How are boilers classified on the basis of pressure ?

Pressure is a very important parameter in boilers.  The boiler and all the connected equipment are designed to withstand the pressure developed by the steam.  Pressure, is an important criterion to classify boilers.
Boilers are classified into

Low to medium pressure boilers
These boilers have an operating pressure of less than 10 bar.  Natural circulation is sufficient for these boilers.  Typical application are in industries.

High pressure boilers
High pressure boilers have an operating pressure of 10 to 14 bar.  They have forced circulation.

Super high pressure boilers
Super high pressure boiler are also used for utility applications.  The operating pressure is above high pressure boilers but generally lesser than 17 bar. 

Super critical boilers
Supercritical boilers have an operation pressure higher than 22.5 bar

Miniature Boilers
These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.