Why are stays not normally required in watertube boilers?

The correct answer is that stays are not normally required in watertube boilers because the pressure parts have no flat surfaces. In watertube boilers, the design typically involves multiple tubes, where water circulates, and the pressure is maintained within these tubes rather than over flat surfaces like in firetube boilers. This tube configuration allows the structure to handle the internal pressure without the need for additional bracing or stays.

In contrast, firetube boilers, which have large, flat surfaces that can become deformed under pressure, often require stays to maintain structural integrity. Consequently, the absence of flat surfaces in watertube designs means that there are fewer structural concerns that would necessitate the use of stays.

Other options may refer to potential issues like space limitations or access problems, but these do not address the fundamental design differences that eliminate the need for stays in watertube boilers. Moreover, while ASME codes provide guidelines for the construction of boilers, the specific design features of watertube boilers inherently reduce the requirement for stays.