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Chemical Engineering Basics - Engineering

Q1:

The most detrimental impurity in high pressure boiler feed water is

A suspended salt

B dissolved salt

C silica

D turbidity

ANS:C - silica

Silica, or silicon dioxide (SiO2), is a common impurity found in water sources used for industrial processes, including boiler feed water. It can enter water sources through natural weathering of rocks and minerals, as well as from industrial pollutants. In high-pressure boiler systems, silica can be particularly problematic due to its tendency to form hard and adherent deposits on heat transfer surfaces. These deposits, often referred to as silica scale or silica fouling, can lead to several operational issues:

  1. Reduced Heat Transfer Efficiency: Silica deposits act as insulating layers on heat transfer surfaces, reducing the efficiency of heat exchange between the hot combustion gases and the boiler water. This reduction in heat transfer efficiency can lead to increased fuel consumption and decreased boiler performance.
  2. Boiler Tube Overheating: In severe cases, thick silica deposits can insulate boiler tubes to the extent that localized overheating occurs. This can result in tube failures, reduced boiler lifespan, and the potential for catastrophic accidents.
  3. Turbine Blade Erosion: Silica particles carried over into the steam can cause erosion of turbine blades, leading to reduced turbine efficiency and increased maintenance costs.
To mitigate the detrimental effects of silica in boiler systems, various strategies are employed, including:
  • Chemical Treatment: Chemical additives, such as phosphate-based compounds, can be added to the boiler feed water to inhibit silica scaling by forming soft, easily removable deposits.
  • Mechanical Filtration: Filtration systems can be used to remove suspended silica particles from the feed water before they enter the boiler system.
  • Blowdown Control: Regular blowdown of boiler water helps control silica concentration by removing concentrated boiler water containing dissolved and suspended impurities.
Overall, effective control of silica levels in boiler feed water is essential for ensuring the reliable and efficient operation of high-pressure boiler systems.