If boiler drain lines are intended to be used both as drains and as blow-off, ______________ valves are required.

The requirement of two valves for boiler drain lines intended to function as both drains and blow-off is based on the need to ensure safe and effective operation. In such systems, a minimum of two valves is mandated to provide a barrier for backflow and to facilitate maintenance without affecting the entire system.

The first valve is typically positioned close to the boiler, allowing for immediate control when draining or blow-off is necessary. This allows operators to release water, steam, or sediments from the boiler safely. The second valve acts as a secondary safety measure, ensuring that there is a reliable means to isolate the boiler from the drain line.

This configuration not only enhances safety for the personnel operating the system but also ensures the protection of the equipment by preventing potential damage from improper drainage or backflow. Additionally, this setup allows for more efficient operations, as it enables the separation of functions—draining and blow-off—so that one can occur without inhibiting the other. Thus, two valves are essential to comply with both safety regulations and operational efficiency in boiler systems.