Characteristics of boiler feed water

Water absorbs more heat for a given temperature rise than any other common inorganic substance. It expands 1600 times as it evaporates to form steam at atmospheric pressure. The steam is capable of carrying large quantities of heat. These unique properties of water make it an ideal raw material for heating and power generating processes. All natural waters contain varying amounts of dissolved and suspended matter and dissolved gases the amount of minerals dissolved in water varies from 30 g/l in sea water to anything from 0.005 to 1500 mg/l in fresh water supplies. Since water impurities cause boiler problems, careful consideration must be given to the quality of the water used for generating steam. The composition of boiler feed water must be such that the impurities in it can be concentrated a reasonable number of times inside the boiler, without exceeding the tolerance limits of the particular boiler design. If the feed water does not meet these requirements it must be pretreated to remove impurities. The impurities need not be completely removed in all cases, however, since chemical treatment inside the boiler can effectively and economically counteract them. Feed-water purity is a matter both of quantity of impurities and nature of impurities: some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts. The purity requirements for any feed-water depend on how much feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feed-water purity requirements therefore can vary widely. A low-pressure fire-tube boiler can usually tolerate high feed-water hardness with proper treatment while virtually all impurities must be removed from water used in some modern, high-pressure boilers. Only relatively wide ranges can be given as to maximum levels of alkalis, salt, silica, phosphates etc, in relation to working pressure. The actual maximum levels must be obtained fro the boiler manufacturer, who will base them on the characteristics of the boiler in question. The following tables are extracts of recommended levels 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 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