Marine Engineering Study Materials

There is a number of ways to produce good quality feed water for boiler plants. Methods such as e.g. reverse osmosis plants or ion exchange plants produce good quality distillate. Also evaporators generally produce good distillate. The important thing is that the distillate used should be clean and without foreign salt contamination.

In practice most distillates used contain minor parts of various salt combinations which can and must be chemically treated away. Furthermore, the distillate may contain dissolved gases like for example oxygen (O₂) and carbon dioxide (CO₂) which may lead to corrosion in the boiler, steam, and condensate system.

Boiler and feed water must be chemically treated in order to avoid corrosion and scaling in the boiler.

The following text regarding feed and boiler water treatment is the normal recommendations for boilers with working pressure below 20 bars. These recommendations should be followed strictly in order to have the best working conditions for the boiler plant and to extend the working life of the plant. The requirements/recommendations of the various values for feed and boiler water are listed in Table 1 below.

Requirements for feed and boiler water
	Unit	Feed water	Boiler water
Appearance	–	Clear and free of mud	Clear and free of mud
Hardness	ppm CaCO3	0 – 5	–
Chloride content	ppm Cl–	<15	<100
“P” alkalinity	ppm CaCO3	–	100 – 150
Total (T) alkalinity	ppm CaCO3	–	<2 x “P” – Alkalinity
PH-value at 25°C	–	8.5 – 9.5	10.5 – 11.5
Hydrazine excess	ppm N2H4	–	0.1 – 0.2
Phosphate excess	ppm PO4	–	20 – 50
Specific density at 20°C	Kg/m3	–	<1.003
Conductivity at 25°C	mS/cm	–	<2000
Oil content	–	NIL	NIL

If hydrazine (N₂H₄) is not used, sodium sulphate (Na₂SO₄) can be used instead, and the excess should be 30 – 60 ppm.

In cases where other kinds of oxygen binding agents are used, it is recommended that an excess of oxygen binding agents can be measured and indicates that no oxygen has been dissolved in the boiler water.

If it is requested to measure the content of dissolved oxygen directly, it is recommended to keep the value < 0.02 ppm.

In addition to the above values, the various water treatment companies will add further demands, depending on the method used for treatment of feed and boiler water.

However, the most important point is that the above values or their equivalents are observed and that a regular (daily) test of feed and boiler water is carried out.

Feed and Boiler Water Maintenance

The following are recommended water maintenance instructions. More exact details concerning analyses and blow downs should be set up together with the supplier of chemicals for water treatment.

Daily

• Analyses of feed and boiler water.

Weekly

• Skimming (surface blow down) according to analyses, but at least once per week (2 minutes with fully open valve).
• Blow down (bottom blow down) according to analyses, but at least once per week (each blow down valve 1 minute in low load condition).

Monthly

• Check the functions for salinity and oil detection systems.

Every six months

• The boiler water side (interior) must be carefully inspected at least twice a year.

Yearly

• Check of the water side of the boiler and hotwell/deaerator for corrosion and scaling.
• Check the chemical pump unit.

Reference:

AQ-2 Boiler Manual by Aalborg Industries