How are tubes secured in a tube sheet in a water tube boiler?

In water tube boilers, tubes are typically secured in a tube sheet through a process known as rolling. This involves rolling the ends of the tubes into the holes of the tube sheet, creating a tight fit that prevents leaks. The flaring of the tubes adds an additional method to enhance the seal by expanding the end of the tube slightly, allowing for a better mechanical connection with the tube sheet.

This method is preferred as it allows for thermal expansion of the tubes while maintaining a secure connection. Moreover, the rolled and flared technique is effective due to the materials used in boiler construction, which need to withstand high pressures and temperatures. The other options presented do not accurately describe the standard practices used in securing tubes within a tube sheet in water tube boilers. Hence, rolled and flared is recognized as the correct method.