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

1-16 API RECOMMENDED PRACTICE 581
4 Basic Concepts
4.1 Probability of Failure (POF)
4.1.1 Overview
Two methods of calculating POF are used within the text: the GFF method and a two-parameter Weibull
distribution method. The GFF method is used to predict loss of containment POF from pressure boundary
equipment. The Weibull distribution method is used to predict POF for PRDs and heat exchanger bundles.
4.1.2 GFF Method
4.1.2.1 General
The POF using the GFF method is calculated from Equation (1.1).
( ) ( )
P t gff F D t
f = ⋅ MS ⋅ f
(1.1)
The POF as a function of time, P f (t), is determined as the product of a generic failure frequency, gff, a
damage factor, D f (t), and a management systems factor, F MS .
4.1.2.2 GFF
The GFF for different component types is set at a value representative of the refining and petrochemical
industry’s failure data (see Part 2, Section 3.3).
4.1.2.3 Management Systems Factor
The management systems factor, F MS , is an adjustment factor that accounts for the influence of the facility’s
management system on the mechanical integrity of the plant equipment. This factor accounts for the
probability that accumulating damage that may result in a loss of containment will be discovered prior to the
occurrence. The factor is also indicative of the quality of a facility’s mechanical integrity and PSM programs.
This factor is derived from the results of an evaluation of facility or operating unit management systems that
affect plant risk. The management systems evaluation is provided in Part 2, Annex 2.A of this document.
4.1.2.4 Damage Factors (DFs)
The DF is determined based on the applicable damage mechanisms relevant to the materials of construction
and the process service, the physical condition of the component, and the inspection techniques used to
quantify damage. The DF modifies the industry GFF and makes it specific to the component under
evaluation.
DFs do not provide a definitive FFS assessment of the component. FFS analyses for pressurized component
are covered by API 579-1/ASME FFS-1 [1] . The basic function of the DF is to statistically evaluate the amount
of damage that may be present as a function of time in service and the effectiveness of the inspection activity
to quantify that damage.
Methods for determining DFs are provided in Part 2 for the following damage mechanisms:
a) thinning (both general and local);
b) component lining damage;
c) external damage (thinning and cracking);
d) stress corrosion cracking (SCC);