The American Society of Mechanical Engineers (ASME) exists to establish the rules and codes for the design, construction and inspection of pressure vessels, or equipment rated for greater than 15 pounds per square inch. Section I of the Boiler and Pressure Vessel Code provides requirements for power, electric and miniature boilers for use in all industries. A boiler is defined as a device that heats water or other liquids to produce steam or vapor for external uses. Other associated pressure vessels such as superheaters and economizers are also considered part of the Section I scope.
All manufacturers of boiler pressure vessels must be certified by the National Board of Boiler and Pressure Vessel Inspectors. All boilers and pressure vessels must bear the National Board stamp on the vessel nameplate. The boiler pressure vessel design calculations must be based on ASME code requirements and documented. The welding procedures, procedure qualifications and welding material identification must also be documented and included in the design package. Each welder used in the construction of a boiler pressure vessel must be certified and possess performance qualification documentation.
Materials of Construction
The materials of construction for a boiler pressure vessel must comply with approved ASME materials. The materials chosen must be chemically compatible with the boiler vessel service and be able to withstand the maximum generated pressures and temperatures. Metal alloy shells have tensile strength requirements based on wall thickness. These maximum allowable working pressure (MAWP) values are de-rated with increases in temperature. The choice of materials contributes to pressure and temperature design decisions.
Testing and Inspection
After the pressure vessel has been built, a certified third-party inspector must approve the final pressure test and document the findings. A licensed professional engineer must provide documentation stating that the pressure vessel has been constructed to standards set forth by the ASME Boiler and Pressure Vessel Code. Boiler pressure vessels must receive an annual internal inspection and, if possible, an external inspection to ensure the integrity of the shell. The testing should include nondestructive test methods such as a liquid penetrating test, magnetic particle test, gamma and X-ray radiography, and ultrasonic test.