Steam turbines extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft.
The steam turbine is a device for obtaining mechanical work from the energy stored in steam. Steam enters the turbine with a high energy content and leaves after giving up most of it. The high-pressure steam from the boiler is expanded in nozzles to create a high-velocity jet of steam. The nozzle acts to convert heat energy in the steam into kinetic energy. This jet is directed into blades mounted on the periphery of a wheel or disc. The shaping of the blades causes a change in direction and hence the velocity of the steam jet. A change in the velocity for a given mass flow of steam produces a force which turns the turbine wheel.
The steam from the first set of blades then passes to another set of nozzles and then blades and so on along the rotor shaft until it is finally exhausted. Each set comprising nozzle and blades is called a stage. The main engine is to be operated within the limits of power, maximum evaporative capacity and pressure of the boilers, and the revolutions per min set out in the commissioning letter issued when the vessel enter service. This is the operating principle of all steam turbines, although the arrangements may vary considerably.
Testing and maintenance of any turbine control system alongside a berth is strictly forbidden under live steam conditions. The main steam stop, guardian and bulkhead stop valves are to be shut prior to maintenance work being carried out. In the case where a simulated test would be unsatisfactory and operation with live steam has to be carried out, then the vessel is to be suitably anchored or stopped at sea, before maintenance or testing is carried out. Running Hours for Turbines and Boilers should be recorded in the “Turbine and Boilers Running Hours” and returned to the Managing Office on a monthly basis.
The steam turbine has until recently been the first choice for very large power marine propulsion units. Its advantages of little or no vibration, low weight, minimal space requirements and low maintenance costs are considerable. Furthermore, a turbine can be provided for any power rating likely to be required for marine propulsion. However, the higher specific fuel consumption when compared with a diesel engine offsets these advantages, although refinements such as reheat have narrowed the gap.
Steam turbine components include:
Turbine Prime Mover
A turbine will act as a prime mover in turbo generator and is fitted on the same shaft as of the alternator’s rotor.
The alternator is used to convert the rotary motion of the turbine to electrical energy and its output is supplied to the main switch board of the ship.
Steam Control Governor
The governor is used to control the speed of the turbine generator during starting, normal operation and shutting down. It controls the quantity of the steam inlet to the turbine generator.
Steam Control Valve
Different pressure control valves are fitted in the steam line and are controlled using governor for the flow of steam from the ship’s boiler system.
The condensed steam, after the turbine is further cooled down, is pumped back to the cascade tank by condensate pump.
Vacuum pump for glands
The steam turbine shaft is provided with glands wherein steam is sprayed at a pressure of 0.3~ 0.5 bar so that the vacuum inside the turbine casing doesn’t drop.
The heat exchanger acts as a condenser to cool down and condense all the steam from the turbine into water so that it can be pumped back to the hot well.
Vacuum pump header tank
A vacuum pump header tank is provided to cool down the vacuum pump as the later deals with high temperature steam.
With the exception of some military vessels, where nuclear reactors can provide large quantities of steam, the use of steam turbines for ship propulsion is mostly now considered to be superseded technology. There are commercial vessels still in operation but, for some operators, it has been viable to convert to diesels engines for improved economy in operation. The most popular market for commercial steam turbines in recent decades has been for the propulsion of LNG carriers, where boil-off gas could easily be used to raise steam. This market reduced, however, as a result of the successful development of large dual-fuel engines, capable of delivering high operational efficiencies running on either LNG or liquid fuels.
Steam turbines are made in a variety of sizes ranging from small <0.75 kW (<1 hp) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500 MW (2,000,000 hp) turbines used to generate electricity.