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

RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-37
5 Component Lining DF
5.1 Scope
A general approach for computing the DF for a component that has a protective internal lining is covered in
this section. It is common practice to construct a component with a material that is known to be subject to
damage in the operating environment, but to protect the material from the environment with a lining that is
resistant as described in Table 5.1. The lining types covered in this section are shown in Table 5.2.
A general approach for lined components involves assessing the severity of damage that would be expected to
occur on the base material, and then give credit for the existence of a lining. The degradation rate of the lining
itself is not addressed. Evaluations of lining effectiveness at preventing damage are based on expert opinion.
5.2 Screening Criteria
If the component has an inorganic or organic lining, then the component should be evaluated for lining
damage.
5.3 Required Data
The data required for analysis are given in Table 4.1, and the specific data required for determination of the
lining DF are shown in Table 5.3.
5.4 Basic Assumptions
All linings afford some degree of protection from the operating environment. Many linings will last for an
indefinite period of time, essentially being immune to damage mechanisms that may otherwise occur. Other
linings will slowly degrade with time and have a finite life. In such cases, the age of the lining (or the years
since the last inspection) becomes important in assigning a factor. Particularly in the case of organic linings,
the assumption is made that the lining is compatible with the environment, has operated within design
temperature limits (including steam out), and was properly applied with appropriate curing.
5.5 Determination of the DF
5.5.1 Overview
A flow chart of the steps required to determine the DF for linings is shown in Figure 5.1. The following
sections provide additional information and the calculation procedure.
The basic approach is that the type of lining and the age or years since last inspection determines a lining
failure factor. This is adjusted for a qualitative description of the lining condition. It is further adjusted based
on the probability of component failure upon lining failure (i.e. if the lining fails, does the component fail
rapidly, or will it be expected to last for a considerable time?). A final credit is made for on-line monitoring
that can provide early detection of a lining failure.
After a lining DF is computed, it is then compared to the thinning DF determined for the base material. The
minimum of the two values is used. The basis for this is that if the thinning damage is small compared to the
lining DF, then it does not yet matter if the lining has failed or not. This also provides a check that lining
failure is not necessarily equated with a component.