Certification Assistance for Canadian Navigators
Canadian Ocean Navigator
EK Steam NotesBrought to you by www.dieselduck.net comments to [email protected]
Disclaimer
Transport Canada has ask us to advise users of this webpage to keep in mind that these questions are not the exact questions found in their exams. Martin's Marine Engineering Page - www.dieselduck.net is not affiliated with Transport Canada and these questions have been gathered from various sources.
Thank you Walt P. for submitting this material.
SATURATED STEAM: is steam which is in physical contact with the boiling water from which it is generated, its temperature is the same as the boiling water and this is referred to as the SATURATION TEMPERATURE. If the vapour produced is pure steam at this temperature, it is called DRY SATURATED STEAM, If the steam contains water ( very fine particles in suspension in the form of mist) it is called WET SATURATED STEAM. Dryness Faction: The quality of wet steam is expressed by its dryness fraction, which is a ratio of the mass of pure steam in a given mass of the steam-plus-water mixture.
EXAMPLE:
2KG of ? steam? tested and found that this composition was 0.06 KG of water and 1.94KG of pure steam, dryness fraction than is 1.94 / 2 = 0.97.
SUPERHEATED STEAM: In order to increase the temperature of steam above saturation temperature without increasing the pressure, it must be removed from contact with the water from which it was generated .
Steam whose temperature is higher than its saturation temperature corresponding to its pressure is termed SUPERHEATED STEAM, whose volume increases approximately in proportion to its increase in absolute temperature.
It is at a higher temperature and greater volume than saturated steam at the same pressure, more heat energy is stored in each kilogram. This extra energy can produce more power in the engines, hence better efficiency.
Saturated steam begins to condense immediately upon contact with engine parts of a lower temperature than the steam, superheated steam contains heat above its saturation temperature therefore initial condensation and power loss is reduced.
ADVANTAGES>>> Engines consume less steam, hence less fuel is required, smaller boiler and fuel capacity is needed.
Less likelihood of water hammer in stream pipes, less initial impact loss and erosion of TURBINE BLADES because of the lack of water,
DISADVANTAGES>>>Superheated steam is not very suited for reciprocating machinery, as these use water to lubricate slide and shuttle valves as well as piston rings, A DESUPERHEATER is usually fitted in the main steam range to supply auxiliaries.
Boilers and associated equipment
Scotch Boilers: seldom in use today as main boiler, mainly used with reciprocating engines.
Donkey Boiler: An auxiliary boiler.
Exhaust Gas and Composite Boilers: Used to recover heat from exhaust gas in motor ships.
Water Tube Boilers: Produces large quantities of high pressure steam for turbine machinery
Boiler attachments
Main Steam Stop Valve - between boiler and main steam pipe, isolates boiler from main steam line. and is therefore always open or closed.
Safety Valves - Spring loaded safety valves are mounted high in the steam space of the boiler. When steam exceeds a predetermined pressure, the valve lifts against the compression springs, which allows the steam to escape through an outlet in the steam chest, connected to the waste steam pipe leading up into the atmosphere.
High Lift Safety Valves- as implied, gives a higher lift than normal thereby allowing the steam to escape through a larger area.
Circulating Valves - at the bottom of the boiler ( fire tube ) connected to the suction side of the feed pumps through which water is drawn when raising steam.
Blow Down Valves - at the bottom of the boiler - connected to the blow down cock on the ships side, when draining the boiler prior to opening for inspection and cleaning.
Scum Valve - this valve is positioned just below water level, An internal pipe leads to a scoop at the top, a few cm., below working level of the water.
Salinometer Cock - fitted in the water space through which water samples are drawn for testing.
Whistle Valve - fitted near the top of the boiler and connected by pipe to the whistle.
Air Cock - on the top of boiler, which when opened allows air to escape when raising steam from cold.
Soot Blowers - fitted in the fire tube boilers, which have super heaters inside the smoke tubes to clean these from soot.Auxiliary Steam Valve - fitted on top of at least one boiler, connected to the auxiliary steam line, which carries steam to steam pumps, winches, and smaller steam driven engines.
Main Feed Check Valve - connected to the main feed line from the feed pumps. this is a screw down non return valve, used to regulate feed water onto the boilers.
Auxiliary Feed Valve - connected to auxiliary feed water line from the pumps. Its main function is to supply feed water while in port.
Feed Water Regulator- this automatically maintains correct water level, and is essential for water tube boilers with high evaporative rates. It relieves the engine room personnel from hand regulating the water level.
Water Gauges Usually a glass tube used to check the water level in boilers.
Water Hammer- is the impact of water on steam pipes and fittings. This can have very disastrous effect on turbine blades etc.
Combustion
The principal combustibles in fuel are carbon and hydrogen. A theoretical minimum of about 14 kg. of air is needed to completely burn 1 kg. of fuel oil,
An excess of the required minimum air is needed to compensate for the conditions under which the fuel is burned. First indication of lack of air is black smoke from the funnel. A lot of heat will be carried away up the funnel if too much air is applied to the combustion process.
Combustion Air Supply
Natural draught of air to the boiler furnace is caused by the hot gasses raising up the funnel, and air from the stoke hold rushes into the furnace to replace air used in up-draught.
Forced Draught - a common forced draught system consists of a large fan in the engine room.
Included in the air ducting is an air pre-heater in the boiler uptake. The furnace front is a closed box with air valves to control the supply. This system has the advantage over natural draught in that it is easier to control the combustion. Air is preheated from hot flue gasses before entering the furnace thereby improving combustion by not requiring heat from within the furnace to reach combustion temperature.
Another method is to install a large fan in the uptake, which pulls the gasses through the boiler and up pushes them up the funnel, thereby inducing more air onto the furnace.
Air Pre-heaters are fitted in boiler uptakes. The most common form is a system of tubes over which the air required for combustion passes on its way to the furnace.
The advantage of preheating, is that heat would normally be lost, is extracted from the flue gasses before escaping up the funnel.
OIL BURNING
Fuel Oil is stored in double bottom tanks and pumped into settling tanks as required, by means of a transfer pump. These tanks are situated in the stoke hold, two tanks each having a capacity of about 12 hours allowing for water to drain to the bottom to be drained off before use. From the settling tank through cold filters into the fuel pressure pump which discharges it at a pressure from 5.5 bar and upwards, depending upon design, through heaters where the temperature of the oil is raised to 90 degrees depending on the class of oil. Through hot filters into the supply line to the sprayers in the boiler furnace.
There is an oil circulating valve on the furnace distribution valve chest of every boiler which allows the oil to be returned to the pump suction so that cold oil laying in the pipe is cleared out for hot oil to take its place before attempting to light a fire.
The fuel transfer pump, fuel pressure pump, heaters and filters are in duplicate with cross connections so that either set can be used while the other set is on standby.
An emergency shut off valve, usually of the quick closing type is fitted between the settling tank and the cold filters, this valve has an extension or remote control to the deck to enable the oil to be shut off during an emergency in the stake hold.
To light the fires, open the settling tank shut off valve, inlet and outlet valves of the cold filter, suction and delivery valves of the fuel pressure pump, inlet and outlet valve of the hot filters and master boiler valve, and ease back the circulating valve on the boiler front. Set the pump away by opening the exhaust and steam valves, and opening steam heater. Open air to furnace. When the thermometer on the boiler front indicates the correct temperature of the oil, have an assistant stand by the pump to maintain desired fuel pressure while the circulating valve is closed, lighted torch inserted into the furnace, furnace oil valve is opened. The supply air adjusted as needed. Good combustion is indicated by a flame, which is short, dazzling bright with no smoke.
Precautions before lighting up - Make absolutely sure there is no oil laying on the bottom of the furnace before lighting up. If there is, get inside and clean it up. Spilt oil gives off gasses, which when mixed with air and heated becomes very explosive. The forced air supply should always be turned on a short while before attempting to light the burners, thereby expelling any explosive gasses up the funnel. If a furnace does not light the first time, get inside the furnace and clean up any spilt oil.
Causes of BAD COMBUSTION - Dirty burners, particles of dirt or other solid matter in the oil supply. Water in the oil. Seawater may find its way into the double bottoms, through leakage in the hull or otherwise.
Incorrect Air Pressure - An insufficient air supply will cause incomplete burning, causing black smoke. An excessive amount of air unnecessarily carries away heat. It is usually indicated by white smoke at the funnel.
MAIN CAUSES OF CORROSION IN BOILERS
Water side of boiler:
Alkalinity too low - should be regularly tested and kept within proper limits. If alkalinity too low, an alkaline chemical i.e. caustic soda may be added
Sea Water - Causes scales to form on the surface and can cause overheating. Chloride testing will indicate this.
AIR - Oxygen content must be kept to a minimum.
BOILER WATER TEST
Density tested with a hydrometer, this will indicate ppm of the sample used.
Alkalinity - A strip of litmus paper immersed in the boiler water sample. If paper becomes red in colour the sample is acidic. If paper turns blue, the water sample is alkaline.
Oil in the water space of the boiler - Oil enters through exhaust of lubricated steam machinery.Steam heated fuel oil systems - fuel oil heaters or steam heating coils in tanks.
Detection and Effect - Oil would be seen in the boiler glass water gauge. Oil adheres to the wall of the tubes and can cause overheating and possible failure.
Gas Side Of Boiler.
Combustion gasses - sulphur in the fuel will burn to form sulphur oxides which will in part combine with water vapour present in the gasses to give Sulphuric Acid vapours. If condensed out upon metal surfaces, can cause corrosion.
When steaming a boiler at reduced load, uptake gas temperatures are low, hence condensation of these acids vapours is most likely to occur.
Dew Point Temperature of sulphoric vapours is roughly that of the water vapours, whereas the dew point of sulphuric acid vapour is approximately 150 degrees C, hence it may condense out upon metal surfaces at or below this temperature.
CLOSED FEED SYSTEM
In modern steam turbine installations with water tube boilers, the feed water circuit from the regenerative condenser to the boilers is completely closed. (Close feed system)
TURBINESSteam Turbine plants consists of essentially of one or two highly rated water tube boilers which supply steam to the turbines, which is geared down to the propulsion shaft.
A Turbine, like the reciprocating engine, is a machine converting heat energy into mechanical energy, but the principles upon which these two engines work is entirely different. In the turbine the rotor coupled to the shaft receives its rotary motion direct from the action of the velocity of the steam impinging on the blades fitted into grooves around the periphery of the rotor, thus the action of the steam in the turbine is DYNAMIC instead of static as the gas in the reciprocating engine.
There are two types of TURBINES, the IMPULSE and the REACTION. In both cases the steam is allowed to expand from a high pressure to a power pressure so that the steam acquires a high VELOCITY at the expense of pressure. It is this velocity that is directed on to curved section of blades, which absorb some of the velocity. The difference is the method of expanding the steam.
IMPULSE TURBINE.
In the IMPULSE TURBINE the high-pressure steam passes into nozzles wherein it expands from a high pressure to a lower pressure and thus the energy in the steam is converted into velocity energy (KINETIC ENERGY). The high velocity steam is directed on to blades fitted around the turbine wheel, the blades are of curved section so that the direction of the steam is changed thereby imparting a force to the blades to push the wheel around.
The best efficiency is obtained when the LINEAR SPEED of the blades is half of the velocity of the steam entering the blades, thus, when one set of nozzles is used to expand the steam from its high supply pressure right down to the final low pressure, the resultant velocity of the steam from the nozzles is very high, and to obtain a high efficiency it means therefore that the wheel should run at a very high rotational speed. Lower speeds, which are more suitable, can be obtained by PRESSURE COMPOUNDING, or VELOCITY COMPOUNDING, or a combination of these called PRESSURE-VELOCITY COMPOUNDING.
In the velocity compounded impulse turbine, the drop in pressure is carried out in stages, each stage consists of one set of nozzles and one bladed turbine wheel, the series of wheels being keyed to the one shaft with nozzles plates fixed to the casing between the wheels.
The pressure-velocity-compounded turbine is a combination of the two.
CONSTRUCTION.
In the MARINE ENGINES where the shaft is required to run in both the ahead and the astern, a separate turbine is necessary.
THE REACTION TURBINE.
In this type of turbine, the steam is expanded continuously through guided blades fixed to the casing and also as it passes through the moving blades on the rotor, on its way from the inlet end to the exhaust end of the turbine. There are no nozzles as in the impulse turbine.
When high pressure steam enters the reaction turbine, it is first passed through a row of guide blades in the casing through which the steam is expanded slightly, causing a drop in pressure with a resulting increase in velocity, the steam being guided on to the blades, thus exerting further force due to reaction. This operation is repeated through each set of guided and moving blades, until the pressure has fallen to exhaust temperature.
This turbine might be known as ? IMPULSE-REACTION? but generally it is referred to as ?reaction? to distinguish it from the pure impulse turbine.
A steam falls in pressure it consequently increases in volume, to accommodate this increasing volume of steam, as it passes through the turbine, each casing and rotor is made progressively larger in diameter from the high pressure end to the low pressure, usually in step with larger area of annulus and longer blades.
ASTERN GEARING.
On board ship a separate astern turbine is required to drive the propeller in the reverse direction. This astern turbine may be mounted ion the same shaft as the ahead turbine, it may be a completely separate unit geared to the main shaft, or the lay-out may be
HP./IP ahead turbines on one turbine shaft, and another parallel shaft carrying the LP ahead and the astern turbines, each shaft bring geared to the main shafting.
GEARING.
The speed at which turbines run to obtain the best efficiency, is very much higher than the economical speed of the ships propeller, therefore to obtain the best performance of both turbine and propeller, reduction gearing is interposed between the turbine shaft and the propeller shaft, to enable both to run at their best speeds.
Single reduction gearing consists of one stage of speed reduction which a pinion on the turbine shaft meshes with a gear wheel on the main shaft.
Double reduction gearing consists of two stages of speed reduction by means of a pinion on the turbine shaft meshing with an intermediate wheel, on the intermediate shaft a second pinion meshes with the main gear wheel on the propeller shafting
The teeth on the gear wheels are helical cut for smooth running, and all the gears are in pairs to balance any axial thrust caused by the gears being helical.
Flexible couplings between turbine and pinion shafts, are to prevent thermal expansion of the turbine or misalignment of its shafts affecting the perfect meshing of the gear wheel teeth.Brought to you by www.dieselduck.net comments to [email protected]
Disclaimer
Transport Canada has ask us to advise users of this webpage to keep in mind that these questions are not the exact questions found in their exams. Martin's Marine Engineering Page - www.dieselduck.net is not affiliated with Transport Canada and these questions have been gathered from various sources.