X-Ray Diffraction
Studies of Steam Turbine Deposits LOHR A. BURKARDT a D CARROLL E. IMHOFF Allis-Chalmers Manufacturing Company, Milwaukee, Wis.
Deposits on Turbine Blading
T"
appearance of deposits in steam turT h e composition of a relatively large number of steam turbine debines leads t o serious reductions in turbine posits as determined by x-ray diffractionanalysis is reported. An effort capacity and efficiency. The nature of these dehas been made to relate the composition of turbine deposits with the posits, the niechanisms of their formation, and temperature at the point of deposition and with the pressure of the the means of avoiding their occurrence are probboiler supplying the turbine with steam. Certain compounds have lems of interest to all concerned with the operabeen found to occur in sequence in steam turbine deposits. tion of steam turbines. Soderberg (7) and Buckland ( 1) describe the appearance of deposits and their effect on turbine capacity and efficiency. Fitze and steam loses part of its contamination in passing through the turLong ( 2 ) recently published their experience with siliceous deposibine, producing the troublesome deposits. tion at 1300 pounds per square inch. They show that siliceous The phases found in turbine deposits are largely crystalline deposition on the turbine blades is a function of the concentration and are normally well crystallized; this would indicate that they of silica in the steam, but that the percentage of the silica carried had been formed from a state of solution. Straub (9) suggested by the steam that is deposited on the blades tends t o reach a that the contamination of steam may occur through a carryover of droplets of boiler water, prevented from evaporating maximum at 60%. They observed a direct relation between the silica concentration of the boiler water in the range of 1.5-14 completely by the presence of sodium hydroxide, so that the salts parts per million and the concentration of silica in the steam. carried by the droplets thus arrive at the turbine in a state of The concentration of silica in the steam was found to be in the liquid solution. The work of Spillner (8), Straub (9, IO),St,raub order of 1% of the silica concentration in the boiler water. This and Grabowski (If), and Norey (6) demonstrates the solubility of salts in steam at high pressure; this indicates another mechais in agreement with the results of Straub and Grabon-ski ( I I ) , who found the silica carry-over to be in the order of lq of the nism by which the steam may become contaminated. The silica concentration of the boiler water using a n esperiniental dissolved state may thus represent either solution in gaseous boiler. steam or in droplets entrained in or mechanically carried by the Efforts made to avoid the occurrence of turbine deposits steam. -4 steam turbine is a "refrigerator" of enormous capacity, and any solute-solvent system passing through it may urfdergo should he guided by knowledge of the mechanism of their formation, which is as yet not fully known. The study of the comtemperature changes in the order of 700" F. and pressure drops plete mechanism of turbine blade deposits may be divided into in the order of 1000 pounds per square inch in a fraction of a second. Such large and sudden changes in the temperaturetwo parts, ( a ) the manner in which the steam becomes contamipressure conditicns cause drastic changes in solubility relations oated with boiler \rater salts and ( b ) the manner in which the 1427
INDUSTRIAL AND ENGINEERING CHEMISTRY
1428
SODIUM SULFATE
III
SODIUM SULFATE
P
1
x
x
ACMITE
x
BURKEITE
x
x
x
L
x s
,
as
x a x
SODIUM METASILICATE
1 )I
X
x
x
SODIUM DlSlLlCATE
'
L
x
x
x
s
a x
x
~
QUARTZ
Vol. 39, No. 11 tains compounds such as calcium carbonate, calcium phosphate. and magnesium hydroxide. I n t h e authors' experience these eo nip o u n d s have been almost invariably a s s o c i a t e d Jvith oily materials. This suggests that the adherence of the5e compounds on turbine hlades depends upon a dhe siv e p r o p e r t i e s (vhich oil in the ~stettni impart-: t o the plitstic niiuture. Tho last group is made up of
x * I X X
x x x x x x x
C R I S TOBALLI T E
Z x
x
x x
x
)L
AMORPHOUS SILICA
x x xxv * x L X x x x x x
1
1 i xn x
s
1
1
,
$
z
,"
L
1
%
f
m I x
x
1
x %
L
XL
m.s
X l l
x
I L
'i
X
x
1
I
l
l
1
1
1
[
'
I
S
1
'
1
'
1
'
water-soluble and -iri~oluhlt5ilicates, and n-atcr-soluble salts such as sodium chloride :ind s o d i u m s u l f a t e . T h i s of deposits has been the major concern of those intvrested in turbine blade deposits; the deposits examined in this study have been limited t o this class. SOURCES OF s A w L I . : s
aiid t i i w \ v certain of the salts out of solutiuii. Sinre t l w wlvent is in intimate contact with the turbine surfaces, supersaturation is relieved by crystallization of the salts on t,he turbine surfaces. Identification of the solid phases and a correlation ,of these phases 17-ith temperature and composition of the steam contamination are necesary parts of the procedure in solving the b part of the problem on the complete mechanism of the formation of turbine deposits. This study was begun as a n attempt to establish a relation between the chemical species of a turbine blade deposit and the temperature at the point of occurrence of the deposit. Such a n effort necessarily involves the identification of the various components of turbine deposits. Goerke ( 3 ) made a similar study, rniploying chemical analysis as a means of identification of the several components. Chemical methods of identification niay be misleading, since they fail to distinguish such compounds as aquartz and amorphous silica, or will indicate the presence of sodium sulfate and sodium carbonate \Then in reality burkeite is present. I n this stud>-x-ray diffraction methods were employed for the identification of the chemical species. Later the scope of the present study n-as extended to seek evidence of a relation between the compounds and the pressure of the boiler producing rhe steam from rvhich the deposits formed. The study is now being continued to see if there is any relation between the compounds found in deposits and the average composition of the boiler water from which the steam was produced, and if there is any rrlation between the compounds found and the composition of the steam from which the deposits a e r e produced. CLASSES O F TURBINE DEPOSITS
Deposits occurring in steam turbines mal- be classified in the following fashion. (a)corrosion or errosion materials, ( b ) waterinsoldble compounds of calcium and magnesium, and (c) silica, silicates, and water-soluble compounds. Corrosion and erosion deposits consist primarily of metallic oxides and, occasionnlly, sulfides. Magnetite, Fe,04, and hematite, Fe201, predominate in this group. The second class con-
A\ total of 199 samples n-as examined in this study. Thtw were obtained from tn-enty-seven p o w r plants. Fourteen of these were utilities, and thirteen were industrial plants. Difficulty is experienced in obtaining complete sets of samples from individual stages of a turbine. Composite sanipltis have little value in this study, and samples from only badly fouled stages also give an incomplete picture. Frequently the amount of sample is small, less than 0.1 gram often being submitted. I n many instances this is necessarily so, since the amount of deposit found on a given stage may be very small. Such small samples do not pcrmit the number of tests xhich would often he desirable. TECHNIQUES
Identifications were made by the x-ray poxder diffraction method. Patterns were taken with unfiltered iron radiation, using circular cameras having a radius of 6.98 cni. Samples were mounted in open wedges and were protected t ~ ycoating with a methacrylate lacquer when thcy appeared liable to change during the time of exposure. The methaw?-late technique proved satisfactory with highly alkaline materials. C'hcniica) analyses ITere also made of a number the deposits. Marly of the analyses are unfortunately incomplete because of the lack of sufficient sample. In general, chemical analyses are unsatisfactory for identification purposes, since they give no evidence for the presence of complex compounds such as hurkeite, Sa2CO3.2Na2SO1,or acmite, S a r 0 , F e r O i 4 S i 0 1 ,and variations in composition from exact forinulas make positive identification difficult. For example, samples containing sodium disilicate frequently show marked variation in the determined sodium oxide-silica ratio. Tablc I gives results obtained by chemical analysis of a number of turbine deposits sho1vn to contain sodium disilicate by x-ray diffraction patterns. The most serious difficulty n.ith x-ray techniques is the lack of diffraction data. Tvio courses are being follon.ed t o mert this difficulty; first, diffraction data are being accuinulnrcd on all compounds that might possibly occur in steam turbines, either using natural minerals or by syntliesizing compounds, and. second,
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
1429
chemical analyses of turbine deposits TABLE I. P.iRTIAL CHEMICAL -kNALYSES O F DEPOSITS COST.4ISIh.G are being examined for suggestions as SODICVDISILICATE to Dossible comoounds. and then at% Equiv. % E q u i r . % Equiv. % Equiv. % Equiv. % Equiv. tempts are 'Ieing made to synthesize the Sodium oxide 25.4 0.82 24.8 0 . 8 0 21.5 0.69 25.7 0.83 10.9 0.35 29.8 0.96 suggested compounds, obtaining from (alkalinity) Total silica 4 9 . 7 1 66 5 7 . 9 1 . 9 3 4 6 . 7 1 . 5 5 4 7 . 5 1 . 5 8 2 2 . 1 0 . 7 4 49 7 1 . 6 6 them x-ray diffraction data which n-ill Soluble silica 2 5 . 0 0 . 8 3 32.5 1 . 0 5 1 2 . 7 0 . 4 1 4 0 . 2 1 . 3 4 1 6 . 6 0 . 5 5 3 2 . 0 1.07 Dermit their identification in deuosits. Two factors tend t o operate against the TABLE 11. OCCCRRESCE D.4TA OX COMPOUiiDS FOUND second course: The amount of sample Av. ordinarily received is too small to pprmit complete chemical Temp. of KO. analysis, and the variations observed make it difficult to dram Locaof Temp. tions Occonclusions from chemical analyses. C R Y S T 4 L FORM4TIOY
With the exception of amorphous silica, which is noncq-stalline, and cristobalite, compounds found in turbine deposits ' are normally \\-ell crystallized; this indicates that the conditions surrounding their occurrence are favorable to crystal formation. Any mechanism postulated for the formation of turbine deposits is thus limited t o conditions favorable to crystal formation. COhIPOUND-TEMPERATURE RELATIONS
The number of compounds thus far identified in this study is rather limited. Table I1 shows the frequency of occurrence, the temperature range, and average temperature of the various compounds found in the samples examined. This table clearly indicates the predominance of siliceous compounds. Silicahearing compounds were found in 90.8mc of the deposits examined. S e s t in frequency of occurrence is sodium chloride, which was found in 1 4 . 6 7 of the samples. The remaining compounds orcur infrequently. T h e temperature a t the point of occurrence of the various compounds in the samples studied is shown in Figure 1. T h e data shown in this figure are arranged essentially in the order of descending average temperatures: for the occurrence of the various compounds. Samples were examined from rows in turbines having a range in temperature from 125-810" F. Sodium sulfate 111and sodium sulfate V may be considered together. Both sodium sulfates are more commonly found as deposits in superheater tubes rather than as turbine deposits. Examples of the occurrence of the sodium sulfates are s h o m in Tables 111, IV, and V. I t seems probable t h a t sodium sulfate is actually deposited as sodium sulfate 111, the high temperature form, since the inversion temperature is only 365" F., and that, in those instances where sodium sulfate V, the stable room temperature form, is found this represents a transformation from sodium sulfate 111. M-here sodium sulfate I11 is found, it would appear that it has been deposited n-ith sufficient foreign material present, most probably in solid solution with it, to stabilize the high temperature fo1.m and thus prevent the inversion from taking place. .icniite, Sa?0,FeyOa.4Si02,occurs somewhat rarely. The formation of acniite may be the result of a reaction between sodium metasilicate or sodium disilicate and hematite, Fe?Oa. It, has heen prepared in the authors' laboratory by heating sodium metasilicate, hematite, and a small amount of water together in a bomb at 400" F. I t may be noted that acniite has been found in the same temperature range as sodium metasilicate and sodium disilicate, and that its average temperature, 549' F., approximates the average temperature, 557" F., for these tn-o compounds combined. =in example of the occurrence of acmite is shown in Table V. Burkeite, Sa?CO3,2Sa2SO4,is also rare. Likc the sodium sulfates, burkeite is more commonly found as a deposit in superheater tubes rather than as a turbine deposit. Deposits which show, upon chemical analysis, an alkalinity relation suggesting the presence of sodium carbonate should he checked for sulfate content. All such samples examined here with x-ray diffraction methods to date have contained burkeite.
Compound a-Quartz ;i-Sbdium disilicate Amorphous silica Sodium chloride Cristobali te Sodium sulfate I11 Sodium sulfate V Acmite Burkeite Sodium metasilicate Unidentified
Range,
Formula
O F .
200-550 41 9-7 10 120-490 200-700 174-360 625-700 564-631 410-744 360-700 535-700
...
Where curFound rences 358 69 546 51 274 48 512 44 270 11 673 9 9 604 8 549 587 7 613 5 ... 23
Per Cent 34.6 25.6 24.1 22.;
5.a
4.5 4.5 4.0 3.5 2.5 11.6
TABLE 111. COMPOSITION OF TURBISE DEPOSITS FROM TURBISE OPERATIXG AT 550-LB. THROTTLE PRESSURE, 700" F. THROTTLE TEMPERATURE Stage Temp., O F. 1 700 2 676 3 650 4 625 5 600 6 575 7 550 8 525 9 500
Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium
Composition metasilicate, sodium sulfate 111, unidentified metasilicate, sodium sulfate 111, unidentified disilicate disilicate disilicate disilicate disilicate disilicate disilicate, sodium chloride
OF TURBISE DEPOSITS FROM TURBISE IV. COJIPOSITIOS OPERATING .IT400-LB. THROTTLE PRESSCRE,680" F. THROTTLE TABLE
TEMPERATURE
Stage 1 2 3 4 5 6 7 8 9 10 11 12
Temp., O F 618 590 562 535 508 * 482 546 430 404 378 352 324
Composition Sodium sulfate V, unidentified Sodium metasilicate, sodium chloride Sodium metasilicate, sodium chloride Sodium metasilicate, sodium chloride Sodium chloride, unidentified Sodium chloride, unidentified Sodium chloride, unidentified Sodium chloride, unidentified Sodium chloride, unidentified a-Quartz, sodium chloride a-Quartz, sodium chloride a-Quartz, sodium chloride
.
TABLE V. COMPOSITIOS O F TURBINE DEPOSITS FROM TURBINE OPERATIXG AT 1250-LB. THROTTLE PRESSITRE, 900 F. THROTTLE T E Y P E R . i T U R E , WITH TURBINE STEAX IVASHED TWCED U R I S G Rczr ISWHICHDEPOSITS FORMED Stage 5 moving 6 movine 7 moving 7 stationary 8 stationary 9 moving 10 moving 11 movine 11 stationary 12 moving 12 stationary 13 stationary 14 moving 14 stationary
Temp., F. 550 510 450 450 410 350 285 259 254
Composition Acmi t e Acmite Scmite Acmite, unidentified Acmite, unidentified a-Quartz a-Quartz Amorphous silica a-Quartz
Sodium chloride occurs rather frequently. Sodium chloride was found across the widest temperature range of any of the compounds. Whereas the other compounds found in this study have been found a t least occasionally in a comparatively pure state, sodium chloride has thus far always been found associated with some other rompound.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1430 Figure 2.
Metasilicate--bietasilicate-
Vol. 39, No. 111
however, the fmding of strong lines of cristobalite in samples (Tables 1-11 and YIII) containing over 90% silica renders this identification certain. In several of the instances where cristobalite !vas found, the turbine from which the samples were obtained was operated n-ith c o n si d e r a b l e . variation in load and hence variation i r k stage temperature. It would appear that cristobalite is derived from amorphous silica under conditions a-hich are sufficient t o initiate crystallization but insufficient t o carry the crystallization t o the stable state, nliicli in this case nould be quartz. Corisiderablc overlap of temperature ranges is observed among the compounds sodium metasilicate, sodium disilicate, a-quartz, and aniorphous silica and cristobalite. Despite the temperature rmge overlap, in deposits from any given turbine the change ironi one of these compounds to another usually occurs across a stage or two and frequently occurs between two adjacent stages. The change from sodium metasilicate to sodium disilicate (Table 111) i.5 sharp, as is the change from sodium disilicate to yuarta (Table I X ) . The change from &-quartz t o amorphous silica is ofieii not so sharp as that betvieen the other members of the
Combinations Observed in the Sodium Silicate-Silica Sequence in Blade Deposits Di-' ' silicate Quartz-Amorphous silica Disilicate Quartz--Xmorphous silica and cristobalite Disilicate &uartz------hmorrihous silica Quartz--4morphous silica and cristobalite Quartz -4morphous ii1ic.a
Trona, SaHCO3,Xa2CO3.2H2O,was found in t ~ t,ulbilie o deposits. One sample vas a composite from five stages having a temperature range of 400-700" F.; the other n-as from tn-o stages having a range of 400-450' F. Enfortunately the deposits were several years old before they were examined. Since it is possible that this compound was formed by a reaction between alkaline components of the sample and carbon dioxide of the air, this compound cannot as yet be definitely placed in the list of those formed in turbines. The compounds sodium metasilicate, 8-sodium disilicatc, LYquartz, cristobalite, and amorphous silica should be considered as a group. since they occur in a sequence in the order givcn, n-ith sodium metasilicate in the higher temperature stages and amorphous silica in the lower temperature stages. I n no instance thus far in this study have these compounds been found in any order than that givm. After the individual members of tliiq group are discussed, evidence of their tendency to occur in ccriei: will be given. Sodium metasilicate is relatively rare. Deposits c.olitaining sodium metasilicate are hygroscopic and strongly alkaliiie. Chemical analysis is liable to suggest the presencc of sodium hydroxide when in reality sodium metasilicate is present in the deposit. @-Sodium disilicate, B-?;aeSi?Oi, the low temperature form of sodium disilicate, was found in 2 5 . 6 5 of the samples studied. The samples are normally n-ell crystallized. They are strongly alkaline and dissolve in water to fbrm a milky solution, the milky appearance being due to colloidal silica formed through the decomposition of the disilicate by ivater. As in thr case of sodium metasilicate, the alkalinity of samples containing sodium disilicate may erroneously suggest the presence of sodium hydroxide. Chemical analysis usually s h o w some approximation of the sodium oxidesilica ratio indicated by the sodium disilicate formula. Some time elapsed betveen the authors' first observation of 8-sodium disilicate and its final identification r i t h the aid of a sample of the low form sodium disilicate supplied by G. W. Morey of the Geophysical Laboratory of the Carnegie Institute of Kashington, D. C. Diffraction data on &sodium disilicate are given in Table VI. a-Quartz, the stable lorn temperature form of crystalline silica, was found in 34.6% of the samples examined. Quartz forms a tightly adherent deposit. It is normally m-ell crystallized and frequently makes up more than 90% of a sample. Amorphous silica n-as observed in 24.191, of the dcpoaits. Like quartz, i t also forms a tightly adherent deposit. Since amorphous silica is noncrystalline, i t yields no x-ray pattern, but evidence of its presence may be obtained by igniting the sample at 950" C. for about a half hour, after xhich a diffractim pattern is taken. This treatment converts amorphous silica to cristobalite, the high temperature form of crystalline silica, which does yield a pattern. Occasionally heating fails t,o bring about the conversion t o cristobalite. I n this case the addition of a mineralizer, such as a trace of sodium hydroxide, follon-ed bv ignition will came the conversion to take place. Cristobalite has been found associated with amorphous silica in turbine deposits. Cristobalite crystallizes poorly in these deposits and therefore yields a poor x-ray pattern. For some time the authors considered its identification questionable;
s?I'Ic'P.
Figure 2 shorn the combinations of these compounds whicb have been observed thus far in this study. As yet the authors havc'iiot observed the complete series in samples from any one turbine. Hankison and Baker ( 5 ) report the analysis of a series of deposits which appears to contain the whole series. I n this i,ics quartz is reported as occurring before sodium silicate, L: situatioii the authors have not encountered. To date, except in those instances where the temperature of the lii.?t stage of the tui,tiine x a s too high, no instance has been observed where eodium disilicate was uot followed by quartz and amorphous silica, or where quartz was not followed by amorphous silica. Table I S illustrates a series of deposits in ivhich sodium disilicate is followed by quartz, which in turn is followed by amorphous silica.
TABLE VI. X-RAYDIFFRACTIOS DATAOK I
d
6.61 3.98 5.46 4.72 4 :7 4.26 1.13 3 94 3.76 3.63 3.52 3.36 3.07 2.96 2.85 2 67 2.58 2 52 2 42 2 35 2 27 2 19 2 14
1.61 1,57 1.53 1 62
'
i.49
2 08
2.05 1.99 1.94 1,93
_-
1- R 4
1.87 1 82 1.78 1.77 1.74 1.70 1.65 1.62
@-SODIUM
d
w H PY
w
1.46 1 45 1 446 1 42 1 399 1 377 1: 334 1.322 1.307 1,297 1.287 1.269 1 259 1.232 1.203 1.184 1.176 1.166 1.155 1.142 1.134 1 111 1.087
w
1 088
R
1.071 1.057 1 044 1.039
s(1) W W
wm Yw VU'
VW
1 030 1 023
1.014
DISILIC-4TE I m
vu w w ni am u. xrri VU
w ni
w rn V W VW
VW
w
\-
V W W
w I\'
Yw VW
vw
Yw
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
This series of deposits shows an intereyting feature. - i t the eleventh stage, where the transformation from sodium disilicate to quartz takes place, the stationary element shows the higher temperature phase, p-sodium disilicate, vhereas the moving element shon-s 3 considerable amount of the ion-er temperature phase, a-quartz. Ai similar situation is seen in t h e fifteenth stage where the transformation from a-quartz to amorphous silica takes place. Here a-quartz, the higher temperature phase predominates in thP deposit from the stationary elenirnt, whereas amorphous silica, the l o w r temperature phase, predominates in the deposit from the moving element. .Inother es:inipl(~of this division of pjiases across stationary and moving elements is shon-n in Tatile I-. I n this series of deposit$, a-quartz appcars in the deposit from tlie stationary element of the eleventh stage, whereas aniorphous silica appears in the moving element of the same stage. Tahle I11 s h o m the sequence of sodium metasilicate and sodium diTilicate. I n anothrr seric-s, illustrated in Table IV, sodium metasilicate n-as not followed b!- sodium disilicate. Hon-ever, in this serirs of deposits, a gap is found betiyeen the occurrence of sodium metasilicate and that of quartz in the rrgion where sodium disilicate n-ould be expected. dn unidentified compound vias found in this gap. I n some instances vhere sodium disilicate n-as not found prcvious t o the occurrence of quartz and amorphous silica. inquiry revealed that a dcxposit had occurred in the stages previous to the Erst one from which samples m r e removed, but t h a t these deposits had not been sanipled because of the small amount present. In one case samples w r e received from a plant and on analysis were found to contain quartz and amorphous silica. Some time later sample. from tlie same plant \rere received from another but id(3ntic:tl turbine, and thi io.5 of deposits contained sodiuni disilicate follo\ved by quartz and amorphous silica. Correspondencc in an effort to find apossible cause for the difference in the two scts of samples revealed that, in the first instance, deposits had occurred n-hich were probably sodium disilicate but had not been sampled. Cases such as this shoTv the necessity of sampling every stage on which a n y deposit appears, if a full picture of the nature of the deposits in a turbine is t o he ob-
710'
565'
Figure 3.
1431
O F TCRBISE 1)EPOSITS FROM TURBINE VII. C~UPOSITION OPER4TING .4T 850-LB. THROTTLE PRESSURE, 900" F. THROTTLE
TABLE
TEMPERATURE
Temp., a F. 710 665
Row 5
6
616
; 9
565 515
10 11
454
12
376 337 295 261 227 198 174
419
13 14
1: 16
17 18
TABLES'III.
CHEJIICAL.$SALYSES TABLE VI1
Sodium Oxjde (hlkalinity).
Soluble Silica,
24.1 27.8 31.5 29.5 29.8
43 1 46 4 16 8
12.;
8
21 20 2-1 40 32 25
%
5 6
11 12
13 14
1.5 10 17 18
OF
Total Silica,
Row 7 8 9 10
Composition 3-Sodium disilicate sodium sulfate 111 &Sodium disilicate: sodium sulfate I11 @-Sodiumdisilicate sodium chloride @-SodiumdiFilicate: sodium chloride &Sodium disilicate, sodium chloride &Sodium disilicate, sodium chloride Q?lartz, sodium chloride p-sodium disilicate Q u a r t z , sodium chloride Quartz, sodium rhloride, amorphous silica Amorphous silica, quartz, cristobalite Amorphous silica, cristobalite, quartz .imorptous silica, cristobalite Amorphous silica, cristobalite Amorphous silica. cristobalite
3.0
1.7 1.2
..
... ... ... ..,
70
54
49 38 75 94 97
I
70
5
16
9 3 96 8 96 1 94 6 95 5 92 8
4 3
..
.. ..
..
DEPOSITSS H O W N Sodium Chloride,
Sodium Sulfate,
...
29.1 3.8
70
70
4.0 19.0 27.6 43.5 19.0 4.0
...
...
...
3 0
... ... ... ...
IN
...
...
...
, . .
... ,..
... ...
...
...
tained. I n other instances, Table I- s h o w , steani wasliing a p pears to have removed the soluble sodiuni metasilicate and disilieate together n-ith any other soluble salts that might have been present. An interesting series of deposits is s h m m in Table VII, with chemical analyses of these dcpcsits s h o m in Table TITI. This series of deposits was taken from the same turbine as rhe series shoim in Table IS, after a 2-year operating interval. Operating conditions were similar during the turbine runs that produced both series of deposits, x i t h one exception. During the Course of the run t h a t produced the deposits s h o m in Table VII, a plugged blowdown line caused evaporator priming, and the r e s u l t a n t c a r r y - o v e r brought about a total dissolved solids concentration n-hich persisted for about a week. I t appears t h a t this condition accounts for the presence of sodium sulfate and sodium chloride found in the series of Table S'II. This set of samples s h o w a n interesting variation in the crystal size of t h e sodium disilicate found in rows 5 t o 11. This variation is shon-n in Figure 3. T h e crystal size of the sodium disilicate increased t o a maximum in row 8 and then decreased in succeeding rows. Maximum crystal size Tvas observed a t a temperature of 565" F., which is not far from the average temperature of deposition, 546" F., found for sodium disilicat e. Further study of the behavior of the sodium silieate-silica system a t elevated temperatures in the presence of steam Variation of Particle Size of Sodium Disilicate at should conti,ibutc much to our underVarious Temperatures
INDUSTRIAL AND ENGINEERING CHEMISTRY
1432
escape'
Vol. 39, No. 11
dctoction bj- x-ray methods. Thus far, however, all of
TABLE IX. COMPOSITIOS OF TURBINE DEPOSITS FROM TCRBISE the highly alkaline hygroscopic deposits examined have given OPERATIKG AT 850-LB. THROTTLE PRESSL-RE, 900’ F. THROTTLE gtmd diffraction patterns with no evidence of eithcr sodium TEMPERATURE
Stage 10 stationary 10 moving 11 stationary 11 moving 12 stationary 12 moving 13 stationary 13 moving 14 stationary 14 moving 15 stationary 15 moving 16 stationary 16 moving
Temp., 454 454
419 419 376 376 337 337 298 298 261 261 227 227
17.
Composition Sodium disilicate Sodium disilicate Sodium disilicate Sodium disilicate, a-quartz a-Quartz a-Quartz a-Quartz a-Quarta a-Quarra a-Quartz a-Quartz, i i i i i o r i ) l i c ) i i s ‘ilica Amorpiic~ussiI.ra, a-quartz Amorphous iilica. a-qnartz Ainorphow -i!ira
hydroxide or its hydrates. Published analyses of turbine deposits such as those of Straub ( I O ) n-hich might be interpreted as containing sodium hydroxide, usually contain considerable .silica, n.hich suggests the prezcnce of sodium silicates rather than sodium hydroxide. Other analyses, both published and madr i n this laboratory, contain an alkaline material xhich cannot he accounted for by either sodium silicates or burkeite. T o datc,. no sample of this type has been available for x-ray diffract ion studies. COIIPOUND-BOILER P R E S S U R E RELATION
TABLE
1. . k I L Y S I S
OF
AIhTERI.41. ( : O S T h I S I S C L-SII)ESTIFIEL) CoirrosEsVr
’3%
Ionic Total silica Soluble silica NatO (alkalinity) c1 SO8 RzOi
29.5 17.8 13.9 12.8 13.2 4.6
Cuinpound
Sodium chloride Sodium sulfate 111 Sodium metaqilicate Burkeire Sodium disilicate Sodium sulfate V Amorphous silica Quartz A c mi t e
As Sodium R u l t s Sodium sulfate Sodium diiilicatr Sodium chloride
“0
21 4 29 9 21 6
K203
‘l,>,L!l
Boiler I’ressure Range, 1.b /Sq. In. 300- G O 0 300- 550 400- 550 380- 900 400-1200 400- 900 330-~1325 350-1325 1200-1 250
4 6 K 4
.4v. 13data from this study to date sodium chlorid(~appc:irs t,o be awociated with Ion-er pressure operation. The authors are awat~c’of witlcncc of the occurrence of sodium chloi,iiic in high opt-ration, although samples n-ere not availaiile for this thcw instances. Sodium sulfate I11 and burkcire Iioth appear to hi with low pressure operation. The appearance of sodium sulfate I-,at ronriderahly higher pressures than that at which sodium sulfatv I11 is found, is interesting. as it duggests that at higher prcssuws sodium sulfate is found in a purer state than a t the l o w r pressures. Acniite has thus far been observed only a t higher pressares. SUMAIARY
UNIDEUTIFIbX) CO\lI’OU\DS
Concitiei alile evidence for the occurrence of a nunibcr of unidentified compounds is a t hand. This is in the forni of diffraction patterns xhich do not conform to any compound on which data are available, and also in the forni of lines n.hich cannot he accounted for in patterns containing identifiable compounds. .Ilthough most of these instancvs have occurred in cases n-here the sample was too small for further investiga,tion, an example of this situation is shown in Tablc S. Uwide thc pattern for sodium chloride, the pattern f r o m this material shon.s lines which have some similarity to those from sodium disilicate as well as additional lines vihich do not corrcspond t o those from sodium disilicatc, sodium sulfate 111, or sodium sulfate V. The efforts being made t o identify thcsc cmmpounds have already been described. Hankison and Baker (j), using optical and x-ray diffraction methods, report the occurrence of magnesium chloride, mag. nesiuni phosphate, magnesium sulfate, sodiuni pliosphatcl, potassium chloride, potassium phosphate, potassium sulffite, and potassium tetrasilicate. As yet thc present authors have not observed these compounds in turbine deposits. Three compounds which might be expected in turbine deposits. sodium hydroxide, sodium carbonate, and sodium bicarbonate, have not as yet’ been found in this study. Sodiuriiliydroxidc, which has frequently been reported on the basis of chemical analysis, niight be present in an amorphous condition and thus
\Vith tlit exccptioii of amorphous silica, thc compounds found in turbine deposits are normally ell crystallized. SIost of the compounds found appear to bear a relation to the temperature a t which thry arc? found. Sodiuni metasilicate, sodium disilicate, quartz, and amorphous silica appear to occur in sequence in tulbine deposits. Little evidence has heen found for a direct. relation h e t w e n boiler operating pressures and the compounda found in turbine deposits. LITERATURE CITED B u c k l a n d , B . 0.. Proc. 2 n d A r i n i ~ a lTVater Conf.,E I L ~ I XSoc. .’ TT7estem P a . , p. 115 (Nov. 1941). Fitze, SI. E., and Long, F. H., Proc. X i d r o r s t P o u w C‘onf., 8. till-8 (.kpril 1 9 4 6 ~ . Gocrke, €I., Elt.ct,.izilafs~irX.schafl,38, 61-6 (1‘339). Hall, It. E . . T r a m . A m . Soc. X e c h . Enyrs., 66, 487-74 (1944). Hankison, L. E.. and B a k e r , SI. D., I b i d . , 67, 317-24 (1945). AIore?., G . K., Am. Soc. Testing .llaterials, Proc., 42, 987 (1942). Sorlei.herg.C . I t . . Potcer, 80, 596-7 (Kov. 1 9 3 6 ) . Spillner.. F.. Chem. Fahrik. 13,S o . 2 2 , 305-16 (Kov. 1940). Strauh, F. G . , Proc. 3 r d Anriual T a t c r C o n f . , Enyrs.’Soc. R’estern Pa., p p , 31-43 ( S o v . 1942). S t r a u h , 17. G.. U n i v . of Ill.. Bull. 282, M a y 1936. Strauh, F. G., and Grabomski, H. .I., Trans. A.S..1I.E., 67, 309-
16 (1945). R E C E I V E Torernber D 27, 1046. Presented before the Divi4on of Water, Sewage, and Sanitation Chemistry a t t h e 110th Meeting of the A \ I E R I C A N C i i ~ m c SOCIETY, a~ Chicago, Ill.
