Watertube tubes are secured by:

Watertube tubes are secured by expanding and flaring because this method ensures a tight, leak-proof seal between the tubes and the header or drum in a watertube boiler system. When the tube is expanded, it is forced against the inner wall of the header, creating a friction fit that holds it firmly in place. Flaring the ends of the tube helps to further secure the connection by creating a larger surface area that enhances the joint's integrity and prevents any potential leaks. This method is particularly effective in high-pressure environments typical of watertube boilers, where reliability and safety are paramount.

In contrast, methods like welding, clamping, or bolting may not provide the same level of reliability or ease of maintenance when dealing with the high temperatures and pressures normally found in boiler applications. Expanding and flaring also allows for easier replacement of tubes if necessary, which is a significant consideration in the maintenance of boiler systems.