船友在线 Ship types | Marine Propulsion | Liquefied Petroleum Gas

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  Gas Carriers Gas carriers take liquid which occupies about 1/600 of the volume it would occupy as a gas. Two different forms are carried; Liquid Petroleum Gas which is mainly propane and butane, and Liquid Natural Gas which is mainly methane. Critical factors in the carriage of gas in liquid form are the boiling point tempo at atmospheric pressure and the critical tempo ( this is the temperature above which the gas cannot be liquified no matter what the pressure. The type of containment vessel u
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  Gas Carriers Gas carriers take liquid which occupies about 1/600 of the volume it would occupy as a gas.Two different forms are carried; Liquid Petroleum Gas which is mainly propane andbutane, and Liquid Natural Gas which is mainly methane. Critical factors in the carriage of gas in liquid form are the boiling point tempo at atmospheric pressure and the critical tempo( this is the temperature above which the gas cannot be liquified no matter what thepressure.The type of containment vessel used for the cargo will differ depending upon thedesired tempo and pressure ( the tempo must always be below the critical ).In general low pressures may be used if the tempo is kept low, alternately highertemperature may be used but higher pressures are required. LNGLNG has a boiling point of -162 o C at atmospheric pressure and a critical tempo at47 bar of -82 o C, suitable containment conditions allow the carriage of LNG at different tempoand pressure. LPG LPG comprises many different gases which have different boiling points and criticaltemperature, carriage requirements vary between atmospheric pressure and 18 bar and -100 o C to -5 o CFor smaller ships carrying LPG, pressurised systems are generally used, theseemploy spherical or cylindrical tanks. However, there is a considerable loss of space. Withhigher pressures ( up to 18 bar) no reliquifacation plant is fitted and no insulation is required.Relief valves are used to protect the system.Recompression of boil off gas may be employed.Systems employing pressurised tanks may be partly or fully refrigerated thusrequiring less strength in the cargo tanks. This reduces weight and cost. Insulation andreliquification plant is required. Partly refrigerated systems have a maximum pressure of about 8 bar and a temperature of about -10 o C. Fully refrigerated have a maximum pressureof about 8 bar but the temperature may be down to -45 o C thus increasing the range of petroleum gas cargoes that may be carried. These systems employ cylindrical or sphericaltanks which must be self supporting.Most shipments of LPG are carried at atmospheric pressure at theire respectiveboiling point. Some typical examples are ethylene -103 o C, propane -42 o C, ammonia -33 o Cand butane 0 o C to- 5 o CLloyds register require that in cases other than for pressurised tanks, for carriageof cargoes below 10 o C the hold spaces should be segregated from the sea by a doublebottom. For below 50 o C the ship should also have longitudinal bulkheads forming the tanksides.Most gas tanks incorporate a method of detecting leakage. When the primary 1  barrier is breached the secondary barrier should capable of confining the leakagefor a minimum of 15 days.In addition, especially for LNG carriers, the inert gas contained in the barrier spaceis sampled and temperature probes fitted. Regular 'cold spot' inspections are carried out onthe secondary barrier.Before designing a gas tank certain criteria set down in the IMO code for shipscarrying bulk gas must be met. These, by giving a set of figures determining a damage to theship, ensure the ships survivability in a collision, grounding etc. The position of the tanks,determined by the type of cargo to be carried, are laid down to prevent the escape of cargounder similar conditions.For systems other than fully pressurised a method of dealing with 'boil off' must befitted. For LPG carriers this takes the form of an on board Fully pressurised The tanks are internally stiffened and constructed of ordinary grade steels as the cargo iscarried at atmospheric temperatures. 2    Alternative tank support arrangements  Tanks are in the form of pressure vessels, cylindrical or spherical.Maximum pressure is about 18 bar and no reliquification plant is provided.Apart from certain areas around the supports insulation is not usually fitted. Relief v/v's are required to safe guard against pressure build up due to boil off. A compressor isprovided to keep the tank system pressurised.Tanks are classed as self supporting, because of the loss of space the system is notpopular and is usually applied to smaller ships Semi-pressurised, partly refrigerated These reduce the cost and weight; tanks are insulated and reliquifaction plant is fitted, maxpressure is 8 bar and minimum tempo about -5 o C. Tank arrangement is similar to the fullypressurised and so there is still the loss of space. Semi-pressurised, fully refrigerated Pressure about 8 bar, and temperatures down to -45 o C. Tanks well insulated andreliquification plant essential. Tank pressurised but it is possible to carry a range to cargoesat different pressures and temperatures. 3  Fully refrigerated Cargoes are carried at atmospheric pressure but at a temperature below the atmosphericboiling point. Very suitable for LNG, but can also be used for LPG and ammonia ( LNG carriersdo not generally have a reliquification plant but LPG carriers may )Prismatic tanks or membrane wall systems may be used. Prismatic tanks are self supporting but they must be tied to the main hull structure. Prismatic tankMembrane tanks Membrane tanks are rectangular and rely on the main hull of the ship for strength.The primary barrier may be corrugated in order to impart additional strength andto account for movement due to change of temperature. Systems vary but the arrangementshown is typical. 4
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