • • •
the slop tank arrangement as per Regulation 15(2) Annex I MARPOL; the Inert Gas System referred to in Regulation 13B (3) Annex I MARPO; and, any crude oil tanker of 20,000 dwt or more; any product carrier of 30,000 dwt or more.
Equipment Inert Gas Systems are required to meet the standards specified in Regulation 62, Chapter 11-2 SOLAS. Equipment regulations for crude oil washing are specified in Resolution A446(XI). Operation Manuals The owner or master of every crude oil tanker of 20,000 dwt or more must submit to the board four copies of the operating manuals for that tanker for the following systems:
• the Inert Gas System; and • the crude oil washing system as per Resolution MEP3(XII).
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Inspections Inspections are required by the Steamship Board under the Canada Shipping Act to verify that a tanker’s construction, equipment fittings and installations and systems are in accordance with the oil pollution regulations. These inspections are to be carried out before a ship is put into service or issued its first Canadian Oil Pollution Prevention Certificate. An oil pollution prevention certificate is valid for a period of five years. (b)
Marine Occupational Safety and Health Regulations
These regulations provide standards which regulate the conditions under which persons work in the marine environment such that this work can be done in a safe and healthy manner. They specify the standards or conditions on a ship that the employer must provide for employees. For example, there must be at least one shower for every ten employees on any ship other than a day ship. Other areas as discussed below are subject to these safety and health regulations. Training Every employer in consultation with a safety and health committee must develop an employee education program respecting hazard prevention and control of the work place (WHMIS). Confined Space Entry Entry into a confined space is not allowed without the prior testing of that space by a qualified person. The O2 content of the confined space atmosphere must not be less than 19.5% nor more than 23% by volume.
1.2
IMO Conventions and Guidelines
A number of international codes and regulations apply to the use of Inert Gas (IG) on tankers that carry crude oil, chemicals and/or liquefied gas. These include SOLAS, MARPOL, and STCW (Seafarers Training Certification and Watchkeeping). (a)
SOLAS
SOLAS is one of the oldest and most important international conventions dealing with maritime safety. SOLAS Regulation 60 Chapter 11-2 requires that crude oil tankers of 20,000 dwt and over be fitted with an Inert Gas System. SOLAS Regulation 62 Chapter 11-2 dictates the design, construction and operating requirements for such an Inert Gas System. (b)
MARPOL 73/78 and 92 Amendments
MARPOL is an important International Maritime Organization (IMO) convention, initially adopted in 1973 and modified by the 1978 protocol, relating to the Tanker Safety and Pollution Prevention (TSPP) Conference (commonly referred to as
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MARPOL 73/78). Regulations covering the various sources of ship-generated pollution are contained in the five annexes of the convention. The Marine Environment Protection Committee (MEPC), formed in 1974, has reviewed various provisions of MARPOL 73/78 and have made some clarifications and amendments. The five annexes include: i)
Annex I (Regulations for the Prevention of Pollution by Oil) Amendments to this by MEPC recognize the Antarctic as a special area and call or a new form of the Oil Record Book for supplements to the IOPP certificate.
ii)
Annex II (Regulations for the Control of Pollution by Noxious Liquid Substances) This came into force on 6 April 1987. In 1989 amendments were made to it by MEPC to make Annex II compatible with Chapter 17/VI and 18/VII of the IBC Code and BCH Code (the amendments came into force in 1990).
iii)
Annex III (Regulations for the Prevention of Pollution by Harmful Substances in Packaged Form) This came into force 1 July 1991. Amendments were made by MEPC incorporating the reference to the IMDG Code.
iv)
Annex IV (Regulations for the Prevention of Pollution by Sewage)
v)
Annex V (Regulations for the Prevention of Pollution by Garbage) This came into force 31 Dec. 1988. MEPC amended Annex V to designate the North Sea, the Antarctic and the Caribbean region as special areas.
The MARPOL Convention deals with a wide range of subjects relevant to pollution from tankers and other marine activities. It covers tanker design, tank size, operational requirements, the documents that must be carried, overboard discharges from machinery and cargo spaces and a host of other oil pollution prevention requirements. It also specifies the requirements for crude oil washing and requires the provision of Inert Gas Systems for every cargo tank and slop tank. (c)
STCW 1978 Convention
The Seafarers Training Certification and Watchkeeping Convention was adopted in 1978. It is primarily a set of standards and training requirements for persons employed in the marine industry. In 1995 IMO held an STCW conference during which amendments were made to the Convention. The 1995 amendments became known as the STCW Code and should be read in conjunction with the Convention. The STCW Code is divided into two parts:
• mandatory standards regarding provisions of the annex to the convention; and • recommended guidance regarding provisions of the STCW Convention and its annex. NOTE: Chapter V of the STCW code deals with standards regarding special training requirements for personnel on various types of tankers (oil, chemical, or natural gas. ãMarine Institute
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(d)
Bulk Chemical Code/BCH Code
The regulations for the construction of and the equipment required on ships that carry dangerous chemicals in bulk is contained in BCH Code. This code was adopted in 1971 and amended in 1972, 1983, and 1985 by the Marine Environment Protection Committee and in 1986 by the Maritime Safety Committee (MSC). Under the provisions of Annex II of MARPOL 73/78 chemical tankers constructed before 1 July 1986 must comply with the BCH Code. Those built subsequently must comply with the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code) for the purposes of MARPOL 73/78 and SOLAS 74. IBC Code The purpose of the IBC Code is to provide an international standard governing the safe carriage by sea of dangerous and noxious liquid chemicals in bulk (See Chapter 17). The Code prescribes the design, construction and standards for ships including the equipment they should carry. Under the provisions of Chapter vii of the International Convention for the Safety of Life at Sea 1974 as amended in 1983, chemical tankers constructed on or after 1 July 1986 must comply with the provisions of the IBC Code. (e)
Gas Carrier Code (IGC Code)
This code provides an international standard for the safe carriage by sea of liquefied gases in bulk. It also prescribes the design and construction standards for ships involved in such carriage and the equipment they should carry to minimize risk to the ship and crew and to the environment. A product may have one or more hazardous properties including flammability, toxicity, corrosivity and reactivity. Further possible hazards may occur due to the products being transported under cryogenic or pressurized conditions. Chapter 9 of the IGC Code details the inerting requirements for gas carriers. Chapter 17 details the special requirements for inerting specific products. These requirements are summarized in Chapter 19 of the IG Code.
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2.0 APPLIED SCIENCE
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In order to fully appreciate the use of Inert Gas Systems on tankers it is important to understand the possible sources of ignition, the role of oxygen in maintaining combustion, how inert gas is produced and the compatibility of inert gas with various products.
2.1
Physical and Chemical Properties of Crude Oil
Crude Oil is a mixture made up of thousands of different compounds containing hydrogen (H) and carbon (C) atoms; hence, the name hydrocarbons. There are also additional substances found in crude oil including nitrogen (N), sulphur (S), oxygen (O), and other trace elements and mineral salts such as sodium chloride. The actual composition of crude oils vary depending on the geological structure from where they originate, due to the percentage of each compound found in the sample. The percentage of each compound helps identify the crude oil and distinguish it from others. A gas chromatograph is used to identify the compounds by their boiling points; and, as each compound has a unique boiling point, it is this temperature which identifies it. The boiling points of some compounds found in crude oil: methane (CH2) propane (C3H8) butane (C4H10) octane (C8H18) tetradecane (C14H30) nonodecane (C19H40)
-162°C -42°C 0°C 126°C 252°C 320°C
The knowledge of the chemical and physical properties of crude oil are important because they affect the safe handling of a product. Chemical Structures As previously mentioned, hydrocarbons consist of the chemical elements hydrogen (H) and carbon (C). Each hydrogen atom can combine with only one other atom, while carbon atoms can combine with up to four other atoms. Hydrogen atom with single bond:
Carbon atom with its four bonds:
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The simplest hydrocarbon molecule consists of one carbon atom and four hydrogen atoms, as shown, in this methane molecule. H CH4
H - C
- H
H Straight chain molecules are called alkanes or normal paraffins. They have the following characteristics:
• they do not react easily with other chemicals; • the vapour is flammable when mixed with air; and • the vapour is usually toxic Branched chain hydrocarbons can have the same number of carbon and hydrogen atoms as straight chain molecules, but with a different molecular structure. These branched chain paraffins, as they are sometimes referred to, exhibit the same general properties as straight chain paraffins. They are also known as isomers. Ring structures or cyclo-paraffins have the same number of carbon atoms as their normal counterpart, but since the structure is bent into a circle, there are two fewer hydrogen atoms. The cyclo-paraffins exhibit the same characteristics as alkanes and isomers. The carbon atoms in the straight chain, branched chain and the ring structured compounds have been joined to other carbon and hydrogen atoms by a single bond. Single bonded compounds are often called saturated hydrocarbons. In some hydrocarbons, the carbon atoms are attached to each other by a double bond. These are called unsaturated hydrocarbons and belong in the olefins series. Olefins are more reactive than paraffins. The type of bond affects the chemical and physical properties of the compound. A further class of compounds are the aromatic hydrocarbons. These occur naturally in crude oil as well as being produced in the refinery. Aromatics are based on the very special cyclic arrangement of benzene, which has six carbon atoms joined by alternating single and double bonds.
H
H
C
Aromatic hydrocarbons carried in tankers include:
H C
• Benzene • Toulene • Xylene Aromatic hydrocarbons are very toxic and must be handled with suitable precautions.
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C
C H C
H
C H
Figure 2 - Benzene
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2.2
Flammability
The introduction of pure oxygen into a flammable vapour mixture increases the mixture’s flammable range creating the potential for a large explosion. For this reason oxygen enriched atmospheres must be avoided. Table 1 - Flammable Limits of Hydrocarbon Vapours in Air and Oxygen* (% by volume)
Hydrocarbon Vapour
Flammable Limits in Air LFL
UFL
Methane
5.3
14.0
Ethane
3.0
12.5
*Propane
2.2
*Butane
Oxygen Lower
Upper
9.5
2.3
55.0
1.9
8.5
1.8
49.0
Pentane
1.2
7.8
Hexane
2.4
7.5
H2S
4.3
45.0
The effect of reducing the oxygen concentration, through displacement by other gases such as carbon dioxide, nitrogen or flue gas, is to narrow the flammable range. Narrowing the flammable range reduces the probability of a fire or explosion. This is the purpose of an Inert Gas System. Flash Point The flash point of a liquid is the lowest temperature at which that liquid will give off sufficient vapour to form a flammable mixture with air. High vapour pressure liquids have low flash points. In a practical sense the flash point indicates that the vapour pressure of the liquid at this particular temperature is sufficient to create a mixture with air corresponding to the lower flammable limit (LFL). The flash point is often taken to represent the point of change from a safe situation to one that is dangerous. The flash point temperature is determined by laboratory testing. Flash points for various petroleum products are found in Table 2. Liquids which have a flash point below 37.8oC are defined as flammable liquids by IMO regulations. Liquids having a flash point of 37.8oC or above are defined as combustible.
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Table 2 - Properties of Typical Petroleum Fractions
Fraction
Flashpoint
Ignition Temperature
Flammable Limits (Percent by Volume in Air) Lower
Upper
Specific Gravity (SG) (Water = 1.0)
Asphalt typical (petroleum pitch)
400°F 204°C
Crude Petroleum
20°F to 9°F 7°C to 32°C
Fuel oil #1 (Kerosene, coal oil, range oil)
100°F 38°C
441°F 229°C
Fuel Oil #2
100°F 38°C
494°F 257°C
1.0 (or less)
Fuel Oil #4
130°F 54°C
505°F 263°C
1.0 (or less)
Fuel Oil #5
130°F 54°C
Fuel Oil #6
150°F 66°C
765°F 407°C
Gasoline (60 Octane)
-45°F -43°C
536°F 280°C
Gasoline (100 Octane)
-36°F -38°C
853°F 456°C
Lubricating oil (motor oil)
300°F to 450°F 149°C to 232°C
500°F to 700°F 260°C to 371°C
Mineral oil
380°F 193°C
Naphtha safety solvent (Stoddard solvent)
100°F to 140°F 38°C to 60°C
300°F to 450°F 149°C to 232°C
1.1
6.0
0.8
Naphtha safety solvent (Stoddard solvent)
100°F to 140°F 38°C to 60°C
300°F to 450°F 149°C to 232°C
1.1
6.0
0.8
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905°F 485°C
1.0-1.1
1.0
0.7
5.0
1.0 (or less)
1.0 (or less)
1.0 (or less)
1.4
7.6
0.8
1.0 (or less)
0.8-0.9
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Vapour Pressure The volatility or the tendency of a crude oil or petroleum product to produce vapour is characterized by its vapour pressure. When a petroleum mixture is transferred to a gas free tank it commences to vaporize: it liberates gas into the space above it. Some of this gas re-dissolves into the liquid and some of it is distributed evenly throughout the space. The pressure exerted by this gas is usually referred to as vapour pressure. The vapour pressure of a compound depends only upon its temperature. The vapour pressure of a mixture depends on its temperature, its constituents and the volume of gas-space in which vaporization occurs (i.e. the ratio of gas to liquid by volume). The highest vapour pressure, which occurs when the ullage space is very small, is called the True Vapour Pressure (TVP) of the liquid at that temperature. The true vapour pressure like the vapour pressure for any vapour/liquid ratio increases as the temperature increases. TVP determines whether or not a petroleum mixture boils. A petroleum mixture boils when it is introduced into a tank if its TVP at the handling temperature is above the prevailing atmospheric pressure. This continues until the TVP falls to atmospheric pressure. The vapour space above crude oil or one of its refined products contains in various proportions the following hydrocarbon gases: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12) and hexane (C6H14). The percentage of each of these constituents in the vapour depends on the nature of the crude or product, its origin and how it has been treated. Table 3 - Typical Composition of Hydrocarbon Vapours in Ullage Spaces (in % by volume)
Hydrocarbon Vapour
Crude Oils Iraq
Iran
Kuwait
Methane
30
2
11
Ethane
11
7
24
Propane
14
18
35
Butane
25
66
18
Pentane
13
6
7
Hexane
9
2
5
Hydrocarbon vapours are generally denser than air or inert gas with the highest concentration of the vapours generally being found at or near the surface of the petroleum product. The concentration of the hydrocarbon vapours decreases as the distance from the surface of the liquid increases. However, this does not hold true where mixing or sloshing of the liquid occurs.
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The depth of the vapour layer is defined as the vertical distance from the surface of the liquid to a point where the vapour concentration drops to 50% by volume. The depth is usually measured in metres.
Figure 3 - Gas Layer Depth
Fire/Explosion The basic difference between a fire and an explosion is shown in Figure 4.. As the combustion process proceeds the rate of energy release is balanced by energy dissipation such that a limiting rate of reaction is reached. In the case of an explosion the products of combustion, and therefore the heat, are not removed from the reaction area (i.e. the temperature rises). As the system is usually confined the pressure rises. Since there is no mechanism for the release of Figure 4 - Fire vs Explosion
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energy the rate of reaction increases exponentially until all the reactants are consumed. For this reason explosions often happen in confined spaces, such as tanks, where pressure cannot be released. To avoid an explosion in a cargo tank it is necessary to eliminate or remove at least one of the following three components:
• the source of ignition; • the hydrocarbon vapour or • the oxygen. The usual practice is to keep sources of ignition away from the cargo tanks. However, it is necessary during certain operations, especially tank washing, to use a different form of control such as reducing the oxygen content of a tank’s atmosphere by the introduction of inert gas. Inert gas is an atmosphere containing 8% or less oxygen by volume.
2.3
Sources of Ignition
Static Electricity While regulations try to limit the number of potential ignition sources in locations sensitive to fire or explosion they cannot eliminate these sources. The major potential ignition source on tankers is from electrostatic charges. Electrostatic charges may result from loading/discharging operations, tank washing, electrical storms, helicopter operations, clothing or aerosol type spraying. There are three basic stages leading up to a potential electrostatic hazard:
• charge separation; • charge accumulation,; and • electrostatic discharge. All three are necessary for electrostatic ignition. Charge Separation Charge separation occurs at the interface between two dissimilar materials when the materials come into contact with each other. The interface may be between two solids, between a liquid and a solid or between two immiscible liquids. At the interface the negatively charged electrons move from material A to material B thereby creating a negative charge on B and a positive charge on A. While the materials stay in contact the charges are relatively small and no hazard exists. However, when the materials become separated a voltage difference develops between them. This gives rise to an electrostatic field. A charged petroleum liquid in a tank produces an electrostatic field throughout the tank both in the liquid and in the ullage space. The charge on a water mist created by tank washing produces a field throughout the tank. If an otherwise uncharged conductor is present in the electrostatic field it has approximately the same voltage as the region it occupies. The field also causes a movement of charge within the conductor with a charge of one sign being attracted
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Inert Gas Systems - 2.0 Applied Science
by the field to one end of the conductor and an equal charge of opposite sign to the other end. Charges separated in this way are known as induced charges and as long as they are separated are capable of producing an electrostatic discharge. Charge Accumulation The degree to which a charge accumulates depends on the conductivity of the associated material. If the material is a poor electrical conductor recombination is impeded and the material accumulates the charge. If the material has a high conductivity the charges can recombine rapidly and counter the separation process with consequently little or no electrostatic charge accumulation. Electrostatic Discharge Electrostatic breakdown between any two points giving rise to a discharge is dependent upon the strength of the electrostatic field in the space between the two points. The field strength near a point of protrusion is greater than the overall field strength; thus, discharges generally occur at points of protrusion. NOTE: Liquids with conductivities of less then 50 picosiemens/metre are considered to be non-conductors and liquids with relaxation times greater than 0.35 seconds are known as static accumulators (e.g. white oils). Liquids having a conductivity exceeding 50 picosiemens/metre are known as static non-accumulators (e.g. black oils and crude oils have conductivities in the range of 10,000-100,000 picosiemens/metre). Generation of Electrostatic Charges (a)
Loading or Discharging Operations An electrostatic charge may develop during several processes:
• as the oil flows through a pipeline system into a tank (charge generation is enhanced if water droplets are suspended in the oil as it flows through the pipes);
• as oil flows through a microspore filter used for aircraft jet fuels (the filters have the • •
ability to charge fuels to a very high degree probably because of charge separation where the fuel contacts the filter surface); during turbulence and splashing in the initial stages of pumping oil into an empty tank,; and and from water droplets entering a tank or from dust particles either entering or being stirred up.
The generally accepted method for controlling electrostatic charge generation in the initial stages of loading is to restrict the flow rate of static accumulator oils. The recommended loading rate for an empty tank is one metre per second (linear velocity) in the branch lines. NOTE: Specific precautions are necessary for loading or discharging static accumulation oils according to the International Safety Guide for Oil Tankers and Terminals.
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~NOTES~
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Inert Gas Systems - 6.0 Operations
6.0 OPERATIONS
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6.1
Operational Cycling of Inert Gas Plants
Petroleum Carriers On oil tankers required to be fitted with inert gas systems, the cargo tanks should at all times be inerted and have a tank atmosphere with an oxygen content not exceeding 8% by volume. During normal operations a variety of processes take place. Purging Purging is the introduction of inert gas into a tank already in the inert condition to:
• further reduce the existing oxygen content of the tank; and • to reduce the existing hydrocarbon vapour content to a level which cannot support combustion if air is subsequently introduced into the tank.
Dilution Method of Gas Replacement in Tanks The replacement of the gas in the cargo tanks is carried out during the following operations:
• inerting; • Purging; and • and gas freeing. To replace the gas in a tank either the dilution or the displacement method is used. The dilution method is a mixing process whereas the displacement method is a layering process. The dilution method is used on most ships: inert gas is introduced
Figure 17 - Dilution Method of Gas Replacement
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into the tanks at a high velocity and it is usually necessary to change the atmosphere three to five times before the tank is fully inerted. Displacement Method of Gas Replacement In the displacement method of gas replacement inert gas is introduced into the tanks at a low velocity (approximately 3 to 5 metres per second). An interface is formed
Figure 18 - Displacement Diagram
between the incoming gas and the outgoing air. As the inert gas settles into the tank the outgoing air is forced out through the purge pipes. Complete inerting using this method requires about 1.5 changes of the atmosphere. Question: Why is it important to take oxygen readings at various levels in a tank?
The following describes the use of the IG System during the indicated operations: (a)
Inerting Gas Free Tanks The inert gas plant is to be started according to the appropriate instructions. All tank hatches are to be closed and the oxygen analyzer checked to insure that it is functioning properly. The vent system is to be opened ( i.e. the stand pipes and the purge pipes on the tanks to be inerted). The tank atmosphere is to be tested at the bottom, the centre and the top of the tank until the oxygen content of the gas leaving the tank is below 8% by volume for a minimum of 30 minutes. The vent system is to be then closed and the tanks pressurized to from 300-750 mm WG. The IG plant is to be shut down according to the appropriate instructions.
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(b)
Discharging Ballast Ballast may be discharged either before or during loading. If ballast is discharged before loading the inerting is done the same as when discharging cargo. If simultaneous loading of cargo and discharging of ballast occurs, the inert gas volume in the tanks being loaded is more than adequate. Inert gas will flow from the cargo tanks to the ballast tanks by interconnecting the IG lines on deck.
(c)
Loading Cargo Question: What is the purpose of opening the mast vents and putting the PV valves in the open or bypass position while loading cargo?
During this procedure the mast vents are to be opened and the PV valves are to be put in the open or bypass position. The deck isolating valve is to be closed and the deck water seal vent opened before the commencement of loading. When the loading is finished the bypass valve is to be closed and the PV valves are to be set to normal operating condition. The deck water seal is to be closed and the system pressurized. (d)
Padding Question: What are some causes of IG leakage?
During a voyage when a tanker is loaded there are many factors which may cause inert gas to escape to the outside atmosphere resulting in a drop in the pressure in the cargo tanks. Padding or the topping-up of the tanks is normally done by using a special topping-up inert gas generator or by starting the ship’s Inert Gas System (depending on the IG equipment the ship carries). During the topping-up operation usually only a small amount of inert gas is required as the ship is in a loaded condition. (e)
Crude Oil Washing Questions: What size crude carriers are required by law to be fitted with IGS and COW? What is the procedure if washing using crude oil from a slop tank?
Many crude carriers now wash their tanks with crude oil while discharging cargo. The use of crude oil as a washing medium can cause the generation of large electrostatic charges in the tanks, especially if the crude oil is not dry. During crude oil washing the IG System must be carefully operated and monitored and the oxygen content of the IG must be kept below 8% by volume.
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(f)
Ballasting The conditions for ballasting are the same as for loading as per ISGOTT 10.6.2. However, when simultaneous loading and ballasting is occurring the pressure in the tanks may be increased or a vacuum may be created so that it may be necessary to adjust the inert gas flow to maintain normal pressure.
(g)
Ballast Passage During a ballast passage all tanks should be kept inerted with the cargo tank atmosphere at a positive pressure of not less than 100 mm WG and an oxygen content of not more than 8% by volume (the only exceptions being those tanks gas freed for entry).
(h)
Tank Cleaning Cargo tanks should be washed when they are in the inert condition and under a positive pressure. The procedures for tank washing should follow those for crude oil washing as per ISGOTT 10.6.8. Before a tank is washed its oxygen content must be determined at a point one metre below the deck and at the middle level of the ullage space. The oxygen content and the pressure of the inert gas being delivered during the wash process should be continually recorded. The oxygen content level should not exceed 8% by volume and a positive pressure must be maintained; otherwise, the washing must be stopped until the desired conditions are restored.
(i)
Purging Prior to Gas Freeing for Entry If it necessary to gas free a tank after washing the tank should first be purged with inert gas to reduce its hydrocarbon vapour content to 2% or less by volume so that during the subsequent gas freeing no portion of the tank atmosphere is brought within the flammable range. The hydrocarbon vapour content must be measured with an appropriate meter designed to measure the hydrocarbon vapour level in an oxygen deficient atmosphere as covered in ISGOTT 10.6.
(j)
Releasing Cargo Tank Pressures for Cargo Measurement or other Operations Frequently cargo owners or receivers or some other authorities may require manual ullaging, water dips and cargo samples before the commencement of a discharge. This practice is quite acceptable and in accordance with the ICS flue gas safety guide if the following conditions are adhered to:
• no cargo or ballasting operations are being undertaken at the same time; • a minimum number of tank openings are uncovered one at a time; and • after the measurements are taken, the tanks should be re-pressurized before discharge begins.
Cargo discharge should not commence until:
• • • • •
cargo tanks and slop tanks are common to the IG main; cargo tank openings and vent valves are closed; the IG main is isolated from the atmosphere; the IG plant is operating; and and the deck isolating valve is opened.
Both the oxygen content and the inert gas deck pressure should be continually recorded throughout the discharge process as per ISGOTT Regulations 62.16.1 and 62.16.2. ãMarine Institute
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Chemical and Product Carriers Chemical Carriers Many cargoes transported on chemical carriers are highly sensitive and can be easily contaminated or thrown off specifications. For this reason, the inert gas used on chemical tankers must be of a high quality and have a low moisture content.
Figure 19 - Membrane Separator
Nitrogen of high purity (95-99%) can be produced using the pressure swing absorption method or the membrane method. Nitrogen produced in this manner remains gaseous throughout. Ship’s personnel should be aware that the exhaust from these methods is rich in oxygen and thus increases the hazard of flammability. Inert gas thus produced is both pure and dry. This helps to prevent the contamination of water/oxygen sensitive cargoes. Modern inert gas generators fitted with coolers and dryers are also quite suitable for use on chemical and liquefied gas carriers. If liquid nitrogen is carried in storage bottles the vacuum should be monitored to avoid excessive boil-off. Nitrogen can cause brittle-fracture to any metal not designed for temperatures below -196°C. The liquid nitrogen can also cause severe cold burns if it comes into contact with the skin.
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Product Carriers Although the basic principles concerning the use of IG on product carriers is the same as for on crude carriers there are certain operational differences. Product tankers may carry petroleum products having a flash point exceeding 60°C without having an IG System fitted or, if fitted, with the tanks having those cargoes in the inert condition (e.g. heavy fuel oils, diesel fuels, gas oils). However, when cargoes with a flash point exceeding 60°C are carried at a cargo temperature higher than their flash points less 10°C, the tanks should be maintained in an inert condition (within 10°C of flash point). Also, any gas freeing operation has to be preceded by a purging operation to lower the hydrocarbon vapour content of a tank to 2% by volume.
Gas Carriers The inert gas used on gas carriers may be carried in storage vessels, be produced on the ship or be supplied from shore. This inert gas should be compatible chemically and operationally, at all temperatures, with the construction materials of the spaces and with the cargo. When the temperature where the IG is stored is below 0°C the IG should not impact negatively on the ship's structure. If IG is produced on board it must have an 02 content at no time greater than 5% by volume (except when subject to the special requirements as covered in Chapter 17 of ISGOTT for cargoes such as vinyl chloride (VCM) which requires an 02 content in the tanks of not more than 0.1% by volume). Such cargoes require the use of a special meter to measure any 02 content level below 1% by volume. The pressure in the IG supply line should not fall below 0.07 bar. Additionally, when IG is produced on board by the process of the fractional distillation of air, which requires the storage of the cryogenic liquefied nitrogen for later release, the gas entering the storage tank should be checked for traces of oxygen. Any IG stored for fire-fighting purposes should be carried in designated containers and should not be used for cargo services. A vessel's operations and equipment manuals should be checked for other specific information on inerting. Gas Carriers (LNG/LPG) The typical ship-generated inert gas produced from flue gases is inadequate for use on LNG and LPG tankers for the following reasons:
• ship generated gases produced by combustion can contain up to 15% CO2 by volume •
which is unsuitable for use with certain cargoes (e.g. ammonia which reacts with CO2 to produce carbonates creating deposits on tank walls and blockages in pipelines); if inert gas is used before the loading of cargo with a temperature below -55oC, the CO2 will freeze and may contaminate the cargo; and
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• to prevent a reaction when preparing to load ammonia after carrying LPG, the inerted tank should be ventilated with air before the ammonia is loaded; and when preparing to load LPG after carrying ammonia, the ammonia concentration should be reduced to 100 ppm by gas freeing with air before inerting.
In accordance with the liquefied gas carrier safety guideline manual the 02 content of tanks should never exceed 5% by volume and should normally be in the range of 2% by volume. Much lower levels of 02 are required when loading oxygen reactive cargoes (e.g. butadiene: 0.2% by volume). Other concerns include the formation of moisture and this moisture freezing in the cargo tanks, due mainly to the cooling of the tanks by cargoes with a low temperature. Before inerting the tanks should be warmed, by using hot gases or some other suitable means, to prevent moisture formation.
Special Inerting Requirements - Combination Carriers The basic principles of inerting gas on combination carriers can be applied in the same way as on an oil tanker. However, differences in design and operation give rise to particular consideration for combination carriers. Special attention has to be paid to OBO carriers especially where “sloshing” or slack tanks could generate electrostatic charges. It is important in such situations that the cargo tanks are maintained in an inert condition. Gas leakage into void spaces or ballast spaces due to shell fractures is another hazard of which personnel should be aware. Any slop tanks should also be maintained under a positive pressure and should be checked every two days to insure that their 02 content is not more than 8% by volume. Otherwise, purging is required until the 02 content is less than 8% by volume. Cargoes, other than liquid carried in other holds, should be isolated by blanks from the oil cargo pipelines and the IG main. If the IG system fails on a product carrier the recommendations of ISGOTT should be followed. Combination carriers include either oil/bulk/ore (OBO) ships and oil/ore (O/O) ships. Both these types of vessels are designed to carry cargoes other than oil. The primary difference between an OBO and an O/O carrier is in their hold construction. The hold in an OBO usually extends the full breadth of the ship with upper and lower hopper tanks and double bottom tanks. The hold in an O/O carrier usually extends half the total breadth of the ship with wing tanks on either side. Double bottoms are always located beneath the cargo hold. On both designs, ballast lines are located in the double bottom tanks. OBO ships usually have cargo lines in the double bottoms while on O/O carriers, the cargo lines are in the wing tanks. Both types of cargo holds have conventional bulk carrier hatches with special sealing arrangements.
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Inert Gas Systems Inert gas systems fitted on combination carriers operate according to the same principles as those fitted on conventional tankers. Differences in design and operation of combination carriers give rise to certain particular considerations. These include: Slack Holds Special attention has to be paid to sloshing of the cargo in slack tanks. Structural damage could result due to the slamming effect of the cargo against bulkheads. The sloshing could also lead to an electrostatically charged mist being formed in tanks containing dirty ballast or in situations where the cargo has water in it. Ballast and Void Spaces To provide access to tank valves and to provide a passage for piping system, void spaces may exist between or below cargo holds. Oxygen deficient atmospheres, and the leakage of inert gas or hydrocarbon vapours into these spaces are hazards personnel should be aware of. Hatch Covers The large hatchway provides for leaks and as such, requires regular maintenance. The hatch cover runways, hatch seals and hatch landing edges should be kept clean. The hatch covers should be tightened according to instruction and as evenly as possible. Leaks at hatch covers permit the escape of inert gas onto the main deck, resulting in a potential safety hazard. This also puts extra demands on the IG system as more frequent top ups are required. Inert Gas Distribution Because of the special construction of combination carriers, the vent line from the cargo hatchway coaming is situated close to the level of the cargo surface. Also, due to the fact that the coamings extend above the main deck and may be partially filled with cargo, the IG main line may lie below the oil level in the hold. Water or oil may enter these lines and prevent an adequate inert gas supply during discharge or tank cleaning. Vent lines should therefore have drain lines at their lowest point which should be checked before operations proceed in the cargo hold. Slop Tanks If slops are kept onboard, the slop tank should always be kept under a positive pressure with inert gas. The slop tank should always be isolated from the main IG line except when purging or inerting, and all cargo lines to and from the slop tank should be blanked. Cargo Change Over Each ship will have a specific procedure relating to cargo change overs. Whether the change is from oil to dry bulk cargo or dry bulk to oil cargo, certain isolation, cleaning and venting procedures are to be adhered to. This is to ensure no environmental hazards are present either by oil contamination, inert gas contamination or dry cargo contamination of the oil cargo system. For obvious reasons, oil and dry bulk cargoes cannot be carried simultaneously. 達Marine Institute
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6.2
IG Plant Failure
The following describes the action to be taken in the event of plant failure. In the event of IG failure when commencing cargo discharge:
• • • •
immediate action must be taken to prevent an ingress of air; all tank operations must be stopped; the deck isolating valve must be closed; and discharging or tank cleaning should not commence or continue until the operation of the Inert Gas System has been restored or an alternative source of IG provided.
If the Inert Gas System fails during discharge the following immediate action must be taken to prevent any air from being drawn into the tanks:
• all cargo and ballast discharge must be stopped; • the vent valve located between the isolating valve and the gas pressure valve must be • • •
opened; immediate action must be taken to repair the malfunction; the situation must be reported to the terminal operator, the harbour authority and any others as required by national or local regulations; and on tankers carrying crude oil, because of the danger of pyrophoric ignition, the IG system must be repaired and restarted or an alternative source of IG utilized before the discharge of cargo or ballast is resumed.
When re-inerting tanks which are not gas free the following precautions must be taken due to the hazard of electrostatic charges (the same procedure as for the initial inerting of non-gas-free tanks):
• no dipping, ullaging, sampling or other equipment should be inserted into a tank until it has been determined that the tank is in the inert condition; and
• no gas sampling system should be introduced into a tank until at least 30 minutes relaxation time has passed; and
• metallic components of the sampling s sampling system should be electrically continuous and securely grounded.
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Inert Gas Systems - 7.0 Systems Design
7.0 VAPOUR EMISSION CONTROL
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7.1
System Designs
Current “Closed Loading” System of Venting The “closed loading” system for controlling cargo vapour represents the latest development in the evolution of cargo tank venting arrangements. All deck openings to the cargo tanks are closed and remain so for the entire transfer of the cargo. The vapours from each cargo tank are directed through vapour collection piping to the manifold connection on deck where a vapour line directs the vapours to shore for processing. The vapours dispelled are those generated by loading operations and by cargo vaporization. On tankers fitted with an IG main the system can be modified as a vapour control main The vapour connection on a vessel must be clearly marked and normally its last metre of piping is painted with red/yellow/red bands and labelled with the word “vapour.” The vapour connection flange must have a 0.5 inch diameter stud at least one inch long projecting outward from top dead centre on the flange face. This is so that it cannot be connected to any other line such as a cargo line.
Figure 20 - Vapour Control Hose Appendix
The manifold must be fitted with a manually operated isolation valve that gives a clear indication of the valve's status. Questions: When a tanker is equipped with a modified IG System and an IG vapour control main there must be a means of isolating the IG supply. How is this done? Why must a vapour connection flange have a 0.5 inch diameter stud at least one inch long projecting outward from the top dead centre of the flange face? ãMarine Institute
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The following covers the different arrangements and components for the gas venting of tanks: (a)
Common Venting One form of single common venting uses the IG main and the IG branch lines from each tank and vents to the outside atmosphere through one or more mast risers or through several high velocity vents. Another type of common venting system uses a separate vent main and vent lines from each tank and vents to the outside atmosphere through one or more mast risers or through several high velocity vents.
(b)
Individual Venting with High Velocity Vents This system makes use of individual vents on each cargo tank utilizing either stand pipes or high velocity vents and a PV valve. No common vent line is used.
(c)
Purge Pipes Many cargo tanks are fitted with purge pipes which protrude into a tank. Each pipe is fitted with a cover which when opened allows vapours to be expelled from a tank. Purge pipes are used when inerting empty tanks but not for venting hydrocarbon vapour or inert gas during loading or ballasting operations. Purge pipes extend down into a cargo tank to within 1 m of the tank bottom; thus, their ends would be covered by the cargo.
(d)
High Velocity Vents These vents are used during loading and ballasting. As the liquid level rises in a tank the atmosphere (IG) of the tank is pushed out at a high velocity (depending on the loading rate) through these vents.
(e)
Mast Risers Mast risers are used to expel IG and/or hydrocarbon vapour at a point well above deck level which insures that the expelled gases pose the least possible safety hazard to personnel on deck.
(f)
Pressure/Vacuum Relief System In the case of a severe under pressure (vacuum) or over pressure in the cargo tanks the PV breakers provide the necessary supplementary flow capacity in addition to that provided by the PV valves. In the open position the PV valves allow the free passage of air and vapours between the tanks and the outside atmosphere. In the closed position the PV valves are designed to lift at a pre-set pressure or vacuum. Therefore, when a PV valve is closed it effectively seals the tank (or tanks) on that line unless a dangerous pressure or vacuum develops.
(g)
Cascading Cascading is a means of utilizing the no longer required inert gas in a cargo tank. If simultaneous loading and deballasting is taking place the inert gas being displaced from the cargo tank by the incoming cargo can be transferred through the IG main into a ballast tank as it is being emptied.
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7.2
Controlling Emissions
Environmental Concerns Regarding the Emission of Noxious Vapours into the Atmosphere Marine vapour control systems are a result of the US Clean Air Act 1970. The goal of this act is to improve air quality by specifying air quality standards regarding primary pollutants (carbon monoxide, sulphur oxides, nitrogen oxides and hydrocarbons). The US federal government has directed state governments to establish their own plans to comply with air quality standards. A number of states were unable to meet the standards and are classified as non-attainment areas resulting in some state enacting legislation to limit the release of hydrocarbon vapours from tankers. Vapours are typically released during loading, ballasting, purging and gas-freeing. The operational and safety implications are significant when using a vapour emission control system. The ship and terminal are connected by a common stream of vapours thereby introducing into the operation a number of additional hazards which have to be effectively controlled. Discussion of some of the main concerns follows. (a)
Mis-Connection of the liquid and Vapour Lines To guard against the possible mis-connection of the ship’s vapour manifold to a terminal loading line a specially painted hose, as mentioned earlier, is used.
(b)
Vapour Over/Under Pressure Due to the ship-to-shore vapour space connection a change on either ship or shore can affect the other. Therefore it is essential to insure that all PV protection devices are fully operational and that loading rates do not exceed the maximum allowable rates. Also, all high and low pressure alarms must be continuously monitored.
(c)
Fire/Explosion/Detonation Due to the ship-to-shore connection a fire or explosion on either the ship or the shore could rapidly spread to the other. A detonation arrestor should be fitted in close proximity to the terminal vapour connection at the jetty head in order to provide primary protection against the transfer or propagation of a flame from the ship to the shore or from the shore to the ship.
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Insert Gas Systems - 8.0 Materials, Maintenance, Inspection and Testing
8.0 MATERIALS, MAINTENANCE, INSPECTION AND TESTING
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8.1
Maintenance Programs
Preventative Maintenance Schedules for the Components of an Inert Gas System The following is a suggested maintenance program as per IMO Inert Gas Systems 1900 Edition: Component Flue gas isolating valves
Preventive Maintenance Operation of the valve Compressed air or steam cleaning Dismantling for inspection and cleaning
Maintenance Interval Before system start-up and at one week Before valve operation After boiler shut down
Water flush
After use
Cleaning of demister
Three months
Dismantling and inspection of temperature probes for inspection
Six months
Full internal inspection
Dry dock
One hour flushing with scrubber water pump
After use
Dismantling of the valve for overhaul, inspector of pipe and overboard end
Dry dock repair period
Vibration check
While running
Flushing
After use
Internal inspection through hatches
After flushing and six months
Dismantling for full overhaul of bearings, shaft tightenings
Two years dry dock
Dismantling of level regulators or float valves for inspection
Six months
Internal inspection
One year
Overhaul auto valves
One year
Moving and lubricating
Before start one week
Internal inspection
One year to 18 months
Operating and lubricating
Six months
Full overhaul and inspection
One year
Deck isolating valve
overhaul
One year
Gas pressure regulating system
Dry air supply
Before start
Gas pressure regulating valve overhaul
As needed
Liquid filled pv brooker
Liquid level when system at ambient pressure
Six months or when opportunity permits
Flue gas scrubber
Overboard pipes and valve from flue gas scrubber
Blowers
Deck water seal
Deck mechanical non-return valve
Pressure/vacuum valves
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8.2
Inspecting and Testing Procedures
IG Scrubber For an IG scrubber the following inspections should be made through the manholes. Checks should be made for the corrosion, fouling and/or damage of:
• • • •
the scrubber shell and bottom; the cooling water pipes and the spray nozzles; the float switches and the temperature sensors; and the internals, trays, plates, and demister filters.
Checks should also be made on non-metallic parts such as:
• internal linings; • Demisters; and • packed beds. IG Blowers For IG blowers an internal visual inspection can reveal damage at an early stage. By fitting two blowers or by supplying and retaining on board a spare impeller with a shaft for each blower an acceptable level of availability is insured. The inspection of blowers should include:
• an internal inspection for soot deposits and signs of corrosion; • an inspection of the functioning of the freshwater flushing arrangements; • an inspection of the drain lines from the blower casing to insure that they are clear and •
operative; and observation of the blower under operating conditions for signs of excessive vibration and imbalance.
Deck Water Seal An inspection of the deck water seal should include the opening of the seal to check for:
• blockage of the venturi lines in the semi-dry type water seals; • corrosion of the pipes and housing and of the heating coils; and • corrosion or sticking of the floats for the water drain, the supply valves and the level monitor.
The deck water seal should also be checked for:
• automatic filling and draining; and • the presence of water carry over (open drain cocks on the inert gas main line) during operation.
Non-Return Valve The non-return valve should be checked for corrosion. Also the condition of the valve seat should be checked and the valve should be tested under operating conditions.
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Scrubber Effluent Line The scrubber effluent line must be inspected internally when the ship is in dry dock. The ship-side distance piece and the overboard discharge valve should be inspected at each dry docking period. Testing Programs to Check the Integrity of the IG System A testing method should be devised to test for the correct functioning of all units and alarms. The following should be checked:
• • • • • • • •
8.3
all alarm and safety functions; the operation of the flue gas isolating valve; the operation of all automatically controlled valves; the functioning of the water seal and of the non-return valve (with a back flow pressure test); the vibration level of the IG blowers; with older systems the deck lines should be examined for leaks; the interlocking of the soot blower; and all measuring equipment, both portable and fixed, should be checked for accuracy by using both air and a suitable calibration gas.
Materials
The materials used in the manufacture of IG System components should be carefully selected. Flue Gas Isolating Valves The materials used in the manufacture of flue gas isolating valves depends on the operational temperature of the flue gas to which the valve will be exposed. For flue gas temperatures below 350oC ordinary cast iron may be used. Valves exposed to temperatures in the range of 350oC-450oC should be made of nodular cast iron. If temperatures are to exceed 450oC the valve material should be Melanite HA or a material of equal quality. The flue gas bellows are prone to corrosion, especially when installed in the horizontal position, and should be made from a nickel-chrome-molybdenum alloy. Most of the damage to the IG scrubbers is caused by the corrosion of their metallic parts. This results from the breakdown of coatings as a result of non-cooled gases impinging on the coated surfaces. Sea water and sulphur dioxide are very corrosive making it difficult to find sufficiently corrosion resistant materials.
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A summary of some experiences with different materials is given in Table 5. Table 5 - Materials for Flue Gas Isolating Valves
Product
Result
Disadvantage
Rubber or glass fibre reinforced epoxy resin lining
Excellent
Must not be exposed to high temperatures
Glass fibre reinforced polyester lining
Limited knowledge available
Cracks develop in sharp corners; not good corrosion resistance
Nickel based alloys
Good
Welding must be done under strict controls
Stainless steel
Satisfactory
Not good in areas of severe corrosion
Aluminum alloys Copper Nickel Alloys Copper-nickel alloys
Suitable for use in cooling valve and spring nozzle Suitable for use in cooling water and spray nozzle
Scrubber Effluent Line and Overboard Discharge Valve The function of the scrubber effluent line is to drain cooling water from the scrubber. This drained water is very corrosive and creates a major problem. A summary of some experiences with different types of piping is given in Table 6. Table 6 - Piping Types
Type of Piping Pipes lined with rubber
Result
Disadvantage
Good
Pipes of aluminum-brass alloys
Corrosion in welds
Pipes GRP
Excellent
Stainless steel using a tar-epoxy internal coating
Good service record
Minor corrosion
IG Blowers IG blowers, especially their rotating parts, are subject to corrosion. The type of coating to be used on these units is of major concern with success depending on the degree of surface preparation. Coal-tar-epoxy coatings on fan casings have been known to blister and deteriorate. Non-ferrous materials are now being used whereby simple washing can prevent the accumulation of deposits. Blower casings which are coated internally with rubber appear to have a longer life than the epoxy coated types. Deck Water Seals Although ordinary sea water is less corrosive than acidic water the problems in the water seal are similar to those of the scrubber. Stainless alloys should be used for deck water seal demister mountings, where fitted, and also for heating coils.
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Water Level Alarm Float Switches Switches manufactured in PVC have been found to be quite satisfactory. IGS Components Most Subject to Corrosion The following Inert Gas System components have been found to be those most subject to corrosion:
• • • • • • •
flue gas isolating valves; the gas bellows; the IG scrubber; the scrubber’s overboard discharge; the IG blowers; the water seals and the non-return valves; and the water level alarm float switches.
Coatings and Coverings Coatings and coverings provide additional protection to IG components. However, they must be properly applied to obtain the best protection. Surface preparation is very important. Problems have developed with the durability of the coatings. In addition to corrosion, overheating can cause damage to coatings.
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APPENDIX A - REFERENCES
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REFERENCES: Canada Shipping Act And Regulations. ISBN 0 660 55696 0. Ministry of Supply and Services, Ottawa: 1990. International Chamber of Shipping. International Safety Guide for the Operation of Oil Tankers and Terminals (revised third edition). ISBN 85609 0264. Witherby & Co. Ltd., London: 1993. International Chamber of Shipping. Tanker Safety Guide Liquefied Gas (second edition). ISBN 0 906270 03 0. Witherby & Co. Ltd., London: 1995. International Maritime Organization. Bulk Chemical Code. ISBN 92 801 1315 1. London. International Maritime Organization. Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk. ISBN 92 801 1302 X. London: 1993. International Maritime Organization. Gas Carrier Code. ISBN 92 801 1222 9. London: 1987. International Maritime Organization. Inert Gas Systems. ISBN 92 801 1262 7. London: 1990. International Maritime Organization. International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk. ISBN 92 801 1315 1. London: 1994. International Maritime Organization. International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk. ISBN 92 801 1277 5. London: 1993. International Maritime Organization. International Convention for the Safety of Life at Sea. ISBN 92 801 1200 7. London: 1992. International Maritime Organization. MARPOL 73/78 Consolidated Edition. ISBN 92 801 1280 5. London: 1992. International Maritime Organization. Model Course 1.02: Advanced Training Program on Oil Tanker Operators. London: 1988. Polytech International (second edition). ISBN 0 906314 054. 1980.
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APPENDIX B - COURSE DESCRIPTION
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COURSE OUTLINE: 1.0 Regulations and Guidelines 1.1
Canada Shipping Act And Regulations
1.2
IMO Conventions And Guidelines
2.0 Applied Science 2.1
Physical and Chemical Properties of Crude Oil
2.2
Flammability
2.3
Sources of Ignition 2.3.1 Static Electricity 2.3.2 Flammability and Explosion Hazards 2.3.3 Tank Coatings
2.4
Inert Gas Production
2.5
Inert Gas Compatibility
3.0 Safety and Health 3.1
Safety 3.1.1 3.1.2 3.1.3 3.1.4
and Health Hazards Health Hazards Toxicity Cargo Hazard Sheets Enclosed Space Entry
3.2
Gas Monitoring Equipment (Fitted and Portable)
3.3
Personal Safety Equipment And Practices
3.4
First Aid and Resuscitation
4.0 Design Parameters and Construction 4.1
General Requirements of an Inert Gas System
4.2
Scrubbers
4.3
Isolating Valves, Water Seals, and Other Non-return Devices
4.4
Inert Gas Blowers
4.5
Inert Gas Distribution Systems
5.0 Instrumentation 5.1
Instruments
5.2
Audible and Visual Alarms and Interlocks
5.3
Testing
6.0 Operations 6.1
Operational Cycling of Inert Gas Plants 6.1.1 Petroleum Carriers 6.1.2 Chemical and Product Carriers 6.1.3 Gas Carriers 6.1.4 Special Inerting Requirements - Combination Carriers
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6.2
I.G. Plant Failure
6.3
Operational Experiences
7.0 Vapour Emission Control 7.1
System Designs
7.2
Controlling Emissions
8.0 Materials, Maintenance, Inspection, and Testing 8.1
Maintenance Programs
8.2
Inspection and Testing Procedures
8.3
Materials
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LEARNING OBJECTIVES: THE EXPECTED LEARNING OUTCOME IS THAT THE STUDENT WILL BE ABLE TO: 1.0 Regulations and Guidelines 1.1
Canada Shipping Act and Regulations
– Discuss the Oil Pollution Prevention Act and Regulations, Canada Labour Code and Occupational Health and Safety Act and Regulations and their application to tankers 1.2
IMO Conventions and Guidelines
– Discuss the International Codes and Regulations as they apply to the use of inert gas in tankers carrying crude oil, chemicals, and liquefied gas.
– Discuss the following IMO regulations and guidelines: SOLAS 74 and Amendments and Amendments to 1992, MARPOL 1978 and Amendments to 1992, SCTW and revisions to Chapter V of the Annex, Bulk Chemical Code, and Gas Carrier Code 2.0 Applied Science 2.1
Physical and Chemical Properties of Crude Oil
– – – – 2.2
Define isomers, paraffins, and aromatics. Define the characteristics of crude oil. Explain the difference in molecular structure of hydrocarbon compounds.
Flammability
– – – – – – – – 2.3
Explain the chemical and physical properties of crude oil.
Define flashpoint, boiling point, and vapour pressure. Discuss the relationship between flashpoint and the lower flammable limit. Define the flammable range. Explain the value of knowing the flammable range of a liquid. Differentiate between a fire and an explosion. State the difference between a flammable and a combustible liquid. Discuss the effect of the addition of oxygen on the flammable range. State the principal method of protection against explosion or fire in cargo tanks.
Sources of Ignition 2.3.1 Static Electricity
– Define the term “static electricity”. – Describe the process of induction. – Explain how static electricity is generated by each of the following: loading or discharging operations, tank washing, line cleaning, gas freeing, electrical storms, helicopter operations, synthetic clothing or materials, or aerosol type spraying.
– Explain the terms “relaxation”, “bonding”, and “insulating” as they pertain to tanker operations.
– Explain why the uncontrolled release of static energy may cause an incendive spark.
– Explain why all metal parts of portable instruments and sampling carriages introduced into a tank must be bonded to the ship’s structure.
– State where to find sampling points for use with portable instruments.
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2.3.2
Flammability and Explosive Hazards
– Explain the effects of high, medium and low vapour pressures on tank atmospheres during loading, discharging, ballasting and tank washing.
– Relate the effect of cargo contamination on flashpoint. – Define the term “pyrophoric ignition”. 2.3.3
Tank Coatings
– Explain how tank coatings can affect gas retention in cargo tanks. 2.4
Inert Gas Production
– Explain the legal requirement for an inert gas system. – Define “inert gas”. – Explain how inert gas is produced using flue gas from the main or auxiliary boiler, inert gas generator, and Stoichiometric Combustion and Fractional distillation.
– Explain the effect of different inert gases on the flammable envelope. – Differentiate between inert gases produced by stoichiometric combustion and those produced from flue gas, and fractional or swing distillation methods.
– Explain that “inert gases” are not necessarily chemically inert. – Explain that gas quality is controlled by constant vigilance during production and all operations are monitored to ensure constant quality control. 2.5
Inert Gas Compatibility
– Explain that all cargoes are not compatible with inert gas produced from boiler or main engine effluent gases.
– Discuss the use of inert gas with chemical cargoes and with low-volatility cargoes. – Explain that oxygen is a very strong contaminant of every chemical gas. – Explain how cargo contamination can occur when using inert gas. 3.0 Safety and Health 3.1
Safety and Health Hazards 3.1.1 Health Hazards
– Discuss the dangers of skin contact, inhalation, and ingestion of hydrocarbon or chemical gases.
– – – – 3.1.2
Discuss the carcinogenic properties of petroleum. Explain the effects on the body of an atmosphere with a low oxygen content. Describe the symptoms of anoxia narcosis.
Discuss the dangers of hydrogen sulphide. Toxicity
– Define “toxicity” and “exposure”. – Explain “threshold limit value” and “short term exposure limit”. – Compare the effects on the human body when exposed is to hydrocarbon gases, hydrogen sulphide, pure oxygen, and inert gas.
– – – – 3.1.3
Describe the acute and chronic effects of toxicity. Discuss the main effect of hydrocarbon gases on tanker personnel. Explain why the absence of smell is insufficient to assume the absence of toxins.
Explain why a combustible gas indicator cannot be expected to measure the threshold limit value accurately. Cargo Hazard Sheets
– Describe the data provided on specific cargo hazard sheets. – Identify other sources where data is provided on specific products. – State where first aid procedures for specific products can be found. ãMarine Institute
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3.1.4
Enclosed Space Entry
– State the requirements and correct procedures for entry into pump-rooms, tanks, and other potentially dangerous spaces.
– Discuss the use of enclosed space entry permits and check lists. – List the procedures regulating entry into enclosed spaces. 3.2
Gas Monitoring Equipment (Fitted and Portable)
– – – – – – – 3.3
List the requirements for a fixed oxygen analyzer. Interpret combustible gas indicator and oxygen analyzer readings. Explain the limitations of explosimeters, which use a catalytic filament. Explain how to calibrate combustible gas indicators and oxygen analyzers. State the procedure for determining the level of toxic gas in a compartment. Describe the maintenance necessary for gas monitoring equipment. Explain, in detail, how to evaluate the atmosphere in a cargo tank or enclosed space.
Personal Safety Equipment and Practices
– Explain the use of self-contained breathing apparatus and tank rescue equipment including airline breathing apparatus.
– Describe standard tank rescue equipment and explain how it is used. – Discuss rescue techniques and the level of supervision needed when effecting a tank entry using rescue equipment.
– Justify the use of protective clothing and equipment. – Debate the pros and cons of personal gas monitors and escape breathing units. – Detail the level of personal hygiene required for effective prevention of health hazards. 3.4
First Aid and Resuscitation
– State the first aid procedures for handling personnel overcome by gas or lack of oxygen. – Describe a resuscitator. – Explain how a resuscitator is used. 4.0 Design Parameters and Construction 4.1
General Requirements of an Inert Gas System
– Describe the general components of an inert gas system and their relationship to each other.
– Explain why inert gas systems on petroleum, chemical, and product carriers follow the same basic pattern.
– Indicate why inert gas systems on gas carriers require additional process equipment but the basic pattern is still the same.
– Identify the valves fitted in the system which isolate one section from another. – Explain why the system must be capable of reducing the oxygen content of each tank to a level at which combustion cannot be supported.
– Explain why the system must be capable of maintaining the oxygen content in any part of any cargo tank at a level not exceeding 8% by volume.
– Explain how the system eliminates the possibility of air entering cargo tanks except when necessary for gas-freeing.
– Discuss the reason for the purging of cargo tanks to eliminate any hydrocarbon or other gases.
– Explain why the system must be capable of delivering inert gas to the cargo tanks at a volume of at least 125% of the rated discharge capacity of the cargo pumps.
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– Identify why the system must be capable of delivering inert gas to the deck supply main at any demanded rate of flow up to the maximum capacity of the blowers and why the oxygen content must not exceed 5% by volume.
– Explain why the inert gas may be treated flue gas from the main or auxiliary boilers, or from one or more separate gas generators or from any other source considered equivalent.
– Identify why stored carbon dioxide is not permitted to be used in inert gas systems on petroleum or chemical tankers.
– Explain that the inert gas system provides a direct pipeline from the hazardous area of the cargo tanks to the engine room, boiler room or gas generator room. 4.2
Scrubbers
– – – –
Explain the purpose of a scrubber and the principles of its operation. Describe the size and throughput requirements for any specific installation. Identify the main and auxiliary cooling water supply. Explain why the use of cooling water by the scrubber must not interfere with the provision of any other essential water supply service on the vessel.
– Identify the corrosive effects of the water leaving the scrubber, especially where the pipe connects to the ship’s hull.
– Differentiate between spray towers, packed plate towers, and venturi scrubbers with specific reference to their efficiency.
– Define the criteria by which the quality of any inert gas made from exhaust gases is judged. – Identify the anticipated cooling effect of the scrubber system on the end product. – Explain the term “carry-over” and the provisions made to counter it. 4.3
Isolating Valves, Water Seals, and Other Non-return Devices
– Discuss the objective for the fitting of isolating valves, water seals, and other non-return devices.
– Describe the action of the regulating valve fitted at the forward bulkhead in the non-hazardous area.
– Explain the function of a deck water seal. – Explain the need for redundancy in the water supply system to a water seal. – Discuss wet, dry, and semi-dry types of water seals and the conditions under which they must continue to operate.
– Describe how water seals are prevented from freezing. – Describe the mechanical non-return device fitted forward of the water seal. 4.4
Inert Gas Blowers
– – – – – 4.5
Discuss the redundancy requirement for inert gas blowers and the alternatives. State the capacity of the inert gas blowers in relation to the redundancy requirements. Describe the characteristics of the blower fans. Explain the arrangements for controlling the flow of inert gas to the distribution system. Describe the re-circulation process for inert gas.
Inert Gas Distribution Systems.
– Explain why the inert gas supply line may be divided into two or more branches forward of the non-return devices.
– Explain why each cargo tank must be supplied by an individual branch line fitted with a stop valve having a locking arrangement.
– Discuss the SOLAS requirements for inerting, ventilation and gas measurement on double hull and double bottom vessels constructed after October 1, 1994.
– Explain the reason for the fitting of pressure/vacuum valves on each cargo tank.
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– Explain why all pipes in the distribution system must be designed to drain naturally under normal conditions of trim.
– Locate the fitting which permits an external supply of inert gas to be connected to the vessel’s distribution system.
– Describe the arrangements for the venting of gases displaced from the cargo tanks during cargo operations.
– Describe the operation of the emergency devices provided to relieve internal tank pressure or vacuum in excess of the test or working pressures of a tank. 5.0 Instrumentation 5.1
Instruments
– Discuss the need for constant measurement of identifiable parameters throughout the inert gas system.
– Describe the operation, calibration, and testing of gas measuring instruments, both fixed and portable.
– Identify the location of the different types of measuring instruments and the parameters measured by each. 5.2
Audible and Visual Alarms and Interlocks
– Detail the installation of the alarms fitted on the scrubber, gas blowers, power supplies, water supplies, supply lines, and deck seals.
– Identify the alarm conditions, which result in the automatic shutdown of the gas blowers and/or the gas regulating valve.
– Describe the conditions under which the automatic shutdown of the cargo pumps may be activated. 5.3
Testing
– Interpret the recommendations of the Instruction Manuals with regard to the testing of alarms as the system is run up and closed down.
– Describe the use of the faultfinding section provided in the Operations Manual. 6.0 Operations 6.1
Operational Cycling of Inert Gas Plants 6.1.1 Petroleum Carriers
– Define the term “purging”. – Explain the difference between the dilution and displacement systems of replacing tank atmospheres.
– Describe the use of the inert gas system during the following: inerting of gas free
6.1.2
tanks, discharge of cargo tank water ballast, loading of cargo, “padding” of cargo tanks on a loaded passage, crude oil washing of cargo tanks while discharging cargo, rinsing of cargo tanks with water, ballasting of the vessel, ballast passage, cleaning of the cargo tanks, purging of a cargo tank prior to gas freeing for entry, and releasing of cargo tank pressures for cargo measurement or other operations Chemical and Product Carriers
– Discuss the operational differences in using inert gases on a chemical carrier. – Discuss the operational differences when using inert gas on a product carrier. 6.1.3
Gas Carriers
– Discuss the procedural and operational uses of inert gases on a gas carrier. 6.1.4
Special Inerting Requirements - Combination Carrier
– Discuss the operational procedures for using inert gas on a combination carrier.
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6.2
I.G. Plant Failure
– Discuss the actions to be taken in the event of plant failure, when commencing cargo discharge, during discharge, and when re-inerting tanks which are not gas-free. 6.3
Operational Experiences
– Discuss the statistical experience with inert gas systems over a number of years with particular emphasis on equipment failures and the frequency of incidents.
– Compare the installation and maintenance costs for flue gas and more recent innovations in marine inert gas systems.
– Discuss the occurrence of fires and explosions in vessels fitted with inert gas systems. 7.0 Vapour Emission Control 7.1
System Designs
– Describe a current “closed loading” system of venting. – Discuss current venting arrangements including the following: common venting, individual venting, purge pipes, high velocity vents, the use of mast risers, pressure/vacuum relief systems and “Cascading”. 7.2
Controlling Emissions
– Relate the current environmental concerns regarding the emission of noxious vapours to the atmosphere.
– Describe a VEC system of loading as may be anticipated in the future for petroleum tankers.
– Discuss the potential safety concerns associated with a VEC system operating in conjunction with an inert gas system. 8.0 Materials, Maintenance, Inspection and Testing 8.1
Maintenance Programs
– Detail the preventive maintenance schedules for components in an inert gas system. – Discuss maintenance frequencies for an inert gas system. – Discuss the cause and effect of equipment failures or malfunctions on an inert gas system. 8.2
Inspection and Testing Procedures
– Describe the inspection routines for the main components in an inert gas system. – Outline the testing programs to check the integrity of the inert gas system’s alarms and interlocks. 8.3
Materials
– Outline the materials utilized in the manufacture of the major components in an inert gas system.
– Identify those components in an inert gas system which are most subject to corrosion. – Detail the additional protection provided to inert gas systems by coverings and coatings.
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