These standards/specifications exist to ensure that certain types of valves are designed in a consistent method meeting certain requirements ensuring that, regardless of brand, the general valve characteristics will remain the same.
ASME - American Society of Mechanical Engineers. Founded in 1880 this organization works to ensure standards are met/improved to ensure safety and efficiency in engineering practices. www.asme.org
ANSI - American National Standards Institute www.ansi.org
Click here for more information on common valve ASME/ANSI Standards
API Standards for Typical Valve Requirements
API - American Petroleum Institute www.api.org
Click here for more information on common valve API standards
More information on other standards including common MSS, CSA, ISO, NACE, GOST standards can be found here
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If someone quotes you 12-14 weeks they probably mean 15 weeks.