Boiler Water Chemistry
Significance of Boiler Deposit Analysis FRANK E. CLARKE AND ROBERT D. HOPKINS U . S. ~WavalEngineering Experiment Station, Annapolis, .Md. water-formed deposits are common causes of overheat failures i n boiler tubes. Solution of deposit problems requires accurate analysis and proper interpretation of the data. The simplest deposit may be difficult to analyze, and complex boiler deposits, such as residues of fuel oil, lubricant, or rust preventive compounds frequently must be extracted and identified. Precipitation of barium and tin compounds with silica or phosphate compounds with iron can paint imaginary pictures of deposits that never existed. Examination of microstructure is an important step in thorough analysis as sources and structures of deposits can vary markedly even though their chemical compositions are almost identical. The analytical scheme should be versatile enough to cope with the most difficult cases, yet flexible enough to handle the simple cases efficiently.
ATERSIDE deposits are a common cause of boiler tube deformities like that shown in Figure 1. The boiler operator looks to the laboratory for diagnosis and correction of such problems. Thorough analysis is an important first step in fulfilling this obligation. Corrective action based on inaccurate or incomplete findings can be as hopeless as medical aid prescribed on false symptoms. Versatility and flexibility are essential characteristics of the thorough analytical scheme. SIMPLE DEPOSITS CAN BE PUZZLING
The difficulty of analytical interpretations cannot always be judged from the number of components involved. Figure 2 is a good example of this fact. I t shows a heavy deposit of almost pure calcium hydroxide (hydrated lime) scale, which caused a fire-row boiler tube to overheat and burst. This unusual, relatively soluble deposit was detected in preliminary spectrographic, x-ray diffraction solubility, and pH tests and thereafter was confirmed by wet chemical analysis. The results led to discovery of lime in the feed water and location of a freshly cemented feed water tank as the source of contamination. Needless to say, routine water testing and treatment had been neglected in this case. In a less thorough analysis, including ignition loss and tests for principal metals and radicals this deposit might have been mistaken for calcium carbonate and the real source of contamination overlooked completely. MISSING COMPONENTS ARE IMPORTANT
~h~ bottle in ~i~~~~ originally contained a tarlike, mud drum sludge which could be removed only by cutting the bottle in two. On first examination, this deposit appeared to be a decomposed product, and it came from a naval boiler particularly susceptible to lubricant contamination. Surprisingly enough, schematic analysis showed it to be a mixture of hydrated ferrous phosphate and iron oxides, which dried to the granular mass shown in the figure. A small amount of supernatant liquid separated from the sludge was relatively acidic and contained high concentrations of sodium chloride and ferrous chloride. Other sea salts, which should have accompanied the sodium chloride, were conspicuously abwnt in the supernatant liquid, as were normal alkali phosphate sludges in the solid phase. This absence of sea salts and normal sludges suggested contamination of the boiler with hydrochloric acid or ferrous chloride. Investigation shoa-ed that the boiler feed water system recently had May 1954
Figure 1. Effect of Waterside Figure 2. Simple Lime Deposit, 2-Inch Tube Scale, 2-Inch Tube
been cleaned with muriatic acid and that flushing had not removed the residual acid completely. Lacking the rather thorough analysis Of the Supernatant liquid and Curiosity about the missing components, this sludge might well have been reported as an iron phosphate-iron oxide mixture without uncovering the
cause
Of
COMPLEX DEPOSITS TEST THE SCHEME
Although unusual simple deposits may test the imagination of the analyst, mixed deposits are the best tests of analytical sequence and techniques. Figure 4 shows a heavy, complex, baked sludge which caused a division wall, boiler tube to fail. This occurred in a reciprocating engine plant equipped with a feed water filter to remove engine lubricant from the condensate. These filters are precoated with a mixture of diatomaceous earth and ferric hydroxide. The latter is prepared by mixing equivalent quantities of barium hydroxide and ferrous sulfate. Failure to
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Figure 3.
Acid Cleaning Sludge (FeP04 X €I@)
obtain chemical equivalence in the mixing will result in contaminating the feed water with the excess chemical. I n addition, the filtering earth generally contain3 a significant amount of soluble aluminum or calcium compounds.
TABLE
1.
C O ~ P O S I T I O SOF COMPLEX
DEPOSIT Per Cent 23
13
12
9 9 8
4 4
2 2 2 9 a
Figure 4. Complex Deposit (Table I)
The probable components of the deposit, as determined from a thorough analysis, are shown in Table I. Some analytical pitfalls are readily apparent. For example, fuming v, ith perchloric acid to dehydrate and separate silica also would precipitate equivalent quantities of barium and sulfa,te as barium sulfate. Failure to treat the resulting silica residue x i t h hydrofluoric acid or t o analyze it othervise would result in an apparent high silica content and loss of the barium and sulfate. Calcium would be reported as phosphate instead of sulfate. To further complicate B might be separated n i t h the analytical picture, an R ~ O group ammonia, without first removing the phosphate. This Tyould include all the iron as either hydroxide or phosphate, as well as considerable amounts of aluminum, calcium, excess phosphate, and excess barium, depending on the p H and the balance of ions. The serious effects of these short cuts can be seen by comparing the major constituents in Table 11. The ‘(thorough scheme” would lead to the proper conclusion that barium hydroxide from the filter coating had leaked into the boiler and consumed the scale preventing phosphate. It would suggest the appropriate corrective measures of reducing the proportion of barium hydroxide to ferrous sulfate in the filter coating and of emphasizing
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Presumably present as complex compounds.
more care in the boiler water hardness test. 011the other hand, the (‘general scheme” Tvould paint an imaginary picture of heavy silicate and phosphate deposits. This could lead to the faulty conclusions that silicate-bearing shore water had been used, that
Figure 5 .
Oil Sludge Balls, 1’/2 Inch
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Boiler Water Chemistry diatomaceous earth filter aid had leaked into the boiler, or that blowdown had been inadequate. While this general analysis admittedly represents a radical departure from thorough analysis, near misses by fairly good schemes can be equally u n d e sirable.
TABLE11. ANALYTICAL VARIATIOX-COMPLEX DEPOSIT
.
Component Baa(PO4) z CaSOa AlzOa Si02 Caa(PO4)z FeaOa
Analyses, % Thorough scheme General scheme 23 0 13 0 12 9 9
8
RaOa
Q
On
31 19 0 24
0
A t proper pH.
PHYSICAL EXAMINATION COMPLETES THE PICTURE
No analytical scheme is completely satisfactory unless it provides for examination of the deposit structure. Many compounds can accumulate as either sludge or scale, depending on the operating condition. For example, siliceous material may enter the boiler as dirt and deDosit as a sedimentarv sand sludge, or it mav enter as soluble silicate and deposit as stubborn silicate scale. In I ,
Figure 6.
Organic Debris, Water Drum
Mixtures of organics and inorganics further complicate the analytical scheme. Sludge balls like those shown in Figure 5 are common results of such mixtures. The sticky material which bonds the ball may be rust preventive compound, paint residue, fuel oil, or lubricant. Such mixtures of adhesives and boiler sludges grow like snowballs if the turbulence in the water drum is just right. Khile the balls themselves rarely cause serious trouble, their presence is a signal of organic matter which may be dangerous otherwise. Identification of the organic requires extraction with various solvents and determination of organic properties such as saponification number, iodine number, specific gravity, and infrared spectrum. I n many cases these searches are rewarded only with disappointment, since distinguishing characteristics can be destroyed by boiler water conditions.
Figure 7.
Lime Scale Structure, 10 x
Figure 8.
Hard Scale Structure (CaSO,), 1OX
ANALYTICAL DEMANDS KNOW NO BOUNDS
Some deposits present analytical problems that are next to impossible to solve. The porous mass shown in Figure 6 came from the water drum of an aircraft-carrier boiler. It was completely organic but resembled nothing that should have been found in a boiler. Organic identification tests suggested synthetic rubber. Investigation of this lead showed that the goggles worn during the boiler overhaul were heavily rimmed with sponge rubber. Gloves and other items used by boiler repairmen can decompose to equally puzzling deposits and they can cause serioue failures. May 1954
Figure 9. . Simple Sludge Structure Cas(PO&, 10 X
Figure 10. Complex Sludge Structure, 1OX
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hydroxide sludges, and a heavy layer of calcium sulfate scale. The presence of calcium hydroxide in the intermediate layer shous that the boiler water was contaminated with lime, as in the earlier example. This caused all of the phosphates to precipitate aE calcium phosphate. When the phosphate ieserve was gone, calcium sulfate formed on the top of the deposit. The iron oxide layer next to the tube was the last t o form and resulted from overheat effects brought on by the other deposits. This history would have been considerably harder to reconstruct without information on the physical structure. Information of thiq type ran be eecured with a simple tripod microscope. THE REACH MAY EXCEED THE GRASP
Kot all identifiable deposit8 can be explained to the satisfaction of the operator. The heavy lead sulfide deposit in the superheater tube of Figure 12 presented no particular analytical problems, even though it was an unusual and uneypected deposit. Explanation was a diff erent matter. Keither contamination of the boiler Jvater with this chemical nor carry-over of the chemical
Figure 11.
Stratified Deposit
the first case, blon-dovn is the answer, n.hile in the second ca'e chemical treatment of the feed wat,er is a t fault. The microstructure of the deposit holds the rlue to it,s method of formation. True scales form from the water directly in place on the heated metal surfaces. They generally consist of columnar crystals growing at' right angles to the surface, as shown in Figures 7 and 8. Scale need not be hard as commonly supposed. The one in Figure 7 was the soft calcium hydroxide scale previously referred to. The one in Figure 8 was a hard calcium sulfate scale. I n contrast to this structure, Figure 9 shous a homogeneous sedimentary sludge of reaction products. Figure 10 is a complex sludge mixture of calcium carbonate, magnesium hydroxide, iron oxide, and silica. Crystalline structure is conspicuously absent in both cases. Structure studies of the type referred to above avoid errors of interpretation that may result from composite sampling. Figure I1 illustrates this point. This deposit contained in order from the bottom up, a thin compact layer of high temperature iron oxide, a thick mixture of magnesium phosphate and calcium
Figure 12. Puzzling Superheater Deposit (PbS)
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Figure 13. Common Dissolved Oxygen Scab, 2-Inch Tuhe
into the superheater should have occurred. It is difficult to explain how such a deposit could have accuniulnt,ed during const'ruction of the superheater. COR'CLUSION
These examples are not meant to imply that a complex analytical scheme must be used on every boiler deposit or even that every deposit need be analyzed. FeiT lahoratoiies would bother to analyze dissolved oxygen corrosion scabs, like that shown in Figure 13, because experience has shown consistently that they have about the same composition. I n many routine or semiroutine analyses, experience also may show that it is necessary t o determine only the few pertinent or major constituents. These facts do not lessen the problems involved in analysis of the relatively large number of complicated "unknown" samnles. The nrudent analyst will eauir, himself with
Xavy Department nor of t h e S a v a l x i v i c e at large.
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