API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2-154 API RECOMMENDED PRACTICE 581
23.6 Determination of the DF
23.6.1 Overview
A flow chart of the steps required to determine the DF for sigma phase embrittlement is shown in Figure
23.1. The following sections provide additional information and the calculation procedure.
23.6.2 Inspection Effectiveness
For this damage mechanism, credit is not given for inspection. However, the results of metallurgical testing
can be used to update the inputs to the DF calculation that may result in a change in this value.
23.6.3 Calculation of the DF
The following procedure may be used to determine the DF for sigma phase embrittlement; see Figure 23.1.
a) STEP 1—Determine the evaluation temperature T min . The material may be evaluated at normal
operating conditions or at a lower temperature such as shutdown or upset temperature.
b) STEP 2—Determine the estimated % sigma in the material. This can be made through comparisons
with materials in similar service or via metallographic examination of a sample.
c) STEP 3—Determine the DF,
sigma from STEP 2.
23.7 Nomenclature
sigma
D
f
is the DF for sigma phase embrittlement
sigma
D
f , using Table 23.3 based on T min from STEP 1 and the estimated %
T min
is the minimum temperature, °C (°F)
23.8 References
See References [66], [67], [68], [69], [70], [72], [73], [74], [75], [76], and [77], in Section 2.2.
23.9 Tables
Table 23.1—Data Required for Determination of the DF—Sigma Phase Embrittlement
Required Data
Administrative controls for upset
management (Yes/No)
Minimum operating temperature under
normal, start-up/shutdown, or upset
conditions, °C (°F)
Amount of sigma
Comments
Are there controls and or awareness training to prevent the coincident
occurrence of low temperatures (upset) at or near design pressures?
This may be the temperature below which the operating pressure is reduced
for purposes of fracture control. If not known, the temperature should be set to
the atmospheric boiling point of the fluid in the component if the fluid is a
liquid.
Estimate of the amount of sigma phase present.
Low (>1 %, <5 %)
Medium (≥5 %, <10 %)
High (≥10 %)