Which system component is critical for safely monitoring a steam boiler?

The water-gauge glass is essential for safely monitoring a steam boiler because it provides a direct visual indication of the water level within the boiler. Maintaining the correct water level is crucial for safe operation; if the water level is too low, it can lead to overheating and potential damage or failure of the boiler. Conversely, if the water level is too high, it can cause issues with steam generation and pressure regulation.

By having a clear, accessible means to observe the water level, operators can take immediate action if they notice it is not within the desired range. This direct observation is vital in preventing accidents, ensuring the boiler operates efficiently, and prolonging the equipment's lifespan.

While other components like the pressure release valve, automatic fill valve, and digital control panel are important for the overall operation and safety of the steam boiler, they do not provide the same direct real-time monitoring of the water level as the water-gauge glass does. Each of these components contributes to system safety, but the water-gauge glass plays a unique and critical role in the immediate visual monitoring of one of the most vital aspects of boiler operation.