Normal or anticipated standard internal pressure difference, i.e. internal minus external pressure, to be experienced by the pipe or piping system which should not exceed the design pressure.
Source: ISO 14692-1:2017, Petroleum and natural gas industries — Glass-reinforced plastics (GRP) piping — Part 1: Vocabulary, symbols, applications and materials, Second Edition, August 2017. Global Standards
Pressure the process system experiences during normal operation, including normal variations.
Source: API STD 521, Pressure-relieving and Depressuring Systems, Sixth Edition, January 2014. Global Standards
Source: API STD 521, Pressure-relieving and Depressuring Systems, Fifth Edition, January 2007 (Addendum May 2008). Global Standards