Environmental Control on Liquefied Gas Carriers

Gas tankers always carry toxic or flammable vapors. This article contains information about Environmental control within cargo tanks, piping system and within hold spaces.

Environmental control within cargo tanks and cargo piping systems

A piping system is to be provided to enable each cargo tank to be safely gas freed, and to be safely purged with cargo gas from a gas free condition. The system is to be arranged to minimize the possibility of pockets of gas or air remaining after gas freeing or purging.

A sufficient number of gas sampling points is to be provided for each cargo tank in order to adequately monitor the progress of purging and gas freeing. Gas sampling connections are to be valved and capped above the main deck.

For flammable gases, the system is to be arranged to minimize the possibility of a flammable mixture existing in the cargo tank during any part of the gas freeing operation by utilizing an inerting medium as an intermediate step. In addition, the system shall enable the cargo tank to be purged with an inerting medium prior to filling with cargo vapour or liquid, without permitting a flammable mixture to exist at any time within the cargo tank.

Piping systems which may contain cargo shall be capable of being gas freed and purged as provided above. Inert gas utilized in these procedures may be furnished from ashore or from the ship.

Environmental control within hold spaces (cargo containment systems other than type C independent tanks)

Interbarrier and hold spaces associated with cargo containment systems for flammable gases requiring full secondary barriers are to be inerted with a suitable dry inert gas and maintained inerted with make-up gas provided by a shipboard inert gas generation system, or by shipboard storage which are to be sufficient for normal consumption for at least thirty days.

Interbarrier and hold spaces associated with cargo containment systems for flammable gases requiring partial secondary barriers are to be inerted with suitable, dry inert gas and maintained inerted with make-up gas provided by a shipboard inert gas generation system or by shipboard storage which are to be sufficient for normal consumption for at least thirty days.

Except as limited by “Special requirements”Special Requirements for LNG and LPG gas carriers, the Society may allow the spaces referred above to be filled with dry air provided that the ship maintains a stored charge of inert gas or is fitted with an inert gas generation system sufficient to inert the largest of these spaces; and provided that the configuration of the spaces and the relevant vapour detection systems, together with the capability of the inerting arrangements, ensure that any leakage from the cargo tanks will be rapidly detected and inerting effected before a dangerous condition can develop. Equipment for the provision of sufficient dry air of suitable quality to satisfy the expected demand is to be provided.

For non-flammable gases, the spaces referred above may be maintained with a suitable dry air or inert atmosphere.

In case of internal insulation tanks, environmental control arrangements are not required for interbarrier spaces and spaces between the secondary barrier and the inner hull or independent tank structures completely filled with insulation material complying with “Materials”Cargo containment system of gas vessel.

Environmental control of hold spaces surrounding type C independent tanks

Spaces surrounding refrigerated cargo tanks not having secondary barriers are to be filled with suitable dry inert gas or dry air and be maintained in this condition with make-up inert gas provided by a shipboard inert gas generation system, shipboard storage of inert gas, or dry air provided by suitable air drying equipment.

Inerting

Inerting refers to the process of providing a non-combustible environment by the addition of compatible gases, which may be carried in storage vessels or manufactured on board the ship or supplied from the shore. The inert gases shall be compatible chemically and operationally, at all temperatures likely to occur within the spaces to be inerted, with the materials of construction of the spaces and the cargo. The dew points of the gases are to be taken into consideration.

Where inert gas is also stored for firefighting purposes, it is to be carried in separate containers and shall not be used for cargo services.

Where inert gas is stored at temperatures below 0 °C, either as a liquid or vapour, the storage and supply system is to be so designed that the temperature of the ship’s structure is not reduced below the limiting values imposed on it.

Arrangements suitable for the cargo carried are to be provided to prevent the back flow of cargo vapour into the inert gas system. The arrangements are to be such that each space being inerted can be isolated and the necessary controls and relief valves etc. are to be provided for controlling pressure in these spaces.

Inert gas production on board

The equipment shall be capable of producing inert gas with an oxygen content at no time greater than 5 % by volume subject to the Special Requirements of “Special requirements”Special Requirements for LNG and LPG gas carriers. A continuous reading oxygen content meter is to be fitted to the inert gas supply from the equipment and is to be fitted with an alarm set at a maximum of 5 % oxygen content by volume subject to the requirements of “Special requirements”Special Requirements for LNG and LPG gas carriers. Additionally, where inert gas is made by an onboard process of fractional distillation of air which involves the storage of the cryogenic liquefied nitrogen for subsequent release, the liquefied gas entering the storage vessel is to be monitored for traces of oxygen to avoid possible initial high oxygen enrichment of the gas when released for inerting purposes.

Inert gas plants are to comply with BKI’s requirements for inert gas systems for Tankers as given in Rules for Machinery Installations (Part 1, Vol. III) Sec.15, D.

An inert gas system shall have pressure controls and monitoring arrangements appropriate to the cargo containment system. A means acceptable to the Society, located in the cargo area, of preventing return of cargo gas is to be provided.

Spaces containing inert gas generating plants shall have no direct access to accommodation, service or control station spaces, but may be located in machinery spaces. If such plants are located in machinery spaces or other spaces outside the cargo area, two non return valves, or equivalent devices are to be fitted in the inert gas main in the cargo area as required above. Inert gas piping shall not pass through accommodation, service or control station spaces. When not in use, the inert gas system shall be made separate from the cargo system in the cargo area except for connections to the hold spaces or interbarrier spaces.

Flame burning equipment for generating inert gas is not to be located within the cargo area. Special consideration may be given to the location of inert gas generating equipment using the catalytic combustion process.

Inert gas plant for liquefied gas carriers above 20 000 tdw

For liquefied gas carriers intended to carry one or more of the products identified with an asterisk (*) in “Minimum requirements”Summary of Minimum Requirements for LNG and LPG tankers or products covered by Rules For Ships Carrying Dangerous Chemicals in Bulk (Part 1, Vol. X) Sec.18, with a flashpoint ≤ 60 °C or oil products with a flash point ≤ 60 °C, the requirements of Rules For Ships Carrying Dangerous Chemicals in Bulk (Part 1, Vol. X), Sec. 9, 3 apply.