Re-use of Steam Condensate as Boiler Feedwater

I

D.

E. NOLL

and H. M.

RIVERS

Hall Laboratories, Inc., Pittsburgh, Pa.

Re-use of Steam Condensate as Boiler Feedwater Modern pretreating facilities, efficient deaerating equipment, proper materials of construction, devices for policing the system, and chemicals for conditioning steam make possible the effective re-use of condensate as boiler feedwater

Ix

THEIR attempt to obtain more energy from fuel and to install equipment for so doing a t lower cost, boiler designers and operators have needed feedwater of constantly improving purity, because pure water is free of scale-forming materials that interfere with heat transfer and solids that require wasteful blowdown for their removal from boilers. Steam condensate, then, looks like a n ideal boiler feedwater. As might be expected, providing for the re-use of steam condensate is not without problems. Pure condensate corrodes the metals with which it comes in contact and may cause serious damage to turbines, heat exchangers, and return line.. Condensate may become contaminated by inleakage a t turbine gland seals, condensers, heat exchangers, process equipment, and reciprocating engines. When the contaminants and corrosion products return to the boiler house, they may cause troublesome deposits in feedwater pumps, closed heaters, and boilers and may cause boiler water to be carried over with the steam. The problems, therefore, are: to prevent the condensate from dissolving the equipment through which it passes and to keep it from becoming contaminated. Figure 1 shows potential sources of trouble resulting from the re-use of condensate in a plant using steam for power, heating, and process.

Warner (74) show that the principal reaction occurs spontaneously for iron whether oxygen is present or not, but that for copper the reaction proceeds to the left unless oxygen is present. The latter fact suggests that there is frequently a significant amount of oxygen in condensate, as corrosion of copper is common. T h e effect of oxygen is to remove hydrogen from the right side of the equation and thus promote corrosion. Measures must therefore be taken to keep oxygen out of the system and to

remove by deaeration oxygen that enters in spite of efforts to keep it out. A corroded nipple, taken from a condensate return line, is shown in Figure 2. The pitting is typical of corrosion aggravated by oxygen. Low pH should be avoided, for a high concentration of hydroxyl ion improves the chances that metal oxides will be formed to plug breaks in the oxide film that normally protects the metal from corrosion. Therefore, carbon dioxide and other acid-forming gases should be kept a t a minimum. Figure 3 shows

r------------HIGH PRESSURE STEAM EXTRACTION

BOILER PROCESS

7 BLOWDOWN

RAW WATER

i

PRETREAT ME YT AND FILTRATION

@

@

@

?I-

1 32

J

SUSGE TANK

CG'LCENSATE RECE VER

Corrosion

Mechanism of Corrosion. Corrosion of the metals commonly used in condensate systems may be represented by the following reactions: 2H20

+ M -+ M(OH)e + Hz

+

2H+

J.

M++

+

+

(0)

4

H20

2H20 Free

2 146

energy

values

compiled

by

Figure 1.

Potential points of trouble resulting from re-use of condensate

Corrosion in wet stages of turbine with pickup of iron Corrosion in condenser with pickup of copper 3. lnleakage of process contaminants 4. Oil pickup in reciprocating engines 5 . Corrosion and deposition in stage heaters 6 . Corrosion in condensate return lines with pickup of iron 7. lnleakage of cooling water and oxygen at condenser a. Introduction of carbon dioxide at softener 9. Deposits in feedwater pump 10. Deposits in boiler tubes 1 1 . Carryover of boiler water with damage to superheater 12. Deposits in turbine resulting from carryover 13. Cooling water attack on condenser tubes

1.

2.

INDUSTRIAL A N D ENGINEERING CHEMISTRY

RE-USE OF WATER BY INDUSTRY grooving in a condensate return line. This attack is attributed to low p H resulting from carbon dioxide in the steam. Figure 4 shows a corroded wheel in the last stage of a condensing turbine. The loss of metal from the wheel is probably the result of both low p H and oxygen attack, combined with erosion by water droplets. Nearly all corrosion troubles in condensate systems stem from the above chemical reaction. The lone exception is the attack on copper by ammonia with the formation of the cuproammonia complex.

Deposits of

Corrosion

Products.

When corrosion occurs in condensate systems, maintenance of corroded equipment is not the only difficulty confronting plant operators. They must also contend with troubles at points where corrosion products settle on the confining surfaces. I n a year's operation a utility plant producing 1,000,000 pounds of steam per hour, with 0.1 p.p.m. of iron in the feedwater, could deposit 1200 pounds of F e s 0 4 in the boiler and preboiler equipment. Accumulations of iron and copper oxides in feedwater pumps can drastically reduce pump efficiency. Similar accumulations in closed feedwater heaters can cut down on heat transfer and increase pressure drops. The most serious consequence of condensate corrosion, however, is the overheating and failure of boiler tubes owing to the insulating effect of oxide deposits. The tube section in Figure 5 is from the generating bank of a boiler operated a t a pressure of 450 pounds per square inch. This tube was ,removed from the boiler before failure occurred, but a blister is clearly evident. Overheating was caused by a l/az-inch layer of iron and copper oxides. At higher boiler pressures, deposition of iron and copper oxides becomes more critical. At 1800 pounds per square inch, for example, the saturation temperature is 165" F. higher than a t 450 pounds. This means that the leeway on overheating is 165' F. less a t the higher pressure and that the tolerable amount of insulating scale is consequently less. Sometimes oxide deposits in boiler tubes have a porous structure that permits boiler water to penetrate through to the metal and not be rinsed away by normal circulation processes. When this happens, boiler water solids, including sodium hydroxide, are concentrated in the interstices. High concentrations of sodium hydroxide strip off the protective iron oxide layer and permit boiler water to attack the steel. A typical example of tube damage caused by a high concentration of caustic under a layer of iron oxide

Figure 2. Nipple corroded b y densate containing oxygen

COR-

Figure 3. Section of condensate return line corroded b y condensate containing carbon dioxide

is shown in Figure 6. The tube section is from a boiler operated a t a pressure of 1350 pounds per square inch. Laboratory examination revealed that the deposits were Fes04.

niques for reducing the carbon dioxide content of boiler feedwater include acid treatment, softening by means of an acid-regenerated cation exchange resin, deionization, dealkalization by means of chloride-regenerated anion exchange resin, and evaporation. When the corrosive action of condensate is accelerated by the presence of oxygen, the difficulty is usually the result of inleakage of oxygen a t points in the system under vacuum, although oxygen may enter with the make-up if the plant does not have a good deaerating heater. Under certain conditions, small amounts of oxygen can be scavenged economically by adding reducing agents, such as sodium sulfite, to the feedwater. I n addition to protecting the boiler from the type of corrosion that is aggravated by oxygen. sodium sulfite aids in preventing oxygen from leaving the boiler with the steam. But it affords no protection to that portion of the system between the point of inleakage and the point in the feedwater system where the sulfite is introduced. Care should be exercised in the use of sodium sulfite as a n oxygen scavenging chemical a t pressures near or above 1000 pounds per square inch, as volatile sulfur compounds usually decompose, and may go off with the steam and reduce the pH of the condensate. Hydrazine, which has been recently introduced as an oxygen scavenger, produces ammonia on decomposition and thus tends to retard corrosion by raising the p H throughout the condensate system. Baker and Marcy (2) have reported that in a boiler operated a t steam conditions of 1350 pounds per square inch and 930' F. the decomposition of hydrazine to ammonia is so great that only a few hundredths of 1 p.p.m. could be maintained in the boiler water. The re-

Keeping Carbon Dioxide and Oxygen Out of System. There are two ways to combat the ravages of conden-

'

sate corrosion and reduce losses caused by deposits: modifying treatment of the make-up, feedwater, or boiler water and making mechanical changes to reduce the concentration of corrosive substances in the steam condensate; and conditioning the steam with materials that inhibit attack by corrosive condensate. Changes in water treatment should be aimed a t reducing the amounts of carbon dioxide and oxygen that leave the boiler with the steam. Many natural waters used as boiler make-up contain bicarbonate, which, if not removed by suitable treatment, will decompose in the boiler water with liberation of carbon dioxide. Sodium carbonate, or soda ash, is often added to boiler water to control boiler water alkalinity, or to a lime-soda softener to precipitate hardness as calcium carbonate. Hall and others ( 5 ) found that the decomposition of sodium carbonate may be as high as 75% at moderate boiler pressures and that the extent of decomposition increases with rising pressure. The carbon dioxide resulting from the decomposition of, carbonate or bicarbonate leaves the boiler with the steam and depresses the p H of the condensate. Where soda ash is used .simply to control alkalinity, replacing it with caustic soda will reduce the concentration of carbon dioxide in the steam; but if bicarbonate occurs naturally in the make-up or if soda ash is added to a softener, extensive changes in the treating method may be required. Tech-

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Figure 4.

Turbine wheel showing effect of corrosion combined with erosion

sulting high concentration of ammonia in the condensate. along with oxygen that may enter through inleakage a t the condenser, represents a combination that is potentially aggressive toward copper alloy condenser tubes. Except in very small boiler installations, it is desirable to provide a deaerating heater to remove carbon dioxide and oxygen from the feedwater. Although the major advantages here involve heat recovery and chemical usage, a good deaerating heater helps to reduce the corrosive action of condensate. Other mechanical means of keeping oxygen in the system at a minimum are the venting of closed heaters. extraction of air from turbines, and deaeration at cocdenser hot wells.

Figure 5 . Boiler iube blistered by overheating resulting from insulating effect of iron and copper oxide de-

Steam Conditioning. Reliance upon equipment and chemicals for trearing make-up, feedwater, and boiler water is not the complete answer to the problem of preventing corrosion; changing treating facilities may be extremely expensive, and some of the chemicals used decompose in high-pressure units. For many plants, steam conditioning is the final answer. I n today’s trend toward higher operating pressures, smaller and smaller amounts of iron and copper corrosion products can be tolerated in boiler feedwater. The steam Conditioning approach to the problem of corrosion, therefore, becomes increasingly important as an economical means of combating the effect of traces of carbon dioxide

Figure 6. Boiler tube damaged b y sodium hydroxide concentrated under layer of iron oxide

posits 2 148

INDUSTRIAL A N D ENGINEERING CHEMISTRY

and oxygen that often cannot be kept out of the system. The allowable amount of corrosion products that may be returned to a boiler with the feedwater is a matter of economic balance between the cost of maintenance and the cost of treatment to reduce corrosion. At pressures above 1000 pounds per square inch the cost of replacing tubes and cleaning dirty equipment usually tips the balance in favor of steam conditioning, if the amounts of iron and copper in the feedwater are not less than 0.02 p.p.m. Few plants that return a high percentage of pure condensate to the boilers, but do not condition their steam, can boast of much less than 0.1 p.p.m. of iron and 0.05 p,p.m, of copper in the feedwater. Steam conditioning consists of adding to the boiler Mater, the feedwater line, or a steam line, volatile materials that either raise the pH of the condensate or form a protective film on metal surfaces in contact with the condensate. I n the former category, ammonia and its salts and volatile neutralizing aminrs, such as cl-clohexylamine and morFholine, have been used with varying degrees of success. Ammonia has been described by Adkins ( 7 ) as a “dual personality,” because it may be beneficial in raising condensate pH, or it may be harmful to copper alloys. Reports from a large number of plants that have fed ammonia suggest that it plays the role of Dr. Jekyll more often than that of Mr. Hyde (3, 7). Ammonia is the cheapest of these chemicals, but because of its high relative volatility it is not always present in sufficient quantity in the liquid phase in regions where steam initially condenses. The neutralizing amines, cyclohexylamine and morpholine, have a much lower relative volatility than ammonia and therefore provide protection against low p H attack in the wet stages of turbines, where ammonia is of little value. Cyclohexylamine and morpholine are extensively used in utility plants where the mechanism of corrosion is not unduly affected by oxygen. In a large plant taking precautions to keep carbon dioxide out of the system, consumption of these amines may be of the order of only 0.5 pound per day. Most of the published data concerning the use of cyclohexylamine and morpholine (8, 72, 73) show that both iron and copper in the condensate can be kept at concentrations less than 0.01 p.p.m. when enough of either of these materials is added to maintain a pH of 8.8 to 9.0. Actual case histories involving the use of cyclohexylamine report the following improvements: Prevention of feedwater pump deposits with subsequent reduction of 2570 in motor amperage (77) $8000 yearly savings in fuel because of increased efficiency of feedwater heaters

RE-USE OF WATER BY I N D U S T R Y 60y0reduction in the weight of boiler deposits (8)

Cyclohexylamine and morpholine appear to be equally effective at the same pH. There is some evidence (4, 6, 73) that morpholine decomposes more readily than cyclohexylamine, although both materials have been used successfully a t superheat and reheat temperatures of 1050" F. Neutralizing amines are effective only when p H is a factor affecting the corrosion mechanism and are economical only when carbon dioxide is a t a low level. I n the majority of plants, particularly low-pressure heating plants, oxygen is a factor to be considered. Boiler plants operating a t pressures less than 800 pounds per square inch commonly treat make-up in a lime-soda softener, where large quantities of carbon dioxide are introduced. The chemicals used for minimizing condensate corrosion in plants of this type are octadecylamine, octadecylamine acetate, and related materials, which afford protection by virtue of their film-forming properties. Many instances are reported of corrosion rates of only a few milligrams per square decimeter per day on steel test strips installed in condensate return lines following the addition of 2 to 3 pounds of a filming amine to the boiler water for each million pounds of steam generated. I n some of these plants corrosion rates before amine treatment exceeded 150 mg. per square decimeter per day. Similar trials with copper test strips have shown reductions in corrosion rates from 10 down to several tenths of a milligram per square decimeter per day. Treatment with filming amines often permits startling savings in maintenance and chemical costs. A case history that illustrates this point involves a n oil refinery that used a high bicarbonate well water as make-up. Sulfuric acid was fed to the water after it was treated in an ion exchange softener regenerated with salt. T h e water was further purified in evaporators, where carbon dioxide was removed. The combined cost of acid and maintenance to the acidfeeding equipment and feedwater lines was so great that the plant was willing to discontinue acid treatment and try to prevent condensate corrosion with a filming amine. This expedient proved successful, and it is now estimated that $10,000 worth of filming amine .is saving the refinery more than $100,000 per year. Because filming amines have thus far found their greatest use in relatively low-pressure steam plants, the pressure and temperature limitations on their use have not yet been ascertained. To date, filming amines have been used

successfully in boilers operating a t a pressure of 1290 pounds per square inch with steam superheated to 950" F. Contaminants Other Than Corrosion Products

Water Inleakage. Impure water may enter condensate from various sources, most commonly hydraulic glands on the low pressure ends of turbines, turbine condensers, and improperly sealed cross connections between the condensate and service water systems. Raw water entering the system can create difficulties with boiler water control, increase the blowdown requirements, and precipitate scale in boiler tubes. Inleakage a t turbine glands usually means that the carbon seal rings have become worn and should be replaced. Using condensate in the glands, so the leaks will have no serious consequence, may provide a temporary solution. Inleakage of raw water at turbine condensers may be caused by failure of the condenser tubes or simply by mechanical defects, such as improper rolling of the tube ends into the tube sheets. Corrosion by the cooling water is usually the cause of condenser tube failures and the difficulties from this source can be minimized by selecting the best alloy for the particular cooling water. Because good heat transfer and corrosion resistance are of primary importance in the selection of condenser tubes, copper alloys are used almost exclusively. Admiralty brass has been found to be the

Figure 7. Upper. lower.

best all-round alloy for condenser tubes, but a great variety of alloy compositions is available for special situations. A sensible way to find the best alloy for a particular plant is to install a number of sample tubes in the condenser and check their performance periodically. Dezincification is probably the most common type of corrosion on the cooling water side of condenser tubes, although in recent years inhibitors such as tin, arsenic, and antimony, added to Admiralty brass, have greatly increased its resistance to this type of attack. However, slime and particles of debris clinging to the surface of the tube may concentrate corrosion in small areas, a phenomenon generally referred to as plug-type dezincification. Figure 7 illustrates both plug-type and layer-type attack. The dark plugs in the uppel picture ale redeposited copper, which is porous and permits leakage of cooling water into the condensate. Cooling water must sometimes be treated to forestall failures that might result in contamination of the condecsate. Chlorination to kill slime-forxing organisms and chemical coagulation to settle out suspended matter can reduce the danger of plug-type dezincification. If there are many mechanical leaks in the condensers and if extremely adverse cooling water conditions lead to frequent tube failures, some plants install a sensing device, such as a conductivity meter, in the condensate return line. When the condensate becomes contaminated beyond some predetermined toler-

Dezincification

Plug-type dezinciflcation in Admiralty brass condenser tube Layer-type dezinciflcation in similar tube

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able limit, the meter sends a signal to an automatic dumping system or gives warning to the operators that the condenser must be repaired. Oil Inleakage. A frequent source of condensate contamination is the lubricating oil used in reciprocating engines. Oil returned to the boiler with the condensate acts as a binder for boiler water sludge that may then stick to tube surfaces and cause overheating. If the oil contains saponifiable matter, the boiler water may foam and cause serious carryover. Carryover can result in superheater tube failures, and fouling of turbines, heat exchangers, and traps. Oil can be removed by passing the oily condensate through leaf-type filters precoated with diatomaceous earth, through anthracite filters employing aluminum hydroxide floc as a filter aid, or chrough simple filters containing coke, charcoal, natural or synthetic sponges, or toweling as the filtering medium. Choice of filtering methods depends upon economic considerations and the degree of oil removal required. A few plants return oily condensate to their lime-soda softeners, where precipitated calcium carbonate adsorbs the oil. Although this expedient removes a good deal of the oil, it may be questionable practice for economic or operational reasons. Oil from reciprocating engines is often removed from the system by mechanical steam separators, or by discarding the exhaust steam (or its condensate) altogether. Process Contaminants. The variety of plant process materials that may contaminate condensate is almost unlimited. Such diverse substances as tar, black liquor, starch, rosin, rubber, sugar, tomato soup, and gravy have been returned to boilers. Some of these contaminants resulted in boiler deposits, some interfered with boiler water testing, and others caused carry-over or corrosion. In the case of contamination with rubber, hydrogen sulfide was evolved with the steam and caused corrosion in the valves in the condensate return system. The plant that found gravy in its condensate used high pressure steam for operating turbogenerators. I n the plant cafeteria low pressure steam was used for heating food in jacketed kettles. To the pressure alarm valve on one of the kettles a swing pipe was attached that could be inserted in a pot of gravy and used for warming. The practice was to tie down the chain on the alarm valve and turn off the inlet valve to the kettle when the gravy became hot enough. With the inlet steam valve turned off, the jacket cooled and sucked gravy in from the pot. The gravy then entered the condensate system through the trap a t the bottom of the jacket. The fat in the gravy was saponified in the boiler, where it caused foaming.

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The subsequent carryover resulted in extremely costly damage to a turbine. I n this instance the solution to the carry-over problem was obvious, but a considerable amount of sleuthing was required before the cause of the problem was found. I n other instances where process contaminants enter the condensate system following failure of a heat-transfer surface owing to the corrosive action of condensate, the feed of neutralizing or filming amines may be the solution. Frequently, only that segment of the system where corrosion is a problem needs to be conditioned. Process materials may enter the condensate system after a heat exchanger has been idle for a prolonged period. If the valve in the steam line is not tight during standby periods, steam can leak into the equipment and keep it moist. Air leaking in through a vent will complete the requirements for a very corrosive environment. When steam heating equipment is not in use, it should be kept completely dry or completely filled with water containing suitable corrosion inhibitors. Sometimes process equipment fails because the material in process is corrosive. If this difficulty cannot be overcome by selecting proper materials of construction, dumping of the condensate may be necessary. When the contaminants are electrolytes, impulses from a pH or conductivity meter may be used to do this automatically. Past, Present, and Future Trends

Thirty years ago boiler operators returned condensate to their boilers only because it wa$*&eap. I t did not need much pretreatment. Efficient deaeration was not required, because boilers were conservatively designed and some oxide accumulations could be tolerated. If deposits became troublesome, the boilers could be cleaned mechanically. Nevertheless, as plants tightened up their systems and the purity of condensate became better, operators and designers realized that they could edge up on pressures and temperatures, which meant more economical operation. Today some boilers are built to operate a t pressures and temperatures that require practically pure water. Evaporation and deionization make it possible to provide high purity make-up to these units, but such pretreatment is costly, so it is mandatory that large quantities of high quality condensate be re-used. Modern pretreating facilities, efficient deaerating equipment, proper materials of construction, devices for policing the system, and chemicals for conditioning steam are all tools that make the re-use of condensate possible. Future trends in the power generation

INDUSTRIAL AND ENGINEERING CHEMISTRY

field focus our attention on atomic energy and operation of boilers at supercritical pressures. Until practical means of converting atomic energy directly into electrical or mechanical energy are found, boilers will remain with us and the associated water problems will not be changed because of this new source of energy. O n the other hand, operation above the critical point may give a new look to the problems involved in the re-use of condensate. There is an indication ( 9 ) that steam at supercritical pressures will pick u p significant quantities of iron oxide, the material that normally protects the boiler steel from corrosion. Partridge (70) has speculated that operators of supercritical pressure boilers may be confronted with the problem of keeping their boilers from dissolving in the steam and turning up later as iron oxide deposited in the turbines. literature Cited

Adkins, S. K., “Factors Influencing Metat Loss in Condensate and Eeedwater Systems,” pp. 81, 82, Proc. 14th Annual Water Conference, 1953. Baker, M. D., Marcy, V. M., Trans. Am. Sod. Mech. Engrs. 7 8 , 299-304 (1936). Cerna, W. W., “German Pow-er Plant Steam Generators and Water Conditioning Systems,” pp. 1-23. Proc. 7th Annual Water Conference, 1947. Commission 6, Rouilles et Corrosion, Activitt du Centre, May 26, 3955, Centre Belge d’Etude et Documentation des Eaux, p. 183 (June 1955). Hall, R. E., Smith, G. W., Jackson, H. A., Robb, J. .4., Karch, H. S., Hertzell. E. A.. Carnezie Institute of Technology,’Minin