Boilers water alkalinity

Where high residual calcium leakage occurs in the FW, the phosphate reserve is lost and the boiler water alkalinity is reduced. This is not the same problem as hideout, which is the apparent loss ofBWphosphate and other salts in higher pressure WT power boilers operating under high load conditions. 

High boiler-water alkalinity tends to increase carryover, particularly in the presence of an appreciable quantity of suspended matter. This effect may be corrected by various methods, dependent on the cause of the high alkalinity. For example, if trisodium phosphate is being added to the boiler water, a less alkaline phosphate, such as disodium or monosodium phosphate will help in reducing alkalinity. 

Phospha.te Treatment. Calcium phosphate is virtually insoluble in boiler water. Even small levels of phosphate can be maintained to ensure the precipitation of calcium phosphate in the bulk boiler water, away from heating surfaces. Therefore, the introduction of phosphate treatment eliminates the formation of calcium carbonate scale on tube surfaces. When calcium phosphate is formed in boiler water of sufficient alkalinity, a particle with a relatively nonadherent surface charge is produced. This does not prevent the development of deposit accumulations over time, but the deposits can be controlled reasonably well by blowdown. 

There are two possible causes. The first could be incorrect control of water treatment and blowdown. This can result in excessive levels of suspended solids in the boiler water, organic matter in the boiler water or high alkalinity. The second can be mechanical. If the boiler is operated below its designed working pressure it will increase the efflux velocity of the steam leaving the water surface area to a point where it may lift the water surface and drop the water level. It is important therefore to give due consideration to the steam load required from the boiler. 

Total solids, alkalinity, silica Organic matter Boiler water... 

In an ideal case the condensate return is high, and the raw water low in dissolved solids, hardness and alkalinity. It is then possible to operate the boiler without external water treatment, relying on conditioning of the boiler water with phosphates, tannins or other chemicals to cope with the small amount of hardness introduced with the raw water. 

The boiler-water standards for shell boilers, as given in BS2486 1978 for 2, 20 and 40 bar, respectively, indicate that some parameters are fairly insensitive to pressure, whilst others are not. Thus, sodium phosphate should fall within the range 50 to lOOmg/kg and sodium sulphite between 30 and 70 mg/kg for each of these pressures. Total alkalinity should have a maximum value of 1 200 mg/kg at 2 bar and 700 mg/kg at both 20 and 40 bar, with the corresponding caustic alkalinity at 350 300 and 200 mg/kg. Silica should be less than 40% of the caustic alkalinity in each case. Total hardness should be undetectable in all cases. Dissolved solids maxima are given as 3 500, 3 000 and 2000 mg/kg, again at 2, 20 and 40 bar. 

These factors severely enhance the risks of condensate system corrosion by carbonic acid (resulting from a breakdown of the alkalinity in the boiler water and carbon dioxide [C02] carryover into the steam) and BW carryover. In addition, boiler operation is more difficult because the possible COC is severely limited, there-... 

NOTE For lower pressure boilers, the maximum permitted level of boiler water silica (as ppm Si02), typically is 0.4 x caustic alkalinity (as ppm CaCOf). This method of control is satisfactory as far as it goes, but it does not solve the problem of the risk of silica deposits in the pre-boiler section. 

We have been able, however, on occasions to use a very simple model to help understand specific plant problems where river water analyses were available and on one occasion to show that at different times the boiler water had (as corrosion evidence suggested) alternated between acidic and alkaline conditions. The model assumes that by 350 C any normally dissociated multi-charged ions will be sufficiently unstable that they will undergo whatever appropriate hydrolysis reactions can reduce their charge to unity. Whether the water goes acid or alkaline then simply depends on whether the total (equivalent) concentration of multiply charged cations exceeds or is smaller than the concentration of multiply charged anions. 

Hot Lime Zeolite-Split Stream Softening. Many raw waters softened by the first two processes would contain more sodium bicarbonate than is acceptable for boder feedwater purposes. Sodium bicarbonate will decompose in (lie boiler water to give caustic soda. Caustic soda in high concentrations is corrosive and promotes foaming. The American Boiler Manufacturers Association has adopted the standard that the alkalinity content should not exceed 20% of the total solids of the boiler water. Split stream softening provides a means for reducing the alkalinity content. 

The presence of acidic gases in steam makes the condensate acidic with consequent corrosion of nielal surfaces. Ill such cases, the corrosion rate can be reduced by feeding to the boiler water chemicals that produce alkaline gases in the steam. The addition of neutralizing and filming amines to boiler water or to condensate to minimize corrosion by condensate and feedwater is discussed later under Control of pH. 

Methods 1 and 2 are intended to control the boiler water pH and to precipitate the calcium and magnesium compounds as a flocculenl sludge, so that they can be removed in the boiler blowdown rather than being deposited on heat-transfer surfaces. Method 1 maintains an excess of hydroxide alkalinity. The effects of alkalinity are discussed later under Steam Purity. Method 3 involves the addition of a complex mctal-chelant compound such as ethylenediainine-tetraacetic acid i a.(FDTA) or niirilolriacelic acid (NTA). In Method 4, as ihe name implies, no solid chemicals are added to the boiler or pre-boiler cycle. The pH of the boiler water and condensate cycle is controlled by adding a volatile amine. 

The recommended phosphate concentration for a given boiler operating pressure is shown in Fig. 8. At the higher pressures, comparatively low phosphate residuals must be maintained in order to avoid appreciable phosphate hideout. Hideout is the term used to identify the phenomenon of the temporary disappearance of phosphate in the boiler water upon increase in load and its reappearance upon load reduction. The recommended alkalinity as a function of pressure is given in Fig. y. 

Mechanical carry-over is the entrainment of small droplets of boiler water in the separated steam. Since entrained boiler-water droplets contain solids in the same concentration and proportions as the boiler water, the amount of impurities in steam contributed by mechanical carry-over is the sum of all impurities ill the boilei water multiplied by the moisture content of the steam. Foaming of the boiler water results in gross mechanical carryover. The common causes of foaming are excessive boiler-water solids, excessive alkalinity or the presence of certain forms of organic mailer, such as oil. 

With long-chain hydrocarbons, such as oil, where hydrophobic and hydrophilic portions of the molecule exist, surface-tension effects will be significant, and may result in foaming and possible acceleration of carryover. The alkalinity of the boiler water may saponify any fatty acids present, producing a crude soap that may foam. Soaps, sulfated oils and alcohol, sulfonated aliphatics and aroma.ics, quaternary ammonium compounds, nonionic organic ethers and esters, and various fine particles, act as emulsifiers that can increase foaming. 

The indophenol method has been applied for determination of nitrogen (as ammonia) in biological materials [31,32], plant materials [33,34], foods [1,2,35], air [36], boiler water [37], and other waters [38-40], organic substances [17,41], refractory alloys [42], tantalum alloys [43], vanadium, titanium, and uranium [21], alkali and alkaline earth metals [22],... 

Beilstein Handbook Reference) AI3-03947 Aminomethyl propanol Aminomethylprop-anol AMP AMP 75 AMP 95 AMP Regular BRN 0506979 Caswell No. 037 Corrguard 75 EINECS 204-709-8 EPA Pesticide Chemical Code 005801 HSDB 5606 Hydroxy-tert-butylamine lsobutanol-2-amine KV 5088 NSC 441. Boiler water treatment ohemioal, corrosion inhibitor, carbon dioxide absorber. Widely used as a buffer and phosphate acceptor in assay of phosphatases. Suitable as buffer for manual and automated determination of alkaline phosphatase using 4-nitrophenyl phosphate as substrate. Solid mp = 25.5° bp = 165.5° d O = 0.934 soluble in CCI4, freely soluble in H2O LDso (rat orl) = 2900 mg/kg. Lancaster Synthesis Co. Lancaster Synthesis Ltd. 

Figure 7.63 Significance of oxygen and chloride content for SCC of an austenitic stainless steel in steam from alkaline-phosphate treated boiler water with intermittent wetting. (From [7.1] after Lee Williams.)...

Recently, a class of alkaline salts called sequestrants has come into general use for water treatment to prevent scale formation and for periodic removal of both water scale and corrosion products. The most useful examples are derived from an organic acid called ethylenediaminetetraacetic acid (EDTA). The sodium salt dissolves water hardness scale, while the ammonium salt is now being used to remove iron oxides and copper from high pressure steam generators. A related compound is nitrilotriacetic acid (NTA). The sodium salt is used in boiler water treatment. 

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