PHOSPHATE. PURPOSE: To remove boiler water calcium as sludge instead of hard scale. * * * * * * * * * * * * *

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1 PHOSPHATE PURPOSE: To remove boiler water calcium as sludge instead of hard scale. * * * * * * * * * * * * * Some calcium is present in the feed water in almost all steam generating plants. In the process of steam generation, calcium is concentrated in company with other components of the boiler water, tending to precipitate eventually as calcium scale (calcium carbonate, calcium sulfate, and/or calcium silicate). In the absence of protective internal treatment, the rate of scale accumulation depends upon the calcium content of the feed water and the rate of steam generation. Calcium scale is extremely objectionable; it may obstruct heat transfer and cause metal to fail by over heating, or it may slough off and then collect in places where it can seriously interfere with heat transfer and/or fluid flow. Phosphate is used specifically to remove calcium from solution so it cannot produce calcium scale. Under proper conditions, the reaction between calcium and phosphate produces a finely divided, nonadherent sludge (calcium phosphate) which remains suspended in the boiler water until removed through blow down. Prevention of calcium scale requires that the boiler water contains a small reserve of soluble phosphate at all times. A carefully designed phosphate test is used to determine this important constituent in the boiler water; and it is most essential that this test be performed on perfectly clear samples which contain no particles of "spent" phosphate in the form of suspended calcium phosphate sludge. The amount of reserve phosphate required in boiler water depends upon the calcium content of the feed water, boiler pressure and the type of boilers involved, blow down procedure, and the chemical feeding method employed. Maximum and minimum phosphate control limits are normally ppm. Loss of the reserve of the phosphate must almost inevitably result in deposition of calcium scale. Therefore, you should never hesitate to increase the feed of phosphate whenever boiler water phosphate concentrations run near or below the specified minimum. Maintaining unnecessarily high phosphate reserves is wasteful of phosphate, a relatively 1

2 expensive treating chemical. Furthermore, if the feed water contains magnesium--as most feed waters do-- high phosphate may result in precipitation of magnesium phosphate, a very objectionable type of adherent sludge. In general, therefore, it is undesirable to maintain phosphate reserves in excess of the recommended maximum limit. Reduce the rate of phosphate addition whenever boiler water phosphate concentrations approach or exceed the upper limit. The purpose of phosphate treatment is to precipitate all boiler water calcium in the form of calcium phosphate sludge. Although calcium phosphate is normally nonadherent, it may come down in adherent form if boiler water alkalinity is too low; therefore, proper conditioning with phosphate also requires that boiler water alkalinity be maintained within specified limits. Prevention of objectionable magnesium phosphate necessitates that a certain relationship exist between boiler water phosphate and silica concentrations; this frequently calls for the use of a silicate chemical in conjunction with phosphate, the purpose being to favor precipitation of desirable magnesium silicate in preference to undesirable magnesium phosphate. A suitable dispersive material is sometimes used to help keep the precipitated sludge from settling out or otherwise adhering to the boiler surfaces. Phosphate may be fed in solution with almost any combination of other water conditioning chemicals. Either continuous or intermittent feeding is appropriate to introduce phosphate solution directly to boilers. Unless feed water hardness is quite low (1 to 2 ppm at most) phosphate should not be fed continuously to feed water systems; the danger of feed line scale deposition would be to great. However, solutions of sodium hexametaphosphate can be fed intermittently in shots to almost any feed water system,provided each shot is added quickly (within 3 minutes) with no " dribbling" at the beginning or end of the feeding period. When phosphate is fed intermittently, the duration of any one shot should not exceed 5% of the time interval between shots. The usual method for feed of phosphate, is to tie the chemical feed pump into the feed water pump. In this manner, the phosphate is fed when the boiler makes up. CHELANTS PURPOSE: To remove cationic deposit forming salts, as calcium and magnesium, 2

3 through the formation of a soluble by-product. * * * * * * * * * * * * * Some cationic salts of calcium, magnesium and in many cases iron, are present in the boiler feed water. These salts will concentrate up in the presence of other boiler water components during the process of steam generation. In that these materials are inversely soluble in water as the water temperature increases (as the water temperature increases, calcium magnesium and iron lose the ability to remain in solution and drop out as a hard solid precipitate). This inverse solubility causes these salts to drop out of solution in the boiler water and form an insulative deposit on the boiler metal surfaces. This insulative deposit will then obstruct heat transfer across the tube which leads to; metal failure due to overheating, fuel wastage, and inhibition of fluid flow. A chelant prevents the formation of these deposits by reacting with the cations (positively charged particles of calcium, magnesium and iron) in the feed water, to form a soluble salt. Hence, if a test tube of hot feed water were to have a scoop of calcium carbonate added to it, it would appear to have a heavy adherent white sludge on the bottom of the test tube. Add to this test tube a sufficient amount of chelant to react with the calcium carbonate and the test tube will appear to have nothing but clear feed water in it. The chelant reaction has created a soluble salt with the calcium and this reaction has made the calcium soluble enough to be brought back into solution. Chelants can therefore provide improved boiler efficiency over most other internal programs, by removing the deposition potential from the boiler water. In addition, a chelant will seek calcium and magnesium in deposits that have previously formed in the boiler. This procedure is called a "CHELANT CLEAN UP" and it essentially is the process whereby old deposits are slowly removed by solubilizing the deposits binders (calcium and magnesium), thus allowing the deposit to slowly slough off of the tubes. Chelant clean up programs are enhanced with the use of dispersants. Dispersants improve the ability of the chelant to remove old deposits by assisting in the suspension of the sludge formed from the chelant clean up of deposits. It is important that the dispersant being used in this case is specific to the type of deposit that is being removed, for optimum performance. The two chelants usually used in boiler water treatment are: EDTA (ethylenediaminetetraacetic acid) and NTA (nitrilotriacetic acid). The molecules that these chelants form with boiler water cations are very stable, so they stay in solution. This removes the cations from availability to react and form; calcium carbonate, calcium sulfate, magnesium hydroxide, magnesium silicate, and iron compounds. The small molecular size of the chelant molecule is what allows the combination of feed water 3

4 cations and chelants to be so stable and as such so soluble in the boiler water. Therefore, this reaction creates a soluble salt rather than a solid precipitant in the boiler. This solid salt formation is one that reduces the level of solids in the boiler. Reduction of solids will then allow the system to reduce the quantity of blow down from the boiler. All of this relates into; less fuel wasted in blow down, a cleaner boiler, and with a cleaner boiler, a more efficient fuel to steam operation. A chelant should be fed through a stainless steel quill. The quill is to be placed in the feed water line, down stream of the feed water pump and at least five feet up stream from any bend in the feed water line. The proper installation will prevent copper loss from the feed water pump impeller as well as any erosion of the feed water piping at elbow bends, while allowing maximum contact between the chelant used and the feed water that is to be treated. Feeding a chelant to the feed water line has the following advantages: * The chelant can react to form a soluble molecule prior to entering the boiler, thus, drastically reducing the potential for boiler deposit formation. * EDTA breaks down at 300 psi and NTA will break down at 900 psi. Their chelant/cation salt complexes are both stable up to 1300 psi. Thus, the maximum benefit from using a chelant can be obtained from the use of feed water line application. To realize the greatest benefit both technically and economically from the use of a chelant, the feed water must be of high quality. The allowable maximum limits for contaminants in the feed water for the effective use of a chelant are: Total Hardness 0-2 ppm Iron 0-2 ppm Must Be Oxygen Free Should the contaminant levels in the feed water be higher than the above limits, then a chelant program will most likely not be economical for use in the boiler water treatment program. Chelants can also be used in combination with other internal treatment programs, based upon the system conditions. In a plant with high quality feed water and good control, a chelant/polymer program would be the most effective. Where feed water quality and control are good, but not of the highest quality, then a phosphate/chelant/polymer program should be used by the boiler plant. Chelant/polymer programs have the potential for being the most effective scale preventative programs for boilers that are supplied with high quality feed water. The polymer (dispersant) 4

5 used in this type of program should be selected based upon the type of contaminants that are found in the particular feed water that is being treated, or the type of deposit that has been found in the boiler. This selection will allow the polymer to compliment the chelant properly. Phosphate/chelant/polymer programs are usually used where the chelant is of a sufficient level to react with the majority of the scale forming agents in the feed water, and the phosphate is then used to tie up the remaining calcium salts, while acting as a product level indicator. This type of a chelant program approach is referred to as "sub stoichiometric" chelant use. The basic concept is that the total chelant requirement for the boiler to be treated is calculated, and then 80% of this chelant requirement is fed. The approach yields practically every benefit achieved with a chelant program, without the possibility of a potential chelant overfeed. The method is also used because of its ease of testing. The phosphate in the product allows for a simple method for the testing of product level, while acting as a buffer against chelant corrosion. Chelants are very different to test for accurately. Due to this problem, many times, a plant will show adequate chelant levels while in reality the chelant demand is much higher than the tests indicate. Even more serious, many of the chelant tests used today will show insufficient levels of chelant when there is actually an overfeed situation occurring in the boiler. In this case, the boiler metal will be eaten away while the operator is increasing chelant feed rates to over come what they think is a scaling condition. With this type of difficulty, many programs are designed for base feed. This uses the quantity of make up or feed water used in the boiler in a day, and the total ppm of cations in the make up or feed water to establish a feed rate for the chelant. In most cases, this procedure is accomplished using a simple chart. In order to simplify matters, many chelant programs go back to the phosphate/chelant/dispersant approach for accuracy of product level. They also use this approach, because of the additional benefit of anion competition between the chelant and the phosphate. this competition creates a condition where the phosphate will act as a buffering agent against chelant attack on the boiler metal, in the event of an accidental chelant overfeed. Chelant use is extremely beneficial, as long as it is used properly and under the proper conditions. DISPERSANTS/SLUDGE CONDITIONERS PURPOSE: To minimize tendency of sludge to adhere to boiler surfaces. To fluidize all solids, thus, enhancing solids removal from the boiler through blow down. * * * * * * * * * * * * * Hardness and other solids entering the boiler will tend to be precipitated as insoluble solid 5

6 particles. The calcium is precipitated as calcium phosphate while the magnesium comes down as magnesium silicate or occasionally, magnesium hydroxide. Representing the desired end products of good internal treatment, the calcium phosphate and magnesium silicate normally occur as finely divided sludge, most of which remains suspended in the boiler water until removed through blow down. If a chelant is utilized, the volume of precipitant will be greatly reduced as EDTA will combine with hardness to form a soluble salt. In any case, proper dispersant program will greatly enhance the effectiveness of any treatment program. In specific cases, sludge may tend to remain in the boiler, either as a shoal or sediment in a header or as an adherent coating on boiler surfaces, especially near the point where mixing of feed water with boiler water occurs. Use of a dispersive represents one means employed to keep sludge from accumulating in boilers. Disperants behave something like this: when particles of sludge first start to form and are still very tiny, they become encased in a sheath of organic material which keeps them from growing larger or combining with one another to form adherent or rapidly- settling masses. These groups attach themselves to an active site in the growing crystalline scale. The presence of the large polymer molecule on the growing crystal alters its growth. The result is crystal distortion and particle repulsion of the subsequently formed colloids in the boiler water. The effectiveness of a polymer is determined by its type, molecular weight and concentration. Generally speaking, the higher the molecular weight of a polymer, the greater its ability to act as a coagulant. On the other hand, the lower the molecular weight, the more a polymer will act as a dispersant. It is the anticoalescent or dispersancy characteristics of the polymer which enable it to function as a boiler water sludge conditioner. The proper application of a synthetic polymer in a boiler water treatment program will change a poorly hydrated, granular and adhesive boiler sludge into a light and fluffy, highly hydrated sludge which is non-adherent to waterside surfaces. Dispersants in general are synthetic polymers. A synthetic polymer is a long chain molecule of repeating units, with each unit being an active group. The most commonly used synthetic dispersants are: POLYACRYLATE Good for calcium carbonate conditioning. Forms a light non-adherent sludge. Can disperse phosphate salts in low alkalinity conditions. POLYMALEIC Crystalline modifier for calcium carbonate and phosphate salts. Excellent clean up material for boilers with calcium or phosphate based scale. Very stable polymer that can 6

7 bring scale off of metal surfaces by pulling the scale binder out of the scale into the boiler water. POLYMETHACRYLATE SULFONATED COPOLYMER CARBOXYLATED COPOLYMER A second generation acrylate that reacts similar to Polyacrylate. Should not be over fed as it could break down. More active than Polyacrylate on calcium salts. Highly charged anionic dispersant that can specifically attach to metallic ions as soluble iron in the boiler. Extremely active dispersant for the suspension of calcium salts. Also, very effective on iron deposits. TERPOLYMER A combination polymer containing, carboxylated, sulfonated and nonionic plane repulsion members. This makes up a dispersant with three dispersant groups. A strong acid (sulfonate) for surface charge repulsion on the particle, a weak acid (carboxylate) for surface charge attachment on the particle, and a nonionic dispersant for broad range dispersancy of particle charge surfaces. It will disperse both hydrated and nonhydrated iron oxide sludge as well as phosphate and calcium salts in both the tricalcium phosphate and the hydroxyapatite forms. Dispersants should be fed on a continuous basis to the boiler feed water in order to be able to continuously supply fresh product to the region of the system where precipitating reactions are taking place. Since there are no satisfactory field tests for the control of a dispersant, they are usually fed in a proportional manner, using the amount of make up water, feed water or in relation to another product which there is a easy field test for. Thus, although the minimum and the maximum limits are specified for the dispersive concentration in the boiler water, treatment is actually based on the feed of (X) number of pounds of dispersant per million pounds of Make Up water, with consideration for the amount of blow down from the boiler. The addition of dispersant is based on: Required dosage in ppm X Millions of pounds of Make Up per Day = #/Day Dispersant Cycles Make Up Cycles are found by: 7

8 Boiler Neutralized Conductivity = Make Up Cycles Raw Water Conductivity 8