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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-83
Initiating Event
Frequency
Equipment
Type
PRD DRRF
Qualifier
Overpressure
Potential
Background and
Comments
Overpressure Scenario—Liquid Overfilling
1 per 100 years
(admin controls)
1 per 10 years (w/o
admin controls
Multiply event
frequency times the
number of
applicable block
valves located in
process flow path
All equipment
including
process tower
(blocked
outlet of liquid
bottoms)
1.0
1.0
Downstream of
rotating equipment
other than positive
displacement type
Downstream of
positive
displacement type
rotating equipment
Deadhead pressure
or 1.3 times the
normal discharge
pressure or bubble
point pressure of the
feed stream at heat
source temperature
(for cases where the
equipment has
internal or external
heat sources),
whichever is greatest
Calculated burst
pressure or estimated
as design margin ×
MAWP
Most centrifugal rotating
equipment will
deadhead at 30 %
above the normal
operating point. Initiating
event frequency should
be adjusted if the
protected equipment is
removed from service
for maintenance or
operational needs (filter
replacement or cyclic
process operation) at a
frequency greater than
the unit turnaround
frequency.
Equipment with internal
or external heat sources
may have a significant
potential for
overpressure as a result
of vaporization of the
contained fluid stream.
Discharge pressure from
positive displacement
pumps will continue to
increase pressure.
Assumption is made that
rupture will occur.
1.0
Downstream of
steam turbines
Steam supply
pressure or bubble
point pressure of the
feed stream at steam
supply temperature
(for cases where the
equipment has
internal or external
heat sources),
whichever is greatest
1.0
Downstream of
process units or
vessels
1.1 × MAWP of
upstream pressure
source vessel