The lowest visible level of water in a boiler must be at least how many inches above the top row of tubes?

The correct answer is three inches above the top row of tubes in a boiler. This requirement is crucial for safe operation and efficient performance of the boiler. Maintaining the water level at this height ensures that the tubes are adequately covered with water during operation, which is vital for proper heat transfer and to prevent overheating or damage to the tubes.

If the water level falls below this minimum threshold, there is a risk of exposing the tubes to steam or a dry fire condition, resulting in overheating and potentially leading to tube failure or boiler explosion. The three-inch margin acts as a safety buffer, allowing for fluctuations in water level due to steam generation and any variations in system operation.

Understanding this requirement helps operators ensure that the boiler is functioning safely and efficiently, mitigating risks associated with inadequate water levels in the system.