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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-57
6
η = = 17.
9
upd
( [ ]) 1
−ln 1−
01310 . .
18
(5.107)
The weighting factors assure a gradual shift from default POFOD data to field POFOD data and do
not allow the characteristic life to adjust upward too rapidly. They will, however, shorten
characteristic life if the device has repeated failures early in its service.
Other points that are not accounted for in the calculation procedure regarding inspection updating
are as follows.
i) Tests conducted at less than 1 year do not get credit.
ii) After a pass, the characteristic life cannot decrease. If the procedure yields a decrease in
characteristic life, this value should not be used. The characteristic life should be kept equal to
the previous value.
iii) After a fail, the characteristic life cannot increase. If the procedure yields an increase in
characteristic life, this value should not be used. The characteristic life should be kept equal to
the previous value.
7) Updating Failure Rates After Modification to the Design of the PRD
Design changes are often made to PRDs that improve the reliability of the device and result in a
change in the failure rate, for example upgrading to a corrosion-resistant material or installation of
an upstream rupture disk. Past inspection data are no longer applicable to the newly designed
installation. In these cases, either a new default curve should be selected per Figure 7.2 or devicespecific
Weibull parameters should be chosen based on owner–user experience, thus generating a
unique curve for the device.
j) Adjustment for Overpressures Higher Than Set Pressure
As discussed in Section 6.1.3, the POFOD curves are based on bench test data where a failure is
defined as any test requiring a pressure greater than 1.3 times the set pressure. Intuitively, one would
expect that at higher overpressures, the probability that the PRD would fail to open goes down
dramatically. A review of the industry failure data supports this. Figure 6.4 shows that as the
overpressure ratio increases, the PRD failure rate reduces significantly.
A conservative approach is to assume that the failure rate is cut by a factor of 5 at 4.0 times the set
pressure and to assume linear interpolation between 1.3 and 4.0 times the set pressure. A factor for
overpressure, F op,j , is introduced in Equation (5.108).
Po,j
F op, j = 10 . for < 1.
3
P
Po,j
F op, j = 0. 2 for > 4.
0
P
1 ⎛ Po,j
⎞
F op, j = 1− ⋅ 1.
3 for all other cases
3. 375
⎜ −
P
⎟
⎝ ⎠
set
set
set
(5.108)
The adjustment factor calculated above cannot be less than 0.2, nor greater than 1.0.