Why are stays not normally required in watertube boilers?

In watertube boilers, stays are not typically necessary due to the design of the pressure parts, which lack flat surfaces. This design feature means that there are no large, flat areas for which stays would be required to maintain structural integrity under pressure. The pressure parts in watertube boilers are usually tubular and constructed to withstand high pressures without the need for additional support from stays.

The absence of flat surfaces eliminates the need for these supportive components because the rounded tubes can distribute stresses more evenly, maintaining the boiler's strength and structural integrity without additional bracing. In contrast, firetube boilers often have larger flat areas that can benefit from stays to prevent deformation and ensure safety.

The other options do not accurately reflect the primary reason for the absence of stays in watertube boilers. Access blockage, limitations of ASME codes regarding stays, and space constraints do not play significant roles in the engineering and design principles that govern watertube boilers.