steam turbines
Steam turbines retain market for LNG carrier propulsion
Despite the growth in popularity of dual-fuel engines, the option of high efficiency boiler and steam turbine plan is still attractive to operators
M
alaysian state-owned oil and gas company
Petronas has recently
placed an order with Hyundai Heavy Industries Company (HHI) of South Korea to build four new 150,000m3 Moss-type liquefied natural gas (LNG) carriers (see also page 33). The contract, which was signed in Seoul in October 2013, includes an option for a further four carriers of the same class and the first deliveries of these new generation Moss type spherical tank carriers are scheduled for late in 2016. A month later, in November 2013, Mitsubishi
Heavy Industries Marine Machinery & Engine Co (MHI-MME) received an order from HHI for four of its Ultra Steam Turbine (UST) plants to power these vessels, with HHI also including the option for a further four units. For MHI, this order is a landmark, being the first sale of USTs outside the Japanese domestic market. The company now plans to promote its marine steam turbine products more actively and increase its stake in the global marketplace.
MHI introduced UST designs with the aim of improving fuel consumption of marine steam turbine plant by introducing a reheat cycle. In this configuration the exhaust from the high pressure steam turbine is routed back through the boiler reheater section, raising temperatures to the same levels as the boiler superheater outlet, before entering the intermediate pressure turbine stage. This additional stage provides increased shaft power and the reheat cycle also increases overall plant efficiency. In designing the reheat boiler, MHI retained the basic needs of high reliability, compact design and minimum weight construction but also took into consideration specific marine operating conditions. Compared with land-based steam turbine plant, marine propulsion systems must be able to cope effectively with low steam flows and frequent load changes during manoeuvring and other varying load conditions. For reliability in service, the reheater tubes must therefore be resistant to failure from the risk of high metal temperatures. From the design options available, MHI selected a dual furnace
www.mpropulsion.com An MHI UST steam turbine plant. Four are on order for Petronas LNGCs at HHI (credit: MHI)
Boilers for MHI’s UST steam turbines (credit: MHI)
approach to maximise reheater reliability. With this arrangement the reheater and reheat furnace are located in the main boiler combustion gas outlet, providing lower gas inlet flow temperatures. For the boiler design, MHI adopted the construction principles of its MB type conventional boiler, adding a downstream reheat furnace in the gas flow path. The main and reheat furnaces are fully water-cooled and of welded wall construction to ensure no leakage can occur from the gas path. To achieve further efficiency increase, the UST design also
includes an increase in the surface area of the superheater through the introduction of a secondary exchanger operating in parallel. Steam temperatures
in this secondary
superheater are higher than in the primary but metal temperatures are controlled as it is shielded from direct heat radiation from the furnace. Combustion gas temperatures are also reduced through heat absorbed in the primary superheater. Overall metal temperatures are therefore held to similar levels as seen in conventional boilers, despite higher steam
Marine Propulsion I April/May 2014 I 41
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