3.1.2 A brittle fracture assessment may be required as part of the assessment procedure when there's need for a brittle fracture assessment as listed below:-
a) A change in process operating conditions, including startup, shutdown and upset conditions, that increase the possibility of low metal temperatures.
b) A process hazards review indicates that process temperatures, including those during start up, shutdown and upset conditions, could be lower than anticipated in the original design.
c) The equipment item is rerated using a lower design margin.
d) A minimum temperature is needed for a hydro test.
e) The equipment is expected to be exposed to a general primary membrane tensile stress greater than 55MPa (8 ksi) at or near ambient temperature and either of the following is true:
3.1.3
The Critical Exposure Temperature (CET ) as used in this Part is defined as the lowest (coldest) metal temperature derived from either the operating or atmospheric conditions at the maximum credible coincident combination of pressure and supplemental loads that result in primary stresses greater than 55 MPa (8 ksi).Note that operating conditions include startup, shutdown, upset and standby conditions. The CET may be a single temperature at the maximum credible coincident combination of pressure and primary supplemental loads if that is also the lowest (coldest) metal temperature for all other combinations of pressure and primary supplemental loads. If lower (colder) temperatures at lower pressures and supplemental loads are credible, the CET should be defined by an envelope of temperatures and pressures..
3.1.4 The Minimum Allowable Temperature (MAT ) is the lowest (coldest) permissible metal temperature for a given material and thickness based on its resistance to brittle fracture. It may be a single temperature, or an envelope of allowable operating temperatures as a function of pressure. The MAT is derived from mechanical design information, materials specifications, and/or materials data using the guidance in this Part