The difference between ANSI B 16.5 150 and ANSI B 16.5 1500 is the maximum allowable nonshock pressure rating at various temperatures. At any standard temperature (minus 20.0 to 100.0 degrees Fahrenheit) the maximum allowable pressure rating for the 150 pipe flange is 290 pounds per square inch gauge, and for the 1500 pipe flange, it is 3,750 pounds per square inch gauge.
The ANSI B 16.5 standards refer to standard dimensions, tolerances and materials for circular steel flanges for pipes, valves, fittings and their accessories. The number listed after the ANSI B 16.5 indicates the strength of the pipe flange. Flanges of the B 16.5 class come in strengths from 150 to 2500. The strength rating refers to the ability of the flange to withstand internal pressure before bursting. The strength ratings are given at various temperatures because the flanges become weaker as the temperature rises above 100 degrees Fahrenheit.
The American National Standards Institute promotes the use of consensus standards in a wide range of industries. ANSI standards are voluntary but have been widely adopted by many industries to ensure consistency across manufacturers. The American Society of Mechanical Engineers certifies manufacturers as complying with ASME standards. For many applications, ANSI and ASME are used interchangeably, namely ASME B 16.5 150 means the same thing as ANSI B 16.5 150.