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Low Temperature Effects in gas carrier - Causes of Brittle Fracture
What is Brittle Fracture ?:
Most metals and alloys become stronger but less ductile at low temperatures (i.e. the tensile and yield strengths increase but the material becomes brittle and the impact resistance decreases) because the reduction in temperature changes the material’s crystal structure.
Normal shipbuilding steels rapidly lose their ductility and impact-strength below 0 degree. For this reason, care should be taken to prevent cold cargo from coming into contact with such steels, as the resultant rapid cooling would make the metal brittle and would cause stress due to contraction. In this condition the metal would be liable to crack. The phenomenon occurs suddenly and is called “brittle fracture”.
However, the ductility and impact resistance of materials such as aluminum, austenitic and special alloy steels and nickel improve at low temperatures and these metals are used where direct contact with cargoes at temperatures below -55 degree is involved.
Contact with LNG chilled to its temperatures of about –160 degree C will damage living tissue.
Most metals lose their ductility at these temperatures; LNG may cause the brittle fracture of many
materials. In case of LNG spillage on the ship’s deck, the high thermal stresses generated can
result in the fracture of the steel.
Avoidance of cold shock to metal
Structural steels can suffer brittle fracture at low temperatures. Such failures can be catastrophic
because, once the steel has become brittle, little energy is required to propagate a fracture once it
has been initiated.
Plain carbon structural steels have a ductile to brittle behaviour transition which occurs generally in
the range +30 degree C to –50 degree C. This precludes their uses as LNG material (carriage temperature –
162 degree C).
The ductile to brittle transition is usually monitored by measuring the energy absorbed in breaking
a notched bar and the transition curve, as shown in illustration “Structural Steel Ductile to Brittle
Transition Curve”, is typical for plain carbon steels.
For this reason, materials which do not show such sharp transition from ductile to brittle fractures
as the temperature is lowered, have found obvious application for use in cryogenic situations such
as liquid Methane carriers. Examples are Invar (36% nickel-iron alloy), austenitic stainless steel,
9% nickel steel and some aluminium alloys such as 5083 alloy.
All of these materials behave in a ductile manner at –162 degree C, so that the chance of an unstable
brittle fracture propagating, even if the materials were overloaded, is negligible.
Fig: STRUCTURAL STEEL DUCTILE TO BRITTLE TRANSITION CURVE
In order to avoid brittle fracture occurring, measures must be taken to ensure that LNG and liquid
Nitrogen do not come into contact with the steel structure of the vessel. In addition, specialist
equipment is provided to deal with any leakages, which may occur.
The manifold areas are equipped with a stainless steel drip tray, which collects any spillages and
drains it overboard. The ship, in way of the manifolds, is provided with a water curtain, which is
supplied by the deck fire main. The fire main must always be pressurized and the manifold water
curtain in operation when undertaking any cargo operations. Additionally, fire hoses must be laid
out to each liquid dome to deal with any small leakages, which may develop, at valves and flanges.
Permanent drip trays are fitted underneath the items most likely to cause problems and portable
drip trays are provided for any other needs.
During any type of cargo transfer, and particularly whilst loading and discharging, a constant patrol
must be conducted on deck to ensure that no leakages have developed.
In the event of a spillage or leakage, water spray should be directed at the spillage to disperse and
evaporate the liquid and to protect the steelwork. The leak must be stopped, suspending cargo
operations if necessary.
In the event of a major leak or spill, the cargo operations must be stopped immediately, the
general alarm sounded and the emergency deck water spray system put into operation.
- Low Cargo Temperature Effects -Ice Formation in gas carrier cargo system
- How to cool down cargo system - a brief guide to Liquefied gas carrier
- Preventive measures against spillage of low temperature cargo in Liquefied gas carrier
- Low Cargo Temperature Effects -What is rollover of Liquefied gas cargo ?
Other info pages:
Preparatory operations for drydocking
Hazards of electrical equipment on board
Discussion prior to cargo transfer in liquefied gas carrier
Handling LPG And ammonia cargo - preparation for loading
The risk of discharging cargo - a brief outline to liquefied gas carriers
The risk of laden voyage - a brief guide to liquefied gas carriers
Connection and disconnection of cargo hoses and hard arms
Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier
Cargo Containment Systems in Liquefied Gas Carriers
cargo emergency shutdown requirement
damage stability guideline for liquefied gas carriers
Various Cargo handling equipments onboard
Cargo hoses connection guideline
Documents accompanying a liquid gas cargo
How LNG transferred from shore to ships cargo tanks ?
Cargo operation guideline onboard a liquefied gas carrier
Cargo piping Systems in Liquefied Gas Carriers
cargo planning requirement
cargo and pumproom safety precautions
cargo stripping guideline
Emergency response for cargo system leaks
Emergency response for cargo tank rupture
Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier
Preparation for cargo transfer
cargo transfer between vessels- safety guideline
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