|
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.
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 aboard.
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.
- 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.
- 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 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 the
Feed Water Treatment unit.
- Boiler feedwater pump
- Boiler water circulation pump
- Steam pressure control
The pressure control loop adjusts the burner load according the steam demand.
- 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.
- 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.
- 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.
- Condenser level control
The level controller actuates the condensate outlet control valve.
- Steam dump control valve
Takes care of excess steam from the waste heat boiler when the steam production
exceeds the steam demand.
- Feedwater tank level control
The level controller actuates the make up water control valve.
|
