Boiler Water Treatment – Boiler Feed Water Properties

Boiler Water Treatment

Boiler water treatment is one type of industrial water treatment which is focused on removing both organic and inorganic elements that can cause damages to the boiler. Water treatment takes places to avoid scaling, corrosion, or foaming. Raw or feed boiler water is treated before the contaminants/impurities reach the boiler. Internal boiler water treatment’s purpose is to prevent dissolving of the boiler and to keep contaminants in certain formations and levels that do not harm it or cause a blowdown.

Water can be easily and quickly heated when exposed to a temperature rise than any other inorganic matter. When it evaporates and turns into steam at atmospheric pressure it dilates by one thousand and six hundred times. While in form of steam, it is charged with a huge amount of heat, and with these special characteristics water has it makes a special element for boilers, heaters and power generators.

Water contains both suspended and dissolved solids and gases in assorted quantities. Varying from 30 grams/liter for seawater and ranging from 0.005 to 1500 milligrams/liter for fresh water. These water contaminants cause erosion and corrosion for boilers and yet other issues, this is when water treatment is considered in order to bring out a certain standard and quality of water to be safely used in generation of steam in steam generators and boilers.

Every boiler has a design that requires a certain level of impurities that cannot be exceeded in the water being used. It is more like a tolerance that is limited and if exceeded it can cause serious deterioration of the boiler. This is why boiler water with high amounts of impurities has to be pretreated and contaminants has to completely removed before being transformed into steam to improve quality.

boiler mechanism reverse osmosis water treatment

boiler mechanism reverse osmosis water treatment

Both quantity of nature of impurities in the boiler water are taken in consideration: for instance hardness, iron and silica are of high concern than sodium salt. Factors that are put in consideration in treatment of feed water are the quantity needed (flow rate) and the design of the boiler; pressure, heat transfer rate, etc. A low pressure fire tube boiler can tolerate high level of hardness with the use of a certain treatment while for modern boilers that use high pressure, contaminants has to completely be removed.

Requirements for boiler water design and requirements include maximum tolerance levels of alkalis, salt, silica, phosphates and other elements in proportion to the pressure at which the boiler will be operated. These boiler water levels must be gotten from the boiler manufacturer, and are basically based on the boiler properties.

Below tables are the recommended boiler water properties / boiler feed water chemical characteristics (levels)  taken from APAVE (Association of electrical and steam unit owners), up to pressures of 100 bar for medium steaming rates and for volumes of water in the chambers sufficient to properly control the blow down rates, and from ABMA (American Boiler Manufacturers Association) in its standard guarantee of steam purity.

Working Pressure (Bar)
0 – 20.7 20.8 – 31.0 31.1 – 41.4 41.5 – 51.7 51.8 – 62.1 62.2 – 68.9 69.0 – 103.4 103.5 – 137.9
Feed water
Dissolved oxygen (measured before oxygen scavenger addition) 0.04 0.04 0.007 0.007 0.007 0.007 0.007 0.007
Total Iron mg/l 0.1 0.05 0.03 0.025 0.02 0.02 0.01 0.01
Total copper 0.05 0.025 0.02 0.02 0.015 0.015 0.01 0.01
Total hardness(CaCO3) 0.3 0.3 0.2 0.2 0.1 0.05 not detectable
Non volatile TOC 1 1 0.5 0.5 0.5 0.2 0.2 0.2
Oily matter 1 1 0.5 0.5 0.5 0.2 0.2 0.2
pH at 25 7.5 – 10.0 7.5 – 10.0 7.5 – 10.0 7.5 – 10.0 7.5 – 10.0 8.5 – 9.5 9.0 – 9.6 9.0 – 9.6
Boiler Water
Silica mg/l 150 90 40 30 20 8 2 1
Total alkalinity (CaCO3) 350 300 250 200 150 100 not specified
Free hydroxide alkalinity CaCO3 not specified not detectable
Specific conductance at 25 without neutralization mS/cm 3500 3000 2500 2000 1500 1000 150 100
Working Pressure (Bar)
0 – 15 15 – 25 25 – 35 35 – 45 40 – 60 60 – 75 75 – 100
Boiler Feed water
Dissolved oxygen (measured before oxygen scavenger addition) mg/l 0.02 (Physical removal of dissolved oxygen)
Total hardness French degrees 0.5 0.3 0.2 0.1 0.05 0.05 0.05
Oily matter mg/l absence 0.05 0.05 0.05
pH > 8.5
Total Iron mg/l not specified 0.05 0.05 0.03
Total copper not specified 0.03 0.03 0.01
Boiler water
M alkalinity French degrees 100 80 60 40 15 10 5
P alkalinity 0.07 M 0.07 M 0.07 M 0.07 M > 0.5 M > 0.5 M > 0.5 M
SiO2 mg/l 200 150 90 40 15 10 5
TDS 4000 3000 2000 1500 500 300 100
Phosphates 30 to 100 31 to 100 20 to 80 21 to 80 10 to 60 10 to 40 5 to 20
pH 10.5 to 12 10 to 11
Make up water Softened or softened and carbonate free Demineralized