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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-45
6 Pressure-Relief Devices (PRDs)
General
Overview
The major concern with PRDs and the main reason that routine PRD inspection and testing is required is that
the device may fail to relieve overpressure events that can cause failure of the equipment protected by the
device, leading to a loss of containment. There are also consequences associated with leakage of PRDs.
A risk-based approach to evaluating PRD criticality to set inspection/testing frequency is covered in this
section. Included in the scope are all spring-loaded and pilot-operated relief valves and rupture disks.
Additional PRD types, such as AST pressure/vacuum vents (P/Vs) and explosion hatches, may be analyzed
provided reliability data in the form of Weibull parameters exist for the PRD type being considered.
It is not the intention of the methodology for the user to perform or check PRD design or capacity
calculations. It is assumed that the owner–user has completed due diligence and the devices have been
designed in accordance with API 521 [7] and sized, selected, and installed in accordance with API 520 [12] . It
is also assumed that minimum inspection practices in accordance with API 576 [9] are in place.
The fundamental approach is to use a demand rate for the device combined with a probability of failure on
demand (POFOD) determined from plant-specific data if available or starting with default data. These inputs are
used to generate a POF as a function of time via a Weibull statistical approach. The consequence of device
failure is determined based on methods outlined in Part 3, but modified to include overpressure demand cases,
the amount of expected overpressure upon PRD failure, and the additional consequences associated with
device leakage. The combination of consequence with a time-based POF results in a risk value that increases
with time between tests. This allows test intervals to be determined based on risk targets.
The flow chart shown in Figure 5.1 illustrates the basic methodology required for the determination of a RBI
inspection and test schedule. The basic data required for the evaluation are listed in Table 5.1.
PRD Interdependence with Fixed Equipment
The risk of a PRD is directly related to the equipment that is protected by the PRD. The consequence
analysis is performed as described in Part 3 for the protected equipment at an elevated pressure
representing the estimated overpressure upon PRD failure to open upon demand.
Failure Modes
There are several failure modes of significance when evaluating the risks associated with PRD failure. For
the PRD, the failure modes are grouped into two categories.
a) Fails to open as designed (FAIL):
⎯ stuck or fails to open (FTO),
⎯ device partially open (DPO),
⎯ opens above set pressure (OASP).
b) Leakage failure (LEAK):