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Boiler feed water

A boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications.

The water circuit of a water boiler can be summarized by the following pictures:

Water circuit of a water boiler

The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure.

Water boiler flowsheet

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment. For more information check the boiler water treatment web page.

In the following table you can find a list of the common boiler feed water contaminants, their effect and their possible treatment.

Find extra information about the characteristics of boiler feed water.

IMPURITY

RESULTING IN

GOT RID OF BY

COMMENTS

Soluble Gasses

Hydrogen Sulphide (H2S)

Water smells like rotten eggs: Tastes bad, and is corrosive to most metals.

Aeration, Filtration, and Chlorination.

Found mainly in groundwater, and polluted streams.

Carbon Dioxide (CO2)

Corrosive, forms carbonic acid in condensate.

Deaeration, neutralization with alkalis.

Filming, neutralizing amines used to prevent condensate line corrosion.

Oxygen (O2)

Corrosion and pitting of boiler tubes.

Deaeration & chemical treatment with (Sodium Sulphite or Hydrazine)

Pitting of boiler tubes, and turbine blades, failure of steam lines, and fittings etc.

Suspended Solids

Sediment & Turbidity

Sludge and scale carryover.

Clarification and filtration.

Tolerance of approx. 5ppm max. for most applications, 10ppm for potable water.

Organic Matter

Carryover, foaming, deposits can clog piping, and cause corrosion.

Clarification; filtration, and chemical treatment

Found mostly in surface waters, caused by rotting vegetation, and farm run offs. Organics break down to form organic acids. Results in low of boiler feed-water pH, which then attacks boiler tubes. Includes diatoms, molds, bacterial slimes, iron/manganese bacteria. Suspended particles collect on the surface of the water in the boiler and render difficult the liberation of steam bubbles rising to that surface.. Foaming can also be attributed to waters containing carbonates in solution in which a light flocculent precipitate will be formed on the surface of the water. It is usually traced to an excess of sodium carbonate used in treatment for some other difficulty where animal or vegetable oil finds its way into the boiler.

Dissolved Colloidal Solids

Oil & Grease

Foaming, deposits in boiler

Coagulation & filtration

Enters boiler with condensate

Hardness, Calcium (Ca), and Magnesium (Mg)

Scale deposits in boiler, inhibits heat transfer, and thermal efficiency. In severe cases can lead to boiler tube burn thru, and failure.

Softening, plus internal treatment in boiler.

Forms are bicarbonates, sulphates, chlorides, and nitrates, in that order. Some calcium salts are reversibly soluble. Magnesium reacts with carbonates to form compounds of low solubility.

Sodium, alkalinity, NaOH, NaHCO3, Na2CO3

Foaming, carbonates form carbonic acid in steam, causes condensate return line, and steam trap corrosion, can cause embrittlement.

Deaeration of make-up water and condensate return. Ion exchange; deionization, acid treatment of make-up water.

Sodium salts are found in most waters. They are very soluble, and cannot be removed by chemical precipitation.

Sulphates (SO4)

Hard scale if calcium is present

Deionization

Tolerance limits are about 100-300ppm as CaCO3

Chlorides, (Cl)

Priming, i.e. uneven delivery of steam from the boiler (belching), carryover of water in steam lowering steam efficiency, can deposit as salts on superheaters and turbine blades. Foaming if present in large amounts.

Deionization

Priming, or the passage of steam from a boiler in "belches", is caused by the concentration sodium carbonate, sodium sulphate, or sodium chloride in solution. Sodium sulphate is found in many waters in the USA, and in waters where calcium or magnesium is precipitated with soda ash.

Iron (Fe) and
Manganese (Mn)

Deposits in boiler, in large amounts can inhibit heat transfer.

Aeration, filtration, ion exchange.

Most common form is ferrous bicarbonate.

Silica (Si)

Hard scale in boilers and cooling systems: turbine blade deposits.

Deionization; lime soda process, hot-lime-zeolite treatment.

Silica combines with many elements to produce silicates. Silicates form very tenacious deposits in boiler tubing. Very difficult to remove, often only by flourodic acids. Most critical consideration is volatile carryover to turbine components.

Source: http://energyconcepts.tripod.com/energyconcepts/water_treatment.htm

The principal difficulties caused by water in boiler are:

References
Water treatment handbook Vol. 1-2, Degremont, 1991
Industrial water conditioning’, BeltsDearborn, 1991
http://www.thermidaire.on.ca/boiler-feed.html







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