Marine Boilers – Marine Engineering Knowledge
What is the meaning of marine boilers ?
Boiler is a pressure vessel used on ships in which the water is heated to evaporate and generate the steam and the unit is so arranged that the generated steam accumulated in it. The two main types are water tube and fire tube.
What is the boiler mounting ?
It is directly fitted to the boiler shell. They are:
- Safety valve
- Main steam stop valve
- Air vent cock
- Two gauge glass level indicators
- Feed check valve
- Scum blow down valve
- Bottom blow down valve
- Salinity cock or test cock
- Man hole doors.
Where are air vent cock located and its purpose ?
The air vent cock is fitted at the top of the steam space of the boiler.
Its purpose are-
- To release air from the boiler either filling the boiler water or raising the steam.
- To allow air to enter in the event of boiler cooling down or blowing down. ( to prevent vacuum effect)
What are the safety arrangements in marine boiler ?
- Safety valve
- Low / high water level alarm
- Too low water level alarm and shut down
- Water level indicators
- Pressure gauge
- Low fuel oil pressure alarm
- Low / high fuel oil temperature alarm
- Flame failure alarm
- Smoke density alarm
- Easy gear arrangement
- Air vent
- Force draught fan stop alarm
- Low / high steam pressure alarm
Why need to blow down the gauge glass ?
Gauge glass blow down is made to know the boiler and gauge glass connection is clear or not and to give the exact water level in the boiler.
Boiler gauge glass blow through procedure ?
- Shut steam and water cocks than open the drain cock.
- After draining water from the sight glass, there is nothing come out, both steam and water cocks are good in order.
- Open the steam cock side, blow out the steam.
- Close the steam cock side, the steam connection is clear.
- Open the water cock side, blow out the water.
- Close the water cock side, the water connection is clear.
- Close the drain cock.
- Open the water cock, water should then gradually come rise up to the top of the gauge glass.
- Open the steam cock, the water in the glass should fall to the level of the water in the boiler.
Why need to blow down the boiler water ?
- Boiler blow down is made to reduce the density of salt and to remove the dissolved and suspended solids, also the floating solid impurities in the boiler system.
- If there are not removed from the boiler water system, foaming, priming, corrosion will occur in the boiler steam space and feed water system.
- See ‘Boiler Blow Down’.
Why scum blow down or surface blow down is carried out ?
- To remove accumulated, suspended and floating solids, impurities and also remove dissolved solid concentration, they hinder the formation of steam.
Why bottom blow down is carried out ?
- To remove suspended solids and residual sludge that have settled at the bottom.
- If these contaminants are not removed regularly they will build up until they hinder the circulation patterns.
What is the purpose of boiler water test ?
- To ensure that proper chemical treatment are maintained at all time.
- To detect the present of contaminants in the water that may be injurious to boiler and system.
Enumerate the boiler water test ?
- Alkalinity test [Phenolphthalein ( p) alkalinity test, Total ( T ) alkalinity test (2 x P)]
- Chloride test
- Condensate PH test
- Amerzine test ( Hydrazine test )
- Excess phosphate test
- Conductivity test (Total dissolve solids test)
- Hardness test
Alkalinity test
- This is to ensure that the boiler water prevent corrosion by neutralization of acidic gases.
(a) Phenolphthalein or ‘p’ alkalinity
- This test is carried out to prevent acidic corrosion
- To test for presence of all of the hydroxide, one half of the carbonate and one third of phosphate present in a water sample.
(b) Total ( T ) alkalinity test
- To determine the amount all of hydroxide, all of the carbonate, and two thirds of the phosphates
Note: Hydroxides and carbonates can co-exist together in a solution but hydroxides and bi-carbonates cannot.
Chloride test
- To know the amount of salt in boiler water.
- To minimize chloride level and to adjust the blow down.
Condensate PH test
- To control condensate PH value within a limit.
- To minimize corrosion in steam and condensate system.
Amerzine test ( Hydrazine test )
- To test for dissolved Oxygen content
- To know reserve hydrazine (N2H4 ) ppm and to prevent corrosion and aeration.
- To minimize oxygen pitting and corrosion in boiler, steam and condensate system.
Phosphate test
- To control the scale formation due to hardness (presence of Phosphate in sample means no hardness salts)
- A reserve of phosphate should be maintained in the boiler water ready to neutralize any hardness salts which may enter.
Conductivity test
- Measure of the total amount of dissolved solids (T.D.S) including the treatment chemicals. (Excessive density leads to priming and or deposits)
- To remove dissolved and suspended solid by blowing down.
Hardness test
- To check for salt causing “hardness”
Note: Hardness test of boiler water are not necessary when the phosphate is above the lower limit of the control range.
What are the advantages of boiler water treatment ?
- To prevent the scale formation in the boiler and feed system
- To prevent corrosion in boiler and feed system.
- To control the sludge formation and prevention of carry over with the steam.
- To maintain the boiler water in alkaline condition and free from dissolved gases.
- To prevent of entry into the boiler of foreign matter such as oil, waste, mill scale, FeO, Cu, Sand, etc.