Marine Electronics and Control Technology lectures

Wednesday, November 30, 2011

Marine Boiler Operation Construction


Boilers and Steam Systems
General Description
The steam generating plant consists of two auxiliary boilers and one exhaust
gas economises Steam is required at sea for fuel, domestic water and cargo
slop tank heating purposes. In port steam is used additionally for driving the
power turbines of the cargo pumps and No. 1 water ballast pump. The steam
demand of the plant, in port, is served by the boilers. At sea, steam demand is
met by circulating boiler water from one of the auxiliary boilers through the
exhaust gas economiser, by one of the boiler water circulating pumps. The
auxiliary boiler acts as a receiver for the steam generated by the economiser.
The economiser is arranged in the main engine exhaust gas uptake to take
waste heat from the main engine exhaust. An auxiliary boiler may be required
at sea in low temperature areas, as well as reduced power operation of the main
engine, such as during manoeuvring or slow steaming on passage when there
will be insufficient waste heat to generate the required steam.
Auxiliary Boiler
No. of sets:
2
Maker:
Hyundai Heavy Industries Ltd
Model:
HMT-50
Type:
Top fired rectangular water tube marine boiler
Evaporation:
50,000 kg/h
Steam Condition:
18 kg/cm2 saturated steam.
Fuel Oil:
HFO up to 700 cSt at 50°C
Safety Valve Setting:
20 kg/cm2
Fuel Oil Consumption:
3,850 kg/h at 100% evaporation
Boiler Associated Equipment
Equipment
Combustion Control Electronic/Air Operated
Feedwater Regulator Electronic/Air Operated
Remote Water Level Gauge
Drum Level Safety System
Steam Jet Oil Burner
Water Level Gauge - Reflex Type
Safety Valve - Full Bore Type
Chemical Dosing Unit
FDFan
FO Pump
FO Heater
Description
General Construction
The boiler is of the two drum rectangular type, with a membrane furnace water
wall connecting steam and water drums.
The furnace consists of gas-tight membrane walls, the downcomer pipes are located outside of the furnace.
The fuel burner unit and associated combustion air inlet, is located in the roof of the furnace with the burner firing downwards using a steam assisted pressure jet burner.
At the furnace bottom, a refractory protects the furnace bottom from the combustion flame.
Combustion gases flow downwards and through the lower part of the division tube wall and the lower section of the generating tube
bank which connect the steam and water drums.
The gases then flow upwards on a return path through the upper part of the generating tube bank to the flue gas box at the top of the boiler.
Radiant heat generates steam in the membrane furnace water wall tubes.
The membrane wall has access doors to allow for furnace inspection and cleaning.
The boiler structure is rigid enough to withstand rolling, pitching and shock loading of the ship operating in a seaway.
The boiler is supported at the water drum and the water wall lower headers, and there are no rigid connections at any other points in order to allow for thermal expansion.
Furnace
Closely spaced water wall tubes of 76.2mm outside diameter, form the membrane walls at the side, roof, except for burner opening, rear, and front of the furnace.
This construction is in order to increase the radiant heat absorption in the furnace and to make it strong enough to withstand vibration.
The furnace is made completely gas-tight by the welded water wall construction.
Situated at the top and bottom of the front and rear walls are water wall headers.
Water enters the bottom headers and rises through the tubes to the top headers due to natural convection.
As the water rises, it is heated until its saturation temperature is reached and it then begins evaporating.
This water- steam mixture is passed to the steam drum via the top headers.
Front and rear water wall tubes connect to steam and water headers at the top and bottom respectively; one end of each top header connects with the steam drum and one end of each bottom header connects with the water drum.
The roof, side and bottom water wall tubes are directly connected to the water and steam drums.
The steam generating bank of tubes, connecting steam and water drums, is located within the furnace.
Boiler Casing
As the furnace of the boiler is made completely gas-tight by the adoption of welded membrane water wall construction, no casing or refractory is required to contain the combustion gases.
Mineral wool insulation is provided on the outer surface of the furnace water walls and this is covered by corrugated galvanised sheets to reduced heat transfer.
The maximum temperature on the casing surface will not exceed 60°C.
Steam Drum and Fittings
The steam and water drums are fabricated using boiler steel plate of all welded construction.
The steam drum has a horizontal perforated baffle plate covering the entire water surface in order to prevent droplets of water rising to the upper part of the steam drum.
A steam separator is provided to completely remove the moisture.
The feedwater pipe enters the steam drum at the rear of the boiler and is attached to an internal perforated feed pipe which extends to the front of the steam drum.
This ensures that there is complete mixing of incoming feed with the existing boiler water and an equalising of temperatures.
The chemical feedwater treatment pipe attaches to the internal feedwater pipe and this also ensures that there is complete mixing of the chemicals before the water reaches the downcomers.
The open ended surface blow off internal pipe extends to the surface of the steam drum to ensure that only floating solids on the water
surface are discharged through this scum blowdown line.
The boiler blowdown connection is fitted to the lower part of the water drum.
Sootblower - Rotary Type
Operating Procedures
Procedure for Preparing the Boiler for Service
The following steps should be taken before attempting to flash up the boiler.
a)  All foreign materials must be removed from internal pressure parts.
b)  All gas side-heating surfaces must be clean and all refractory be in good condition.
c)  The furnace bottom and the burner wind box must been cleaned of oil and other debris.
d)  All personnel not involved must remain clear of the boiler.
e)  All manhole covers must be securely tightened.
f)   Inspect safety valves and ensure that gags have been removed and easing levers are in good condition.
g)  Open root valves for all instruments and controls connected to the boiler and check that they work as intended.
h) Open the vent valve of the steam drum.
i) Open all pressure gauge valves and check to ensure that all valves on the pressure gauge piping are open.
j) Check and close all blow-off valves and drain valves.
k) Fill the boiler until water level appears 25 to 50mm high in the gauge glasses.
Allow for swell in level after firing.
1) Check the operation of gauge glasses.
Remote reading instruments will not work correctly until the boiler is under pressure and so they must not be relied upon.
Raising Pressure With No Steam Available from the Other Boiler or Economiser
With the boiler water at the correct level and other checks made as above:
a) Set up the fuel system for diesel oil and circulate the fuel until all heavy fuel has been discharged from the fuel lines.
Ideally the fuel system should have been flushed through with diesel oil prior to the previous shutdown.
b)  Set the burner for air atomising, using an air pressure of 5 kg/cm2 and fuel pressure of 3 kg/cm2.
Purge the furnace with the forced draught fan for one minute with vanes fully open.
c)  Reduce the air pressure at the windbox to between 10 and 20mm WG and close recirculating valve.
d)  Light the burner using the pilot burner and adjust air and fuel pressure to ensure stabilised combustion by using the furnace
observation port and smoke indicator.
e)  When raising the pressure, keep the burner firing for 5 minutes and out of service for 15 minutes repeatedly at the lowest fuel oil
pressure (2.5kg/cm2) for one hour.
Again, repeatedly light and shut down the burner to raise pressure as recommended on the pressure raising curve supplied by the manufacturer. A guideline would be to aim for lkg/cm2 after 2 hours firing, 5kg/cm2 after 2.75 hours firing and 12 kg/cm2 after 3.25 hours firing.
f)   When the drum pressure has risen to about 2 kg/cm2, close the drum vent valve.
g)  Drain and warm through all steam supply lines to ancillary equipment before putting the boiler on load.
h) Supply steam to one of the HFO service tanks.
When the tank is of sufficient temperature to be pumped by the HFO pump, supply steam to the HFO heater and prepare to change over from DO to HFO firing.
The HFO must be thoroughly circulated through the system to ensure it is at the correct temperature for good combustion.
When firing on HFO, check the combustion and adjust the fuel and air as required, then continue pressure raising.
(Note ! Caution must be exercised when operating with diesel oil due to its lower flash point. Diesel oil must not be heated above 40°C and there is a greater risk of leakage compared with HFO.)
i) At working pressure, switch to automatic operation.

Raising Pressure with Steam Available from the Other Boiler or Economiser
a)  Start the forced draught fan, open the inlet vanes and purge the furnace.
b)  Ensure that the HFO system is correctly heated then start the HFO burning pump and circulate oil through the heater and burner
manifold, open the recirculating valve and discharge the cold HFO in the line.
(Note ! At normal sea going condition, the boiler fuel system should be continually circulating heated HFO.)
c)  Reduce the air pressure at the windbox to between 10 and 20mmWG.
d)  Close the recirculating valve.
e)  Light the burner and adjust the air and fuel pressure to ensure stabilised combustion, using the furnace observation port and
smoke indicator.
Boiler pressure must be raised gradually over a period of hours in accordance with the manufacturer's instructions.
The recommendations are the same as in item e) in the section; Raising Pressure With No Steam Available
f)   When the drum pressure has risen to about 2 kg/cm2, close the drum vent valve.
g)  Drain and warm through all steam supply lines to ancillary equipment before putting the boiler on load.
Shutting Down
a)  Operate sootblowers before shutting down the boiler whenever possible.
b)  Shut down the burner.
c)  Continue operation of the forced draught fan for a short while after shutting down, keeping an air pressure of 150mm WG at burner inlet and purge the furnace of combustible gases.
d)  Maintain the water level visible at about 50mm in the gauge glass and when the boiler is closed raise the water level 70mm to 120mm above the normal water level.
e)  Open the drum vent valve when the boiler pressure reaches about 2 kg/cm2.
f)   Change the fuel system to diesel oil and circulate back to the tank.
(Note ! If steam is to remain available from the other boiler or economiser, the boiler HFO system should remain in use and there is no need to change to diesel oil.)
g)  When fuel oil has been purged, shut down the fuel system.
After the boiler has been shut down for 4 hours the forced draught fan may be used to assist cooling down should immediate access be required. However, to avoid the risk of damage to refractory, allow the boiler to cool down under natural means if possible.
! CAUTION
Do not attempt to cool down the boiler by blowing down or by filling with cold water.