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

5-80 API RECOMMENDED PRACTICE 581
Initiating Event
Frequency
Equipment
Type
PRD DRRF
Qualifier
Overpressure
Potential
Background and
Comments
Overpressure Scenario—Control Valve Fail Close at Outlet
1 per 10 years [14]
for fail-closed
control valves
Exchangers,
fin fans,
reactors,
piping or
drums, or
rotating
equipment
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 per 50 years for
fail-open control
valves
Multiply event
frequency times the
# of applicable
control valves
located in process
flow path
1.0
1.0
Downstream of
steam turbines
Downstream of
process units or
vessels
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
1.1 × MAWP of
upstream vessel
source pressure
Process tower
with fired
heater heat
source
1.0
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. This
applies to the blocked
vapor outlet line only;
see liquid overfilling
case for blocked
liquid/bottoms outlet.
Process
tower, all
other heat
sources
1.0
Bubble point pressure
of the feed stream at
heat source
temperature
This applies to the
blocked vapor outlet line
only; see liquid
overfilling case for
blocked liquid/bottoms
outlet
Heaters 1.0
Calculated burst
pressure or estimated
as design margin ×
MAWP
Added increase in
potential overpressure
with fired/radiant heat
transfer. Assumption is
made that rupture
occurs.