Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
BFW Systems - BFW Treatment
February 27, 2005
Total Dissolved Solids (TDS)
Coordinated Phosphate Programs
Miscellaneous Treatment Methods
The pressure and design of a boiler determine the quality of water it requires for steam generation. Municipal or plant water of good enough quality for domestic use is seldom good enough for boiler feed water. Makeup water to a boiler is nearly always treated to reduce contaminants to acceptable levels. In addition, corrective chemicals are added to treated water to counteract any adverse effects of the remaining trace contaminants. The sequence of treatment depends on the type and concentration of contaminants found in the water supply and the desired quality of the finished water to avoid the three major boiler system problems - deposit, corrosion and carryover.
The design of a water treatment system is left to the experts, however, a basic understanding of the methods and processes is required to properly evaluate bids and evaluate existing water treatment programs.
Deposits, particularly scale, can form on water tubes as the equilibrium conditions in the water are upset by external forces such as heat. If water is is contact with a hot surface and the solubility of the contaminant is lower at higher temperatures, the contaminant will precipitate from the water to form scale on the hot surface. The most common components of boiler deposits are calcium phosphate, calcium carbonate, magnesium hydroxide, magnesium silicate, various forms of iron oxide, silica absorbed on the previous mentioned precipitates and alumina.
Deposits are a serious problem because they reduce heat transfer which can lead to boiler tube failure. The maximum temperature for the boiler tubes in generally in the range of 480oC to 540oC. Water circulating through the tubes normally conducts heat away from the tubes preventing the metal temperature from exceeding the maximum allowable temperature. The insulating effect of the layer of deposit reduces heat transfer and raises the metal temperature above the point at which failure can occur.
The most common form of corrosion is due to attack of steel by oxygen. Oxygen attack is accelerated by high temperature and by low pH. Another form of corrosion is alkali attack which can occur where caustic is concentrated. Direct attack by steam on the boiler metal can occur at elevated temperatures according to the following reaction:
4H2O + 3 Fe -> Fe3O4 + 4 H2
This type of corrosion can occur in areas of restricted water flow where the entire metal surface is blanketed with steam. Monitoring the levels of hydrogen in the steam is a simple method of detecting this form of corrosion.
Carryover is generally a mechanical problem caused by an upset in boiler
operation, broken baffle or mist eliminator. Carryover may also be caused
by the volatility of certain boiler water salts, such as silica and sodium
compounds. Foaming may also result in carryover from the boiler to
the steam system.
There are 3 basic means of treatment for the control of deposits, corrosion and carryover.
a) External Treatment - Treatment of water - makeup, condensate or both before it enters the boiler, to reduce or eliminate chemicals (such as hardness or silica), gases or solids.
b) Internal Treatment - Treatment of the boiler feed water, boiler water, steam or condensate with corrective chemicals.
c) Blowdown - Control of the concentration of chemicals in the boiler water by bleeding off a portion of the water from the boiler.
The following unit operations are used alone or in combinations with others to treat water supplies:
a) Direct Addition
c) Solids/Liquid Separation
f) Ion Exchange
i) Membrane Separation
These 9 unit operations are used to treat water in 7 broad categories of impurities:
i) Suspended Solids
v) Total Dissolved Solids (TDS)
vi) Organic Matter
Suspended solids are generally treated by filtration, precipitation or coagulation/flocculation. Removal of suspended solids is usually the first step required because other treatment methods require low levels of suspended solids. For example, water to be processed by ion exchange requires less than 10 mg/l of suspended solids to prevent fouling of the exchanger beds.
The treatment of hardness also known as water softening is the removal of calcium and magnesium from water. The removal of hardness is accomplished by precipitation (partial removal), ion exchange, and to a lesser degree by evaporation, degasification and membrane separation.
Some water softening processes are sodium ion exchange, split-stream, partial lime (cold) and hot lime-zeolite. Some methods like sodium ion exchange only treat hardness while others like hot lime-zeolite also are effective in reducing alkalinity, silica and TDS.
It is desirable to have some alkalinity in boiler water, so complete removal of alkalinity is not required except in demineralization. Some alkalinity is also needed to provide optimum pH in the feed water to prevent corrosion of piping and equipment. Alkalinity may be present in the bicarbonate (HCO3-), carbonate (CO3-) or hydroxide (OH-) forms. The bicarbonate form will be prevalent if the water is zeolite softened and in the carbonate form if lime softened.
The control of alkalinity is accomplished by direct addition, coagulation/flocculation, precipitation (partial removal), ion exchange, evaporation and membrane separation.
Through a series of reactions, bicarbonate and carbonates breakdown at boiler operating temperatures to form CO2. The CO2 leaves the boiler with the steam and redissolves when the steam condenses to produce carbonic acid. This can be controlled by chemical addition to the boiler or directly into the steam to control the condensate pH in the range of 8.5 to 9.0.
Coagulation/flocculation, solids/liquid separation, precipitation and adsorption all provide partial removal of silica. Ion exchange, evaporation and membrane separation and also used to remove silica. The degree to which silica is reduced depends on the end use of the steam. If there is a condensing turbine, the low levels of silica (<0.05 mg/l) achieved by demineralization is required.
The methods used to control TDS are: precipitation, ion exchange, evaporation and membrane separation. Direct addition methods may actually increase the level of TDS in the water
Sodium zeolite softening increases TDS by exchanging sodium ions for the lower equivalent weight calcium or magnesium ions.
Partial removal of organics is achieved by coagulation/flocculation. precipitation, adsorption, ion exchange, evaporation and membrane separation. Problems related to organic matter are generally the result of the formation of organic acids.
Removal of gases such as H2S, CO2 and O2 is accomplished by direct addition, precipitation, adsorption, degasification and membrane separation. The most common method of reducing gas concentrations in boiler feed water is the use of a deaerator which produces a CO2 free effluent and O2 concentrations in the range of 0.005 to 0.01 mg/l. If further reduction of O2 is required, an oxygen scavenger (volatile oxygen-reducing compounds) such as sulfite, hydrazine or hydrazine substitutes can be added to completely eliminate O2.
Scale formation within a boiler is controlled by one of four chemical programs: coagulation (carbonate), phosphate residual, chelation or coordinated phosphate.
In this process, sodium carbonate, sodium hydroxide or both are added to the boiler water to supplement the alkalinity supplied by the makeup, which is not softened. the carbonate causes precipitation of calcium carbonate, magnesium hydroxide and magnesium silicate. This method is limited to boilers (usually firetube) using high-hardness feed water and operating below 250 psi. In addition, some form of sludge conditioner must be used and blowdown rates are high due to high suspended solids. Coagulation programs are slowly becoming obsolete because of high cost.
When operating pressures exceed 250 psi, the high concentration of sludge associated with coagulation programs is undesirable. For a phosphate program to be effective, the feed water should contain less than 60 mg/L of hardness.
In this process, sodium phosphate is fed into the BFW or directly into the steam drum. Insoluble precipitates are formed, primarily hydroxyapatite (Ca10(PO4)6(OH)2), magnesium hydroxide, magnesium silicate and calcium silicate.
These programs produce high suspended solids and often the addition of sludge conditioners/dispersants are required.
A chelate is a low molecular weight molecule, soluble in water and similar to an ion exchanger. The most common chelating agents are sodium salts of ethylene diamine tetraacetic acid (EDTA) and nitrilotriacetic acid (NTA). These chelating agents form complex ions with magnesium and calcium which are soluble. Since less sludge is produced, blowdown rates are reduced. Because of their high cost, chelate programs are limited to feed water containing low hardness. They are also limited to boiler operating pressures below 1500 psi.
Chelates can also react with oxygen in the water resulting in higher dosages to control hardness and hence higher operating costs.
When solids and free caustic alkalinity must be minimized, a coordinated phosphate program is used. This program differs from the regular phosphate program in that the phosphate is added to provide a controlled pH range in the boiler water as well as to react with calcium should hardness enter the boiler. Combinations of disodium phosphate with trisodium or monosodium phosphate are added to control pH without the presence of free OH-. To achieve this, the feed water must be extremely pure and of consistent quality. A dispersant must be added to treat deposits so they don't interfere with heat transfer. This program is ideal for high pressure, high heat transfer rate boilers.
Boiler feed water, regardless of the type of treatment program used still contains measurable concentrations of impurities. To maintain reliable boiler operation the concentration of each component of the boiler water must be limited to certain maximums. This is accomplished by blowing down water from the boiler.
agents, dispersants and oxygen scavengers are also added as part of an internal
treatment process to condition boiler water.
Anitfoaming agents, dispersants and oxygen scavengers are also added as part of an internal treatment process to condition boiler water.
A high percentage of carryover is caused by foaming in the steam drum. Antifoaming agents reduce foaming and help maintain steam purity.
Chemicals are used to condition boiler water particles so they do not form larger particles which can settle out and form scale and deposits. Dispersants keep the particles small so they remain suspended at the liquid velocities encountered in a boiler.
Deaeration reduces oxygen in the water to low levels but does not completely eliminate it. Application of sulfite, hydrazine or hydrazine-like compounds after deaeration , scavenges the remaining oxygen and maintains a reducing condition in the boiler water. Hydrazine has the advantage of leaving with the steam and is then available in the condensate to protect against oxygen corrosion in the condensate system.
The recovery of condensate saves costs in terms of energy, fresh makeup water and water treatment chemicals. The kind of treatment process required depends on what the steam has been used for and how the condensate has been handled. In acid plants, the condensate will generally be contaminated with corrosion products and in-leakage of hard water. High flow rate condensate 'polishers' (sodium ion exchangers) are often used to treat condensate prior to reusing the water.