Which type of valve is located between the boiler and the water column and must be locked and sealed open?

The type of valve that is located between the boiler and the water column and must be locked and sealed open is a stop gate valve. This valve is specifically designed to isolate the boiler from other systems, allowing for maintenance or repair without affecting the operation of the boiler itself. When locked and sealed in an open position, it ensures a continuous flow of water to the boiler, which is crucial for maintaining proper water levels and preventing issues such as low water conditions that could lead to potential damage or safety hazards.

The stop gate valve's design allows for minimal pressure drop and smooth flow, making it suitable for such critical applications. The locking and sealing mechanisms prevent accidental closure, which could disrupt the operation of the boiler and introduce risks.

Understanding the application of each type of valve is important in boiler operations. For instance, the relief valve is designed to relieve excess pressure for safety, the ball valve is suitable for on-off control but may not always be appropriate for installation in the specific context described, and the check valve prevents backflow but does not serve the same purpose as the stop gate valve in this situation. This leads to the conclusion that the stop gate valve is the required component in this context.