The impact of boiler water salinity on steam moisture content

In my opinion, as the salt content increases, vapor-liquid azeotropy is likely to occur inside the furnace, making it difficult to separate steam from the furnace water, and thus causing the steam to contain water.

Steam containing salt can lead to a decline in quality, and scaling in the units that use steam at the back, especially in turbine blades, can cause accidents.

2) Impact of boiler water salinity. If the impurity content in the feed water is high, continuous evaporation in the drum, along with inadequate blowdown, will reduce the surface tension of the solution, causing the diameter of the bubbles formed during evaporation to decrease as well. As the salt concentration in the boiler water continues to increase, the viscosity of the liquid between adjacent bubbles rises, making it difficult for the bubbles to merge and grow larger. The bubbles become smaller, and their relative movement through the water slows down. At this point, the vapor content in the aqueous solution at the bottom of the drum increases, causing the water volume to expand and the height of the steam space to decrease. As a result, more water is carried in the steam. The smaller the diameter of the bubble, the higher the internal excess pressure, and the more water droplets are ejected when it bursts ； As the strength of the bubble liquid film increases, the bubbles can only burst when the liquid film is very thin. The thinner the liquid film, the smaller the droplets formed upon rupture, and the easier they are to be carried away by steam. Different refineries use various water treatment processes; reverse osmosis is commonly used as the primary treatment method, while mixed-bed (or anion/ cation exchange) systems are employed for secondary treatment. Some refineries opt for the more advanced EDI (electrodeionization) technique. The quality of the water produced varies from one refinery to another, and in some cases, the water supply does not even meet the required standards, namely a hardness level of less than 2.0 μmol/L. Furthermore, the amount of condensate water recovered by various refineries is increasing, with most of this condensate water remaining untreated. Theoretically, as long as the heat exchange equipment does not leak, the condensate water formed by the condensation of steam with higher purity has a better quality than desalinated water, and can achieve a theoretical level of purity – neutral water. However, due to production factors, the condensate from refining units is likely to become contaminated. However, condensate generally only has high levels of oil and iron, which has a significant impact on the safe operation of the boiler but little effect on the quality of steam. Figure 1 shows the relationship between the steam salt content Sq and the boiler water salt content Sls. Initially, there was a linear relationship between the two, indicating that as the salt content in the boiler water increased, the amount of water carried by the steam remained unchanged; it was only the increase in salt content in the water that led to an increase in salt content in the steam. However, when the salt content in the boiler water increases to a certain level, the salt content in the steam suddenly rises. The two are no longer in a linear relationship. The salt content in the boiler water is referred to as the critical salt content, Slslj. At this point, the height of the steam space in the drum decreases significantly, and the number of tiny water droplets in the steam space increases. Figure 1 Relationship between steam salinity and boiler water salinity (evaporation rate D1 > D2). The maximum allowable salinity level of the boiler water is determined by the operating pressure and the separation devices present in the drum. For steam generation equipment with a drum pressure of 4.4 Mpa and equipped with an internal cyclone separator, the salt content in the boiler water generally does not exceed 800–1200 mg/kg.

Thermochemical tests are conducted on the steam generation drum or boiler drum to determine the maximum salt content in the boiler water, and the parameters of the boiler water are kept at a level that is some distance away from the critical salt content.

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