Seeking advice on boiler feedwater issues

In neutral or slightly acidic conditions, the calcium and magnesium ions present in the water readily react with carbonate ions to form magnesium carbonate and calcium carbonate; at a pH level of 9 or higher, it is difficult for carbonates to form.

From previous studies, it seems that carbonate precipitation is less likely to occur under acidic conditions, while it tends to form more easily under alkaline conditions. I’m not sure if I misunderstood this

Standards of the People’s Republic of China ** GBl576—2001 Standards for Water Quality in Industrial Boilers Replaces GBl576—1996 1. Scope The standards for water quality in boilers specify the requirements regarding water quality during the operation of industrial boilers. The boiler water quality standards apply to fixed steam boilers and dual-purpose steam and water boilers that use water as the working medium, with a rated outlet steam pressure of 2.5 MPa or less; they also apply to fixed pressurized hot water boilers and atmospheric pressure hot water boilers that use water as the working medium. II. Boiler Water Quality Standards 1. For feed water in steam boilers and dual-purpose steam and water boilers, external chemical water treatment is generally required, and the water quality shall meet the specifications in Table 1. Table 1 Items Feed Water Boiler Water >1.0 >1.6 >1.0 >1.6 Rated steam pressure, MPa ≤1.0 ≤1.0 ≤1.6 ≤2.5 ≤1.6 ≤2.5 Suspended solids, mg/L ≤5 ≤5 ≤5 Total hardness, mmol/L ≤0.03 ≤0.03 ≤0.03 Without superheater 6-26 6-24 6-16 With superheater Total alkalinity, mmol/L ≤14 ≤12 pH (25°C) ≥7 ≥7 ≥7 10-12 10-12 10-12 Dissolved oxygen, mg/L ≤0.1 ≤0.1 ≤0.05 1) The basic unit for hardness in mmol/L is c(1/2Ca2+, 1/2Mg2+); the same applies hereafter. 2) The basic units for alkalinity in mmol/L are c(OH-), 1/2CO2-3, and HCO3-, the same applies hereafter. 　 For boilers that have low requirements regarding steam quality and do not come equipped with superheaters, the upper limit for the alkalinity value can be appropriately increased, provided that the operating unit obtains approval from the local boiler and pressure vessel safety supervision authority. 3) Deoxidization is required when the rated evaporation capacity of the boiler is 6 t/h or more. For boilers with a rated evaporation capacity of less than 6 t/h, deoxidization measures should be applied to the feedwater in case of localized corrosion; the oxygen content in the feedwater for boilers supplying steam to turbines should be less than or equal to 0.05 mg/L. 4) If it is difficult to determine the dissolved solids, conductivity or chloride (Cl-) measurement can be used for indirect control; however, the ratio relationship between dissolved solids and conductivity, or between dissolved solids and chloride (Cl-), should be determined through experiments. This ratio should also be rechecked and adjusted regularly. 5) The relative alkalinity of fully welded structure boilers can be left uncontrolled. 6) Only for fuel and gas boilers. 2. Steam boilers and dual-purpose steam and water boilers with a rated evaporation capacity of 2 t/h or less, and a rated steam pressure of 1.0 MPa or less (provided that there are no special requirements regarding the quality of steam and water) can also employ in-boiler chemical treatment. However, strict supervision must be exercised over scaling, corrosion, and water quality in the boiler; proper dosing, sludge removal, and cleaning procedures must be carried out, and the water quality must meet the requirements specified in Table 2. Table 2: Parameters for feedwater of water boilers – Suspended solids, mg/L: ≤20; Total hardness, mmol/l: ≤4; Total alkalinity, mmol/l: 8–26; pH (25°C): ≥7, 10–12; Dissolved solids, mg/L: <5000. For pressure-bearing hot water boilers, external water treatment is required. For non-frame-type pressure-bearing hot water boilers with a rated power of 4.2 MW or less, as well as atmospheric-pressure hot water boilers, internal chemical treatment can be used; however, close monitoring of scaling, corrosion, and water quality is necessary, and proper chemical treatment must be carried out, with the water quality meeting the requirements specified in Table 3. Table 3: Treatment methods – Chemical treatment inside the boiler vs. chemical treatment outside the boiler. For feed water and boiler water: Suspended solids, mg/L: ≤20, ≤5; Total hardness, mmol/L: ≤6, ≤0.6; pH (at 25°C): ≥7, 10–12, ≥7. Dissolved oxygen, mg/L: ≤0.1. Oil content, mg/L: ≤2, ≤2. 1) The pH value of the boiler water is maintained between 10 and 12 by adding chemicals. 2) The feed water for pressure hot water boilers with a rated power of 4.2 MW or more should be deoxygenated; for pressure hot water boilers with a rated power of less than 4.2 MW and for atmospheric pressure hot water boilers, the feed water should be deoxygenated as much as possible. 4. For DC (through-flow) boiler feed water, external chemical treatment should be employed, and the water quality shall comply with the standards specified in Table 1 for a rated steam pressure of greater than 1.6 Mpa and less than or equal to 2.5 Mpa. 5. The water quality specifications for waste heat boilers and electric heating boilers shall meet the requirements of boilers of the same type and with the same parameters. 6. The water quality testing methods shall be carried out in accordance with Appendix A (the appendix to the standards).

I hope there’s a reasonable explanation; waiting

Boiler feed water is slightly alkaline; this is because the deionized water used in boilers is usually slightly alkaline, and also because slightly alkaline water tends to form a thin layer of scale on metal surfaces at high temperatures, which serves to protect the boiler drums and furnace tubes. However, if the scale layer inside the boiler drum and furnace tubes is thick, mechanical or chemical cleaning is required to prevent the scale from falling off and blocking the furnace tubes, which could lead to tube rupture accidents. For reference! This post was last edited by clguan on 2009-4-1 16:27.]

Thank you. I’m not sure if it’s due to the action of hydrazine in removing oxygen, or perhaps its ability to form complexes with metals

Generally, the pH level should be kept above 7; if it is below 7, the acidity increases after the water is concentrated in the boiler, which can cause corrosion of metal pipes. Additionally, in alkaline conditions, calcium and magnesium ions form permanent precipitates with hydroxide ions, which are then removed from the boiler through waste discharge. This prevents the formation of carbonate deposits on the boiler tube walls, thereby avoiding localized overheating that could lead to explosions. For such technical issues, it’s best to seek help in the boiler-related forums

Since dissolved oxygen is inevitably present in boiler water (even after physical and chemical deoxygenation), and oxygen is a strong oxidizing agent whose oxidizing activity increases with rising temperature, and since boiler water operates at high temperatures, oxidation corrosion easily occurs in boilers. The products of oxidative corrosion in metal equipment are all alkaline, and an alkaline environment can reduce the occurrence of such corrosion. Therefore, the boiler feed water must be alkaline, with a pH level generally kept above 9; however, it should not be too high, as this can increase the brittleness of the equipment.

Everyone’s analysis is good; I’ve learned from it.

A dense protective gas envelope of iron oxide tetrahydrate that is formed in a slightly alkaline environment. It will dissolve if it is too high or too low.

Replying to the post on floor 4: The water quality standards have been updated; they are now in accordance with GB/T1576-2008, and the corresponding requirements have also been modified

Carbon steel is prone to corrosion in acidic conditions, while excessive alkalinity can lead to scaling. However, in the presence of dissolved oxygen, the higher the steam pressure, the greater the alkalinity required.

1. Weakly alkaline water often forms a protective film on metal surfaces. 2. If the pH is too low, hydrogen evolution occurs in the water, leading to a sharp increase in corrosion rate. 3. In alkaline solutions, hydrazine has strong reducing properties, making it effective for removing oxygen. (Mainly)

It is mainly to prevent scaling caused by calcium and magnesium ions

Because acidic water has a strong corrosive effect, when the pH value is below 7, ion concentration on the separation surface inside the boiler drum leads to an even lower pH value, which corrodes the drum and the blowdown pipes.