Questions & Answers (Boiler) Part-3

41.	Boilers Alarm and Trip Functions

The most important alarm and trip functions on marine boilers

Function	Audible and visual	Action
Low water level	Alarm
Low Low water level *	Alarm	Burner stop
High water level	Alarm
High High water level	Alarm	Feedwater pump stop
Low steam pressure	Alarm  (overridden on slave boiler?)
High steam pressure	Alarm	Burner stop
Low feedwater pressure	Alarm	Stand-by pump start
Low fuel oil pressure	Alarm	Stand-by pump start
Low fuel oil temperature	Alarm
High fuel oil temperature	Alarm
Low combustion air pressure	Alarm	Burner stop
Low atomizing steam pressure	Alarm	Burner stop
Flame failure **	Alarm  (overridden during light-up)	Burner stop
Burner not in firing position	Alarm	Burner stop

42. Significance of regular blow down:

                1)            To reduce the water level in the boiler to prevent carry over or priming.

                2)            To reduce corrosion.

                3)            To remove sludge or mud from the boiler.

                4)            To reduce solid particles.

                5)            To reduce the dissolved solid.

                6)            To reduce oil by scum blow down.

                7)            To reduce chloride.

43.How regular partial below down of a boiler helps to reduce the boiler water problem?

Boiler water problem means presence of various salts, oil, gas, oxygen, scum & even priming.

1. Regular partial blow down reduce the boiler water level as well as total dissolved solid which are precipitated from salt by using chemical & chloride. 
2. It also removes sludge & prevents priming. Thus regular partial below down of boiler helps to reduce boiler water problem.

44. With respect to contamination of boiler water by fuel oil explain why the presence of oil is hazardous?

Oil will effect to the heating surface & causes:

    - Over heating

    - Poor heat transmission.

    - Efficiency loss.

    - Oil may cause acid corrosion.

    - It may causes priming. 

45. Source of oil contamination:

1. Leaky steam heating coil of heater.
2. Leaky heating line of fuel oil tank.
3. Excess lubrication of machinery
4. Condenser (steam leakage sea water gets entry.
5. If the cascade tank is not covered properly dirt’s, debris get entry.

46. Step to be taken after identification the contamination (Oil) :

1) Boiler load is to be reduced.
2) If boiler pressure can not be maintained then open the exhaust by pass v/v
3) Identify and isolate the leaky component.
4) Cascade tank scum blow down should be done.
5) Also boiler scum blow don should be done.
6) Continuously add coagulants & potentially
7) Alkaline chemical should be added to maintain alkalinity for protection from acid corrosion.
8) Anti-foam should be added to prevent carry over of water.
9) Check for any more, any more oil spot is visible or not.
10) Then carry out other tests of boiler water to ensure that other parameters are within the limit.
One short system / single short system:
Na2CO3 + H2O ->NaOH + H2O
NaOH + Na2HPO4 –> Na2PO4+H2O
Na2PO4+CaSO4 -> Ca3(PO4)2+Na2SO4
The chemical is added to make the boiler water more alkaline known as BNT corn starch is used to combine the sludge together.
No other treatment is required. 

47.What is caustic embrittlement? How to prevent it ?

It is inter crystalline fracture. It is cause by high concentration of caustic soda and the material under stress. The stress corrosion cracks follow the grain or crystal boundaries of the material and failure.
Sodium sulphate or sodium nitrate is used for the prevention of caustic embrittlement.
It can be found in highly stress area in boiler. Such as tube and tube plate connection, riveted head, seam and boiler mountings.

48. Erosion

It is a mechanical attack on the metal surface which may be due to a disturbance in the flow of the fluid over the metal surfaces, resulting in a loss of metal. Suspended abrasive matters in the fluid can increase the rate of metal losses.

49. Corrosion:

It is the deterioration of metals due to oxidation. The present of water in an acidic condition provides the electrolyte required for corrosion action.

Two forms of corrosion:
Direct chemical attack:
It occurs when metal at high temperature comes into contact with air or other gases, resulting in oxidation or sulphidation of the metal.
Electro-chemical action:
Galvanic action, this being set up when two dissimilar metals are placed in an electrolyte. The noble of the two metals form a cathode to the base metal which , forming the anode, is wasted away.

50. Caustic embrittlement.

Caustic embrittlement is a form of inter crystalline cracking, which results from a solution of sodium hydroxide or caustic soda, becoming more and more concentrated at the bottom of a crack or fissure (narrow opening) which may be the result of fatigue, in the boiler plate or furnace.
The plate must be stressed intention (Wi fNi f x et eto u 117m b n f, so that wastage take place at the bottom of crack, the plate weakens, the crack extends to expose new metal to the caustic action and thus it proceed.
Caustic embrittlement will only occur when there is a high caustic alkalinity that is when the ratio of NaOH to the alkalinity is high. To keep this ratio at the safe level the sodium sulfate to sodium hydroxide should be maintained above 2:5. The sodium sulfate comes out of solution in high sodium hydroxide concentration and by doing forms a protective layer on the surface of the plate.
Also keep the lowest T.D.S level. (Total dissolved suspended & solid).

51. What do you do for preventing of oxygen?

The oxygen-scavenging chemical which is used for DE-aeration the water are usually sodium sulphate or hydrazine.
Excessive dosage of hydrazine could lead to steam and condensate line corrosion due to ammonia being produced as the excess hydrazine decomposed.
In high pressure boiler, by using the sodium sulphite, the sulphite can break down to give hydrogen sulphide which can attack steel brass and copper.

52.Why water testing is essential in water management?

1) To ensure efficient steam production.
2) To get better efficiently of boiler.
3) To minimize heat loss through expensive blow down from boiler.
4) To maintain clean & scale free heat transfer surface.
5) To prevent metal loss due to corrosion.
6) To reduce the total dissolves solid.
7) To prevent formation of deposit in steam condenser system. 8) TO keep the water in alkaline condition.

53. Safety device of boiler gauge glass: 

1) Two gauge glass.
2) Glass is very thick.
3) Guard protect from accidents damage.
4) A board painted white with diagram block.
5) Water side ball, steam side orifice.
6) Drain cock.
7) All handle must be pointed vertically down ward in normal working condition.
8) Remote operation is provided with gauge glass.

54. If boiler water level low, what are the reasons?

The reasons are-
1) Evaporation & consumption rate is higher than the feed rate.
2) Cascade tank empty.
3) Regulator not functioning properly.
4) Safety valve blowing.
5) Blow down v/v leaking.
6) Boiler tubes leaking.
7) Boiler feed p/p is not building / vapour lock.
8) Heating coil is leaking.

9) Faulty gauge glass.

55. If the boiler water is low, what to do?

At port:
1. Stop firing.
2. Blow through the gauge glass to check the water level.
3. If the water level is low then
4. Inform C/E
5. Stop feed p/p running.
6. Shut main steam stop v/v
7. Stop T/A by consulting with duty officer.
8. Stop C. O. P by informing duty officer.
9. Stop purifier if running
10. Go for a check.
At sea:
It water level is below the gauge glass, it is unsafe to put water.
1) Inform C/E.
2) Stop feed p/p if running.
3) Inform bridge & reduce engine RPM.
4) Shut main steam stop v/v.
5) Stop purifier if running.
6) Go for a check.

56. Indication of tube leaking:

1) White smoke.
2) Boiler water low level alarm due to excessive leakage, if the feed P/P cannot cover.
3) cascade tank water level low.
4) Feed P/P running continuously.
5) Change flame shape.
6) Noise.

57. Purpose of blowing down a boiler.

1) Reduce water level to prevent carryover or priming.
2) Reduce total dissolved solid.
3) Reduce chloride.
4) Remove sludge.
5) Remove oil by scum blow down.
6) Boiler empty.

58. If chloride is found too much in your boiler water what will you do?

1. Blow down frequently
2. Reduce the boiler load to minimum.
3. If highly contaminant shut down completely and wash out.
4. Trace and find out the fault and remedies.

59. What do you understand by composite, alternative, and economizer boiler?

Composite boiler:
If exhaust gas and oil fired can be used the same time, it is termed the composite boiler. In this type a separate nest of tube for exhaust gas is provided, situated above the return tubes from the fire furnace. Uptake from the tube nest are separated.
Alternative boiler: 
If the exhaust gas and the oil fire are arranged to be used only at a time, the exhaust gas boiler term as the alternative boiler. This unit can be oil fire heated by the exhaust gases alternatively. It required only one uptake.
Economizer:
In this system a separate exhaust gas economizer is connected to an oil fired auxiliary boiler or an accumulator by means of piping and a set of circulation pump. Exhaust gas is used as heating medium and it has no steam space.

60. What is meant of accumulation pressure?

Accumulation pressure is the rise in boiler pressure which take place when the spring loaded safety valve lift due to the increase loading caused by further compression of the spring.

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References:

Marine Engineers' World (Facebook)

Bangladesh Marine Academy

Manual Book: AALBORG MISSION OM 1200

Hand Notes of Mr. Min Zar Tar