-Boiler
PANEL DISCUSSION.
Water Chemistry
..
What’s New in Boiler Water Chemistry J
A. A. BERK, iMODERATOR
E. P. PARTRIDGE,
Zndustrial Water Branch,
COMODERATOR
Bureazc of Mines, College Park, M d .
Hall Laboratories, Pittsburgh, Pa.
T h e purpose of this session is to present an omnibus report on recent developments in the field of boiler-water chemical engineering. A panel of experts is ready to summarize for you the newer literature and, in some instances, unpublished information on chemical cleaning of boilers; on preparation of the boiler feedwater; on treatment to prevent corrosion, encrustation, and carry-over j and on chemical treatment to prevent corrosion by condensed steam. The evolution of the modern steam generator has been made possible by a partnership of advances in mechanical engineering design, progress in physical metallurgy, and new chemical engineering skills in water conditioning. It is an indication of this evolution that embrittlement cracking of boiler seams, relatively recently an important topic of panel discussions, can now be covered briefly as a minor aspect of boiler corrosion that we have learned to control. The emphasis now is on economy of fuel through clean boilers and clean turbines; the preservation of the expensive boiler and its auxiliaries from corrosion, fouling, and over-heating j and the dependable availability of the steam-generating and steam-utilizing plant. Each of the experts will summarize “what’s new” in his assigned topic.
Internal Cleaning of Boilers J. R11. MALONEY Hall Laboratories, Chicago, I l l .
R
EMOVING scales and deposits formed on the evaporative metal surfaces of steam generating units has been successfully accomplished for the past several years by means of inhibited acid solvents. Because of the dangers involved in most chemical cleaning practices plus the difficulties encountered in adding large volumes of acid to the unit and controlling both temperature and acid strength, this work is usually assigned to specialists engaged in acid cleaning operations. In general, t h e chemical cleaning procedure is done in four major steps: May 1954
1. Sufficient inhibited acid solvent is added to the boiler through a lower boiler coniiectioii and the proper acid strength and temperature are maintained. 2. The chemical solvent remains in the unit from 6 to 8 hours. This is appropriately termed the “soaking period.” 3. The boiler is drained and sufficiently rinsed with clear water. 4. Finally, metal surfaces are neutralized by means of a 3to 4-hour boil with an alkaline solution.
The basic solvent used in removing the common constituents found in boiler deposits is approximately a 5 % solution of hydro-
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chloric acid. However, the concentration varies somewhat depending on the type and amount of deposit that is to be removed. Sufficient inhibitor is mixed with the acid solvent to prevent any appreciable acid attack on the metal surfaces. All boiler deposits are not equally susceptible t o the action of any one standard cleaning solution; so it is frequently necessary to use auxiliary chemicals along with the inhibited acid solution. Some of these additives are classified as intensifiers or wetting agents. Their usual role is to increase the solubility of the deposits encountered and to increase the ability of the solvent to wet and penetrate the scale. For example, where boiler deposits have a high content of silica or complex silicates, it is usually necessary to add hydrofluoric acid or fluorides to the solvent. Carbonaceous matter that is occasionally present in boiler deposits may complicate the cleaning technique further. Here, chemical cleaning in several stages may be required to do a satisfactory job. Preliminary boil-out using an oxidizing-alkaline solution followed by normal treatment with inhibited hydrochloric acid has been successfully used where boiler deposits had high ignition losses, indicating deposits high in organic matter. I t is impossible in such a short discussion to point out the many complexities that are encountered in chemically cleaning boilers. However, the term "acid cleaning" does not adequately describe the cleaning procedures so often found necessary in ridding boilers from unwanted deposits and/or scale. DEVELOPBIENTS IN NEW IRHIBITORS
Acid solvents have a greater effect on disintegrating boiler deposits a t high temperatures, but at the same time the danger of attacking boiler metal is increased since many of the inhibitors used today break down under high temperatures and their effectiveness is considerably lessened. For this reason, the temperature of the cleaning solution in contact with the boiler metal is usually held between 130' and 150" F. Newly developed acid inhibitors are reportedly stable up to 190" F., and they still provide adequate metal protection. Inhibitors having a higher breakdown temperature decrease the outage time on the boiler in addition to doing a more satisfactory job of deposit removal. Where stressed boiler metal has different crystalline structures, it is necessary to use an inhibitor that will provide adequate protection in the stressed areas of the boiler. hTewlydeveloped surface amino inhibitors, reportedly, are more effective in protecting such boilers than the older, more common, nitrogen-sulfur coal tar inhibitors when different crystalline structures are encountered. Acid inhibitors have yet to be developed that ndl completely check acid corrosion of boiler metal. However, in the majority of cases, such attack is considered inconsequential. AUXILIARY BOILER DRAINS
The time required for draining chemical solvents and the following rinse waters from the boiler has been sizably lessened by employing auxiliary boiler drains. These are especially effective where the only existing boiler drains are small blowdown valves. Special manhole or handhole covers equipped with a siphon attachment have been successfully employed to speed up the chemical cleaning program. I n addition to trimming down on boiler outage time this special equipment provides an additional surge of the cleaning solvent through the boiler tubes.
Although protective iron oxide film formations are essential to the future life of the boiler, it is important in many instance3 that the amount of such afterdeposits be controlled. Displacement of the cleaning solvent and the water rinse with nitrogen has been a successful tool in many recent acid chemical cleanings. The prevention of the air contact with boiler metal by the nitrogen displacement method considerably reduces the afterrusting problem encountered in many instances. Phosphoric acid has been used in place of hydrochloric acid in some acid cleaning techniques where rust and mill scale removal from the boilers were involved. The principal advantage of phosphoric acid cleaning is that the solvent can be boiled in the unit by direct firing of the boiler with negligible attack of the metal. Phosphoric acid exhibits other properties that offer merit as a chemical cleaning solvent. However, it seems doubtful that phosphoric acid cleaning will prove adequate in removing complex boiler scale offtimes encountered. Too, since phosphoric acid costs almost five times as much as hydrochloric acid, it appears that hydrochloric acid will remain the basic solvent for some time to come. CHEMICALLY CLEANING S E W BOILERS
Before new boilers are put into operation, thorough cleaning is required to remove oils and greases found on the metal surfaces. Some of the older cleaning methods have not been entirely satisfactory-that is, harmful sludges may be formed that intermingle with the oil and grease already present and make repeated cleanings necessary. Unpredictable results in removing undesirable organic materials from new boilers indicate that a more efficient alkaline boil out is necessary, A high porered alkaline commercial detergent, essentially a mixture of metaphosphate and silicate, has bcen used with a great deal of success in many recent applications and appears to be a much added improvement over some of the older techniques. Usually, one cleaning is sufficient to remove all the greases and oils present in a new boiler. The most important factors that should be considered in using this material for cleaning new boilers are: 1. Sufficient mixture must be used so that all calcium is precipitated as phosphate, all magnesium is precipitated as silicate; all oil is saponified or dispersed; and all dirt is dispersed. An alkalinity of at least 275 p.p.m. as sodium hydroxide must be maintained during the boil-out. 2. Sufficient circulation must be obtained so that all surfaces are thoroughly flushed during the boiling period.
The silica concentration may be unusually high when the boilc is filled and placed in service owing to residual silicate from the cleanser. Usually, this is unimportant when raw or treated water is used as boiler make-up. But, where boiler pressures exceed 900 pounds per square inch gage, if feed water is mostly all condensate and steam turbines are operated, relatively high concentrations of silica in the boiler water, even temporarily, are obiectionable. In such a case, a second alkaline boil-out, omitting the special cleanser, is necessary for the complete removal of the silicate. Usually, the second alkaline boil-out will remove all the sili ceous matter left in the boiler. However, should the alkaline boil out fall short of trimming the silica concentration to a safe limit, then a magnesium compound such as epsom salt (MgS047H20) could be added to the boiler to precipitate the soluble silica excess as the soft, easily removed sludge, magnesium silicate.
AFTERRUSTIlVG
When the boiler is being drained of the acid and water rinses l"o1lowing the chemical cleaning, the metal surfaces become exposed t o acid and water vapors in the presence of air. This situation brings about initial rusting of the boiler metal, with the rust varying in composition according t o the temperatures involved. 984
What are the names of new inhibitors? What is the best inhibitor for hot hydrochloric acid? How much is required? Wha.t are specific inhibitors, and how much is used to inhibit, say 5% hydrochloric acid? What 5Jms have amino inhibitors
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 5
Boiler Water Chemistry available? Are the normal alkyl fatty amines effective inhibitors? J. M. MALONEY: Most of the questions are related, and some involve trade secrets known only to the specialists and the cleaning companies who do this type of work. I don’t have a list of all the inhibitors. Perhaps there is somebody in the audience who could provide a t least a partial answer to some of the questions. FRANK E. CLARKE, Naval Engineering Experiment Station, Annapolis, Md.: In my opinion, use of 190” F. cleaning acid is dangerous. It does dissolve deposits faster and some inhibitors are effective a t this temperature. On the other hand, i t is risky to assume that a boiler can be filled with 190” F. solution without esceeding that temperature and thereby damaging the inhibitor at some time during the operation. K e prefer to acid clean a t approximately 170” F. All the principal manufacturers of inhibitors market products that are effective a t this temperature. Dowell A-25 or A-30 and Rodine 101 are typical products. Good inhibitors will provide adequate protection a t acid concentrations up to 20% by weight. ADDITIONAL COMMENT: I have tested quite a few inhibitors for various chemical companies. In general, amines of above about 8 carbon atoms are quite effective. I think the dibutyls are good materials. If the person who asked that question would like to talk to me, I might be able to give him some specific information. Do you recommend acid cleaningnew boilers? J. M. MALONEY: If one is interested in seeing what imperfections are in the boiler metal, that is a good m y of finding them out. Others who have been disturbed with long-lived iron oxide deposits prefer t o acidize a new boiler. Black water present in the boiler water for a long while would be another reason for acidizing the new unit. More people do notxlo it than do it, as far as I know. Whether I recommend it or not would be very hard to say. R. F. ANDRES, Dayton Power and Light Co., Dayton, Ohio: We acid clean every new unit. We have found that there are as high as 1000 to 1600 pounds of mill acale in a new boiler and feel
that this material must come out of the boiler in some way. We have calculated the cost of blowing down a boiler to remove mill scale and find that it is cheaper to acid clean the boiler than it is to blow it down. We have acid cleaned boilers and have been able to reduce the black water condition in a matter of about 72 hours to the point where the boiler water is crystal clear. We feel this has a definite merit with regard to keeping our turbines clean. Dismantling a turbine is an expensive proposition that some companies have been faced with owing to iron oxide carryover from a boiler newly placed in service. So, as a matter of insurance, we are acid cleaning all new boilers. It was mentioned that phosphoric acid is about five times the cost of hydrochloric. Is this on the basis of 100% acid in each case? J. M. MALONEY: I don’t have the relative costs of the two acids, but for an equivalent acid cleaning of a particular boiler, phosphoric acid is approximately five times more expensive, based on the figures of Purcell and Whirl. [“Phosphoric Acid Cleaning of Boilers,” Trans. Am. SOC.Bfech. Enyrs., 73, 135-9 (1951).] Which is best t o use, caustic soda, phosphates, or sodium carbonate for neutralizing? J. M. MALONEY: Without saying which one is best, let me say that sodium carbonate or soda ash is the most popularmore so than caustic soda, because it is safer to handle and easier to get into t$e boiler. ,4s far as phosphate neutralizing is concerned, I am not familiar enough with the procedure to answer. What has happened to the old remedy of cleaning boiler tubes with potato peelings? Is there any scientific foundation for such a cleaning procedure? E. P. PARTRIDGE: I think that goes back to the bunch of potato peelings that were inadvertently left in a water boiler by a workman who had been sent t o clean out the scale. You can find the story in an old German engineering publication of about 1830 or 1840. But I do not believe that anybody ever used anything like that specifically to clean out a boiler. The potatoes were put into the boiler while i t was in service or just before it went into service to prevent scale formation.
Preparation of Feed Water Make-up S. B. APPLEBAUM Cochrane Corp., Philadelphia, Pa.
URING the last few years a number of new ion exchange
1)water treating methods have been developed to supplement the older process. These are: 1. Silica removal demineralizing2. Hot lime zeolite 3. Salt splitting by anion exchange following sodium zeolite; this reduces alkalinity or silica without demineralizing. SILICA TO LER 4NCES
Table I shows silica tolerances in the concentrated boiler saline for various boiler pressures suggested by Babcock & Wilcox Co. engineers ( I ) to minimize turbine silica deposits. Table I1 gives silica tolerances in the make-up water (for 5 to 10% boiler blowoff obtained by dividing silicon dioxide in Table I by 10 to 20). May 1954
The residual silica by the three prevailing methods is: Cold process precipitation Hot lime zeolite Demineralizing
Residual SiOz, P.P.N. 2 to 3 0 . 5 to 1 . 0 0.05 t o 0 . 2
Therefore, for boiler pressures exceeding 800 to 1000 pounds per square inch demineralizing is the usual optimum method
TABLEI. SILICA TOLERANCES IN CONCENTRATED BOILER SALINE Operating Boiler Pressure, Lb./Square Inch Gage 600-800 801-1000
1001-1500 Above 1500
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Max. Silica in Boiler Salines, P.P.M. 25
16 5 3
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