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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-77
Overpressure Demand Case
Table 6.2—Default Initiating Event Frequencies
Event Frequency
EF j
(events/year)
DRRF j
(See Notes 2
and 3)
Reference
1. Fire 1 per 250 years 0.0040 0.10 [12]
2. Loss of cooling water utility 1 per 10 years 0.10 1.0 [12]
3. Electrical power supply failure 1 per 12.5 years 0.080 1.0 [12]
4a. Blocked discharge with
administrative controls in place
(see Note 1)
4b. Blocked discharge without
administrative controls (see Note 1)
5. Control valve failure, initiating event
is same direction as CV normal fail
position (i.e. fail safe)
6. Control valve failure, initiating event
is opposite direction as CV normal
fail position (i.e. fail opposite)
1 per 100 years 0.010 1.0 [13]
1 per 10 years 0.10 1.0 [13]
1 per 10 years 0.10 1.0 [14]
1 per 50 years 0.020 1.0 [14]
7. Runaway chemical reaction 1 per year 1.0 1.0
8. Heat exchanger tube rupture 1 per 1000 years 0.0010 1.0 [15]
9. Tower P/A or reflux pump failures 1 per 5 years 0.2 1.0
10a. Thermal relief with administrative
controls in place (see Note 1)
10b. Thermal relief without administrative
controls (see Note 1)
1 per 100 years 0.010 1.0
1 per 10 years 0.10 1.0
Assumed same as
blocked discharge
Assumed same as
blocked discharge
11a. Liquid overfilling with administrative
controls in place (see Note 1)
11b. Liquid overfilling without
administrative controls (see Note 1)
1 per 100 years 0.010 0.10 [12]
1 per 10 years 0. 10 0.10 [12]
NOTE 1 Administrative controls for isolation valves are procedures intended to ensure that personnel actions do not compromise the
overpressure protection of the equipment.
NOTE 2 The DRRF recognizes the fact that demand rate on the PRD is often less than the initiating event frequency. As an example,
PRDs rarely lift during a fire since the time to overpressure may be quite long and firefighting efforts are usually taken to minimize
overpressure.
NOTE 3 The DRRF can also be used to take credit for other layers of overpressure protection such as control and trip systems that
reduce the likelihood of reaching PRD set pressure.
NOTE 4 Where the Item Number has a subpart (such as “a” or “b”), this clarifies that the overpressure demand case will be on same
subpart of Table 5.3.