A boiler, which includes drums, tubes, flues, ducts, auxiliary equipment, and their associated supports, is subject to continual stress resulting from expansion and contraction and from elevated temperatures when it is in service. Boilers must be made adequately strong and with suitable materials to withstand these forces and temperatures. Boiler design requires a detailed study of the forces that are exerted on the various parts and of the temperatures to which they will be exposed. In addition, because of the corrosive properties of substances contained in fossil fuels and in water, attention must be given to the selection of the proper materials to ensure a high availability of the boiler system.

Pressure-vessel construction codes adopted by federal, state, and local jurisdictional authorities play an important part in determining safety requirements and thus the construction features of pressure vessels. The most widely used of these is the ASME Boiler and Pressure Vessel Code, published by the American Society of Mechanical Engineers. This code gives excellent guidance and basic requirements to the boiler designer.

This code sets rules of safety for the design, fabrication, and inspection of boilers, pressure vessels, and nuclear power plant components. The purpose of these rules is to assure that life, equipment, and property are safely protected during normal and abnormal service conditions. This requires a margin for safety for these service conditions, as well as the expected deterioration over the life of the plant.