Marine Boilers Steam and Water

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Marine Boilers Steam and
Water
N.B. this is just an overview; lots of valves and other accessories are
missing.
Fuel Oil system
Steam flow diagram for an exhaust gas boiler and two oil-fired auxiliary marine
boilers
In this marine steam boilers system, the circulation pump "J" runs continuously. The
auxiliary steam boiler serves as steam separator for the entire system. The system is
thus kept in hot condition and is ready to start instantly. If the steam from the exhaust
boiler exceeds the steam demand onboard, a steam dump valve automatically dumps
the surplus steam to a dump condenser.
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The energy content of the exhaust gases is utilized optimally in this marine boiler
system. In addition to generating superheated steam for driving a turbo-alternator, the
exhaust gas boiler also supplies steam for other requirements on board.
Some things to think about
The outside temperature of the tubes of an Exhaust Gas Economizer of forced
circulation type, the most common type today, is only a few °C higher than the water
inside the tubes. The water temperature must therefore not be below 130°C if low
temperature corrosion shall be avoided. Since ships mostly are equipped with open
feed water systems where the feed water temperature is not higher than 50-80°C this
feed water may be mixed with saturated water from the Oil Fired Boiler or from a
steam separator drum, in proportions so that the water entering the EGE is minimum
130°C. As the feed water from the open feed water system contains oxygen,
installation of a de-airator in this case is ecommended in order to avoid oxygen
corrosion inside the tubes.
However, the most common way is to feed the EGE with saturated water from the
OFB linked with EGE, since the temperature of the saturated water entering the EGE
is reduced to 130°C in an externas heat changer by the feed water having a
temperature of 50-80°C.
If the steam is used for production of electric energy by a turbo alternator, a
superheater may be incorporated and located on the gas inlet side of the EGE.
Except for tankers that mostly are provided with large OFB's producing steam for the
cargo pumps, the main part of the operating ships are furnished with an OFB and an
EGE with capacities which are generally large enough to maintain only the bunker
heating and the domestic heating.
However, where there are energy enough in the exhaust gases, it should be
economically justified to instalI a turbo-alternator set. When the EGB then is designed
for a high rate of heat recovery from the gases, the EGB wilI be larger in size and it
wilI thus have a larger water volume inside the tubes.
Due to increased steam production and water volume of the EGE, the OFB, steam
receiver, has to be enlarged in order to obtain larger steam space and to withstand
level variations which occur when the main engine starts and stops. When the main
engine starts, and consequently the steam production takes place, the water in the
EGB will expand and will be pressed from the EGE to the OFB where the water leivel
increases. On the contrary, when the main engine stops, the steam production
interrupts. The circulating water pump will then fill up the EGE with water that
results in a decrease of the water level in the OFB. In case of a too small OFB
compared with the steam production and the water content of the EGE, the water level
variations in the OFB will be unacceptable and they will cause repeated, undesired
high and low level alarms.
A. Economizer section of the Waste Heat Recovery Boiler
Preheats the circulating water before it enters the evaporator tubes. The
counterflow principle is applied, i.e., the hottest exhaust gasses meet the
heated feed water.
B. Evaporator section of the Waste Heat Recovery Boiler
Water evaporates and emulsion of steam and water flows back to the boiler.
The counterflow principle is not applied since the evaporated steam rises in
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C.
D.
E.
F.
G.
H.
I.
J.
K.
the tube bank and it would be disadvantageous to let the steam rise against the
water flow.
Superheater section of the Waste Heat Recovery Boiler
Superheats the steam for the turbine. The counterflow principle is applied, i.e.,
the hottest exhaust gasses meet the superheated steam.
Heat exchanger
Preheats the boiler feedwater.
Condenser
The exhaust steam from the turbine and excess steam from the steam system
condense and recycle.
Condensate pump
Cooling water pump
Seawater.
Make-up water pump
From softener unit.
Boiler feedwater pump
Boiler water circulation pump
Alternator
Deliver electric power to the ships different el-consumers.
1. Steam pressure control
The pressure control loop adjusts the burner load according the steam demand.
2. Water level control
A simple control loop will do for a boiler with large amount of water and
relatively small steam output. To minimize shrink and swell at start and stop
of the burner it would be wise to have two setpoints for the water level. A
lower level (abt. 40%) when the burner is stop and a higher (abt. 50%) when
it's firing.
3. Economizer inlet temperature control
The feedwater is pre-heated in order to increase the efficiency of the plant.
The circulating water to the exhaust gas boiler heats the feed water and the
three-way valve on the inlet to the heat-exchanger controls the temperature.
The economizer inlet temperature must never fall below 135°C to avoid
corrosion on the economizer tubes.
4. Condenser pressure control
An absolute pressure transmitter and a controller adjust the cooling-water to
the condenser to protect the condensate from being cooled down more than
necessary.
5. Condenser level control
The level controller actuates the condensate outlet control valve.
6. Steam dump control valve
Takes care of excess steam from the waste heat boiler when the steam
production exceeds the steam demand.
7. Feedwater tank level control
The level controller actuates the make up water control valve.
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