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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-51
b) Categories of Service Severity
The failure rates of PRDs are directly related to the severity of service in which they are installed.
Different categories of service are established in the PRD module as a function of the fluid tendency to
induce PRD failure due to corrosion, fouling, plugging, or other effects. Temperature has also been found
to be a factor in determining the severity of service. The categories of service severity (MILD,
MODERATE, or SEVERE) are linked to specific failure tendencies (and default Weibull cumulative failure
distribution curves) and are described in Table 6.5.
It is important to note that a fluid that is classified as being a MILD service group for the fail to open
failure mode is not necessarily a MILD service for the leakage failure mode. As an example, industry
failure data show that cooling water, which is known to be a dirty/scaling service, has one of the highest
failure rates for the fail to open case and therefore may be classified as SEVERE for the FAIL case.
Conversely, PRDs in cooling water service have not demonstrated a significant amount of leakage
failures and therefore may be classified as MILD for the leak case. Another example is steam, where
industry data indicate that steam should be classified as MILD for the fail to open case but classified as
SEVERE for the leak case. Steam is known to be a leaking service due to the erosive nature of the hightemperature
steam.
c) Default POFOD vs Time in Service
1) General
Table 6.6 provides the default Weibull parameters for failure to open for conventional spring-loaded
pressure-relief valves (PRVs), balanced bellows PRVs, pilot-operated PRVs, and rupture disks.
These parameters were determined using industry failure rate data. The majority of the available
data indicated successful performance during the interval that the PRD was in service. The
successful test points are referred to as suspensions and were included with the failure data in
determination of the Weibull parameters.
Weibull parameters are provided for the three categories of PRD service severity—MILD,
MODERATE, and SEVERE—as discussed in Section 6.2.4 b. These values, when substituted into
the Weibull cumulative failure density function, F(t), given by Equation (5.90), provide the default
POFOD curves for each of the PRD types listed in the table.
For example, Figure 6.2 provides the default Weibull cumulative failure distribution curves used for
spring-loaded conventional PRVs using the Weibull function to describe the three categories of
service severity.
Note that the units for the POFOD data presented in Figure 6.2 are failures/demand as these data
were established from bench tests of actual PRDs, not from continuous service data. POFOD
should not be confused with POF (failures per year) that includes the demands on the PRD (see
Section 6.2.3) and the probability that the protected equipment will fail (see Section 6.2.5).
The cumulative failure distribution curves shown in Figure 6.2 and the Weibull parameters
presented in Figure 6.6 are the default values based on the category of service severity of the PRD
being evaluated. These base values are defaults and should be overridden if the owner–user
provides site-specific data as explained in Section 6.2.4 c3.
Presence of an Upstream Rupture Disk
Rupture disks are often installed in combination with PRVs to isolate the valve from process
conditions and corrosive or fouling fluids that can reduce the probability that the valve will open
upon demand. API 520, Parts 1 and 2 provide additional information related to the use and
installation of rupture disks upstream of PRVs.