When securing a boiler, as steam pressure drops to 10-15 psi, which of the following should be open?

Opening the boiler vent when steam pressure drops to 10-15 psi is crucial for several reasons. As the pressure within the boiler decreases, venting allows for the safe release of any remaining steam and prevents the buildup of vacuum pressure. This process is essential to maintain the integrity of the boiler and avoid potential structural issues caused by negative pressure.

Additionally, keeping the vent open helps to prevent the formation of a vacuum that could impede the safe operation of the boiler. This is particularly important in maintaining a safe environment around the boiler system and ensuring that it can be adequately serviced or turned off without risk of damage.

The other options do not serve this specific function or may not be appropriate for this pressure condition. For example, the safety valve is designed to open automatically to relieve excess pressure, but it operates primarily when the pressure is too high, not necessarily when it's dropping. The drum outlet and feedwater inlet are involved in different operational aspects of the boiler and are not intended for managing pressure loss in the same way that venting is.