What are the process conditions for EDTA boiler cleaning?

Thread Content

I. Determination of EDTA concentration The selection of the EDTA concentration is crucial for optimizing the EDTA-based furnace cleaning technique; an inappropriate concentration can directly affect the effectiveness of cleaning and passivation. The EDTA concentration is generally calculated theoretically based on the amount of scale in the tube sample, and then 1.5 is added to maintain a minimum residual concentration in the cleaning solution. However, due to the poor representativeness of the tube samples, it is difficult for this approach to reflect actual cleaning conditions, so multiple verifications are necessary. Theoretical calculations can be done first, followed by empirical verification, and finally confirmed through small-scale tests. Practice has shown that this method can eliminate theoretical calculation errors. II. Selection of the pH value for cleaning: At different pH values, EDTA exists in solution mainly in the forms of H4Y, H3Y-, H2Y2-, HY3-, and Y4-. When the pH is between 5.5 and 8.0, it exists in the forms of H2Y2- and HY3-, and at this time it has a strong capacity to chelate iron ions ； When the pH is between 8.0 and 9.0, it exists in the forms of HY3- and Y4-, and the passivation effect of the solution is best at this time. Therefore, the appropriate pH range for controlling EDTA furnace cleaning is 5.5 to 9.0. III. Selection of cleaning temperature: At room temperature, EDTA forms complexes with iron oxide at a slow rate; the higher the temperature, the faster this complexation occurs. However, the corrosion-inhibiting efficiency of most corrosion inhibitors decreases as the temperature rises. Therefore, to achieve an optimal cleaning effect, it is necessary to select the appropriate temperature – the temperature that yields the best cleaning results and also the temperature at which EDTA begins to decompose. As shown in the figure on the left, within the lower temperature range, the corrosion inhibitor exhibits a better corrosion-inhibiting effect. For example, in aqueous solutions at 20–80°C, sodium benzoate provides good corrosion inhibition for carbon steel; however, in boiling water, it can no longer prevent the corrosion of carbon steel. This is because the effectiveness of sodium benzoate requires the presence of dissolved oxygen, and the amount of dissolved oxygen in boiling water is very low, which affects the corrosion-inhibiting effect of benzoic acid ； Furthermore, bubbles in boiling water may destroy the protective film formed by iron and sodium benzoate. As the temperature rises, the adsorption capacity of the corrosion inhibitor decreases, thereby accelerating metal corrosion. Therefore, as the temperature rises, the corrosion inhibition rate decreases significantly. Therefore, during cleaning, the boiler is ignited to maintain the cleaning temperature at 110–135°C. This serves both to heat and to maintain the thermodynamic cycle. EDTA decomposes when the temperature is above 150°C. IV. Selection of corrosion inhibitors: Due to the high cleaning temperature using EDTA and the wide range of pH values in the cleaning solution, conventional corrosion inhibitors are not suitable for this purpose. Traditionally, corrosion inhibitors based on composite formulations containing MBT have been used, but practice has shown that such inhibitors have many drawbacks, such as poor solubility – they must first be dissolved in concentrated NaOH – which results in complicated preparation steps and prolongs the time required for formulating the solution ； After the cleaning was completed, an inspection of the cut tubes revealed a small amount of yellow crystals attached to the tube walls ； During the medication preparation process, solids precipitate and tend to accumulate in the medication mixing box ； When the temperature of the cleaning solution is below 120°C, it remains turbid. Practice has shown that imidazoline-based cathodic corrosion inhibitors offer satisfactory performance. Imidazoline appears as a brownish-black viscous liquid; it is insoluble in water but soluble in ethanol and propanol. For example, the TPRI-6 type corrosion inhibitor is a corrosion inhibitor formulated from imidazoline and other components. Due to the shared electron pair in imidazolines, they exhibit a strong adsorption effect on metal surfaces, forming a dense protective film. Adding a small amount of TPRI-6 type corrosion inhibitor to the cleaning medium is sufficient to protect the metal. Furthermore, organic corrosion inhibitors with composite formulations such as XJ-225, urethophene, YHH-1, Lan826, CM-991, etc., are all good options to use when performing EDTA cleaning. V. Cleaning time: An excessive cleaning time can lead to the formation of noticeable coarse metal crystals on the metal surface, as well as secondary rust formation. It also results in an uneven protective layer, thereby increasing corrosion of the boiler’s metal surface – this is what is commonly referred to as \"over-cleaning\"” ； If the cleaning time is too short, the deposits in the cleaning system cannot be removed easily, resulting in insufficient cleaning performance. Therefore, the cleaning time must be strictly controlled, and chemical monitoring of the cleaning solution should be carried out promptly until the iron content in the solution no longer increases significantly, indicating that the scale samples in the pipeline section have been cleaned. VI. Cleaning flow rate: The cleaning flow rate should not be too high or too low. An excessively high cleaning flow rate can accelerate the dissolution of sediments, but the corrosion-inhibiting effect of the inhibitor decreases as the flow rate increases ； At the same time, the diffusion rate of various impurities in the cleaning agent (such as Fe3+) increases, which in turn accelerates the corrosion rate. If the cleaning flow rate is too low, it is not possible to ensure uniform flow of the cleaning solution throughout all parts of the cleaning system, which in turn affects the cleaning efficiency ； It is also possible for cleaning product buildup or \"air pockets\" to occur in certain areas that are being cleaned; this not only prevents effective cleaning of those areas, but it also makes it difficult to rinse away the waste liquid resulting from the cleaning process.