Natural gas supply apparatus

#This invention relates to apparatus for supplying natural gas fuel for the purpose of heating the boilers of tankers that transport liquefied natural gas (LNG).

#An LNG tanker is conventionally propelled by one or more steam turbines. The tanker is thus provided with boilers to raise the steam. About 50% of the fuel that is required to be burnt in order to raise the steam provided from the LNG storage tanks themselves of the tanker. Since liquid natural gas has a boiling point at a temperature of about minus 162° C., even though the storage tanks are thermally insulated, the boiling point of liquid natural gas is so low that there is inevitably a continuous boil-off of LNG from the tankers, albeit at a modest rate. Conventional operation is to employ such natural boil-off to meet a part of the requirements for fuel of the boilers. The balance is met from a stock of a heavy hydrocarbon liquid fuel, sometimes referred to as “bunker oil”.

#It is however required that the tanker should have the capability of generating sufficient thermal energy from the combustion of the natural gas alone to raise all the steam required for its propulsion. Accordingly, the apparatus for supplying natural gas to the burners associated with the boilers additionally includes a forced vaporiser. In the tanker apparatus that has been used for the past 20 years there is a natural gas compressor which receives part of its feed from the ullage space of the LNG storage tanks and the remainder from the forced vaporiser. The forced vaporiser has an outlet temperature in the order of minus 40° C., chosen to be well above the dew point of the natural gas so as to ensure that there is no precipitation of liquid droplets in the inlet to the compressor. On the other hand the temperature of the natural gas taken from the ullage space of the tanks tends to be in the range of minus 100° C. to minus 140° C., the latter temperature obtaining when the tanker is fully laden with LNG, the former temperature obtaining when the tanker returns from making a delivery, a small amount of LNG having been retained for purposes of keeping the storage tanks at a suitable low temperature. In practice, therefore in this conventional apparatus the compressor has to be arranged so that it can operate at any natural gas inlet temperature in the range of minus 40° C. to minus 140° C. At the lower temperature the adiabatic work required to compress the gas is in the order of 20 to 25 kJ/kg whereas at the higher temperature it is in the order of 50 to 60 kJ/kg. The motor which drives the compressor therefore has to be able to cope with a wide range of duties. Typically, the motor is either provided with a frequency converter or is of a plural speed kind. Other disadvantages arise. In particular, it is necessary to employ a mist separator downstream of where the gas from the forced vaporiser is mixed with the natural boil-off gas so as to ensure that no particles of liquid are carried into the compressor, such particles being a potential source of liquid erosion damage to the impeller of the compressor.

#An apparatus according to the invention will now be described by way of example with reference to the accompanying drawings in which:

#FIG. 1 is a schematic flow diagram of a first embodiment of the apparatus; and

#FIG. 2 is a schematic flow diagram of a second embodiment of the apparatus.

#Referring to FIG. 1 of the drawings, an LNG storage tank 2 is located on board an ocean-going tanker (not shown). The tank 2 is vacuum-insulated or has another form of thermal insulation associated therewith so as to keep down in flow of heat from the ambient environment into the liquid therein. The tank 2 has an ullage space 4 and contains a volume 6 of LNG. Since LNG boils at a temperature well below ambient, notwithstanding the thermal insulation of the tank 2, there is a continuous evaporation of the LNG into the ullage space 4. The resulting vaporised gas flows out of the tank 2 though an outlet 8 and passes along a pipeline 10 to a compressor (or blower) 12. The compressor 12 is driven by an electric motor 14. The motor typically has a single speed and does not employ a frequency converter. The pressure of the gas is raised by operation of the compressor 12. The heat of compression is not nearly sufficient to raise the temperature of the gas being compressed to ambient temperature, such temperature being required for the operation of the boilers. The resulting compressed gas therefore passes through a gas heater 16 in which it is heated by steam (or other heating medium, e.g. hot water) so as to raise its temperature to approximately ambient temperature. To avoid overheating the gas, temperature control is provided by a valved by-pass line 20 extending from upstream to downstream of the gas heater 16. As a further measure of temperature control, the gas heater has on its upstream side a flow control valve 18 which is able to set so as to select the outlet temperature therefrom. The resulting heated gas passes into a conduit 22 in which it mixes with a second flow of vaporised natural gas.

#The second flow of vaporised natural gas is formed by employing a forced vaporiser 24 to vaporise a flow of LNG taken by a submerged pump 26 from the volume 6 of LNG within the tank 2. The outlet of the pump 26 communicates with the forced vaporiser 24 via a riser 28. A pressure control valve 48 opens a pipe 50 to allow liquid to be returned to the storage tank 2 for different flow rates through the vaporiser 24. The forced vaporiser 24 has an enlarged superheating section to enable an outlet temperature of plus 20° C. to be readily achievable. The forced vaporiser 24 has an arrangement of valves similar to that associated with the gas heater 16. Thus, there is a first flow control valve 30 on the upstream side of the vaporiser 24 to set the outlet pressure of the heater 16 so that it is equal to the outlet pressure of the compressor 12, and a valved by-pass line 32 extending from upstream to downstream of the vaporiser 24 to control the outlet temperature of the vapour. The vaporiser 24 is typically of a kind which employs steam heating to raise the temperature of the fluid flowing therethrough.

#The mixture of the gas supplied from the heater 18 and that supplied from vaporiser 24 passes through a master flow control valve 38 into burners 40 associated with boilers 42 in the engine room 44 of the tanker. The master flow control valve 38 is located in operation, the heat generated by firing of the burners 40 raises steam in the boilers 42. Steam is used to drive a turbine (not shown) that provides propulsive power for the tanker.