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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-81
Initiating Event
Frequency
Equipment
Type
PRD DRRF
Qualifier
Overpressure
Potential
Background and
Comments
1 per 10 years [14] for
fail-closed control
valves
Overpressure Scenario—Control Valve Fail Open at Inlet, Including the HP/LP Gas Breakthrough Case
1 per 50 years for
fail-open control
valves
Multiply event
frequency times the
# of applicable
control valves
located in process
flow path
All equipment
types
1.0 N/A
Use the upstream
source pressure
Overpressure potential
is a function of the
pressure ratio across the
control valve
Overpressure Scenario—Runaway Chemical Reaction
1 per year All equipment 1.0
Calculated burst
pressure or estimated
as design margin ×
MAWP
This overpressure
scenario should be
based on a thorough
review of the wide
variety of potential
initiating events and
mitigation measures
associated with the
reactor system
installation.
The DRRF and the
potential overpressure
associated with failure of
PRD to open upon
demand should be
chosen based on a risk
assessment.
Per shell study, 50 % of
all vessel ruptures are
attributed to reactive
overpressure case.
Overpressure Scenario—Tube Rupture
1 per 1000 years
(9 × 10 -4 per
exchanger per [15] )
Exchangers—
HP gas in
tubes, LP
liquid in shell
1.0
Normal maximum
operating pressure of
the high-pressure side
of the exchanger
Likelihood of shell
rupture is increased
when high-pressure
tubeside gas enters lowpressure
shellside liquid
Overpressure Scenario—Tower P/A or Reflux Pump Failure
1.0
1 per 5 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
4.0 × MAWP (rupture)
Assumption is made that
rupture occurs
All other
process
towers
1.0
Bubble point pressure
of the feed stream at
heat source
temperature
Overpressure Scenario—Thermal/Hydraulic Expansion Relief