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

2-156 API RECOMMENDED PRACTICE 581
23.10 Figures
STEP 1: Determine the evaluation temperature:
• Normal operating temperature
• Shutdown temperature
• Upset temperature
STEP 2: Determine the estimated Sigma from:
• Comparisons with other materials
• Metallographic examination
STEP 3: Determine the Damage Factor using
Table 23.3.
Figure 23.1—Determination of the Sigma Phase Embrittlement DF
24 Piping Mechanical Fatigue DF
24.1 Scope
The DF calculation for piping components subject to mechanical fatigue is covered in this section.
24.2 Description of Damage
Fatigue failures of piping systems present a very real hazard under certain conditions. Properly designed and
installed piping systems should not be subject to such failures, but prediction of vibration in piping systems at
the design stage is very difficult, especially if there are mechanical sources of cyclic stresses such as
reciprocating pumps and compressors. In addition, even if a piping systems are not subject to mechanical
fatigue in the as-built condition, changing conditions such as failure of pipe supports, increased vibration
from out of balance machinery, chattering of relief valves during process upsets, changes in flow and
pressure cycles, or adding weight to unsupported branch connections (pendulum effect) can render a piping
system susceptible to failure. Awareness of these influences incorporated into a management of change
program can reduce the POF.
24.3 Screening Criteria
If both of the following are true, then the component should be evaluated for susceptibility to mechanical
fatigue.
a) The component is pipe.
b) There have been past fatigue failures in this piping system or there is visible/audible shaking in this piping
system or there is a source of cyclic vibration within approximately 15.24 m (50 ft) and connected to the
piping (directly or indirectly via structure). Shaking and source of shaking can be continuous or intermittent.
Transient conditions often cause intermittent vibration.
24.4 Required Data
The basic component data required for analysis are given in Table 4.1, and the specific data required for
determination of the DF for mechanical fatigue are provided in Table 24.1.