What kind of surfaces require staying in a firetube boiler?

In a firetube boiler, the surfaces that require staying are typically flat surfaces. This is due to the structural requirements and the stresses experienced by the boiler components during operation. Flat surfaces, such as those found in various components like manways and access panels, need additional support to ensure they can withstand the pressure exerted by the water and steam.

Flat surfaces, compared to other shapes, may be more prone to deformation under pressure without proper reinforcement. The staying process involves the addition of supports or stays to maintain the integrity and shape of these surfaces under the operational conditions of the boiler.

Curved surfaces, such as those found in cylindrical sections, may inherently provide more strength and resistance to pressure due to their geometric shape, which efficiently distributes the pressure. Ellipsoidal surfaces also share these beneficial structural properties. Therefore, staying would not be as critical for curved or ellipsoidal surfaces compared to flat surfaces where additional reinforcement is necessary.

The mention of surfaces that are "not covered in water" refers to those parts of the boiler that may not have the same structural requirements since they do not experience the same internal pressure exerted by water and steam.