Marine steam turbines operating procedure
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.
The steam turbine requires a considerable period for warming-through prior to any manoeuvring taking place. The high-speed operation of the turbine and its simply supported rotor also require great care during manoeuvring operations.
Warming-through a steam turbine
First open all the turbine-casing and main steam-line drain valves and ensure that all the steam control valves at the manoeuvring station and around the turbine are closed. All bled steam-line drain valves should be opened. Start the lubricating oil pump and see that the oil is flowing freely to each bearing and gear sprayer, venting off air if necessary and check that the gravity tank is overflowing.
Obtain clearance from the bridge to turn the shaft. Engage the turning gear and rotate the turbines in each direction.
Start the sea water circulating pump for the main condenser. Then start the condensate extraction pump with the air ejector recirculation valve wide open.
Fig: Energy conversion in a steam turbine
Open the manoeuvring valve bypass or 'warming through' valve, if fitted. This allows a small quantity of steam to pass through the turbine and heat it. Raising a small vacuum in the condenser will assist this warming through. The turbines should be continuously turned with the turning gear until a temperature of about 75°C is reached at the LP turbine inlet after about one hour. The expansion arrangements on the turbine to allow freedom of movement should be checked.
Gland sealing steam should now be partially opened up and the vacuum increased. The turning gear should now be disengaged.
Short blasts of steam are now admitted to the turbine through the main valve to spin the propeller about one revolution. This should be repeated about every three to five minutes for a period of 15 to 30 minutes. The vacuum can now be raised to its operational value and also the gland steam pressure. The turbines are now ready for use. While waiting for the first movements from the bridge, and between movements, the turbine must be turned ahead once every five minutes by steam blasts. If there is any delay gland steam and the vacuum should be reduced.
Once warmed through, the turbine rotor must not remain stationary more than a few minutes at a time because the rotor could sag or distort, which would lead to failure, if not regularly rotated.
Astern operation involves admitting steam to the astern turbines. Where any considerable period of astern running occurs turbine temperatures, noise levels, bearings, etc., must be closely observed. The turbine manufacturer may set a time limit of about 30 minutes on continuous running astern.
Emergency astern operation
If, when travelling at full speed ahead, an order for an emergency stop or astern movement is required then safe operating procedures must be ignored.
Ahead steam is shut off, probably by the use of an emergency trip, and the astern steam valve is partly opened to admit a gradually increasing amount of steam. The turbine can thus be brought quickly to a stopped condition and if required can then be operated astern.
The stopping of the turbine or its astern operation will occur about 10 to 15 minutes before a similar state will occur for the ship. The use of emergency procedures can lead to serious damage in the turbine, gearbox or boilers.
Manoeuvring revolutions are usually about 80% of the full away or full speed condition. Once the full away command is received the turbine can gradually be brought up to full power operation, a process taking one to two hours. This will also involve bringing into use turboalternators which use steam removed or 'bled' at some stage from the main turbines.
Checks should be made on expansion arrangements, drains should be checked to be closed, the condensate recircuiation valve after the air ejector should be closed, and the astern steam valves tightly closed,
Prior to arriving at a port the bridge should provide one to two hours' notice to enable the turbines to be brought down to manoeuvring revolutions. A diesel alternator will have to be started, the turboalternator shut down, and all the full away procedure done in reverse order.
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