Which property of boiler water can cause general waterside thinning of tubes when it is too low or too high for a long period?

The correct answer is alkalinity. Alkalinity is a critical property of boiler water that helps to buffer the water's pH and stabilize it within the appropriate range. When alkalinity levels are either too low or too high, this can lead to imbalances that affect the corrosive properties of the water. Specifically, low alkalinity can result in insufficient buffer capacity, which can lead to pH drops that promote corrosion. Conversely, excessively high alkalinity can lead to scaling and other deposit formations, which can create uneven water flow and localized overheating, ultimately contributing to thinning of the tubes.

Maintaining proper alkalinity in the boiler water is essential for preventing corrosion and ensuring the longevity and efficiency of the boiler system. Regular monitoring and adjustment of alkalinity, along with other chemical properties, are important practices for machinists and operators to ensure the safe and efficient operation of the boiler.