-Boiler Water Chemistry
Deposits in Boilers C . JACKLIN National Aluminate Co., Chicago, 111.
A
S SIZE, pressure, temperature, and heat transfer rates of
steam generators continue to increase, the emphasis grows on longer operation of unit's without internal deposition and subsequent service interruption. The general trend in many older established industries is to replace several older, small boilers with a single high capacity boiler equipped with fully automat'ic controls to reduce labor costs and maintain high operating efficiency at, all times. In many new plants that require large amounts of process stearn: the boilers are designed to produce steam for both power generation and process needs. In the utility field the trend is toward single boiler, single turbine units operating a t high pressures and steam generating rates. It is evident from these trends that water conditioning must be properly planned and carefully controlled to keep these boilers free from deposits for long operating periods. For it is only by operati g t,hese units for long periods between scheduled outages that the designed savings can I)e obtained. For purposes of discussion, waterside deposits in boilers can be divided into three general groups based on the quality of feed water entering the boiler. 1. Partial or no external treatment of feed water supply 2. Efficient external treatment or evaporation 3. Very low make-up units where the major source of deposits is from internal corrosion of the condensate system and the boiler itself.
insoluble form, foster rapid ci ystal growth of the precipitate, :rnd minimize the number of colloidal or seinicolloidal particles present when the boiler water passes over the heat transfer surfaces. Finally, these crystals must be conditioned so they remain suspended in the boiler water as R flocculent and free flowing sludge. Unconditioned or improperly conditioned sludges tend to collect in locations rchere circulation iates are low and form packed IaTers of deposit on metal surfaces whirh may interfere with circulation and heat transfer.
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DEPOSIT T H I C K N E88 - I N C H E S
Figure 2.
Increase in Tube Metal Temperature Caused by Different Deposits
Heat transfer, 280,000 B.t.u./sq. ft. /hour; pressure, 1500 1b.lsq. inch
Figure 1. Partly Collapsed Tube That Failed While Analcite Scale Was Forming Tube was sectioned to determine thermocouple position
Different types of deposits are found in each of these groups and different treatments are required for their specific prevention. INTERNAL TREATMEXT OF BOILER WATER
I n the first group where feed waters receive no external treatment or only partial treatment, large amounts of potential scale and sludge forming materials enter the boiler. The problem in this group is to inhibit the precipitation of these materials while they are passing through feed water pumps and lines and then precipitate them in the main body of the boiler water before they reach a heat transfer surface. At this last mentioned stage it is important t o precipitate these scale forming materials in an
May 1954
A similar condition of coating or packing of suspended sludge may occur on normally active heat transfer surfaces during periods of lor3 load or banking of the boiler. If these solids have not been properly conditioned, they will remain in place when evaporation rates are increased and form a &mly adherent deposit. Repeated build-up of layers of deposit on heat transfer surfaces of this type can cause severe overheating and failure of boiler elements. Continuing research and testing programs have led to improved organic materials for conditioning these sludges to keep them fluid and free flowing and still not interfere with the importaht functions of precipitation and crystal growth. The new developments in this field have been confined to refinements and careful control of the details of existing treatments rather than the introduction of radically new or different materials and methods. EXTERNAL TREAT.1IENT OF FEED WATER
The next group of deposits is found in those installations in which the deposit forming solids in the feed water are reduced to a low figure by efficient external softening processes or evapora-
INDUSTRIAL AND ENGINEERING CHEMISTRY
989
tion before the feed water enters the boiler. The capital investment' in these installations is high and the steam is often used for producing power and in large scale continuous processes w-hich require uninterrupted steam supplies. The prevent'ion of deposits on metal surfaces in these units is especially iniport'ant since high availability and reliability are essential t'o justify the capital investment, and to avoid costly shutdowns of production equipment. The chemical requirements of these units are considerably smaller than the first group, but they are more exacting. Boiler pressures and heat transfer rates are generally higher: and periods between scheduled shutdoms are longer. The concentration of potential deposit and sludge forming solids present in the feed water entering these boilers is generallv quite low. However, the actual pounds of solids entering a boiler will in time add up t o a troublesome deposit if t,hey are not properly treated, since these boilers usually evaporate large quantities of feed water. Internal treat,ment to prevent scale deposits in t,hese boilers must do two things. I t must rapidly precipitate. t o an insoluble form, the small amounts of hardness which enter the boiler, and it must condition these nreciDitated sludges to make them nonadherent. Various phosphates are used for precipitation. conditioning o f these phosphat,e sludges i? accomplished by the use of organic dispersant? which have been specially processed t'o improve their stability a t the high temperature in the boiler water. The int,roduction of electronic induction heat'ing as a method of applying heat to experimental boilers has made possihle precisely controlled laboratory tests of organic dispersants under presssure and heat, transfer conditions which closely simulate artual hoiler conditions. As a result of these tests, processing conditions have been modified t o obtain dispersants having greater effectiveness and stability. The limiting conditions of pressure and heat) transfer which these dispersants will n-ithstand have also been det ermincd. The effect, of different, scale deposits on tube m e h l tcmpera-
tures at, different pressurrs and heat transfer rates was also studied recently in an induction heated experimental boiler. Heat t'ransfer coefficients of the four deposit8 tested were found to increase in the order of analcite, magnesium phosphate, magnetic iron oxide, and calcium phosphate (Figure 2). During this serieii of tests, an actual tube metal failure due t,o overheating caused by analcite scale format,ion was produced in the laboratory (Figure 1). CONDENSATE ALKALINITY
I n the third group of boilers where make-up and leakage t o t'he system are very low, iron and copper picked up from the turbine and condemate &em represent a major portion of the deposit forming materials entering the boiler. In some instances organic dispersants have been effective for keeping these materials suspended in t,he boiler water so they could be removed through the hlowdorm system. The inore recent approach to t,he solution of this problem is to add volatile ainines to raise the p€I of the condensate in all parts of t,he system. Since iron pickup hp pure -rater is at a minimum when the pH is about 9.0, it is both pract~icaland economical t,o control the amount of iron pickup by this mct,hod. The exact pK which should be carried depends on several factors and should be determined by careful evaluation of these factors for each individual system. Recent experimental st'udies of these volatile amines have shown that, at least one of them may be relat,ively stable at pressures up t o 2500 pounds per square inch and steam temperatures up to 1200' F. This means that an effective method of controlling iron pickup is available to practically all steam generating plants in operation today. Improvements in the conditioning of feed water by modern external treating met'hods have not been t8he final answer to wat,er conditioning. Each of these improvements has emphasized the need for careful attention to the entire water-steam cycle to ensure that the problems are not eliminat'ed from one spot, only to be transferred to another.
Corrosion in the Boiler R . F. ANDRES Dayton Power and L i g h t Co., D a y t o n , Ohio
scuision piesents a resume of some recent theoiies
TH and dl I developments S in the field of chemical treatment of boiler wateis for corrosion control. The chemical reactions of iron in aqueous media are both complex and controversial For this papcr the electrochemical explanation for corrosion of iron in water based on the following general equation is used. 3Fe
+ 4H2O .+ FesOa -+ 4H2
>himixing corrosion on boiler metal surfaces is still pi edicated on controlling this reaction through the formation and maintcnaiire of a continuous adherent protective magnetic iron oyide film of minimum thickness. This protective film formation is quite rapid i f the water in contact with the metal surface is a k a line and free of disqolvcd oxygen. Coriosion OCCUIY if, and on11 if, the protective film is destroyed ( 9 ) . Any disruption of the continuity of this protective film either by mechanical means or chemical rcnriion tends to promote corrosion. Alechaniral de990
stiuction of the film may result from erosion, tcrnperature changes, stresses, etc., and chemical destruction of hlms may result fiom action of dissolved oxygen, carbon dioxide, galvanic coi iosion. excessive caustic concentration, and reaction of iron nith supelheated Jtearn. The solutions to the problems of chemical film destruction present a very real challenge to chemists and engineers involved in preventing internal boiler corrosion It has long been recognized that an1 chemical treatment that tends to leave boilei water osygrn free, maintains proper alkalinity in rontacr with all wetted surfaces, and reduces or eliminates sludge and scale deposits is a nisterial aid in reducing internal boiler coirosion. Troublesome boiler deposits may be formed from thrre distinct sources-salts noniinally present or added t o boileI reed water, inetals and metallic oxides resulting from corrosion and Prosion in the preboiler cycle, and metal oxidee formed intrmdllq in the hoiler from corrosion of boiler metal. The prevention and elimination of sealrs and sludges formed from
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. S
