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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-49
API 581 provides estimates for the PRD initiating event frequencies, EF j , based on the various relief
overpressure demand cases that the device is providing protection. Examples of the initiating event
frequencies are provided in Table 5.2. Additional background on the default initiating event frequencies
is provided in Table 5.3.
b) Credit for Other Layers of Protection
It is recognized that the actual demand rate on a PRD is not necessarily equal to the initiating event
frequency. The concept of a demand rate reduction factor, DRRF j , is introduced here to account for the
difference in the overpressure demand case event frequency and the demand rate on the PRD.
Many pressure vessel installations include control systems, high integrity protective instrumentation,
shutdown systems, and other layers of protection to reduce the demand rate of a PRD. Credit can be
taken for operator intervention to reduce the probability of overpressure.
The DRRF j is used to account for these additional layers of protection. The DRRF j may be determined
rigorously for the installation as a result of a layer of protection analysis (LOPA) or use the estimated
value provided in Table 5.2.
Another example of where credit may be taken using the DRRF j is for the fire overpressure demand
case. A good estimate for the initiating event frequency of a fire on a specific pressure vessel is 1 every
250 years (0.004 events/year). However, due to many other factors, fire impingement from a pool
directly on a pressure vessel rarely causes the pressure in the vessel to rise significantly enough to
cause the PRD to open. Factors reducing the actual demand rate on the PRD include fire proofing,
availability of other escape paths for the process fluid, and fire-fighting efforts at the facility (to reduce
the likelihood of overpressure).
c) Calculation of Demand Rate
The demand rate (DR) on the PRD is calculated as the product of the initiating event frequency and the
DRRF j in accordance with Equation (5.90):
DR j = EF j ⋅ DRRF j
(5.90)
The subscript j in Equation (5.90) signifies that the demand rate on a PRD is calculated for each
applicable overpressure demand case.
Typically, a PRD protects equipment for several overpressure demand cases and each overpressure
case has a unique demand rate. Default initiating event frequencies for each of the overpressure cases
are provided in Table 5.2. Additional guidance on overpressure demand cases and pressure relieving
system design is provided in API 521 [2] . An overall demand rate on the PRD can be calculated in
Equation (5.91):
DR
total
ndc
= ∑ DR j
(5.91)
j=
1
If the relief design basis of the PRD installation has not been completed, the list of applicable
overpressure demand cases may not be available, and it may be more appropriate to use a simple
overall average value of the demand rate for a PRD. An overall demand rate for a particular PRD can
usually be estimated from past operating experience for the PRD.
d) Owner–User Experience