Boiler

Boiler Water Treatment

The treatment and conditioning of boiler feed water must satisfy three main objectives:

Continuous heat exchange

Corrosion protection

Production of high quality steam

External treatment is the reduction or removal of impurities from water outside the boiler. In general, external treatment is used when the amount of one or more of the feed water impurities is too high to be tolerated by the boiler system in question. There are many types of external treatment (softening, evaporation, deaeration, membrane contractors etc.) which can be used to tailor make feed-water for a particular system. Internal treatment is the conditioning of impurities within the boiler system. The reactions occur either in the feed lines or in the boiler proper. Internal treatment may be used alone or in conjunction with external treatment. Its purpose is to properly react with feed water hardness, condition sludge, scavenge oxygen and prevent boiler water foaming.

External treatment

The water treatment facilities purify and deaerator make-up water or feed water. Water is sometimes pretreated by evaporation to produce relatively pure vapor, which is then condensed and used for boiler feed purposes. Evaporators are of several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point. Sometimes to increase the efficiency the vapor from the first tank is passed through coils in a second tank of water to produce additional heating and evaporation. Evaporators are suitable where steam as a source of heat is readily available. They have particular advantages over demineralization, for example, when the dissolved solids in the raw water are very high.

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

The sketch illustrates the principle of water treatment for steam boilers.

The illustration is general. There are various possibilities of combination to be determined in each individual case.

Process water

Process water covers the wide range of boiler feed water, cooling water for heat exchangers or engine, chemicals dilution, etc…It should typically have a conductivity ranging from 0,1 to 50 uS/cm, with little to no hardness to avoid scaling in heating system. Oxygen and carbon dioxide should be removed to prevent corrosionDepending on your application, the water quality requirements can vary:

Boiler feed water characteristic

Cooling water quality

Tap water or fresh groundwater are the most widely used source of water to produce process water.

Our process water treatment plant can combine various technology, depending on the purity required:

Source Quality required Technology applied

500-2000 uS/cm 5-20 uS/cm Reverse Osmosis

< 5 uS/cm 2-pass reverse Osmosis

< 1 uS/cm 2-pass reverse Osmosis + Mixed bed

< 500 uS/cm < 5 uS/cm Ion exchange

< 1 uS/cm Ion exchange + Mixed bed

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