Water quality control issues in drum boilers
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I would like to ask the experts: Our plant has several waste heat steam drums with pressure ratings of 3.5 MPa, 1.7 MPa, and 0.3 MPa respectively. What are the different standards for controlling the quality of boiler water in steam drums with these various pressure ratings? I would also like to ask for your expert advice: In our plant’s boiler feedwater system, the recycled steam condensate is first subjected to oil and iron removal before being sent to a common deaerator for deoxygenation. After that, an organic amine solution is added to adjust the pH of the feedwater to between 8 and 9.5, and it is then supplied to various steam drums. The users of these steam drums add trisodium phosphate as well. The problem is that the phosphate ion concentration in the water inside these steam drums is extremely high. What effects does an excessive phosphate ion concentration have on water quality and equipment? What methods can be used to control the phosphate content in the boiler water, while ensuring that its pH level remains within the acceptable range?
A certain amount of phosphates should be maintained in the boiler water to prevent calcium and magnesium scaling. The phosphate level in boiler water should not be too low or too high; the excess amount of phosphate in the boiler water must be kept within specified limits. Control the amount of phosphate ions added to the boiler water, or increase wastewater discharge
High levels of phosphate ions can combine with calcium and magnesium ions to form phosphate scale. Once such scale forms, the heat transfer efficiency of the boiler is significantly reduced, and it is difficult to remove this phosphate scale; therefore, it is necessary to control the level of phosphate ions.
Sodium triphosphate is added to each boiler drum because it can react with soluble calcium and magnesium salts in water, which are prone to forming scale, to produce insoluble precipitates such as calcium phosphate Ca3(PO4)2 and magnesium phosphate Mg3(PO4)2 that remain suspended in the water, thereby preventing scale formation in the boiler. However, the dosage does not need to be too high; it should be determined based on the hardness of the boiler water through calculations (using the KSP of calcium phosphate, with a slight excess) ； Furthermore, since the pressure levels of different bubble boilers vary, the amount of water discharged regularly also differs; as a result, the hardness of the boiler water and the phosphate concentration also change. Therefore, for bubble boilers with different pressure levels, separate trisodium phosphate dosing systems should be installed to apply different amounts of this substance.
Thank you, the answer was very professional. I would like to ask again what “KSP of calcium phosphate” means? How can the dosage of trisodium phosphate be calculated based on the hardness index of the boiler water? Could you please explain in detail? Thank you!