8/10/2019 boiler treatment

1/22008 Teledata Marine Solutions. AllM A R I N E S O L U T I O N S

aritime leadership. All t he way. 12008 Teledata Marine Solutions Ltd. All rights reserved .

MEO Class IV - GEKAuxiliary Boilers

Boiler Water Treatment S cale Formation and CorrosionWhen water containing dissolved and suspended solids is evaporated in a boiler, the solidsconcentrate and scales are formed.

The formation of scale is a complex process. The physical formation of scales depends on

constituents in boiler water and the prevailing conditions.

Mechanics of scale formation

When a steam bubble is formed on a plate which is hot, the plate under the bubble becomesoverheated locally. The plate is insulated momentarily by the steam bubble from the water. Thesalts in solution are in contact with the plate. The periphery around the bubble is overheated.

With the movement of water, these solids are reabsorbed, but some amount insoluble salts areleft behind and stick to the heating surface. The adhered insoluble salts forms a scale.

If the solubility of salts decreases with increase in temperature, then scale is formed.

Scale deposits impede heat transfer and restrict ow of water leading to local overheating andsubsequent failure. One way of controlling formation of scale is by regular blow down of theboiler.

Oxygen corrosion

The presence of oxygen in the feed water is harmful for the metals in the system. Oxygen infeed water gives rise to feed line corrosion. This results in the formation of corrosion productssuch as oxides of iron and copper. Effective thickness of the metals in the feed system getsreduced.

Oxygen not used up in the feed line corrosion reaches the boiler along with the feed water. It willbe carried over with the steam. But when oxygen attack does take place in the boiler, it causeslocalised deep pitting. Deep pitting leads to boiler tube failure.

Consequence

Corrosion in the feed line mainly occurs due to iron and copper oxides. These oxides getdeposited on steam generating tube surfaces. It restricts heat and leads to increase in thetemperatures of tube metal. Therefore localised heating takes place resulting in tube failure.

8/10/2019 boiler treatment

2/22008 Teledata Marine Solutions. AllM A R I N E S O L U T I O N S aritime leadership. All t he way. 22008 Teledata Marine Solutions Ltd. All rights reserved .

MEO Class IV - GEKAuxiliary Boilers

Boiler Water Treatment S cale Formation and CorrosionComparison between scale formation and corrosion

When hard scale deposits on the heating surfaces, it forms a thick insulating layer. It restrictsthe ow of heat from the heating surface. The result is a steep temperature gradient across theheating surface. This results in inef cient heat transfer and subsequent tube failure.

It has been estimated that a scale of 3 mm thickness increases the fuel consumption by 16%and 6 mm scale by 50%. This proves that the over heating effect is not a straight line gradientbut is exponential.

As compared to scale formation, acidic corrosion due to chlorides is less dangerous, because thescale formation is rapid when compared to chloride corrosion. Therefore, hardness is limited toa low value of 5 ppm. Chlorides should be less than 20 ppm for boilers operated above 30 bar.The chloride content should be as less as possible for low pressure boiler to Enhance the life andperformance of the boiler.

Scale thickness (mm)3 6

Effect ofoverheating

Fuel oilconsumption

(ltrs / hr)

50

40

30

20

10