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

5-48 API RECOMMENDED PRACTICE 581
Although consequences associated with PRD overpressure cases are greater than those associated with the
fixed equipment operating at normal pressure, this is tempered by the fact that the use of realistic PRD
demand rates and accurate PRD failure rate data results in a low frequency of occurrence.
PRD POF (FAIL)
Definition
For a PRD, it is important that the definition of failure be understood, since it is different than failure of other
equipment types. A PRD failure is defined as failure to open during emergency situations causing an
overpressure situation in the protected equipment, resulting in loss of containment (failures/year). Leakage
through a PRD is also a failure. This type of failure is discussed in Section 6.3.
Calculation of POF to Open
The fundamental calculation applied to PRDs for the fail to open case is the product of an estimated
overpressure demand case frequency (or demand rate), the probability of the PRD failing to open on
demand, and the probability that the protected equipment at higher overpressures will lose containment.
A PRD protects equipment components from multiple overpressure scenarios. Guidance on overpressure
demand cases and pressure relieving system design is provided in API 521 [2] . Each of these scenarios (fire,
blocked discharge, etc.) can result in a different overpressure, P o,j , if the PRD were to fail to open upon
demand. Additionally, each overpressure scenario has its own demand rate, DR j . Demand cases are
discussed in more detail in Section 6.4.3, Table 6.2, and Table 6.3. The expression for POF for a PRD for a
particular overpressure demand case is as shown in Equation (5.89).
prd
f ,j
P P DR P
= fod,j ⋅ j ⋅ f ,j
(5.89)
prd
The subscript j in the above equation indicates that the POF for the PRD, P
f ,j
, needs to be calculated for
each of the applicable overpressure demand cases associated with the PRD.
The POF (loss of containment) of the equipment component that is protected by the PRD, P f,j , is a function
of time and the potential overpressure. API 581 recognizes that there is an increase in probability of loss of
containment from the protected equipment due to the elevated overpressure if the PRD fails to open during
an emergency event.
Each of the terms that make up the POF for the PRD shown in Equation (5.89) is discussed in greater detail
in the following sections.
a) Section 6.2.3—PRD Demand Rate, DR j .
b) Section 6.2.4—PRD POFOD, P fod,j .
c) Section 6.2.5—POF of Protected Equipment as a Result of Overpressure, P f,j .
PRD Demand Rate
The first step in evaluating the probability of a PRD failure is to determine the demand rate (demands or
events/year) placed on the device.
a) Default Initiating Event Frequencies