Why are stays not typically needed in watertube boilers?

Stays are structural supports used to prevent distortion of boiler components under pressure. In the context of watertube boilers, the design and construction of the pressure parts significantly influence the necessity of stays. In watertube boilers, the pressure parts such as the water-filled tubes and headers have a cylindrical shape that inherently resists the forces that might cause distortion. This cylindrical geometry, in conjunction with the tube's ability to handle internal pressure, means that these elements do not have the flat surfaces—a characteristic of firetube boilers where stays are often necessary to maintain structural integrity.

In firetube boilers, flat surfaces can experience bending moments which may require additional support. However, since watertube boilers utilize a more complex arrangement of tubes that rely on the strength of the tubes and headers to withstand operational pressures, stays are unnecessary and can complicate the design. Therefore, the different structural characteristics of the pressure vessels in watertube boilers is the primary reason stays are not typically needed.