BLOW DOWN. PURPOSE: To control the concentration of one or more natural components of boiler water. * * * * * * * * * * * * *

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1 BLOW DOWN PURPOSE: To control the concentration of one or more natural components of boiler water. * * * * * * * * * * * * * Boiler feed water usually contains dissolved or suspended materials introduced with raw or treated make-up water or picked up by condensate returns. These solids concentrate as evaporation proceeds in the boiler. Unless something is done to keep the boiler water solids from concentrating excessively, serious difficulties may result. The boiler water may take on a foaming character which causes water to be entrained in the out going steam; this is the most immediate effect of excessive solids concentrations. Sludge may build up abnormally to such an extent that it interferes with the heat transfer, hampers circulation, or necessitates an undue amount of boiler cleaning. Boiler water silica, if allowed to concentrate without restraint, may participate in formation of siliceous scale, or it may leave the boiler as an entrained or volatilized contaminant in the steam, especially in high pressure boilers. Blow down represents the most direct -- often the only-- means for effectively controlling the concentration of solids in boiler water. When properly controlled, blow down removes solids in the concentrated boiler water solution at exactly the same rate with which they are introduced. Blow down is regulated to avoid exceeding some specified maximum concentration of either total dissolved solids, iron silica or alkalinity. suspended solids, Although the permissible maximum concentrations are lower with increase in boiler pressures, there actually is no simple rule for judging what the limiting concentrations should be in any given boiler. It depends upon steam pressure, steam temperature (whether saturated or super heated), the type of boiler involved, steam purification facilities in the steam drum, method of blow down (whether manual or continuous), water level control, the nature of fluctuations in boiler load, the purposes for which steam is used, and the kind of turbines involved, if any -- plus, of course, the kinds and the amounts of impurities in the boiler feed water. 1

2 As a guide for blow down control in your particular boilers, maximum and minimum concentrations have been recommended for one or more components of the boiler water. You may be controlling blow down directly on the basis of the test for boiler water alkalinity, silica, or solids (dissolved, suspended, or total). Or you may be testing for some reference constituent such as chloride, sulfate, or total electrolytes (conductivity); by regulating blow down so as to keep the reference constituent within some specified concentration range, you automatically maintain proper control over other constituents in the boiler water (dissolved solids, suspended solids, total solids, or silica, whichever the case may be. In any case, the blow down rate must be increased whenever the maximum limiting concentration is approached or exceeded. This may sometimes call for emergency manual blow downs, for operating with concentrations that are to high can be seriously detrimental to boiler and plant performance. Minimum limiting concentrations are specified to avoid undue wastage of water, heat and treating chemicals through blow down. In general, the range of permissible concentrations is fixed at the highest level considered to be safe for the particular boilers involved. Antifoam materials are sometimes used to allow an increase towards the tolerance for higher solids concentrations. This allows a reduction in blow down losses. Antifoam treatment is economically justified for this purpose if the cost of such treatment does not off set the resulting savings in blow down. The risk of serious consequences, however, can not too great if there is a sudden and unavoidable interruption in antifoam feed. Blow down losses can be reduced, also, by the use of pre-treating equipment to remove raw water constituents which otherwise would have to be removed through boiler blow down. This is a very important economic consideration in the selection of pre-treatment equipment. Good blow down control is one of the most important requirements in boiler water conditioning. If blow down is not regulated properly, it will be almost impossible to maintain satisfactory control over the chemical treatment program. Blow down has a direct influence over the concentrations of all constituents in the boiler water. Take particular pains to stay within the limiting concentrations specified for blow down. A continuous blow down valve is the preferred method for the control of boiler water component build up. This will gradually remove boiler water on a continuous basis, thus preventing the radical change in boiler water chemistry as seen during excessive manual bottom blow downs. Manual bottom (sludge) blow downs are still required to remove the heavy sludge, even with continuous blow down systems. Boiler makers usually provide instructions covering manual blow down from drums and headers. 2

3 Heavy manual blow downs should be avoided where practical. Mud drum connections should be "tried" at least every 24 hours. It is generally poor practice to blow down water wall headers while the boiler is in full load service. Continuous blow down represents the most effective means for controlling boiler water solids concentrations. It: Provides ideal boiler water samples for testing and control purposes. Permits close regulation of blow down without wide fluctuations in concentration. Makes possible the use of blow down heat recovery equipment--an important economic factor in many plants. 3

4 ALKALINITY CONTROL PURPOSE: To aid in formation of desirable sludge and to maintain a protective coating of iron oxide on steel surfaces. * * * * * * * * * * * * * The total alkalinity of boiler water includes all dissolved components that are capable of reacting with acid: These are most commonly hydroxide (OH),carbonate (CO), bicarbonate (HCO), phosphate (PO), and silicate (Sio). Among these alkaline components, the hydroxide is most important from a chemical control standpoint; it is measured directly by ph determination or by titration. Boiler water must contain a certain amount of caustic (i.e. Hydroxide alkalinity in order to accomplish three things: 1. To maintain a protective coating of iron oxide over the metal and thereby prevent certain types of corrosion. 2. To provide a proper environment for precipitation of desirable sludge materials. 3. To keep silica in solution so as to avoid deposition of siliceous scales. Excessively high concentrations of hydroxide alkalinity may result in foaming difficulties or caustic corrosion of the boiler steel. Caustic alkalinity may also cause caustic embrittlement of boiler steel under some conditions. Alkalinity control is correct when the boiler water contains enough hydroxide to secure the desired benefits without inviting trouble associated with aggressively high concentrations of caustic. The P alkalinity test provides a sound basis for control of the alkalinity in boiler water. ph determination is also widely used for this purpose where a low level alkalinity is to be maintained. Hydroxide alkalinity in boiler water may come from a variety of sources. It may be added directly in the form of caustic soda (sodium hydroxide). When soda ash is fed as a water conditioning chemical, it 4

5 decomposes under boiler steaming conditions, producing sodium hydroxide (plus carbon dioxide which goes off with the steam). Sodium hydroxide and/or sodium carbonate may be present as excess treating chemicals in the effluent from a lime-soda softener; the use of such pre-treating equipment automatically provides the alkali required for alkalinity control in the boiler water. Most raw waters contain bicarbonate (HCO). So do effluent waters from sodium cycle and from most sodium-acid cycle zeolite softeners. Under boiler water conditions, bicarbonate decomposes first to carbonate and then to hydroxide. In all cases it is the boiler water alkalinity, as measured by the p alkalinity test or ph value, that concerns us, regardless of whether this hydroxide is added as such or represents the decomposition product of soda ash or alkaline components in raw water or treated makeup water. Maximum and minimum P alkalinity values (and/or ph) limits are specified for proper control of boiler water alkalinity in your plant. Alkalinity concentrations below the specified minimum may result in corrosion difficulties or the formation of objectionable sludge or scale. Whenever boiler water alkalinities fall below the recommended minimum. do not hesitate to increase the feed of alkali (caustic soda or soda ash) or to increase the total alkalinity of the treated water if a softener is involved. Alkalinities above the specified maximum may result in foam-type carry over or caustic corrosion, and they may complicate protective measures employed to prevent caustic embrittlement. When boiler water alkalinities rise above the recommended maximum, reduce the feed of alkali (caustic soda or soda ash) or reduce the total alkalinity of the treated water if a softener is used. In an emergency, increase the blow down rate if necessary to bring boiler water alkalinity under control. Sodium hydroxide is the preferred approach, as the break down components do not form carbon dioxide. Carbon dioxide will carry off the steam and form carbonic acid in the condensate return lines. For this reason, sodium carbonate is not generally used in todays boilers. It is never recommended for use by good water treatment consultants. 5

6 ANTIFOAM MATERIALS Purpose: To minimize the amount of solids carried by the steam * * * * * * * * * * * * * Many different factors - some chemical, some mechanical -- entrained boiler water in the steam leaving the boiler. "Foaming" is a general term applied to carryover, which is caused by objectionable concentrations of certain components of the boiler water. Foam aggravating substances such as excessive concentration of mineral solids, high alkalinity, or some types of organics) steam bubbles do not allow steam bubbles to collapse or coalesce as rapidly as they should. They rather collect as a relatively stable layer of foam on the surface of the water. When this foam layer becomes thick enough to invade the active steam separation section of the boiler, masses of foam are sucked off with the steam and carryover results. Antifoam agents (polyalkylene glycol) are used specifically to speed up the coalescence of steam bubbles and thereby minimize the tendency for foam to accumulate in any part of the boiler.antifoam treatment is often effective in preventing carryover caused by uncontrollable feed water contamination or by high concentrations of boiler water solids, which for physical or economic reasons, can not be adequately controlled by normal blowdown. Even when no foaming problem exists, the cost of antifoam treatment can be justified because of savings resulting from reduced blowdown and the maintenance of higher boiler water solids. Under certain operating conditions, some types of boilers may have a tendency to allow steam bubbles to collect in downcomer tubes and thereby cancel part of the hydrostatic head required for proper circulation of water through the boiler. Antifoam treatment may improve circulation in such cases. An antifoam may be beneficial by producing a higher quality boiler water that is ideal for uniform steaming and by encouraging steam bubbles to coalesce with the greatest possible speed. It also can minimize the carryover effects of mechanical or operating factors for which there is no immediately practical or economic remedy. Of course, the best approach to problems of this nature is through physical changes, which correct the difficulty at its source. 6

7 Whenever possible, antifoam should be fed continuously, although shot feeding is permissible if additions are made at frequent intervals. Antifoam can be fed in solution along with other chemicals used for internal treatment of boiler water. Avoid over-feed of antifoam in that gross over-treatment may actually cause boiler water to foam. Another consideration is that starting with a full boiler antifoam treatment program to boilers that contain foam may result in low water because of to the sudden coalescence of steam bubbles in certain banks of tubes. When the application is to be intermittent, feed about one-fourth of the required amount durng the first hour. Follow this with one-third the second hour, one-half the third hour, three-fourths the fourth hour and the full treatment requirement for each hour there after. Unlike most other water conditioning chemicals, antifoam loses its effectiveness rather rapidly under boiler steaming conditions, therefore, feed antifoam continuously or in shots at frequent intervals. Take special precautions to assure that the antifoam treatment is not interrupted once it gets under way. 7