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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-89
Table 6.13—Default Weibull Parameters for Probability of Leakage (POL)
Fluid
Severity
Conventional PRVs 1
Balanced Bellows
PRVs 1 Pilot-operated PRVs 2 Rupture Disks 3
β η def β η def β η def β η def
Mild 1.6 17.5 1.6 16.0 1.6 17.5 1.6 17.5
Moderate 1.6 15.5 1.6 14.0 1.6 15.5 1.6 17.5
Severe 1.6 13.1 1.6 11.5 1.6 13.1 1.6 17.5
NOTE 1
The ηdef parameter values are increased by 25 % for conventional and balanced PRVs that have soft seats.
NOTE 2 The ηdef parameter values for pilot-operated valves are currently based on the conventional PRV data, since there are
currently no failure rate data to support otherwise.
NOTE 3
Without any failure rate data for rupture disks, the conventional PRV values for Mild service were used.
Table 6.14—Potential Consequences of Pressure Vessel Overpressure
Accumulation
(% over MAWP)
10 %
16 %
21 %
50 %
90 %
300 %
Significance [2]
ASME code allowable accumulation for
process upset cases (non-fire) protected by a
single relief device
ASME code allowable accumulation for
process upset cases protected by multiple
relief devices
ASME code allowable accumulation for
external fire relief cases regardless of the
number of relief devices
ASME standard hydrostatic test pressure
(may be 30 % on new designs)
Minimum yield strength (dependent on
materials of construction)
Ultimate tensile strength (dependent on
materials of construction)
Potential Consequence
No expected consequence at this
accumulation level
No expected consequence at this
accumulation level
No expected consequence at this
accumulation level
Possible leaks in associated instrumentation,
etc. Medium consequence.
Catastrophic vessel rupture, remote
possibility. Significant leaks probable. Failure
of damaged vessel areas (corrosion, cracks,
blisters, etc.) likely. High consequence.
Catastrophic vessel rupture predicted. Highest
consequence.