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

5-78 API RECOMMENDED PRACTICE 581
Table 6.3—Overpressure Scenario Logic
Initiating Event
Frequency
Equipment
Type
PRD DRRF
Qualifier
Overpressure
Potential
Background and
Comments
Overpressure Scenario—Fire
— Modified by
industry data that
indicate demand
rates on the order
of 1 per 400 years
1 per 250 years
See Lees [13] page
A7-7, states major
fire at plant 1 every
10 years
All equipment
types
0.1
Industry
experience
justifies this
value
N/A
Calculated burst
pressure or estimated
as design margin ×
MAWP
— The DRRF factor of
0.1 recognizes the
industry experience
that relatively few
vessels exposed to
a fire will
experience a PRD
opening
— Assumption is
made that in those
rare cases where a
PRD would open
during a fire,
rupture will occur if
the PRD failed to
open upon demand
Overpressure Scenario—Loss of Cooling
1.0
1 per 10 years
Process tower
with fired
heater heat
source
Consider LOPA
or risk reduction
analysis
associated with
loss of flow
controls on the
fired heater
Heat source to tower
is a fired heater
Calculated burst
pressure or estimated
as design margin ×
MAWP
Assumption is made that
rupture occurs
All other
equipment
with internal
or external
heat sources
1.0
Bubble point pressure
of the feed stream at
heat source
temperature
Overpressure Scenario—Electrical Power Failure
1.0
0.08 per year (1 per
12.5 years) power
supply failure per
table on page 9/30
of [13]
Process tower
with fired
heater heat
source
Process tower
and other
equipment
with internal
or external
(non-fired)
heat sources
Consider LOPA
or risk reduction
analysis
associated with
loss of flow
controls on the
fired heater
1.0
Heat source to tower
is a fired heater
Calculated burst
pressure or estimated
as design margin ×
MAWP
Bubble point pressure
of the feed stream at
heat source
temperature
Assumption is made that
rupture occurs