MARINE PROPULSION STEAM TURBINE

Main Feed Pump  Provides water to the boilers at a sufficient rate and pressure to ensure boiler level is maintained  Typically: Multistage, high speed, centrifugal pump. May be either steam or electric driven  Discharge pressure up at least 10% higher than boiler pressure

Summary  The four phases of the steam cycle  The components of the cycle  Why do we superheat  Purpose of a vacuum

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Boilerwater treatment deals with the various scale and corrosion causing salts and entrained gases by suitable chemical treatment. This is achieved as follows: 1. By keeping the hardness salts in a suspension in the solution to prevent scale formation. 2. By stopping any suspended salts and impurities from sticking to the heat transfer surfaces. 3. By providing anti-foam protection to stop water carry-over. 4. By eliminating dissolved gases and providing some degree of alkalinity which will prevent corrosion.

The following salts, if present in dissolved form in the feed water will cause formation of scale  Calcium Carbonate  Calcium Bi-carbonate  Calcium Phosphate  Calcium Sulphate  Magnesium Hydroxide  Magnesium Phosphate  Magnesium Chloride These dissolved salts make up what is called the “hardness” of the water.

 The bicarbonates of calcium and magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as “temporary hardness”.  The chlorides, sulphates and nitrates are not decomposed by boiling and are known as “permanent hardness”.  Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the boiler feedwater.

 Adding various chemicals into the feedwater system and then testing samples of boiler water with a test kit.  The test kit is usually supplied by the treatment chemical manufacturer with simple instructions.  For high-pressure watertube boilers various phosphate salts are used, such as trisodium phosphate, disodium phosphate and sodium metaphosphate.  Coagulants are also used which combine the scale forming salts into a sludge and stop it sticking to the boiler surfaces. Sodium aluminate, starch and tannin are used as coagulants.  De-aeration of the boiler water is achieved by chemicals, such as hydrazine, which combine with any oxygen present.

Common dissolved gases are:  Oxygen  Carbon Dioxide Oxygen enters the boiler water through leakage of turbine glands and also from make up water. CO2 is formed due to chemical reaction of carbonates and bi-carbonates of Calcium salts.  Hydrazine treatment Hydrazine ( N2H4) is a ammonia based compound. It reacts with O2 and forms water and releases nitrogen gas. Nitrogen gas being inert in nature does not harm the boiler material and is removed by air ejectors from the condenser.

 Hydrazine, if it is injected in little excess amount then it releases small amount of ammonia which dissolves in water and raises alkalinity.  This increased alkalinity neutralises the carbonic acid which is formed by the dissolved CO2.  Hence hydrazine treatment can remove both dissolved gases ( O2, CO2) from the boiler water.  The carbonic acid so formed is neutralised by keeping the boiler water slightly alkaline. (Ph > 9.2)

 Foaming is caused by high density or high alkalinity of boiler water.  It can be reduced by adding small amount of anti-foaming chemicals in the form of polymides or polyamides.  These chemical primarily reduce the surface tension of water and thereby minimise formation of bubbles.













BOILER OPERATION The procedure adopted for raising steam will vary from boiler to boiler and the manufacturers' instructions should always be followed. A number of aspects are common to all boilers and a general procedure might be as follows:

PREPARATIONS 1. Check all uptakes are clear to ensure exhaust gas can flow thro boiler; dampers operated and in correct position. 2. All vents, alarm, water and pressure gauge connections should be opened. 3. The superheater circulating valves or drains should be opened to ensure

Cont’d 4. Check and ensure all other boiler drains and blow-down valves are closed. 5. Filled the boiler to slightly below the working level with hot de- aerated water. 6. Close all header vents as water is seen to flow from them. 7. Ensure that economiser is full of water and all air vented off.

Cont’d 8. Check operation of the forced draught fan and where exhaust gas air heaters are fitted they should be bypassed. 9. The fuel oil system should be checked for the correct positioning of valves,etc. The fuel oil should then be circulated and heated.

RAISING STEAM 1. Start the forced draught fan and pass air through the furnace for several minutes to 'purge' it of any exhaust gas or oil vapors. 2. Close air slides at every register except for the “lighting-up” burner. 3. The operating burner can now be lit and adjusted to provide a low firing rate with good combustion.

4. Ensure that the fuel oil pressure and forced draught pressure are matched to ensure good combustion with a full steady flame. 5. The superheater header vents may be closed once steam issues from them. 6. Close the steam drum vent when the pressure reached 2.1 bar.

Time Taken.  The boiler must be brought slowly up to working pressure in order to ensure gradual expansion and to avoid overheating the superheater elements and damaging any refractory material.  Boiler manufacturers usually provide a steam raising diagram in the form of a graph of drum pressure against hours after flashing up.

7. Warm thro the main and auxiliary steam lines and then the drains closed. 8. Blow through water level gauges and checked for correct reading. 9. Safety valves should be lifted and released using the easing gear when the pressure is about 3 bar below normal working pressure.

10. Once at operating pressure the boiler may be put on load and the superheater circulating valves closed. 11. Close all other vents, drains and bypasses. 12. Check water level in boiler closely and observe that the automatic water regulating arrangements operate correctly.

Boiler water level control system Problems of “swelling” and “shrinkage” overcome by measuring ‘steam flow’. Variations in feed pressure reduced by measuring ‘feed flow’

TURBINE OPERATION  steam turbine requires a considerable period for warming- through before manoeuvring.  high-speed operation of the turbine and its simply supported rotor also require great care during manoeuvring operations.

WARMING-THRO PROCEDURES 1. Open all the turbine-casing and main steam-line drain valves. 2. Ensure that all the steam control valves at the manoeuvring station and around the turbine are closed. 3. All bled steam-line drain valves should be opened.

Cont’d 4. 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. 5. Obtain clearance from the bridge to turn the shaft. Engage the turning gear and rotate the turbines in each direction. 6. Start the sea water circulating pump for the main condenser. 7. Start the condensate extraction pump with the air ejector recirculation

Cont’d 8. 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. 9. Raising a small vacuum in the condenser will assist this warming through. 10. 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. 11. The expansion arrangements on the turbine to allow freedom of movement

Cont’d 12. Gland sealing steam should now be partially opened up and the vacuum increased. 13. The turning gear should now be disengaged. 14. 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 3 to 5 minutes for a period of 15 to 30 minutes. 15. The vacuum can now be raised to its operational value and also the gland steam pressure. The turbines are now

Important Note:  While waiting for the first movements from the bridge, and between movements, the turbine must be turned ahead once every 5 minutes by steam blasts.  If there is any delay, gland steam and the vacuum should be reduced.

MANOEUVRING  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 OPERATIONS From full speed ahead, an order for an emergency stop or astern movement is required then safe operating procedures must be ignored. 1. 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. 2. The turbine can thus be brought quickly to a stopped condition and if required can then be operated astern. 3. 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

FULL AWAY 1. Manoeuvring revolutions are usually about 80% of the full away or full speed condition. 2. Once the full away command is received the turbine can gradually be brought up to full power operation, a process taking 1 to 2 hours. 3. This will also involve bringing into use turbo- alternators which use steam removed or 'bled' at some stage from the main turbines. 4. Checks should be made on expansion arrangements, drains should be checked to be closed, the condensate recirculation valve after the air ejector should be closed, and the astern steam valves tightly closed,

ARRIVAL PORT  Prior to arriving at a port, the bridge should provide 1 to 2 hours‘ notice to enable the turbines to be brought down to manoeuvring revolutions.  A diesel alternator will have to be started, the turbo-alternator shut down, and all the full away procedure done in reverse order.

Turbine L.O. System

 A common lubrication system is used to supply oil to the turbine, gearbox and thrust bearings and the gear sprayers.  The turbine, rotating at high speed, requires a considerable time to stop. If the main motor driven lubricating oil pumps were to fail an emergency supply of lubricating oil would be necessary.  This is usually provided from a gravity tank, although main engine driven lubricating oil pumps may also be required.  A lubricating oil system employing both a gravity tank and an engine driven pump is shown.  Some of the oil also passes through an orifice plate and into the gravity tank from which it continuously overflows (this can be observed