Answer Construction Question 2
Question
2: A. List SIX hazards that arise with the carriage of liquefied gas in
bulk.
B. Describe, with the aid of a sketch, the details of construction of
a prismatic cargo tank within a gas carrier designed to carry liquefied
gas(LPG).
Answer: A. 1. Personnel Hazards: Toxicity (poisoning); Asphyxia (suffocation); Low temperature (frostbite) and Chemical burns.
2. Reactivity: A liquefied gas cargo may react in a number of ways; with water to form hydrates, with itself, with air, with another cargo or with other materials. The most common form of self-reaction is polymerisation which may be initiated by the presence of small quantities of other cargoes or by certain metals. Polymerisation normally produces heat which may accelerate the reaction. Some cargoes can react with air to form unstable oxygen compounds which could cause an explosion. Cargoes can react dangerously with one another.
3. Corrosivity: Some cargoes and inhibitors may be corrosive. Corrosive liquids can also attack human tissue and care should be taken to avoid contact reference should be made to the appropriate data sheets. Instructions about the use of protective clothing should be observed.
4. Boil-off & Low-Temperature Effects: Cargo can easily boil-off if adequate cooling is not effected of due to improper insulation.
5. Brittle Fracture: Most metals and alloys become stronger but less ductile at low temperature(i.e the tensile and yield strengths increase but the material becomes brittle and the impact resistance decreases)
6. Ice Formation: Low cargo temperatures can freeze water in the system leading to blockage of, and damage to pumps, valves, sensor lines, spray lines etc. Ice can be formed from moisture in the system, purge vapour with incorrect dew point, or water in the cargo.
7. Rollover: Rollover is a spontaneous rapid mixing process which occurs in large tanks as a result of a density inversion. Stratification develops when the liquid layer adjacent to a liquid surface becomes denser than the layers beneath, due to boil-off of lighter fractions from the cargo. This obviously unstable situation relieves itself with a sudden mixing, which the name 'rollover' aptly describes.
2. Reactivity: A liquefied gas cargo may react in a number of ways; with water to form hydrates, with itself, with air, with another cargo or with other materials. The most common form of self-reaction is polymerisation which may be initiated by the presence of small quantities of other cargoes or by certain metals. Polymerisation normally produces heat which may accelerate the reaction. Some cargoes can react with air to form unstable oxygen compounds which could cause an explosion. Cargoes can react dangerously with one another.
3. Corrosivity: Some cargoes and inhibitors may be corrosive. Corrosive liquids can also attack human tissue and care should be taken to avoid contact reference should be made to the appropriate data sheets. Instructions about the use of protective clothing should be observed.
4. Boil-off & Low-Temperature Effects: Cargo can easily boil-off if adequate cooling is not effected of due to improper insulation.
5. Brittle Fracture: Most metals and alloys become stronger but less ductile at low temperature(i.e the tensile and yield strengths increase but the material becomes brittle and the impact resistance decreases)
6. Ice Formation: Low cargo temperatures can freeze water in the system leading to blockage of, and damage to pumps, valves, sensor lines, spray lines etc. Ice can be formed from moisture in the system, purge vapour with incorrect dew point, or water in the cargo.
7. Rollover: Rollover is a spontaneous rapid mixing process which occurs in large tanks as a result of a density inversion. Stratification develops when the liquid layer adjacent to a liquid surface becomes denser than the layers beneath, due to boil-off of lighter fractions from the cargo. This obviously unstable situation relieves itself with a sudden mixing, which the name 'rollover' aptly describes.
B.
Fully refrigerated Cargo is carried at atmospheric pressure and at a
temperature at or below the boiling point. The system is particularly
suitable for the carriage of LNG but is also used extensively for LPG
and ammonia. Vessels designed for LNG do not usually carry
reliquefaction plant but LPG and ammonia vessels may gain from its use.
The tank structure may be of prismatic form or of membrane construction.
Prismatic
tanks are self-supporting, being tied to the main hull structure by a
system of chocks and keys. They make excellent use of the available
space.
Membrane tanks are of rectangular form and rely on the main hull structure for their strength. A very thin lining (0.5 mm to 1.2 mm) contains the liquid. This lining must be constructed of low expansion material or must be of corrugated form to allow for changes in temperature. The lining is supported by insulation which must therefore be load-bearing.
Membrane tanks are of rectangular form and rely on the main hull structure for their strength. A very thin lining (0.5 mm to 1.2 mm) contains the liquid. This lining must be constructed of low expansion material or must be of corrugated form to allow for changes in temperature. The lining is supported by insulation which must therefore be load-bearing.
For
both membrane and prismatic tanks having a minimum temperature less
than — 50°C either nickel steel or aluminium must be used. In both cases
secondary barriers are required if the minimum temperature is less than
— 10°C. Thus, in the event of leakage from the primary container, the
liquid or vapour is contained for a period of up to 15 days. If the
minimum temperature is higher than — 50°C, the ship's hull may be used
as a secondary barrier if constructed of Arctic D steel or equivalent.
Independent secondary barriers may be of nickel steel, aluminium or
plywood as long as they can perform their function.
Several types of insulation are acceptable, such as balsa wood, polyurethane, pearlite, glass wool and foam glass. Indeed, the primary barrier itself may be constructed of polyurethane which will both contain and insulate the cargo. Usually, however, the primary barrier is of low-temperature steel or of aluminium, neither of which become brittle at low temperatures. Care must be taken throughout the design to prevent the low-temperature liquid or vapour coming into contact with steel structure which may become brittle and hence fracture.
Several types of insulation are acceptable, such as balsa wood, polyurethane, pearlite, glass wool and foam glass. Indeed, the primary barrier itself may be constructed of polyurethane which will both contain and insulate the cargo. Usually, however, the primary barrier is of low-temperature steel or of aluminium, neither of which become brittle at low temperatures. Care must be taken throughout the design to prevent the low-temperature liquid or vapour coming into contact with steel structure which may become brittle and hence fracture.
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