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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-67
For a multiple device installation, the probability of leakage for any one specific PRD does not increase.
However, since the number of devices increases, the probability of a leak and its associated consequences
does increase in proportion to the number of devices protecting the system.
Estimation of PRD Leakage Rate
An analysis of industry bench test data shows that approximately 8.4 % of the PRVs tested had some
leakage on the bench stand between 70 % and 90 % of their set pressure. An additional 6.6 % of the PRVs
tested leaked at pressures below 70 % of their set pressure. An additional 2.4 % of the tested PRVs leaked
significantly below 70 % of their set pressure. A summary of the estimated leakage rates used for the
consequence calculation is provided in Table 6.16.
As shown in Table 6.16, a leakage rate of 1 % of the PRD capacity (calculated at normal operating
conditions) is used for minor or moderate leaks. For a stuck open PRD, the leakage rate is assumed to be
25 % of the PRD capacity, as given in Equation (5.118).
Two leak cases are evaluated. The first case handles minor or moderate leakage, , and represents
90 % of all of the potential leakage cases, per Table 6.16. A stuck open case results in a leakage
so
consequence, C l , and makes up 10 % of all possible leakage cases.
For mild and moderate leaks, 1 % of the rated capacity of the PRD,
see Equation (5.117).
prd
W c
mild
C l
, is the basis for the leakage rate;
prd
lrate mild = 0. 01⋅ Wc
(5.117)
For the stuck open or spurious open case, the leakage rate is estimated per Equation (5.118).
prd
lrate so = 0. 25⋅ Wc
(5.118)
prd
W c
The rated capacity of the PRD, , can usually be found on the PRD datasheet. It can also be calculated
using the methods presented in API 520, Part 1 [8] .
Estimation of Leakage Duration
The leakage duration, D mild , is estimated for the mild or moderate leakage case as shown in Table 6.13. The
durations shown make the assumption that mild leakage from larger PRDs will be discovered sooner than
leakage from smaller PRDs. The stuck open category is handled differently. For the stuck open case, it is
assumed that immediate repair to the PRD is required and that the time to isolate the PRD will be within 30
minutes. Therefore,
30 min
D so = = 0.
021 days
60 min hr ⋅24 hr day
(5.119)
Credit for Recovery of Leaking Fluid
API 581 recognizes that the cost of lost inventory is not as severe when the unit has a flare recovery system
installed or the discharge from the relief devices is to a closed system. A recovery factor, F r , is applied to the
leakage consequence as defined by the following cases.
F = 0.
5 if the PRD discharges to flare and a flare recovery system is installed
r
F = 0.
0 if the PRD discharges to a closed system
r