Working Of A Marine Steam Turbine Onboard A Merchant Ship

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Most would want to know why is steam still being used as a means of propulsion at this day and age. It is only because of the nature of the cargo carried on board LNG carriers i.e Liquefied Natural Gas – Methane, reports Team bhp.

Marine Steam turbines are the primary means of propulsion on board LNG carriers though they are rapidly going obsolete with the advent of gas burning diesel engines. Most LNG carriers built pre 2015 were with steam propulsion. The generators on board are also steam turbine driven called Turbine Generators with Diesel generators as a back up.

Most would want to know why is steam still being used as a means of propulsion at this day and age. It is only because of the nature of the cargo carried on board LNG carriers i.e Liquefied Natural Gas – Methane.

The cargo keeps continuously evaporating, technically called boiling off and this needs to be controlled. The BoG produced is 0.1% when the ship is in Ballast condition & 0.15% when she is loaded. This percentage is of the total cargo carrying capacity of the ship in CBM.

What better way than burn the gas and produce steam to propel & power the ship?

A brief overview of the process flow is as follows: Both the boilers are MITSUBISHI MB-4-KS

  • BoG generated is sucked by a compressor and sent to a heater. (-130C to 40C)
  • From the heater it is sent to the boilers for burning after mixing with pre-heated air.
  • The heat produced inside the boiler is used to further produce super-heated steam.
  • This superheated steam is used for the turbines (propulsion & power).Auxiliary steam is used for heating purposes.
  • The exhaust steam is condensed in a condenser and re-used, so its a closed loop system.

The Main steam turbine is a KAWASAKI UA-360 Cross compound, impulse, double reduction geared marine turbine output rated at 23757 kW / 32300 PS

  • The HP turbine – 4860 rpm, 10 stages
  • The LP turbine – 3222 rpm, 8 stages AHEAD, 2 stages ASTERN
  • The Propeller – 84.8 rpm
  • The reduction gear is a tandem articulated, double reduction, double helical type.

1st reduction pinion – 1st reduction gear – 2nd reduction pinion – Main gear – propeller shaft – Propeller

The thrust bearings are located in the forward side of both LP & HP turbines.

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Source: Team bhp