Boiler design pressure

Perhaps the most widely accepted boiler and pressure code in the world is the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
This is an internationally recognized standard governing the design and construction of heating and power boilers and unfired pressure vessels.

Section IV—Heating Boilers:

The industry standard for high pressure vs. low pressure is 15 psi — boilers operating at 15 psi or lower are classified as low-pressure boilers.
Any boiler operating at a pressure higher than 15 psi is considered a high-pressure boiler.
Relationship between pressure and temperature.

This standard applies to steam boilers generating steam at more than 15 psi and hot water boilers generating hot water at temperatures over 250 F or pressures higher than 160 psi.
Section II—Materials: