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INDUSl'RIrlL A N D ESGINEERILYG CHEMISTRY
elice. Considering the carcass stocks, the 1-year natural tensile is slightly more than original, while the 4-day oven test s h o w a slight decrease. The 2-year and 8-day oven tests correspond quite closely; however, the 3-year and 12-day tests show a large variance. TENSILE PRoDvcT-In this case the softening effect of the heat of the oven is counterbalanced by the extra decrease in tensile strength. This produces a satisfactory agreement for the tread compounds and also the carcass compounds for 1 and 2 years and 4 and 8 days. As before. the 3-year and 12dag carcass-stork tests check very poorly.
Vol. 21, No. 11
Summary of Results
In standard guanidine tread and carcass compounds, test results obtained from the 4- and 8-day periods in the Geer oven at 70" C. correspond approximately to 1 and 2 years of natural aging. Data obtained from the 12-day period correspond with 3 years of natural aging for tread compounds, but results showed no relation between these same periods for camas3 stocks. The tensile product is the criterion which shows the best correspondence between Geer oven tests and natural aging tests on the tread and carcass compounds used.
[END OF SYMPOSIUM]
Zeolite Water-Treating System of the Beacon Street Heating Plant' J. H. Walker and Leo F. Collins T H E DETROIT EDISONC O M P A N Y , DETROIT, hlICH
The Beacon Street plant is the newest of several HE: problem of feedof the water from the mains district heating plants operated by the Detroit Edison water treatment is of is given in Table 1. Company. The factors involved in the choice of a e x t r e m e importance Choice of Feed-Water Treatzeolite softening system followed by partial neutraliin a district heating plant. ment System zation with acid are outlined in this article, which Such a plant, generally describes the complete feed-water treatment system, located in a congested section Serious consideration was gives the operating experiences and results of three of a large city, would obvig i v e n s e v e r a l methods of seasons, and presents cost data for a full year of service. ously require a much smaller t r e a t m e n t out of the large investment for a given can u m b e r of commercial syspacity if it could avail itself of the recent advances in boiler tems available. Internal treatment with sodium carbonate, design and firing methods made in the power-plant field than conditioning by the use of barium salts, the hot-process limeif it had to be content with the older and more conservative and-soda system, and zeolite softening were given the greatmethods of operation. The modern power plant, however, est weight as practicable for the Beacon Street plant. with a feed-water make-up of less than 5 per cent, is able to Internal treatment with sodium carbonate had been used maintain desirable boiler-water conditions by the use of in other plants of the company since 1916 with some success. eyaporators for this make-up, while the dist.rict heating plant, This method, however, produces large quantities of carbonate with nearly 100 per cent make-up, must face the problem of sludge in the boiler, and it has not entirely prevented the attempting to obtain equally good conditions by other means. burning out of tubes. I n solving this problem, the development of a relatively exTable I-Typical Analysis of Detroit Water pensive but very efficient feed-water treatment system is (March, 1929) justified when it allows the continuous operation of boilers st P. p . m. high st,eaming rates, since the charges against the treatment Total dissolved solids (on evaporation). . . . . . . . . . . . . . . . . . 107.0 system are more t,han offset by the reduction in total plant ~ ~ ~ $ 4 , ' s " ~:;:$::~: : :: :: 1 : . . . . . . . . . . . . . O.O Dissolved oxygen., . . . . . . . . . . . . . . . . . . . . . . . . investment charges. Free carbon dioxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 I n designing the Beacon Street plant, therefore, the object p~ vSiu ........................................ 7.6 . . . . . . . . . . . 25.7 was not the devcloprnent of the most economical feed-water ........... 6.9 So-iium.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . treatment system, but rather the development of the system Silica. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . which would give the best possible boiler-water conditions. Iron and aluminum oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Ricarbonate.. . . ................................. 89.0 The primary consideration was that the system Rhould give Carbonate. . . . . . . . . . . . . . . . . . . . . .......... 0.0 Sulfate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.7 as nearly scale-free conditions as possible in the boi!er itsclf. Chloride. . . . . . . . . . . . . . . . . . . . . . ..... 6.6 It has been shown that: as a result of the extremely high rates of heat transfer produced across surfaces with Barium salts, because of their poisonous nature, Were ruled modern firing methods, even an extremely thin layer of scale out as a conditioning agents as a of public policy, inasmay cause failure due to overheating of the tube metal. Corsupplied by the plant was to be for much as rosion, embrittlement, and foaming were recognized as the cooking purposes in open equipment. other major problems involved in the choice of the method of The hot-process lime-and-soda system was carefully confeed-water treatment. sidered, but to the engineers responsible for the design and The make-up water a t t'he Beacon Street plant is taken from operation of the it did not appear that this treatment the city water mains, which are supplied from the Detroit would invariably produce scale-free conditions in the boiler, River at the foot of Lake st. Clair, the water being treated while the hot sedimentation tanks which are a necessary part with alum, filtered, and chlorinated. A typical analysis of such a system wollld have been awkward because of the enormous sizes required. 1 Received August 12, 1929.
T
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November, 1929
INDUSTRIAL AND ENGI.l'EFRISG CHEMISTRY
Zeolite softeiiiiig appeared to be ideal from tlie standpoint of elimination of scale problems, but it substituted the problems of possible embrittlement and possible foaming due to high concentrations of dissolved substances in the boiler water. However, it seemed probable that these attendant difficulties could be successfully met, and the certsir,ty of m l e prevention determined the choice of this system. The danger of embrittlement as the result of high caustic :ilkalinity produced in the boiler mater by the decomposition :it boiler temperatures of the sodium bicarbonate content of the zeolite-softened feed water mas obviated by the use of an :ifter-treatment of sulfuric acid ( 2 ) . Thiq partisl neutralization not only reduced the potential alkalinity of the water going to the boiler, but it also increased the sulfate-ion concentration to well above the value necessary to maintain the wlfate-alkalinity ratio proposed for the prevention of emOrittlement (1, 2 ) . The use of acid following zeolite treatment was also expected t o show a favorable effect in reducing the carbon dioxide content of the steam, as the result of laboratory tests made before the system was finally designed (3). This carbon dioxide in the outgoing steam WRS believed to contribute t o the slow corrosion taking place in pipe lines and in the radiators of consumers' heating systems, and a reduction in the amount liberated was therefore of some importance. A system of continuous blow-down with heat recovery from the wasted water was designed to limit the concentration of dissolved solids in the boiler water. Arrangement of Feed-Water Treatment System
1021
upward-flow principle. The actual relative merits of the up-flow and down-flow systems are hidden behind a smoke screen of commercial considerations, but operating esperience has substantiated the original decision that there seemed to be no fundamental objection to the up-flow s p tem. The zeolite is of the greensand type, with a rated exchange value of 3375 grains of calcium carbonate pcr cubic foot. The rate of flow a t normal load through the 41-inch zeolite bed is 4.25 gallons per minute per square foot of area, or 63,000 gallons per hour through each tank. Thr total output of each tank between regenerations is 445,000 gallons, and the normal rate of flow may be increased to a permissible maximum of 75,000 gallons per hour per tank. The brine required for regeneration is supplied t o each zeolite tank by a simple piping system from a central overhead tank where it is made up from rock sa!t. Figure 2 shows one of thc zeolite tanks from the control-valve side. Although the controller governing the rate of flow of raw water to the zeolite softeners might have operated with sufficient exactness t o allow the establishment of a fixed rate oi acid feed, it was felt that an automatic proportioning deviccb was needed, which also would cut off the acid feed immediately if the flow of water should stop. Figure 3 shows :I diagram of the proportioning device developed forthis purpose This acid feeding system was designed to introduce 5 pel' cent sulfuric acid through a small coil of perforated lead pipc into the stream of water leaving the zeolite softener, and to maintain the rate of feed of this acid exactly proportional to the rate of flow of the water. Commercial concentrated sulfuric acid is diluted to a concentration of 5 pcr cent in :I central tank (not shown in Figure 3), and the dilute acid i. pumped through lead-lined pipe to a constant-head tank at each zeolite softener. The outlet of this constant-head tank
The Beacon Street plant went into service in the fall of 1926 with two boilers. It now contains four boilers (including one in the course of erection a t the time of writing), each boiler having approximatelj. 41,NO square feet of heating surface. As these boilers are capahle of ACID HEAD TANKS^. being operated at 300 per cent NOTCH METER of rated capacity, each one will d e l i v e r 4 0 5 , 0 0 0 pounds of steam per hour. The boilers NOTCH FLOW are of the W type and are fired with underfeed stokers. I n the original installation there were two zeolite softening tanks. A third is shortly to be installed, and the completed plant of ten or twelve boilers mill probably require five or six softening units. The complete water c y c l e is shown diagrammatically in Figure 1. T h e c i t y m a t e r flows through the zeolite tanks under the pressure existing in the mains. I n order t o insure a constant rate of flow regardless of fluctuations in the water p r e s s u r e , Venturi-type flow controllers are used. A f t e r leaving the zeolite tanks, the water is dosed with acid by a BLOW OFF RECIRCULA?ING PUMP system which will be describad Figure I-Diagram of C o m p l e t e Feed-Water T r e a t m e n t S y s t e m of t h e Beacon Street P l a n t i n more detail in a later section, and then flows t o a concrete storage reservoir. From is a small orifice on the end of a piece of rubber hose, the level the reservoir the water is lifted by booster punips to a head of the orifice being controlled by a float resting on a mercury tank, from which it flows by gravity to a flow-equalizing tank column. The height of this mercury column is in turn conof the 1'-notch type, and thence to the deaerating heaters, trolled by a Venturi nozzle in the soft-water line from thr, t h e boiler-feed pumps, and into the boiler itself. zeolite tank. Since a stuffing box is not necessary with this The zeolite tanks are 10 feet in diameter and 25 feet long, construction, the float moves practically without friction, and aet with their axes horizontal. They are constructed on the since the flow of water through the Venturi throat and the
Huw of acid tlrrough the uriiice 1,iotIi follow a square root fuiiction, these rates of flow are always maintained directly proportional. The error introduced by the displacement nf tlje niereury in the lomr leg of the device is made negligible hy the large diameter of t,he chanrhcr.
teiit is entirely autoruntic. Occasional adjustment of t,lic blow-down is necessary from time to time, but this operat.ion involves little effort. Improvement of the System
An ititerest.iirg a i d novel feature developed sitice the plant wiit into operation is tlie rccirculatioii of a small percentagc t,lic water from the boilers back t o tlie fecil-wat,er heater. lie first year’s operat,ion disclosed the fact t,Iiat tlre boilor l e d water, after acid feed arid de-aeratioo, still contained :I considcrnble aiiiount of bicarhoiiute; ani1 the fact that sorue ,:orrosioii wits taking plnee in tlie boilers which appeitred to ire diie to a direct actidn betweeir carbon dioxide and iron. since ferrous carbumate was found to have been formed over ilie corroded areas. This corrosion ha.s heeir coinplctely overcoine by the expedient of recircliiating t,o the water spdcc in the de-aerating heater sonic of tlie alkaliiie boiler water taken from the blor-off system, This converts any hicarbunate present. into normal carhonato. The amount of water recirculated is varied according to tlie alkalinity of the blowoff water to maintairi a pII value in the shell of tlre de-aerator of about 9.8; jt averages ahout 5 per cent of t.lie boiler feed, this amount of recirculation hoiiig in addition t o the 4.5 per -cnt of ;xt.ual Ijlm-down going to tlic sewer.
T h e de-aera~~iiig heaters are of tlie Cuchiane type. nncl itre ;nnong the largest units of this type yet attempted. They contaiii ii large nunilier of trays over which tlic water flows by gravity while it is swept liy a countcrcnrrent of steam to re-
The coritinuiius blow-down systcni, designed tu limit tlic concentratioti of dissolved solids in the boiler water while conserving the Iimt in the wasted water, is operated a t tlic preseiit time witli a rate of blou-down of approximately 4.5 per cent of tire rate of the boiler feed. This ratio maintaiiis a conceiitration in the boiler water of from 2000 to 3000 p. p. m. of dissolved substances. The arratigement of the blow-down system is shown in Figure 4. Tire flow of wat.cr from each of tlie two mud-drums of each boiler is regulated by a needle valve, water at hoileu temperature and pressure pasing the valve flaslies, tlie mixture of water and steam passing to a separating tank. This tauk is vented into the exhaust system of the plant whicli carries a pressure of approximately 2 poutvis gage. A constant level of water is maintained in the separating timk by i~ float-controlled valve, and the liot water leaving this tank passes tlirougli a lieat exchanger, where cold feed water on ils way to the de-aerating heaters absorbs its heat. The blowdown then passes thruugh a V-notch meter to tlie sewer. A feature of the blow-down system is the use of 1.eeds and Xdrthrup recording coiidoctivit,y meters on the boiler wnt.cr to check the conceiitration of dissolved solids. Since the rehtive amounts of the suhsttrnces in thc feed water remaio practic;rlly constant, the conduci,ivity of the boiler water is N measure of t.he coilcentration of these sulistanees, and offers a convenient m c m s of keeping a continnous record of this X’i*ure 3 ~4Maaranrof Acid-Pmportlonins Device factor. For the water coiiditiorrs at the Beacon Street plant, tlie concentrat,ion limits of 2000 t,o 3000 p. p. in. correspond trr I t WRS considered advisable, after operation had coma conducti%.ity range of froiii 3400 to 3700 reciprocal megruenced, to augment the staodard feed of 40 parts of sulfuric ohms. acid per million parts of wat,er treated by a small amount of Method of 1:ontrol of system pliosplioric acid. The us0 OF this acid is in no way essential The treatiiig system as a whole, and the acid feed in pur- to the operation of tlie system, but it is added for the purpose ticuiar, have been handled wit11 corisiderable caution, hut no of precipitating the small residual amount of calcium ion in serious troubles have been experienced. The daily operation the water. This comes down as tricalcium phohosphate, wliieli is adequately carried on by the regular operating crew under deposits in a thin film on the boiler surfaces and probably has tlie direction of a cliernist. Except for the mixing of the some effect in nreventine corrosion. At tlie nresent time 4.5 acid and brine solutions. the regoneration of the zeolite, and parts of phosphoric acidy are fed per million’parts of water, the measurement of pH on the water after the acid feed, the in addition to the amount of sulfuric acid stated above
November, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1023
dioxide and about 10 per cent of the half-bound carbon dioxide from the zeolite-softened and partially acidified water. They Three seasons of operation have brought to light a fen7 remove most of the dissolved oxygen, but have not, up to the troubles which could be classed as minor ones. The most present time, consistently removed it to the expected degree serious of these is the mild but active corrosion which is ap- during periods of operation a t full load. Table I1 shows the water conditions a t various points in the parent in the zeolite tanks and piping system. This is not cycle. The change in pH from one point to another is particularly interesting. The passage of water through the zeolite softener increases the pH from 7.6 to 8.1; introduction of acid causes it V E N T TO A T U O S to drop again to 6.2 during its passage to the OR EXHAUST H E A D E R storage tanks and on to the de-aerating heaters, in these heaters the removal of carbon dioxide brings the pH up to 8.6, and the addition of recirculated water from the blow-down steps it up still farther to 9.8; in the boiler the further removal of carbon dioxide gives a value for the boiler water of 11.3; but this carbon dioxide passing out with the steam to the mains gives the TO WEAD T A N K O R H E A T E P S steam condensate a pH of 6.0. . - .. --There has never been any serious amount of scale in the boilers of the Beacon Street plant. No tube burn-outs have been experienced during -WATER FROM BOOSTER PUMPS three seasons of operation a t steaming rates up to 300 per cent, with this maximum maintained TO S E W E R for periods of several hours at si time. Operating Experience and Results
VM1'CY U
